Identification of a fifth antibacterial toxin produced by a single Bacteroides fragilis strain

ABSTRACT

Bacteroidales are the most abundant Gram-negative bacteria of the healthy human colonic microbiota, in many individuals comprising nearly 50% of their colonic bacteria. Numerous species and strains of gut Bacteroidales are present simultaneously at high concentration in this ecosystem. Studies are revealing that gut Bacteroides have numerous antibacterial weapons to antagonize closely related members. In this study, we identify a new diffusible antibacterial toxin produced by B. fragilis 638R, designated BSAP-4. This is the fifth antibacterial toxin produced by this strain and the second toxin of this strain with a membrane attack complex/perforin domain (MACPF). We identify tthat he target molecule of sensitive cells is a β-barrel outer membrane protein (OMP) with calycin-like domains. As with other MACPF toxins, the gene encoding the target in sensitive strains is in the same genetic region as bsap-4 is in producing strains. Comparison of B. fragilis strains shows there are two sensitive variants of this OMP that are 87% similar to each other and 50% similar to the resistant OMP. Unlike other MACPF toxins, there are numerous B. fragilis strains that encode the resistant OMP without bsap-4. Several OMP variants from strains that are BSAP-4 resistant under the conditions of our assay confer BSAP-4 sensitivity to B. thetaiotaomicron when constitutively expressed. Using a reporter assay, we show that the BSAP-4 receptor gene is differentially expressed in sensitive and resistant strains leading to apparent BSAP-4 resistance under the conditions of our assay, despite harboring the BSAP-4 target gene

Importance The intestinal microbiota is a diverse microbial ecosystem that provides numerous benefits to humans. The factors that govern its establishment and stability are just beginning to be elucidated. Identification and characterization of antimicrobial toxins produced by its members and their killing range is essential to understanding the role of antagonism in community composition and stability. Here, we identify a fifth antimicrobial toxin produced by a single B. fragilis strain and identify its target. The finding of such a large number of toxins that antagonize competing members suggests that this feature substantially contributes to the fitness of these bacteria. In addition, these toxins may have applications in genetically engineered gut bacteria to allow engraftment or to antagonize a potentially pathogenic member.