Diverse toxic effectors are harbored by vgrG islands for interbacterial antagonism in type VI secretion system

The type VI secretion system (T6SS) is considered as one of the key competition strategies by injecting toxic effectors for intestinal pathogens to acquire optimal colonization in host gut, a microenviroment with high-density polymicrobial community where bacteria compete for niches and resources. Enterotoxigenic Escherichia coli (ETEC), a major cause of infectious diarrhea in human and animals, widely encode T6SS clusters in their genomes. In this report, we first identified VT1, a novel amidase effector in ETEC, significantly hydrolyzed D-lactyl-L-Ala crosslinks between N-acetylmuramoyl and L-Ala in peptidoglycan. Further study showed that the VT1/VTI1 effector/immunity pair is encoded within a typical vgrG island, and plays a critical role for the successful establishment of ETEC in host gut. Numerous putative effectors with diverse toxin domains were found by retrieving vgrG islands in pathogenic E. coli, and designated as VT modules. Therein, VT5, a lysozyme-like effector widely encoded in ETEC, was confirmed to effectively kill adjacent cells, suggesting that VT toxin modules may be critical for pathogenic E. coli to seize a significantly competitive advantage for optimal intestinal colonization. To expand our analyses for large-scale search of VT antibacterial effectors based on vgrG island, >200 predicted effectors from 20 bacterial species were found and classified into 11 predicted toxins. This work reports a new retrieval strategy for screening T6SS effectors, and provides an example how pathogenic bacteria antagonize and displace commensal microbiome to successfully colonize in the host niches through a T6SS-dependent manner.