Isolates of Klebsiella pneumoniae are responsible for opportunistic infections, particularly of the urinary tract and respiratory tract, in humans. These bacteria express type 3 fimbriae that have been implicated in binding to eucaryotic cells and matrix proteins. The type 3 fimbriae mediate binding to target tissue using the MrkD adhesin that is associated with the fimbrial shaft comprised of the MrkA protein. The formation of biofilms in vitro by strains of K. pneumoniae was shown to be affected by the production of fimbriae on the bacterial surface. However, a functional MrkD adhesin was not necessary for efficient biofilm formation. Nonfimbriate strains were impaired in their ability to form biofilms. Using isogenic fimbriate and nonfimbriate strains of K. pneumoniae expressing green fluorescent protein it was possible to demonstrate that the presence of type 3 fimbriae facilitated the formation of dense biofilms in a continuous-flowthrough chamber. Transformation of nonfimbriate mutants with a plasmid possessing an intact mrk gene cluster restored the fimbrial phenotype and the rapid ability to form biofilms.

The YPS3 gene of Histoplasma capsulatum encodes a protein that is both surface localized in the cell wall of H. capsulatum and released into the culture medium. This protein is produced only during the pathogenic yeast phase of infection and is also expressed differentially in H. capsulatum strains of different virulence levels. In this study, we silenced the YPS3 transcript by using an interfering-RNA strategy and examined the silenced mutants for phenotypic differences in vitro and during infection. The mutants showed no growth defect during in vitro culture in a defined medium at 37°C and appeared to have normal virulence in a RAW 264.7 murine macrophage-like cell line. In a C57BL/6 mouse model of infection, however, the mutants caused significantly decreased fungal burdens, particularly in the peripheral phagocyte-rich tissues of livers and spleens. This defect in organ colonization was evident within 3 days of infection; however, it appeared to be exacerbated at later time points.

The YPS3 gene of Histoplasma capsulatum encodes a protein that is both resident in the cell wall and also released into the culture medium. This protein is produced only during the pathogenic yeast phase of infection and is also expressed differently in H. capsulatum strains that differ in virulence. We investigated the cellular localization of Yps3p. We demonstrated that the cell wall fraction of Yps3p was surface localized in restriction fragment length polymorphism class 2 strains. We also established that Yps3p released into the G217B culture supernatant binds to the surface of strains that do not naturally express the protein. This binding was saturable and occurred within 5 min of exposure and occurred similarly with live and heat-killed H. capsulatum. Flow cytometric analysis of H. capsulatum after enzymatic treatments was consistent with Yps3p binding to chitin, a carbohydrate polymer that is a component of fungal cell walls. Polysaccharide binding assays demonstrated that chitin but not cellulose binds to and extracts Yps3p from culture supernatants.

The YPS3 locus of the dimorphic fungus Histoplasma capsulatum encodes a secreted and surface-localized protein specific to the pathogenic yeast phase. In this study we examined this locus in 32 H. capsulatum strains and variants. Although protein production is limited to a select group of strains, the North American restriction fragment length polymorphism class 2/NAm 2 isolates, the locus was present in all the strains we examined. The YPS3 gene is well conserved in its 5 and 3 regions but displays an intragenic hypervariable region of tandem repeats that fluctuates in size between strains. This feature is similar to that seen with genes encoding several cell surface proteins in other fungi.