Myeloperoxidase (MPO), which is located within neutrophils capable of producing hypochlorous acid, is active in vitro against bacteria and fungi. However, MPO-deficient persons are usually healthy. To define the in vivo contribution of MPO to early host defense against pulmonary infections, MPO-deficient and control mice were intranasally infected with various fungi and bacteria, and the number of residual microorganisms in lungs was compared 48 h later. MPO-deficient mice showed severely reduced cytotoxicity to Candida albicans, Candida tropicalis, Trichosporon asahii, and Pseudomonas aeruginosa. However, the mutant mice showed a slight but significantly delayed clearance of Aspergillus fumigatus and Klebsiella pneumoniae and had comparable levels of resistance to the wild type against Candida glabrata, Cryptococcus neoformans, Staphylococcus aureus, and Streptococcus pneumoniae. These results suggest that the MPO-dependent oxidative system is important for host defense against fungi and bacteria, although the effect varies by pathogen species.

Protein phosphatases are critical for the regulation of many cellular processes. Null mutants of 21 putative protein phosphatases of Candida albicans were constructed by consecutive allele replacement using the URA3 and ARG4 marker genes. A simple silkworm model of C. albicans infection was used to screen the panel of mutants. Four null mutant (cmp1⌬, yvh1⌬, sit4⌬, and ptc1⌬) strains showed attenuated virulence in the silkworm model relative to that of control and parental strains. Three of the mutants, the cmp1⌬, yvh1⌬, and sit4⌬ mutants, had previously been identified as affecting virulence in a conventional mouse model, indicating the validity of the silkworm model screen. Disruption of the putative protein phosphatase gene PTC1 of C. albicans, which has 52% identity to the Saccharomyces cerevisiae type 2C protein phosphatase PTC1, significantly reduced virulence in the silkworm model. The mutant was also avirulent in a mouse model of disseminated candidiasis. Reintroducing either of the C. albicans PTC1 alleles into the disruptant strain, using a cassette containing either allele under the control of a constitutive ACT1 promoter, restored virulence in both infection models. Characterization of ptc1⌬ revealed other phenotypic traits, including reduced hyphal growth in vitro and in vivo, and reduced extracellular proteolytic activity. We conclude that PTC1 may contribute to pathogenicity in C. albicans.The opportunistic fungal pathogen Candida albicans, a member of the normal human microflora, can cause superficial or life-threatening systemic infections, particularly in immunocompromised patients. Virulence determinants of C. albicans include the yeast-to-hypha transition, adherence to host receptors, and the ability to produce a variety of secreted hydrolytic enzymes, such as aspartic proteinases, phospholipases, and lipases. In addition, genes encoding metabolism and stress response proteins of this pathogen are important for pathogenicity (9,16,22).It would be of interest to determine whether other, previously uncharacterized C. albicans genes have a role in virulence. However, screening of whole families of C. albicans genes for virulence-related properties using a mammalian model of infection, such as the mouse model, would pose issues both of ethics and of practical costs. A number of substitute, invertebrate models of microbial virulence more suitable for screening experiments have been developed, including the use of Caenorhabditis elegans, Drosophila melanogaster, and Galleria mellonella (10, 18, 35, 41), silkworms (20), and locusts (32). The silkworm model of C. albicans infection has been used for the quantitative evaluation of antifungal agents, with results equivalent to those in a mouse model. In addition, the silkworm infection model has been used successfully to identify and evaluate uncharacterized genes required for the virulence of Staphylococcus aureus (28,29).The fungus must adapt to stresses encountered in vivo, such as various host defense mechanisms and/or microenvironmental changes in...

CgAUS1 appears to function as a sterol transporter that may contribute to lower azole susceptibility in the presence of serum and to protect C. glabrata against azole toxicity in vivo.

Summary Saccharomyces cerevisiae

Oxygen metabolites generated by myeloperoxidase (MPO) and nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase contribute to microbial killing by phagocytes. To compare the importance of the 2 enzymes for host defense, MPO-deficient (MPO(-/-)) mice and NADPH-oxidase-deficient mice with chronic granulomatous disease (CGD mice) were intraperitoneally infected with 3 different doses of Candida albicans, and their infection severity was analyzed. CGD mice had increased mortality and exhibited increased tissue fungal burden in a dose-dependent manner, whereas normal mice showed no symptoms. Of interest, at the highest dose, the mortality of MPO(-/-) mice was comparable to that of CGD mice, but at the lowest dose, it was the same as that of normal mice. At the middle dose, the number of fungi disseminated into various organs of the MPO(-/-) mice was comparable to that of the CGD mice at day 6 of infection, but it was significantly lower at day 14. These results suggest that MPO and NADPH-oxidase are equally important for early host defense against a large inoculum of Candida.