Highlights d Tlde1 is an antibacterial T6SS effector related to L,Dtranspeptidases d Tlde1 does not form crosslinks between peptide stems d Tlde1 has L,D-carboxypeptidase and L,D-transpeptidase exchange activity d Tlde1 reduces the availability of peptidoglycan precursors and impairs synthesis

Bacteria commonly live in dense polymicrobial communities and compete for scarce resources. Consequently, they employ a diverse array of mechanisms to harm, inhibit and kill their competitors. The cell wall is essential for bacterial survival by providing mechanical strength to resist osmotic stress. Because peptidoglycan is the major component of the cell wall and its synthesis is a complex multi-step pathway that requires the coordinate action of several enzymes, it provides a target for rival bacteria, which have developed a large arsenal of antibacterial molecules to attack the peptidoglycan of competitors. These molecules include antibiotics, bacteriocins and contact-dependent effectors that are either secreted into the medium or directly translocated into a target cell. In this review, we summarize the diversity of these molecules and highlight distinct mechanisms to disrupt the peptidoglycan, giving special attention to molecules that are known or have the potential to be used during interbacterial competitions.

A commensal microbial community is established in the mammalian gut during its development, and these organisms protect the host against pathogenic invaders. The hallmark of noninvasive Salmonella gut infection is the induction of inflammation via effector proteins secreted by the type III secretion system, which modulate host responses to create a new niche in which the pathogen can overcome the colonization resistance imposed by the microbiota. Several studies have shown that endogenous microbes are important to control Salmonella infection by competing for resources. However, there is limited information about antimicrobial mechanisms used by commensals and pathogens during these in vivo disputes for niche control. This review aims to revisit the steps that Salmonella needs to overcome during gut colonization—before and after the induction of inflammation—to achieve an effective infection. We focus on a series of reported and hypothetical antagonistic interbacterial interactions in which both contact‐independent and contact‐dependent mechanisms might define the outcome of the infection.

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The type VI secretion system (T6SS) is a molecular machine that Gram-negative bacteria have adapted for multiple functions, including interbacterial competition. Bacteria use the T6SS to deliver protein effectors into adjacent cells to kill rivals and establish niche dominance. Central to T6SS mediated bacterial competition is an arms race to acquire diverse effectors to attack and neutralize target cells. The peptidoglycan has a central role in bacterial cell physiology, and effectors that biochemically modify peptidoglycan structure effectively induce cell death. One such T6SS effector is Tlde1a from Salmonella Typhimurium. Tlde1a functions as an LD-carboxypeptidase to cleave tetrapeptide stems and as an LD-transpeptidase to exchange the terminal D-alanine of a tetrapeptide stem with a noncanonical D-amino acid. To understand how Tlde1a exhibits toxicity at the molecular level, we determined the X-ray crystal structure of Tlde1a alone and in complex with D-amino acids. Our structural data revealed that Tlde1a possesses a unique LD-transpeptidase fold consisting of a dual pocket active site with a capping subdomain. This includes an exchange pocket to bind a D-amino acid for exchange and a catalytic pocket to position the D-alanine of a tetrapeptide stem for cleavage. Toxicity assays in Escherichia coli and in vitro peptidoglycan biochemical assays with Tlde1a variants, correlate Tlde1a molecular features directly to its biochemical functions. We observe that the LD-carboxypeptidase and LD-transpeptidase activities of Tlde1a are both structural and functionally linked. Overall, our data highlights how an LD-transpeptidase fold has been structurally altered to create a toxic effector in the T6SS arms race.