Growth inhibition of pathogenic yeasts by Pseudomonas aeruginosa in vitro: clinical implications in blood cultures.

The interaction between yeasts and bacteria may have clinical implications in polymicrobial septicaemia. The in vitro effect of Pseudomonas aeruginosa on five pathogenic yeast species, Candida albicans, Candida tropicalis, Candida parapsilosis, Candida glabrata and Cryptococcus neoformans, was investigated. Yeast inhibition assays were performed in an aerobic blood culture medium, inoculated with yeast cells (inoculum 1-10 CFU ml-1) and bacterial cells (inoculum 10-10(7) CFU ml-1). Interactions between P. aeruginosa and yeasts were determined after incubation by enumeration of pure and mixed cultures. Growth of all isolates tested was completely or partially inhibited by P. aeruginosa in blood culture medium, the phenomenon depending on the yeast genus and bacterial inoculum. Suppression of fungal growth was also observed in bacterial culture filtrate. This in vitro antifungal activity may preclude yeast recovery from blood cultures in mixed infections.