Avirulence of Candida albicans auxotrophic mutants in a rat model of oropharyngeal candidiasis

Candida albicans Biofilms and Human Disease

In humans, microbial cells (including bacteria, archaea, and fungi) greatly outnumber host cells. Candida albicans is the most prevalent fungal species of the human microbiota; this species asymptomatically colonizes many areas of the body, particularly the gastrointestinal and genitourinary tracts of healthy individuals. Alterations in host immunity, stress, resident microbiota, and other factors can lead to C. albicans overgrowth, causing a wide range of infections, from superficial mucosal to hematogenously disseminated candidiasis. To date, most studies of C. albicans have been carried out in suspension cultures; however, the medical impact of C. albicans (like that of many other microorganisms) depends on its ability to thrive as a biofilm, a closely packed community of cells. Biofilms are notorious for forming on implanted medical devices, including catheters, pacemakers, dentures, and prosthetic joints, which provide a surface and sanctuary for biofilm growth. C. albicans biofilms are intrinsically resistant to conventional antifungal therapeutics, the host immune system, and other environmental perturbations, making biofilm-based infections a significant clinical challenge. Here, we review our current knowledge of biofilms formed by C. albicans and closely related fungal species.

The Candida albicans ATO Gene Family Promotes Neutralization of the Macrophage Phagolysosome

Candida albicans is an opportunistic human fungal pathogen that causes a variety of diseases, ranging from superficial mucosal to life-threatening systemic infections, the latter particularly in patients with defects in innate immune function. C. albicans cells phagocytosed by macrophages undergo a dramatic change in their metabolism in which amino acids are a key nutrient. We have shown that amino acid catabolism allows the cell to neutralize the phagolysosome and initiate hyphal growth. We show here that members of the 10-gene ATO family, which are induced by phagocytosis or the presence of amino acids in an Stp2-dependent manner and encode putative acetate or ammonia transporters, are important effectors of this pH change in vitro and in macrophages. When grown with amino acids as the sole carbon source, the deletion of ATO5 or the expression of a dominant-negative ATO1(G53D) allele results in a delay in alkalinization, a defect in hyphal formation, and a reduction in the amount of ammonia released from the cell. These strains also form fewer hyphae after phagocytosis, have a reduced ability to escape macrophages, and reside in more acidic phagolysosomal compartments than wild-type cells. Furthermore, overexpression of many of the 10 ATO genes accelerates ammonia release, and an ato5Δ ATO1(G53D) double mutant strain has additive alkalinization and ammonia release defects. Taken together, these results indicate that the Ato protein family is a key mediator of the metabolic changes that allow C. albicans to overcome the macrophage innate immunity barrier.

A URA3 null mutant of Candida albicans (CAI-4) causes oro-oesophageal and gastric candidiasis and is lethal for gnotobiotic, transgenic mice (Tgepsilon26) that are deficient in both natural killer and T cells

Current data suggest that functional URA3 genes are necessary for the full pathogenesis of Candida albicans. Herein it is shown that a putatively avirulent URA3/URA3 null mutant of C. albicans (CAI-4) can colonize the murine alimentary tract, invade oro-oesophageal and gastric tissues with yeasts and hyphae, evoke a granulocyte-dominated inflammatory response, and kill transgenic mice that are deficient for both natural killer cells and T cells. Because C. albicans-colonized (gnotobiotic) mice lack a viable prokaryotic microbiota, this study also demonstrates that the gut microbiome is not required to supply the mutant's nutritional needs. The gnotobiotic murine model described herein can be used to assess the capacity of C. albicans mutants to colonize and infect cutaneous, mucosal and systemic tissues and kill the susceptible host via a clinically common, natural route of infection; namely the alimentary tract.

Animal models of mucosal Candida infection

Rodent models of oral, vaginal and gastrointestinal Candida infection are described and discussed in terms of their scientific merits. The common feature of all experimental mucosal Candida infections is the need for some level of host immunocompromise or exogenous treatment to ensure quantitatively reproducible disease. A growing literature describes the contributions of such candidiasis models to our understanding of certain aspects of fungal virulence and host response to mucosal Candida albicans challenge. Evidence to date shows that T-lymphocyte responses dominate host immune defences to oral and gastrointestinal challenge, while other, highly compartmentalized responses defend vaginal surfaces. By contrast the study of C. albicans virulence factors in mucosal infection models has only begun to unravel the complex of attributes required to define the difference between strongly and weakly muco-invasive strains.

RT-PCR analysis of Candida albicans ALS gene expression in a hyposalivatory rat model of oral candidiasis and in HIV-positive human patients

ALS gene expression was studied in the hyposalivatory rat model of oral candidiasis and in clinical specimens collected from HIV-positive patients to assess similarities in expression patterns between the model system and clinical isolates. Two Candida albicans strains, SC5314 and OY-2-76, were used in the rat model system and infection progressed for 3 or 5 days. The strains produced similar oral lesions at 3 days. At 5 days, strain OY-2-76 produced more superficial lesions containing relatively more yeast forms compared to invasive hyphal forms observed for strain SC5314. For all infections, the most severe lesions were observed on the tongue and gingiva overlying the mandible. ALS transcripts were easier to detect by RT-PCR later in infection and under other conditions where more fungal cells were present. Expression of ALS1, ALS2, ALS3 and ALS4 was observed in rats infected for 3 days with ALS5 and ALS9 transcripts detected after 5 days of infection. Expression of ALS6 was observed in a single specimen from a 5-day infection while ALS7 transcript was never found. Expression of all ALS genes was observed in oral clinical material collected from HIV-positive patients although ALS6 and ALS7 transcripts required an extra PCR amplification step to be detected. Overall, the patterns of ALS gene expression were similar between the rat model and human clinical specimens, suggesting that the model would be useful for studying the phenotype of al delta/al delta mutant strains.

Expression of firefly luciferase in Candida albicans and its use in the selection of stable transformants

The infectious yeast Candida albicans is a model organism for understanding the mechanisms of fungal pathogenicity. We describe the functional expression of the firefly luciferase gene, a reporter commonly used to tag genes in many other cellular systems. Due to a non-standard codon usage by this yeast, the CUG codons were first mutated to UUG to allow functional expression. When integrated into the chromosome of C. albicans with a strong constitutive promoter, cells bioluminesce when provided with luciferin substrate in their media. When fused to the inducible promoter from the HWP1 gene, expression and bioluminescence was only detected in cultures conditioning hyphal growth. We further used the luciferase gene as a selection to isolate transformed cell lines from clinical isolates of C. albicans, using a high-density screening strategy that purifies transformed colonies by virtue of light emission. This strategy requires no drug or auxotrophic selectable marker, and we were thus able to generate stable transformants of clinical isolates that are identical to the parental strain in all aspects tested, other than their bioluminescence. The firefly luciferase gene can, therefore, be used as a sensitive reporter to analyze gene function both in laboratory and clinical isolates of this medically important yeast.

The SAT1 flipper, an optimized tool for gene disruption in Candida albicans

The construction of Candida albicans mutants by targeted gene disruption usually depends on the use of nutritional markers for the selection of prototrophic transformants from auxotrophic host strains, but it is becoming increasingly evident that this strategy may cause difficulties in the interpretation of mutant phenotypes. Here, we describe a new method for inactivating both alleles of a target gene in C. albicans wild-type strains to obtain homozygous null mutants. The SAT1 flipping method relies on the use of a cassette that contains a dominant nourseothricin resistance marker (caSAT1) for the selection of integrative transformants and a C. albicans-adapted FLP gene that allows the subsequent excision of the cassette, which is flanked by FLP target sequences, from the genome. Two rounds of integration/excision generate homozygous mutants that differ from the wild-type parent strain only by the absence of the target gene, and reintegration of an intact gene copy for complementation of mutant phenotypes is performed in the same way. Transformants are obtained after only 1 day of growth on a selective medium, and integration into the target locus occurs with high specificity after adding homologous flanking sequences on both sides of the cassette. FLP-mediated excision of the SAT1 flipper cassette is achieved by simply growing the transformants for a few hours in medium without selective pressure, and nourseothricin-sensitive (NouS) derivatives can easily be identified by their slower growth on indicator plates containing a low concentration of nourseothricin. We demonstrate the use of the system by deleting the OPT1 gene, which encodes an oligopeptide transporter, in the C. albicans model strain SC5314. The null mutants became resistant to the toxic peptide KLLEth, and reintroduction of an intact OPT1 copy restored susceptibility. The SAT1 flipping method provides a highly efficient method for gene disruption in C. albicans wild-type strains, which eliminates currently encountered problems in the genetic analysis of this important human fungal pathogen.

Developing animal models for polymicrobial diseases

Although polymicrobial diseases are not a new concept for microbiologists, they are experiencing a resurgence of interest owing to the development of suitable animal models and new molecular techniques that allow these diseases to be studied effectively. This broad review provides an excellent introduction to this fascinating topic.

Examples are included of each type of polymicrobial disease and the animal models that are used to study these diseases are discussed. In many instances, schematics for the animal model are presented.

Viral co-infections including bovine viral diarrhoeal viruses, porcine reproductive and respiratory syndrome, mixed hepatitis virus infections and HIV co-infection with hepatitis virus are discussed, together with attempts to model these diseases in animals.

Viral and bacterial co-infections are reviewed with a special focus on otitis media and the rodent models that have been used to probe this important childhood illness.

Of the polybacterial diseases, periodontitis is one of the best understood and a clinically relevant rodent model is now available. This model, and the role of biofilm formation in periodontitis are examined.

Fungal infections of humans are often referred to as 'opportunistic' but in fact these infections are often fungal co-infections with viruses such as HIV and fungal mixed co-infections. The roles of these infections in disease and the rodent models used to study them are discussed.

Parasite co-infections are thought to have a role in the severity of malaria and the severity of Lyme arthritis. These diseases and attempts to model them are evaluated.

Finally, co-infections that are associated with virus-induced immunosuppression are discussed, together with their animal models.

Polymicrobial diseases involve two or more microorganisms that act synergistically, or in succession, to mediate complex disease processes. Although polymicrobial diseases in animals and humans can be caused by similar organisms, these diseases are often also caused by organisms from different kingdoms, genera, species, strains, substrains and even by phenotypic variants of a single species. Animal models are often required to understand the mechanisms of pathogenesis, and to develop therapies and prevention regimes. However, reproducing polymicrobial diseases of humans in animal hosts presents significant challenges.

Characterization of Candida albicans colony morphological mutants and their hybrids by means of RAPD-PCR, isoenzyme analysis and pathogenicity analysis

A wild-type strain of Candida albicans (S1, ATCC 10261) was used to obtain stable auxotrophic colony morphological mutants (mutant M5 producing only true hyphae and mutant M2 containing 90 % blastospores and 10 % pseudohyphae) by induced mutagenesis. A hybrid was produced by somatic hybridization between these 2 mutants. Out of the isolated 10 clones, 2 stable hybrid clones were chosen and characterized: clone VI. 1M produced rough colonies containing a new, extended cell type (never observed in natural isolates), exhibited unipolar budding, did not form a germ tube, and possessed 12 chromosomal bands. All other features (antifungal and stress sensitivity, adhesion ability, pathogenicity, and isoenzyme and RAPD patterns) were similar to those of mycelial mutant M5. In contrast, the characteristics of clone VI.9S were similar to those of morphological mutant M2.

Calcineurin is essential for virulence in Candida albicans

Calcineurin is a conserved Ca(2+)-calmodulin-activated, serine/threonine-specific protein phosphatase that regulates a variety of physiological processes, e.g., cell cycle progression, polarized growth, and adaptation to salt and alkaline pH stresses. In the pathogenic yeast Cryptococcus neoformans, calcineurin is also essential for growth at 37 degrees C and virulence. To investigate whether calcineurin plays a role in the virulence of Candida albicans, the major fungal pathogen of humans, we constructed C. albicans mutants in which both alleles of the CMP1 gene, encoding the calcineurin catalytic subunit, were deleted. The C. albicans Delta cmp1 mutants displayed hypersensitivity to elevated Na(+), Li(+), and Mn(2+) concentrations and to alkaline pH, phenotypes that have been described after calcineurin inactivation in the related yeast Saccharomyces cerevisiae. Unlike S. cerevisiae calcineurin mutants, which exhibit reduced susceptibility to high Ca(2+) concentrations, growth of C. albicans was inhibited in the presence of 300 mM CaCl(2) after the deletion of CMP1, demonstrating that there are also differences in calcineurin-mediated cellular responses between these two yeast species. In contrast to C. neoformans, inactivation of calcineurin did not cause temperature sensitivity in C. albicans. In addition, hyphal growth, an important virulence attribute of C. albicans, was not impaired in the Delta cmp1 mutants under a variety of inducing conditions. Nevertheless, the virulence of the mutants was strongly attenuated in a mouse model of systemic candidiasis, demonstrating that calcineurin signaling is essential for virulence in C. albicans.

Ura-status-dependent adhesion of Candida albicans mutants

Gene disruptions in the diploid opportunistic human fungal pathogen Candida albicans are usually created using multiple rounds of targeted integration called the 'ura-blaster' method. Resulting heterozygous and homozygous null mutants can be auxotrophic (Ura(-)) or prototrophic (Ura(+)) for uracil biosynthesis. Here we demonstrate that the Ura-status of otherwise isogenic mutants affected the adhesion of C. albicans. Moreover the effect of Ura-status on adhesion was also dependent on the null mutant background, the nature of the underlying surface and the carbon source for growth. Therefore the Ura-status is not neutral in determining adhesive properties of C. albicans mutants that are generated via the ura-blaster protocol.

Gene regulation and host adaptation mechanisms in Candida albicans

The yeast Candida albicans is a harmless member of the normal microflora on the mucosal surfaces of most healthy persons, but it can cause severe opportunistic infections in immunosuppressed patients. To become a successful human commensal and pathogen, C. albicans has evolved host adaptation mechanisms on different levels. The regulated expression of virulence and other genes in response to environmental signals allows an optimal adaptation to new host niches during the course of an infection. In addition, C. albicans is able to switch between different cell types in a reversible and apparently random fashion. Phenotypic switching involves the coordinated regulation of phase-specific genes, and the resulting generation of selected, pre-programmed cell types may represent an additional strategy to adapt to certain host environments. Finally, C. albicans produces genetically altered variants at a high rate. This microevolution ensures survival when the pathogen encounters new adverse conditions, as exemplified by the development of stable drug-resistant variants under the selection pressure caused by antimycotic therapy. Thus, rather than the possession of single dominant virulence factors, it is its remarkable versatility that makes C. albicans the most important fungal pathogen of humans.

Virulence genes in the pathogenic yeast Candida albicans

In recent years, the incidence of fungal infections has been rising all over the world. Although the amount of research in the field of pathogenic fungi has also increased, there is still a need for the identification of reliable determinants of virulence. In this review, we focus on identified Candida albicans genes whose deletant strains have been tested in experimental virulence assays. We discuss the putative relationship of these genes to virulence and also outline the use of new different systems to examine the precise effect in virulence of different genes.

Isolation of a Candida albicans gene, tightly linked to URA3, coding for a putative transcription factor that suppresses a Saccharomyces cerevisiae aft1 mutation

A pathogen such as C. albicans needs an efficient mechanism of iron uptake in an iron-restricted environment such as is the human body. A ferric-reductase activity regulated by iron and copper, and analogous to that in S. cerevisiae, has been described in C. albicans. We have developed an in-plate protocol for the isolation of clones that complement an aft1 mutation in S. cerevisiae that makes cells dependent on iron for growth. After transformation of S. cerevisiae aft1 with a C. albicans library, we have selected clones that grow in conditions of iron deficiency and share an identical plasmid, pIRO1, with a 4500 bp insert containing the URA3 gene and an ORF (IRO1) responsible for the suppression of the iron dependency. IRO1 does not show homology with AFT1 or with other sequences in the databases. Northern analysis demonstrates constitutive expression of IRO1. CAI4, a C. albicans strain isolated as Deltaura3, also has a deletion of the 3' half of IRO1, and displays in YNB medium similar phenotypic characteristics to S. cerevisiae aft1 mutant strains. Therefore, we consider IRO1 as a gene of C. albicans involved in the utilization of iron. However, in extreme conditions of iron deprivation, CAI4 seems to activate alternative mechanisms of iron uptake that allow a better growth than the wild strain SC5314. Analysis of its predicted protein sequence is in agreement with a role of Iro1p as a transcription factor.

Effect of INT1 gene on Candida albicans murine intestinal colonization

BACKGROUND

Increased intestinal colonization with Candida albicans is believed to be a major factor predisposing immunocompromised and postsurgical patients to systemic candidiasis, although the mechanisms facilitating C. albicans colonization remain unclear. Because previous studies have linked the C. albicans INT1 gene to filament formation, epithelial adherence, and mouse virulence, experiments were designed to evaluate the effect of INT1 on intestinal colonization.

MATERIALS AND METHODS

Mice were orally inoculated with either the parent strain (CAF2, INT/INT1), an int1 heterozygote (CAG1, INT1/int1), an int1 homozygote (CAG3, int1/int1), or a reintegrant (CAG5, int1/int1 + INT1), and sacrificed 3 and 7 days later for quantitative analysis of cecal C. albicans.

RESULTS

Following oral inoculation with 10(3) C. albicans, only small numbers of each strain were recovered from the cecal flora of normal mice. However, in mice pretreated with oral antibiotics, cecal colonization of each strain was increased (P < 0.01). In addition, cecal colonization was reduced for all int1 mutant strains compared with the parent strain (P < 0.05). By light microscopy, all four C. albicans strains were easily observed in the ileal lumen as both budding yeast and filamentous forms, although only occasional yeast forms appeared adherent to the intestinal epithelium.

CONCLUSIONS

C. albicans readily colonized and replicated in the ceca of antibiotic-treated mice. The presence of two functional copies of INT1 appeared to facilitate C. albicans cecal colonization, suggesting that intestinal colonization may be another virulence factor associated with INT1 and that the gene product may be an attractive target to control C. albicans intestinal colonization.

Control of filament formation in Candida albicans by polyamine levels

Candida albicans, the most common fungal pathogen, regulates its cellular morphology in response to environmental conditions. The ODC gene, which encodes ornithine decarboxylase, a key enzyme in polyamine biosynthesis, was isolated and disrupted. Homozygous null Candida mutants behaved as polyamine auxotrophs and grew exclusively in the yeast form at low polyamine levels (0.01 mM putrescine) under all conditions tested. An increase in the polyamine concentration (10 mM putrescine) restored the capacity to switch from the yeast to the filamentous form. The strain with a deletion mutation also showed increased sensitivity to salts and calcofluor white. This Candida odc/odc mutant was virulent in a mouse model. The results suggest a model in which polyamine levels exert a pleiotrophic effect on transcriptional activity.

Avirulence of Candida albicans CaHK1 mutants in a murine model of hematogenously disseminated candidiasis

Deletion of both alleles of the Candida albicans CaHK1 gene, which causes cells to flocculate when grown at pH 7.5, a pH comparable to that of mammalian blood, abolishes the ability of the yeast to establish a successful infection in a murine model of hematogenously disseminated candidiasis. Within 72 h all mice inoculated with the parental C. albicans strain had died. The mice infected with either the heterozygote or revertant strain, either of which harbors only one functional CaHK1 allele, also succumbed to the infection, although survivors were observed for up to 16 days postinfection. However, mice inoculated with the Deltacahk1 null strain survived for the course of the infection. These results indicate that CaHK1 is required for the virulence of C. albicans in a murine model of hematogenously disseminated candidiasis. In contrast, CaHK1 is not required for the virulence of C. albicans in a rat model of vaginal candidiasis.

Adhesion of Candida albicans mutant strains to host tissue

Adhesion of Candida albicans to host cells is believed to represent a fungal virulence factor and a significant step in the development of candidiasis. As C. albicans strains may differ in their in vitro adhesion ability we initiated a study to investigate whether mutant strains differ in this respect from their parent wild-type. We assessed the in vitro adhesion of C. albicans CBS562 and two mutants obtained by mutagenesis with N'-nitrosoguanidine: a histidine auxotroph, SAG5, derived from CBS562, and a respiratory-deficient strain (a petite mutant), SAR1, derived from SAG5. The adhesion was tested in vitro using two target cell systems: (1) exfoliated human buccal epithelial cells (BEC); and (2) human keratinocyte tissue line cells (HaCaT cells). Adhesion to BEC was evaluated microscopically and that to HaCaT cells by a direct ELISA technique. The results indicated a 54% reduction in adhesion to BEC for SAG5 and 30% for SAR1 as compared to the wild-type, and a 25% reduction in adhesion to HaCaT cells for SAG5 and 20% for SAR1. To verify whether the prototrophy restores the adhesion ability, we complemented the his-negative auxotroph by transforming the strain with the HIS4 gene. Then we assayed the adhesion to BEC of the complemented his-negative mutant in comparison to that of the wild-type, the his-negative mutant (SAG5) and the plasmid-cured transformant. The adhesion values of the complemented his-negative strain were similar to those of the wild-type, whereas the values of the plasmid-cured strain were similar to those of SAG5.

Linkage of adhesion, filamentous growth, and virulence in Candida albicans to a single gene, INT1

Adhesion and the ability to form filaments are thought to contribute to the pathogenicity of Candida albicans, the leading cause of fungal disease in immunocompromised patients. Int1p is a C. albicans surface protein with limited similarity to vertebrate integrins. INT1 expression in Saccharomyces cerevisiae was sufficient to direct the adhesion of this normally nonadherent yeast to human epithelial cells. Furthermore, disruption of INT1 in C. albicans suppressed hyphal growth, adhesion to epithelial cells, and virulence in mice. Thus, INT1 links adhesion, filamentous growth, and pathogenicity in C. albicans and Int1p may be an attractive target for the development of antifungal therapies.

A neutral trehalase gene from Candida albicans: molecular cloning, characterization and disruption

A neutral trehalase gene, NTC1, from the human pathogenic yeast Candida albicans was isolated and characterized. An ORF of 2724 bp was identified encoding a predicted protein of 907 amino acids and a molecular mass of 104 kDa. A single transcript of approximately 3.2 kb was detected by Northern blot analysis. Comparison of the deduced amino acid sequence of the C. albicans NTC1 gene product with that of the Saccharomyces cerevisiae NTH1 gene product revealed 57% identity. The NTC1 gene was localized on chromosome 1 or R. A null mutant (delta ntc1/delta ntc1) was constructed by sequential gene disruption. Extracts from mutants homozygous for neutral trehalase deletion had only marginal neutral trehalase activity. Extracts from heterozygous mutants showed intermediate activities between extracts from the wild-type strain and from the homozygous mutants. The null mutant showed no significant differences in pathogenicity as compared to the wild-type strain in a mouse model of systemic candidiasis. This result indicates that the neutral trehalase of C. albicans is not a potential target for antifungal drugs.

Avirulence of Candida albicans FAS2 mutants in a mouse model of systemic candidiasis

Disruption of both alleles of the Candida albicans FAS2 gene abolishes the ability of the organism to establish infection in a murine model of systemic candidiasis. Within 72 h all mice inoculated with 10(6) CFU of the parental C. albicans strain had died. In contrast, all animals inoculated with the mutant strain CFD2 survived for the course of the experiment (21 days). Animals infected with either mutant strain CFD1 or CFD3, in which only one FAS2 allele was disrupted, also succumbed to infection, but mortality was not observed until 4 days postinfection and survivors remained for up to 20 days postinfection. The results demonstrate that FAS2 is required for successful C. albicans infection.

Signal transduction through homologs of the Ste20p and Ste7p protein kinases can trigger hyphal formation in the pathogenic fungus Candida albicans

The CST20 gene of Candida albicans was cloned by functional complementation of a deletion of the STE20 gene in Saccharomyces cerevisiae. CST20 encodes a homolog of the Ste20p/p65PAK family of protein kinases. Colonies of C. albicans cells deleted for CST20 revealed defects in the lateral formation of mycelia on synthetic solid "Spider" media. However, hyphal development was not impaired in some other media. A similar phenotype was caused by deletion of HST7, encoding a functional homolog of the S. cerevisiae Ste7p protein kinase. Overexpression of HST7 partially complemented the deletion of CST20. Cells deleted for CST20 were less virulent in a mouse model for systemic candidiasis. Our results suggest that more than one signaling pathway can trigger hyphal development in C. albicans, one of which has a protein kinase cascade that is analogous to the mating response pathway in S. cerevisiae and might have become adapted to the control of mycelial formation in asexual C. albicans.

Attenuated virulence of uridine-uracil auxotrophs of Aspergillus fumigatus

Aspergillus fumigatus mutants that are deficient in the de novo UMP biosynthesis pathway because of a mutation in the pyrG gene encoding orotidine-5'-phosphate decarboxylase (and therefore auxotrophic for uridine or uracil) were evaluated in a murine model of invasive aspergillosis. These mutants were entirely nonpathogenic, and mutant conidia remained ungerminated in alveolar macrophages. Both the germination and virulence defects could be restored by supplementing the drinking water of the animals with uridine. DNA-mediated transformation of one of the pyrG mutants with the Aspergillus niger pyrG gene also restored virulence. These results suggest that uridine and uracil are limiting in the lung environment, thus preventing conidium germination and hence virulence of the pyrG mutants.