Bacillus velezensis NC-B4 as a promising antifungal agent for biocontrol of Candida auris.

Introduction: Candida auris, known as the "super fungus", is commonly existed in hospital. The treatment of C. auris infection is difficult for its multi-drug resistance and difficult to accurately detect. The use of synthetic antibacterial agents has caused major problems such as drug-resistance and environment pollution and negatively affects non-target species. Microbial biocontrol agents (probiotics) are needed for fungal infection. Bacillus and related genera produce a variety of bioactive substances. As probiotics, it has been widely studied in the field of medicine and is a novel microbial factor for biological control.

Methods: B. velezensis NC-B4 was isolated using gradient dilution method. Then it was identified by phylogenetic analysis and physiological and biochemical characteristics. The antibacterial mechanism of NC-B4 was explored by detecting cellulase, protease and genomic analysis. Then antimicrobial effects were analyzed by detecting the growth and biofilm of C. auris BJCA001. Finally, the cytotoxicity and the protective effect on mice were analyzed by cell line and mouse systemic infection models.

Results: We isolated B. velezensis NC-B4, which showed cellulase, protease enzyme activity and antimicrobial effects against human pathogenic fungi by inhibiting the growth of Candida auris, Cryptococcus neoformans, Candida albicans and mycelial fungus. B. velezensis NC-B4 inhibited the biofilm formation and growth of C. auris. B. velezensis NC-B4 has a protective effect against the toxicity of Candida auris in A549 cell line and mouse systemic infection models. The complete genome of B. velezensis NC-B4 was 3.93 Mb with a 46.5% G+C content and possessed the macrolactin H, bacillaene, fengycin, difficidin, bacillibactin and bacilysin biosynthesis cluster, which known as key factors in biological control.

Discussion: The results of the present study indicated that B. velezensis NC-B4 has antimicrobial properties for its cellulase, protease and antibacterial secondary metabolites, thereby inhibiting the growth of pathogenic bacteria and the formation of biofilms. B. velezensis NC-B4 is expected to be developed as a source for probiotics or new antibiotics.