ABSTRACTBacteria interact with each other in nature and often compete for limited nutrient and space resources. However, it is largely unknown whether and how bacteria also interact with human fungal pathogens naturally found in the environment. Here, we identified a soil bacterium,Bacillus safensis, which potently blocked several keyCryptococcus neoformansvirulence factors, including formation of the antioxidant pigment melanin and production of the antiphagocytic polysaccharide capsule. The bacterium also inhibitedde novocryptococcal biofilm formation but had only modest inhibitory effects on already formed biofilms or planktonic cell growth. The inhibition of fungal melanization was dependent on direct cell contact and live bacteria.B. safensisalso had anti-virulence factor activity against another major human-associated fungal pathogen,Candida albicans. Specifically, dual-species interaction studies revealed that the bacterium strongly inhibitedC. albicansfilamentation and biofilm formation. In particular,B. safensisphysically attached to and degraded candidal filaments. Through genetic and phenotypic analyses, we demonstrated that bacterial chitinase activity against fungal cell wall chitin is a factor contributing to the antipathogen effect ofB. safensis.IMPORTANCEPathogenic fungi are estimated to contribute to as many human deaths as tuberculosis or malaria. Two of the most common fungal pathogens,Cryptococcus neoformansandCandida albicans, account for up to 1.4 million infections per year with very high mortality rates. Few antifungal drugs are available for treatment, and development of novel therapies is complicated by the need for pathogen-specific targets. Therefore, there is an urgent need to identify novel drug targets and new drugs. Pathogens use virulence factors during infection, and it has recently been proposed that targeting these factors instead of the pathogen itself may represent a new approach to develop antimicrobials. Here, we identified a soil bacterium that specifically blocked virulence factor production and biofilm formation byC. neoformansandC. albicans. We demonstrate that the bacterial antipathogen mechanism is based in part on targeting the fungal cell wall, a structure not found in human cells.
Faculty Opinions recommendation of Disarming Fungal Pathogens: Bacillus safensis Inhibits Virulence Factor Production and Biofilm Formation by Cryptococcus neoformans and Candida albicans.
