Nucleic Acid Probes for the Detection and Identification of Fungi

The inventions described and claimed herein relate to the design and composition of two nucleic acid probes capable of detecting many different fungal organisms in clinical, food, environmental and other samples. The inventions described and claimed herein also relate to the design and composition of 21 probes capable of specifically detecting and identifying Aspergillus fumigatus, Blastomyces dermatitidis, Candida albicans, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, Aspergillus flavus, Aspergillus glaucus, Aspergillus niger, Aspergillus terreus, Candida glabrata, Candida guilliermondii, Candida kerr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Pseudallescheria boydii, and Sporothrix schenckii in clinical, food, environmental and other samples.

Fungi are eukaryotic microorganisms that are universally distributed. While in nature fungi play a major role in the decomposition of plant materials, they are also responsible for spoilage of food, beverage and pharmaceutical preparations. Out of an estimated 100,000 species of fungi described by mycologists, approximately 150 species are pathogenic to man and animals. The increasing incidence of AIDS and the development of newer treatments for hematologic malignancies and organ transplants has lead to an increase in the number of immunocompromised patients. These patients have a high risk of developing fungal infections, which if not rapidly diagnosed and treated are capable of causing death in a matter of days. The number of antifungal drugs is limited and their toxic side effects on the patient are much higher than that of comparable antibacterial therapy. A rapid diagnosis of fungal infection and start of treatment is critical in these patients. Books by Kwon-Chung and Bennett, along with Sarosi and Davies, provide an overview into the medical importance of fungi.

Fungal organisms are identified by morphology and nutritional characteristics. Fungi may take anywhere from two days to several weeks to grow in culture and often the same organism can take radically different forms depending on the growth conditions. This makes timely identification difficult even for the classically trained expert and impedes the treatment of patients where rapid identification of genus and species is of medical advantage.

The incidence and distribution of major pathogenic fungi varies by geographic location. Aspergillus fumigatus, Blastomyces dermatitidis, Candida albicans, Coccidiodes immitis, Cryptococcus neoformans, Histoplaisma capsulatum, Paracoccidioides brasiliensis, Pseudallescheria boydii and Sporothrix schenkii represent some of the leading causes of mycotic infections.

Aspergillus fumigatus is among the top three causes of systemic fungal infection treated in hospitals. It usually affects patients with organ transplants, acute leukemias and bums and can be rapidly fatal if not diagnosed quickly. With over 150 species of Aspergillus present in the soil, air and water, accurate detection of Aspergillus fumigatus becomes extremely important. Aspergillus clavatus, Aspergillus flavus, Aspergillus fumigatus, Aspergillus glaucus, Aspergillus nidulans, Aspergillus niger, Aspergillus ochraceus, Aspergillus terreus, Aspergillus unguis and Aspergillus ustus represent a majority of Aspergillus species seen in clinical specimens and their presence can cause diagnostic difficulties. Aspergillus flavus, Aspergillus fumigatus and Aspergillus niger have been linked with disease in humans, with Aspergillus fumigatus being the predominant pathogen in North America. A few immunologic tests exist for Aspergillus fumigatus but these have limited sensitivity and specificity. There are also reports of development of polymerase chain reaction based tests for Aspergillus fumigatus based on the amplification of the Asp fl antigen gene and a ribosomal intergenic spacer (Spreadbury et. al.). The Spreadbury technique is based on the PCR amplification of a 401 bp fragment spanning the large subunit rRNA/mtergenic spacer region. This relies on a pair of primers to specifically amplify DNA from Aspergillus fumigatus only, and is of no utility in identifying other fungi.

Blastomyces dermatitidis is present in the soil, usually in bird droppings and animal feces. Infections often occur at construction sites and the ensuing lung infiltration and pneumonitis are usually fatal in immunocompromised patients. Diagnosis by culture may take weeks, and the organism is occasionally mistaken for other fungi. Existing immunological diagnostic tests are unreliable, and there is a need for rapid and reliable DNA based diagnostic tests. Similarly, Histoplasma capsulatum exists in the soil and is known to have infected at least 20% of the population of North America. Most infections start in the lung and resolve spontaneously, but may occasionally spread to other organs. AIDS patients represent a growing number of cases of Histoplasmosis. Diagnosis is difficult as immunological tests are often negative during the first 4-6 weeks of infection. Coccidioides immitis is found in abundance in the soil in Southwestern United States. Dust storms, farming, building construction, earthquakes and even hiking have been linked with outbreaks of disease. Lung infection followed by cavitation and disseminated miliary coccidioidomycosis are seen. Meningitis is usually lethal, and as with other fungi, mortality is highest in debilitated hosts. Four serotypes of Cryptococcus neoformans cause disease in humans. The incidence of this disease is growing rapidly, with up to 10% of HIV infected people developing cryptococcosis. DNA probes capable of detecting all 4 serotypes are required for the early diagnosis and treatment for life threatening infections like cryptococcal meningitis. A report by Stockman et. al. discusses commercial tests for Histopkama, Blastomyces, Coccidioides, and Cryptococcus based on the 18S rRNA (Gen-Probe, Inc., San Diego, Calif.). The authors report sensitivities ranging from 87.8 to 100% and a specificity of 100%. One drawback of these probes is that these are used on rRNA extracted from fungal cultures. As some fungi may require up to 3 weeks to grow in culture, this technique cannot be used to expedite diagnosis until a culture becomes available.

Candida albicans is one of the most common causes of fungal infection in humans. It is present in the respiratory, gastrointestinal and female genital tract of healthy individuals, and acts as an opportunistic pathogen in debilitated individuals on steroid or chemotherapy. Diabetes mellitus and indwelling catheters are other predisposing causes. Immunocompromised hosts show rapid hematogenous spread of fungi. Morbidity and mortality in untreated cases is high. Candida glabrata, Candida guilliernondii, Candida ken, Candida krusei, Candida lusitaniae, Candidaparapsilosis and Candida tropicalis are also known to cause disease in humans. DNA probes capable of identifying these individual species would eliminate the need for multiple blood cultures and lengthy biochemical speciation.

Recent advances in molecular techniques have led to the approach of microbe detection and identification based upon the DNA sequence of ribosomal genes. Commonly used detection techniques include either direct amplification of the ribosomal DNA (rDNA) genes by the polymerase chain reaction, or reverse transcription of the ribosomal RNA (rRNA) into complementary DNA (cDNA) followed by polymerase chain reaction amplification of the cDNA. Ribosomes are composites of unique rRNA and protein species that function in the translation of messenger RNA into protein. Evolutionary studies are consistent with the interpretation that all extant life has evolved from a single organism. Thus, all cellular organisms contain rRNA and these rRNAs are related by evolution. The evolutionary process is such that each species of organism appears to have unique regions of sequence in its ribosomal genes. The presence of these unique species specific regions allows one to design DNA probes that under conditions of hybridization will specifically bind to, and identify the polymerase chain reaction amplified DNA from only one species of fungus. For the purposes of this application, the word "primer" is used to mean a nucleotide sequence which can be extended by template-directed polymerization, and "probe" is used to mean a nucleotide sequence capable of detecting its complementary sequence by hybridization. Also, for the purpose of this application, the phrase "nucleotide sequence" is intended to include either DNA or RNA forms or modification thereof. Furthermore, those versed in the art will recognize that primer sequences can be used as probes and vice versa. The use of nucleic acid hybridization to detect specific nucleic acid sequences of interest is also described by Kohne (U.S. Pat. No. 4,851,330, 7/1989).

In prokaryotes and eukaryotes, ribosomal RNA and the corresponding rDNA genes are identified by the size of the RNA. The sizes are related in terms of sedimentation velocity or S values. Thus, for prokaryotes the values are 5S, 16S, and 23S; and for eukaryotes the values are 5S, 5.8S, 18S and 28S. Because all ribosomes perform the same function which is essential for cell viability, ribosomal sequences are largely conserved, yet certain regions of each ribosomal species are subject to more variation without consequence to function. It is these hypervariable regions that allow one to identify different species amongst members of the same genus. As noted in the referenes, there are several reports where 5S, 18S and the intergenic spacer between 5.8S and 28S rDNA have been used for the detection and identification of fungi (Holmes et. al., Hopfer et. al., Lott et. al., Maiwald et. al., Makimura et. al., Mitchell et. al., Nakamura et. al.). Holmes et. al. describe a PCR test based on the co-amplification of the 5S rDNA and an adjacent nontranscribed spacer region. This identifies only Candida albicans and detects other Candida species without identifying individual organisms. Hopfer et. al. and Maiwald et. al. both use universal primers to amplify 18S rDNA from several fungi including Candida sp., Aspergillus fumigatus, Cryptococcus neoformans and Trichosporon sp. These amplicons are digested with restriction enzymes and the cut fragments are sized by gel electrophoresis. This restriction fragment length polymorphism pattern enables them to identify most but not all organisms. This technique can be used on amplified DNA from a pure fungal culture. As clinical samples such as sputum usually contain multiple fungal organisms, this technique has little utility in diagnosis as multiple overlapping fragments obtained from a mix of fungi would be nearly impossible to interpret. Lott et. al. use the 5.8S RNA and the internal transcribed spacer (ITS2) to identify and speciate Candida albicans and related Candida species. Makimura amplifies a 687 bp fragment from the 18S rDNA of 25 medically important fungi and uses these in the diagnosis of Candida albicans in clinical samples. Mitchell uses nested PCR to amplify 5.8S and internal transcribed spacer (ITS) to identify Cryptococcus neoformans. No subsequent testing is done to verify the identity of the amplified DNA. Nakamura et. al. use 18S primers to detect Aspergillus fumigatus infections of the lung. Most protocols given in these references can only be used to detect an extremely limited number of fungi from a clinical specimen. Hopfer et. al. and Maiwald et. al. can identify multiple organisms from pure cultures, but their utility for clinical specimens containing multiple fungal species is limited at best.

United States patents have been issued to Weisburg et. al. for probes developed for the detection of 18S small subunit ribosomal RNA sequences in fungi. These probes will detect fungi from many species, but cannot be used easily to identify any single species. United States patents have also been issued to Milliman for probes developed for the specific detection of the bacteria Staphylococcus aureus based on the 23S ribosomal sequence (U.S. Pat. No. 5,292,874) and Histoplasma capsulatum based on the 18S ribosomal sequence (U.S. Pat. No. 5,352,579). Hogan et. al. (European Pat. App. 0,272,009) describe one fungal probe for 18S rRNA and three fungal probes for 28S rRNA sequences. None of the 28S probes described by Hogan et. al. is related to any of the probes described in our invention. All probes claimed in our invention can be mapped within the first 900 base pairs of a 28S gene. The probes described by Hogan et. al. are located further 3' on the 28S sequence, between base pairs 1000 and 2000 (these numbers are comparable to the primary sequence of Saccharomyces cerevisiae 28S rRNA gene. Genbank accession number: J01355). Leclerc et. al. have published reports analyzing the phylogenetic relationship between fungi based on partial DNA sequences of several fungal 28S genes sequenced by them. Some of the organisms claimed to have been sequenced by Leclerc are the same as some organisms sequenced by us. These are Sporothrix schenckii, Pseudallescheria boydii, Blastomyces dermatitidis, Histoplasma capsulatum and Chrysosporium sp. Leclerc et. al. have not published any sequence data in their report, and to the best of our knowledge, these sequences have not been made available in the GenBank. The reverse-complement sequence of their sequencing primer 401 (TCCCTTTCAA CAATTTCACG) overlaps our SEQ ID NO: 1 (GTGAAATTGT TGAAAGGGAA) by 19 nucleotides and their sequencing primer 636 (GGTCCGTGTT TCAAGACGG) overlaps our SEQ ID NO: 2 (GACTCCTTGG TCCGTGTT) by 10 nucleotides. We are aware of no reports in the literature of variable regions from 28S rRNA genes of fungi being used as targets for the development of species specific diagnostic probes.

As discussed above, most present techniques for the molecular detection of fungi rely on the use of highly specific primers for the PCR amplification of only one fungal species. Those that employ "Universal" primers for a PCR amplification of DNA from multiple organisms, use post-PCR amplicon identification techniques that are useful only on pure cultures of fungi. These are not able to identify fungi from a clinical specimen containing multiple fungal organisms. Our first aim was to develop "Universal" primers for the 28S gene. These primers would be capable of amplifying in a PCR, 28S rDNA from most fungi. Our subsequent aim was to develop species specific probes for fungi of interest, that would be used to analyze our "Universal" 28S amplicon. These species specific probes would be able to detect the presence of fungi of interest even in situations containing mixed fungal species.

One aspect of this invention is to provide nucleic acid primers capable of detecting 28S sequences from DNA or RNA of most fungi. These would be used as "Universal" primers in a polymerase chain reaction to amplify 28S sequences from any fungus present in clinical, food, environmental or other samples.

A second aspect of this invention is to provide nucleic acid probes capable of detecting and identifying, by nucleic acid hybridization, the pathogens Aspergillus fumigatus, Blastomyces dermatitidis, Candida albicans, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, Aspergillus flavus, Aspergillus glaucus, Aspergillus niger, Aspergillus terreus, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Pseudallescheria boydii, and Sporothrix schenckii by use of any of several different formats.

Nucleic acid probes and primers are described for detecting fungi that cause disease in humans and animals, as well as spoilage of food and beverages. These probes can detect rRNA, rDNA or polymerase chain reaction products from a majority of fungi in clinical, environmental or food samples. Nucleic acid hybridization assay probes specific for Aspergillus fumigatus, Blastomyces dermatitidis, Candida albicans, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, Aspergillus flavus, Aspergillus glaucus, Aspergillus niger, Aspergillus terreus, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Pseudallescheria boydii, and Sporothrix schenckii are also described.

FIG. 1 represents the relative position of the sequences described on the 28S subunit of fungi.

Our first objective was to develop nucleic acid primers for use in a polymerase chain reaction to amplify 28S genes from all fungi likely to be present in a clinical sample. This amplified DNA would then be amenable to probing with several different species specific probes. Each one of these species specific probes would, under conditions of hybridization, anneal to 28S ribosomal DNA from only one species of fungus, thereby detecting and identifying the species of fungus present in the clinical sample. The 28S gene was selected as a target because it had regions that were conserved among fungi and these would provide potential annealing sites for "universal" fungal probes. The ribosomal 28S genes were also expected to have hypervariable regions that would be unique enough to provide sites for species specific probes. The large rRNA gene is called the 23S rRNA gene in prokaryotes and 28S in eukaryotes. This designation is based on the length and therefore the sedimentation coefficient of these rRNA molecules. Fungal large subunit rRNAs vary in size among different organisms and are often referred to as being 25S, 26S or 28S. Since fungi are eukaryotes, and to maintain uniformity in this application, we shall refer to fungal large subunit rRNA as 28S rRNA.

Published sequences from Cryptococcus neoformans, two Candida albicans, Saccharomyces cerevisiae and two Schizosaccharomyces pombe 28S genes are approximately 3.5 kilobases in length (Genbank accession numbers: L14068, L28817, X70659, J01355, Z19136 & Z19578). These four sequences were aligned, and a region of sequence variability was found clustered between coordinates 200 and 700 from the 5' end of these genes. As an initial starting point, two nucleic acid primers P1 (ATCAATAAGC GGAGGAAAAG) SEQ ID NO:24 and P2 (CTCTGGCTTC ACCCTATTC) SEQ ID NO:25 (see FIG. 1), capable of hybridizing to all 4 of the above mentioned organisms and not to human 28S sequences (GenBank accession number: M11167), were designed and used under low stringency hybridization conditions in a polymerase chain reaction to amplify approximately 800 base pairs of DNA spanning this hypervariable region from the following 34 fungi that were obtained from the Mayo Clinic fungal collection: Acremonium sp., Aspergillus clavatus, Aspergillus fumigatus, Aspergillus glaucus, Aspergillus nidulans, Aspergillus niger, Aspergillus ochraceus, Aspergillus terreus, Aspergillus unguis, Aspergillus ustus, Beauvaria sp., Bipolaris sp., Blastomyces dermatitidis, Candida albicans, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candidaparapsilosis, Candida tropicalis, Chrysosporium sp., Cladosporium sp., Coccidioides immitis, Cryptococcus neoformans var. neoformans, Curvularia sp., Geotrichum sp., Histoplasma capsulatum, Mucor sp., Penicillium sp., Pseudallescheria boydii, Saccharomyces cerevisiae, Sporothrix schenkii and Trichosporon beigelii.

DNA was extracted from the fungi listed above by the following method. A loopful of fungal culture was scraped off a culture plate using a sterile inoculation loop. The fungus was added one milliliter of sterile water in a 1.5 ml Sarsted (Newton, N.C.) screw cap microcentrifuge tube. This tube was placed in a boiling water bath for 20 minutes in order to lyse the fungus and release DNA from the cells. Two microliters of this whole cell lysate was used in a PCR to amplify 28S rDNA. All PCR amplifications were carried out as hot-start reactions in a 50 ul reaction volume using Perkin-Elmer (Norwalk, Conn.) 0.5 ml thin-wall polypropylene tubes and a Perkin-Elmer thermal cycler. Reagents added to the tube initially were 2.5 ul of 10.times.PCR buffer (100 mM tris pH 8.3, 500 mM KCl, 15 mM MgCl.sub.2), 5.0 ul of 50% glycerol/1 mM cresol red, 8.0 ul of dNTP mix (1.25 mM each of dATP, dGTP, dTTP and dCTP), 12 picomoles of each nucleic acid primer and sterile water to make up a volume of 25 ul. A wax bead (Ampliwax Gem-100, Perkin-Elmer) was added and the tubes heated to 77.degree. C. for 1 minute and cooled to room temperature to form a wax barrier. 2.5 ul of 10.times.PCR buffer, 5.0 ul of 50% glycerol/1 mM cresol red, 0.2 ul Taq polymerase (AmpliTaq 5 U/ul, Perkin-Elmer) and 15.3 ul of sterile water was added to the tube along with 2.0 ul of DNA from the fungal whole cell lysate described above. 50 cycles of thermal cycling was carried out at 94.degree. C.--30 sec, 40.degree. C.--1 min, 72.degree. C.--2 min. The amplified DNA was purified from a low melt agarose gel by tris buffered phenol pH 8.0, phenol/chloroformlisoamyl alcohol (25:24:1 by vol.) and 3 ether extractions, followed by isopropanol precipitation and 70% ethanol wash.

We completely sequenced both strands of DNA amplified from the organisms listed above. All sequencing was carried out on an Applied Biosystems 373A sequencer. Every nucleotide in the sequences generated was verified and confirmed by examining the complementary nucleotide from the second strand sequence. We had now created a novel database consisting of nucleic acid sequences spanning a variable region of the 28S rDNA from a diverse collection of medically important fungi.

While the complete sequences for Candida albicans, Cryptococcus neoformans and Saccharomyces cerevisiae 28S genes had previously been published and deposited in GenBank, it was not obvious, nor had it been defined, whether any regions of sequence identity among these three organisms would also be conserved among all fungi of interest. DNA sequences from all the fungi in our novel 28S database had to be analyzed in order to develop "Universal" 28S probes. All sequences were subjected to extensive manipulation to identify optimal relative alignments in order to identify regions of similarity for use as "Universal" probes. The selected probe sequences had to meet several important criteria besides the condition of being present in 28S genes from most fungal species. Each probe sequence required an appropriate thermal profile, secondary structure and utility in a DNA amplification reaction. These probes were optimized to work for PCR amplification in pure cultures of fungus, as well as in the presence of DNA from multiple sources as in the case of clinical specimens. The probes were also designed to facilitate direct sequencing of the amplified DNA. Our analysis led to the discovery of the oligonucleotide probes listed in (SEQ ID NO: 1) and (SEQ ID NO:2). (For their location, see FIG. 1.) The successful identification of these two probes ((SEQ ID NO: 1) and (SEQ ID NO:2)) completed our first objective to develop nucleic acid probes that would hybridize to, and detect 28S rRNA and rDNA from a majority of fungi (FIG. 1 and Table 1). As shown later in this application, the novel sequence information generated by the use of our "Universal" probes allowed us to develop species-specific probes ((SEQ ID NO:3) to (SEQ ID NO:23)) capable of identifying 19 different disease-causing fungi.

Probes SEQ ID NO: 1 and SEQ ID NO: 2 were used to successfully amplify (Table 2) and sequence DNA spanning this variable region from the following 49 organisms: Acremonium sp., Aspergillus clavatus, Aspergillus flavus, Aspergillus fumigatus, Aspergillus glaucus, Aspergillus nidulans, Aspergillus niger, Aspergillus ochraceus, Aspergillus sp., Aspergillus terreus, Aspergillus unguis, Aspergillus ustus, Beauvaria sp., Bipolaris sp., Blastomyces dermatitidis, Blastoschizomyces sp., Candida albicans, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Chrysosporium sp., Cladosporium sp., Coccidioides immitis, Cryptococcus neoformans var. neoformans, Cryptococcus neoformans var. gattii, Cryptococcus terreus, Cryptococcus laurentii, Curvularia sp., Filobasidiella neoformans, Filobasidium capsuligenum, Filobasidium uniguttulatum, Fusarium sp., Geotrichum sp., Histoplasma capsulatum, Malbranchea sp., Mucor sp., Paecilomyces sp., Penicillium sp., Pseudallescheria boydii, Rhizopus sp., Saccharomyces cerevisiae, Scopulariopsis brevicaulis, Scopulariopsis brumptii, Sporothrix schenkii and Trichosporon beigelii. This sequence information generated by the use of probes SEQ ID NO: 1 and SEQ ID NO: 2 expanded the size of our database consisting of fungal 28S sequences. All amplified DNA was sequenced across both strands from a minimum of two different isolates of each organism to ensure accuracy of the data generated.

This list of fungi sequenced by us represents organisms responsible for most cases of subcutaneous and deep mycotic infections in humans and also includes saprophytes (non-pathogenic fungi) commonly encountered in clinical isolates. Since the two probes (SEQ ID NO: 1 and SEQ ID NO: 2) hybridize to 28S rDNA from all the fungi listed above, they are capable of diagnosing the presence of a majority of fungi that are likely to be present in a clinical specimen. They are believed to be primers for universally detecting fungi.

Probes listed in SEQ ID NO: 1 and SEQ ID NO: 2 were also checked for their potential ability to hybridize to, and amplify (in a polymerase chain reaction) 23 S sequences from bacteria by searching for hybridization sites among the 539 bacterial 23S genes listed in GenBank. Bacterial 23S rDNAs do not have suitable hybridization sites for SEQ ID NO: 1 and SEQ ID NO: 2 and these two probes should not be able to amplify bacterial DNA under stringent conditions.

Our second objective was to develop species specific probes, which under hybridization conditions, would detect Aspergillus fumigatus, Blastomyces dermatitidis, Candida albicans, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, Aspergillus flavus, Aspergillus glaucus, Aspergillus niger, Aspergillus terreus, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Pseudallescheria boydii, and Sporothrix schenckii. We used our database of fungal 28S nucleic acid sequences to create a multiple sequence alignment of all the organisms that we had sequenced. Every individual sequence was subjected to intensive comparison with all other sequences in our database in order to discover unique regions of sequence that would be present only in the fungus of interest, and would be absent in all other fungi. When unique stretches of sequence were identified, these were further analyzed for thermal profile and secondary structure. Each probe constructed by us will, under conditions of hybridization, specifically hybridize to and detect, nucleic acid sequence from the unique region of only one specific target fungus. Those versed in the art will recognize that specification of a single-stranded DNA sequence implies the utility of the complementary DNA sequence, as well as the two equivalent RNA sequences. Furthermore, sequences incorporating modification of any of the moieties comprising the nucleic acid (i.e., the base, the sugar or the backbone) are functional equivalents of the sequence. It should also be recognized that these additional sequences can potentially serve as probes or primers.

The nucleic acid sequences for these species specific synthetic probes are listed in SEQ ID NO: 3 to SEQ ID NO: 23. There are two probes specific for Cryptococcus neoformans, two probes specific for Sporothrix schenckii, and one probe each for Aspergillus fumigatus, Blastomyces dermatitidis, Candida albicans, Coccidioides immitis, Histoplasma capsulatum, Aspergillus flavus, Aspergillus glaucus, Aspergillus niger, Aspergillus terreus, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida Armsei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis and Pseudallescheria boydii 28S rRNA and rDNA. (See Tables 3-6 and further discussion below.)

All species specific probes developed by us are novel and to the best of our knowledge have not been reported in the literature. While all 28S genes sequenced by us had several regions that were different among the various species analyzed, the regions that would function best as species specific probes under conditions of hybridization were not obvious. Extensive analysis of each 28S sequence yielded several potential probe sites. These were studied in detail to enable the selection of optimal unique sites for each probe, based on the need to obtain optimal hybridization characteristics under the test conditions. The highly specific hybridization characteristics of all probe sequences developed by us were then validated by experimental results. The prior existence in GenBank of sequences for Candida albicans and Cryptococcus neoformans 28S genes was in itself not sufficient to enable even an individual versed in this field to develop specific probes for either of these two organisms. We had to obtain novel 28S sequence from Candida albicans, Candida glabrata, Candida guilliernondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Cryptococcus neoformans var. neoformans, Cryptococcus neoformans var. gatti, Cryptococcus terreus, Cryptococcus laurentii, Filobasidiella neoformans, Filobasidium capsuligenum and Filobasidium uniguttulatum before we were able to identify potential regions for the development of species specific probes for these two fungal organisms that would not cross react with the others listed above.

Our modification of the Chomczynski technique (see Example 2, below) allows us to obtain DNA from any clinical specimen, irrespective of source (see Table 8 for a variety of clinical specimens tested), within a 3 hour period. The PCR amplification and subsequent probing can be accomplished with ease within a 24 hour period. The final identification is therefore possible in a day as opposed to several days or weeks required by traditional methods. This speed and sensitivity of diagnosis can make a difference between life and death in debilitated patients battling fungal diseases of undetermined cause. Rapid diagnosis will allow physicians to immediately direct their therapy towards curing the identified causative fungus, rather than wait for days or weeks while the patient succumbs to an unknown fungus.

Our probes have the ability to pick out the correct target organism even in a mixed fungal infection because of their high level of specificity. The methods of Hopfer et. al. and Maiwald et. al., do not allow identification of individual species in a mixed fungal infection because restriction fragment length polymorphism results are nearly impossible to interpret when multiple organisms contribute to the restriction fragments. Their method can therefore only be used on a pure culture, and this also does not save any diagnostic time, because the fungus first has to be grown in culture.

The probes developed by us allow rapid species identification of a large number of pathogenic fungi by using multiple probes against only one PCR amplified fragment of DNA. Coupled with our modified DNA extraction technique and our ability to accurately diagnose in the case of mixed organisms, this strategy can provide the greatest amount of diagnostic information in the shortest amount of time. This diagnostic strategy is also amenable to automation, which can result in even greater savings in time, money and effort.

The sequences and the complement of the sequences claimed in this disclosure, along with any modifications to these sequences, may potentially be utilized in assays for the identification of fungi based on several existing methodologies, as well as future improvements and alterations of this technology. These techniques include, but are not limited to, assays based on hybridization, ligation, polymerization, depolymerization, sequencing, chemical degradation, enzymatic digestion, electrophoresis, chromatography and amplification. Furthermore, all such variations ultimately are based in some selection or amplification process, some ligand or some nucleic acid moiety that recognizes or utilizes the sequences (SEQ ID NO: 1) to (SEQ ID NO:23) claimed in this application. Such variations include but are not limited to use of a variety of linear or exponential target amplification schemes, such as, any of the myriad forms of PCR, the ligase chain reaction, Q-beta repliase, etc.; direct detection of species-specific nucleic acid purified or extracted from pure fungal culture using a probe selected from the group (SEQ ID NO: 3) to (SEQ ID NO: 23); use of the complementary DNA forms of (SEQ ID NO: 1) to (SEQ ID NO:23); use of the RNA forms of these sequences and their complements; and use of derivatives of these DNA or RNA sequences by the addition of one or more moieties from a variety of labels including nucleic acid sequences, proteins, signal generating ligands such as acridinium esters, and/or paramagnetic particles. These techniques may be utilized with DNA, RNA or modified derivatives used as either the target or the detection molecule.

Further variations of the invention that utilize any of the named sequences will be apparent to those with ordinary skill in the art. The following examples illustrate various aspects of the invention but are not intended to limit its usefulness.

Testing probes SEQ ID NO: 3 to SEQ ID NO: 23 for hybridization specificity.

Probes listed in SEQ ID NO: 3 to SEQ ID NO: 23 were tested for specificity against their target organisms. Probe SEQ ID NO: 5 for Candida albicans was the first one tested against a panel of fungi taken from the Mayo Clinic collection. 28S rDNA from Acremonium sp., Aspergillus clavatus, Aspergillus flavus, Aspergillus fumigatus, Aspergillus glaucus, Aspergillus nidulans, Aspergillus niger, Aspergillus ochraceus, Aspergillus terreus, Aspergillus unguis, Aspergillus ustus, Aspergillus sp., Beauvaria sp., Bipolaris sp., Blastomyces dermatitidis, Candida albicans, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Chrysosporium sp., Cladosporium sp., Coccidioides immitis, Cryptococcus neoformans var. neoformans, Curvularia sp., Fusarium sp., Geotrichum sp., Histoplasmna capsulatum, Mucor sp., Penicillium sp., Pseudallescheria boydii, Rhizopus sp., Saccharomyces cerevisiae, Scopulariopsis breviacaulis, Sprothrix schenkii and Trichosporon beigelii was amplified in a polymerase chain reaction using oligonucleotide probes SEQ ID NO: 1 and SEQ ID NO: 2. All PCR amplifications were carried out as hot-start reactions in a 50 ul reaction volume using Perkin-Elmer (Norwalk, Conn.) 0.5 ml thin-wall polypropylene tubes and a Perkin-Elmer thermal cycler. Reagents added to the tube initially were 2.5 ul of 10.times.PCR buffer (100 mM tris pH 8.3, 500 mM KCl, 15 mM MgCl.sub.2), 5.0 ul of 50% glycerol/1 mM cresol red, 8.0 ul of dNTP mix (1.25 mM each of dATP, dGTP, dTTP and dCTP), 11 picomoles of each nucleic acid primer and sterile water to make up a volume of 25 ul. A wax bead (Ampliwax Gem=100, Perkin-Elmer) was added and the tubes heated to 77.degree. C. for 30 seconds and cooled to room temperature to form a wax barrier. 2.5 ul of 10.times.PCR buffer, 5.0 ul of 50% glyceroyl/1 mM cresol red, 0.2 ul Taq polymerase (AmpliTaq 5 U/ul, Perkin-Elmer) and 15.3 ul of sterile water was added to the tube along with 2.0 ul of DNA from the fungal whole cell boiled lysate described above. 50 cycles of thermal cycling was carried out at 94.degree. C.--30 sec, 50.degree. C.--1 min, 72.degree. C.--2 min. Five microliters of polymerase chain reaction mix from each sample was run on a 5% polyacrylamide gel to visually confirm the successful amplification of 28S rDNA from each fungus listed above. 40 ul of the remaining amplified 28S rDNA was denatured in 1 N NaOH, and half of this denatured rDNA was slot blotted on to a positively charged polysulphone based membrane equilibrated in 0.5N NaOH. The membrane was air dried for 15 minutes and baked in a vacuum oven at 80.degree. C. for 30 minutes. Amplified rDNA from each species was now bound and immobilized at a separate spot on the membrane. The free binding sites on the membrane were blocked by incubating the membrane for 3 hours at 40.degree. C. in hybridization buffer (100 ml of hybridization buffer was made using 1 g non-fat milk powder, 6 g NaH.sub.2 PO.sub.4, 7 g SDS, 200 ul 0.5M EDTA and adjusted to pH 7.2 with NaOH). The specific probe for Candida albicans (SEQ ID NO: 5) was end-labeled with radioactive phosphorus using .sup.32 P ATP and T4 polynucleotide kinase. 50 picomoles of this probe was added to 70 milliliters of hybridization buffer and the membrane was probed at 40.degree. C. overnight. The membrane was washed in hybridization buffer at 40.degree. C. for 15 minutes followed by a wash in 2.times.SSC at 40.degree. C. for 15 minutes. The membrane was then exposed on x-ray film for at least 1 hour. The oligonucleotide probe SEQ ID NO: 5 only hybridized to amplified 28S rDNA from Candida albicans (see Table 3) Under these hybridization conditions, probe SEQ ID NO: 5 is extremely specific for Candida albicans. The sequence of oligonucleotide probe SEQ ID NO: 5 differs from the sequences of other species of Candida by as few as 1 or 2 bases, but these mismatches are sufficient to prevent stable hybrids from forming with the other Candida species.

Probes SEQ ID NO: 3 to SEQ ID NO: 23 were tested for specificity, as described above for the Candida albicans probe SEQ ID NO: 5, against the same panel of fungi listed in the preceding paragraph. The positively charged polysulphone based membrane probed with Candida albicans probe SEQ ID NO: 5 was washed in 0.5N NaOH at 40.degree. C. for 10 minutes to remove all bound Candida albicans probe. The membrane was sequentially probed with all probes listed in SEQ ID NO: 3 to SEQ ID NO: 23. For each subsequently tested probe, the membrane was blocked for at least 30 minutes, probe hybridization was carried out at 40.degree.-42.degree. C. for at least 3 hours, and post-hybridization washes were done in 2.times.SSC for 20 minutes. The membrane was stripped between probings by washing in 0.5 to 1.0N NaOH at 40.degree.-42.degree. C. Results are listed in Tables 3 to 6.

As shown in Tables 3 to 6, each probe listed in SEQ ID NO: 3 to SEQ ID NO: 23 specifically hybridizes to only one target fungal 28S nucleic acid sequence. This specificity is essential for identifying a given species of fungus in clinical specimens containing mixed fungal organisms with a high level of reliability. The 39 organisms listed in these Tables represent a majority of organisms that are commonly isolated from clinical samples. While we have developed 21 species specific probes (SEQ ID NO: 3 to SEQ ID NO: 23) that identify a total of 19 individual organisms, the additional organisms listed in the test panel were used to ensure that our probes did not have any cross-reactivity with other fungi likely to be present in a clinical specimen. The ability to accurately and reliably diagnose, and identify to a species level, this large a number of pathogens is unmatched by any other report. The fact that we can achieve this by probing DNA amplified by a single pair of "Universal" probes (SEQ ID NO: 1 and SEQ ID NO: 2) is highly advantageous as it saves time, money and effort by providing the ability to test a single amplified target with 21 different probes (SEQ ID NO: 3 to SEQ ID NO: 23).

A GenBank search was carried out with all probes listed in SEQ ID NO: 3 to SEQ ID NO: 23 in order to determine whether similar gene sequences were present in the database. 28S sequences for Candida albicans and Cryptococcus neoformans are already present in GenBank, and as expected, the probes for Candida albicans and Cyptococcus neoformans correctly identified the 28S sequences from these two organsims. Ten other probes also matched DNA sequences from a variety of genes not related to the 28S gene (Table 7). This was expected because short stretches of sequence identity can often be found for any query sequence in unrelated genes from the same or a different organism. This observation is known to those versed in this art. In all cases, sequences that matched a probe sequence were not located within the 28S rRNA genes. Our probes are used to analyze 28S DNA that has been previously amplified in a polymerase chain reaction with our probes SEQ ID NO: 1 and SEQ ID NO: 2. Under stringent conditions, these two probes only amplify DNA from fungal 28S rRNA genes. Therefore no amplified DNA from the non-28S genes listed in Table 7 will be available for the hybridization of probes SEQ ID NO: 3 to SEQ ID NO: 23. The presence of related sequences in non-28S, unamplified genes will not be detected and will, thus, not have any effect on the sensitivity or the specificity of our detection and identification strategy.

Use of method in example 1 to test clinical specimens for specific fungal organisms.

Clinical samples taken from the respiratory and gastrointestinal tract of healthy individuals almost always contain fungal flora. Most of these fungi are non-pathogenic, but may give false positives on traditional immunochemical diagnostic tests for pathogenic fungi.

We obtained 44 specimens from diverse sources ranging from sputum and incision drainage tubes, to intervertebral disc and lung biopsies. Traditional smear and culture results showed that all 44 contained at least 1 type of fungus. In order to test the efficacy of our probes, we extracted DNA from all 44 clinical samples and used probes SEQ ID NO: 1 & 2 in a polymerase chain reaction to amplify fungal 28S sequences present in these samples.

DNA was extracted from all clinical samples by our modification of the technique of Chomczynski and Sacchi which originally described the use of acid guanidinium thiocyanate-phenol-chloroform to preferentially extract RNA from cells and tissues. We replaced room temperature cell lysis by boiling lysis, and acid guanidinium thiocyanate-phenol-chloroform extraction by alkaline phenol-guanidine thiocyanate to preferentially extract DNA from cells. 1.5 ml Sarsted (Newton, N.C.) polypropylene screw cap tubes with o-ring seals were used for the extractions. 200 ul of specimen was added to 500 ul of GPT reagent (6M guanidine thiocyanate dissolved in 50 mM tris pH 8.3 mixed with an equal volume of phenol buffered in tris pH 8.0). This was mixed by vortexing and immediately placed in a boiling water bath for 15 minutes. The tubes were spun in a microcentrifuge for 5 seconds and 250 ul of chloroform/isoamyl alcohol (24:1 by volume) was added and mixed by vortexing. The liquid phases were separated by centrifugation for 10 minutes and 450 ul of aqueous (upper) phase was transferred to a fresh tube. The aqueous phase was mixed with 500 ul of 100% isopropanol and placed at -20.degree. C. for at least 1 hour. At the end of this period the tubes were centrifuged for 15 minutes and the supernatant removed without disturbing the nucleic acid pellet. The pellet was washed with 500 ul of ice-cold 70% ethanol to remove traces of GPT reagent by gently inverting 2 times and then centrifuged for 5 minutes. The ethanol was removed and the pellet dried in a speed vac for 10 minutes. The pellet was resuspended in 25 ul of sterile deionized water and 5 ul was used in a 50 ul PCR amplification. The PCR was carried out as a hot-start reaction using the thermal cycling conditions for probes SEQ ID NO: 1 and SEQ ID NO: 2 described in example 1. Gel electrophoresis showed that probes SEQ ID NO: 1 and SEQ ID NO: 2 successfully amplified DNA from all 44 specimens.

The amplified DNA from each specimen was transferred to a positively charged polysulphone based membrane. We radioactively labeled our species specific probes SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 6 and SEQ ID NO: 7, and sequentially probed the membrane to test for the presence of 28S rDNA from Aspergillus fumigatus, Candida albicans, Coccidioides immitis and Cryptococcus neoformans respectively. Membrane blocking, probe hybridization and washes were done exactly as described in example 1. The results are shown in Table 8.

No false positives were observed, indicating a specificity of 100% for these 4 probes in the clinical specimens tested. 10 out of 12 culture positive samples for Aspergillus fumigatus, and 11 out of 13 samples of Candida albicans were identified, indicating a detection sensitivity of about 85% for these two probes. Additionally, two out of two Coccidioides immitis and two out of two Cryptococcus neoformans were correctly identified (detection sensitivity of 100%). As seen by these results, the probes described in this invention can be used on a diverse variety of clinical specimens with excellent efficacy.

DNA sequence based identification of unknown fungal organisms.

Another utility of our probes is in the rapid DNA sequence based identification of a pure culture of fungus. Probes SEQ ID NO: 1 and SEQ ID NO: 2 are used in a polymerase chain reaction to amplify 28S rDNA from an unknown fungus. Probes SEQ ID NO: 1 or SEQ ID NO: 2 are then used as sequencing primers to obtain DNA sequence from this amplified 28S DNA belonging to the unknown fungus. This DNA sequence is compared to the fungal 28S DNA sequences in our database, and a sequence match at, or overlapping any one of the probe sequences in SEQ ID NO: 3 to SEQ ID NO: 23 will confirm the identity of the fungus. This technique cannot be used directly on clinical samples, as these usually contain DNA from more than one fungus, and the DNA sequence generated will consist of overlapping sequences of several organisms. This technique has utility in rapidly and reliably identifying colonies of a single fungus on culture plates, clinical specimens, food, pharmaceutical, environmental or other samples containing only one species of fungus.

Capture and identification of target DNA or RNA

All primers and probes described in this invention disclosure may be labeled with any detectable moiety including, but not limited to radioisotopes, enzymes, antigens, antibodies, chemiluminescent reagents and fluorescent chemicals. Additionally, these probes may be modified without changing the substance of their purpose by terminal addition of nucleotides designed to incorporate restriction sites or other useful sequences. These probes may also be modified by the addition of moieties (including, but not limited to para-magnetic particles, biotin, fluorescein, dioxigenin, antigens, antibodies) or attached to the walls of microtiter trays to assist in the solid phase capture and purification of these probes and any DNA or RNA hybridized to these probes. Fluorescein may be used as a signal moiety as well as a capture moiety, the latter by interacting with an anti-fluorescein antibody.

A typical utility of these modifications would be as follows. Primers SEQ ID NO: 1 and SEQ ID NO: 2 would be utilized to amplify 28S rDNA from a sample, if present, as described previously. Primers would be modified so as to contain a biotin moiety at their 5' ends. A streptavidin solid phase, such as a paramagnetic particle, would be used to separate PCR products, if present, from the reaction mixture. The amplified target may be subsequently hybridized to a third probe ((SEQ ID NO: 3) to (SEQ ID NO: 23) or their complements) attached to a detectable moiety to determine which species of fungus is present in the given sample. Multiple probes, each labeled with a different detectable moiety may be used at one time to analyze the amplified target.

Alternatively, Primers SEQ ID NO: 1 and SEQ ID NO: 2 would be utilized to amplify 28S rDNA from a sample, if present, as above. In a separate reaction, individually, either SEQ ID NO: 1 or SEQ ID NO: 2 would be modified by attachment to a solid phase capture moiety, such as a paramagnetic particle, and SEQ ID NO: 3 to SEQ ID NO: 23 (or their complements) would be modified by addition of a detectable moiety. Alternately, in the amplicon, any sequences delimited by SEQ ID NO: 1 and SEQ ID NO: 2 with the exception of the SEQ ID NO: 3 to SEQ ID NO: 23, may be used in the design of a capture probe. One of the probes attached to a solid phase (SEQ ID NO: 1 and SEQ ID NO: 2) or any other appropriately designed sequences and one of the probes modified by attachment to a detectable moiety (SEQ ID NO: 3 to SEQ ID NO: 23 or their complements) would be hybridized together, in solution, to products of the PCR, if they had been generated. The hybrids, if present, would be captured from the solution, and analyzed by a method appropriate to the detection moiety. Detection of the hybridized probe would indicate which species of fungus was present in the given sample. Multiple probes, each labeled with a different detectable moiety may be used at one time to analyze the amplified target.

Species-specific amplification of fungal DNA

Another utility of the probes described in this invention is their usage as primers in the direct detection of a specific fungal species by virtue of a nucleic acid amplification reaction. In this embodiment, one primer is a universal one, such as (SEQ ID NO: 1) or (SEQ ID NO:2), and the other is a species-specific primer selected from the group consisting of (SEQ ID NO:3) to (SEQ ID NO: 23) or the complements thereof. One variation of this approach is the substitution of (SEQ ID NO: 1) or (SEQ ID NO:2) with any functional sequence located in proximity to the species-specific primer.