Systematic identification of Salmonella T6SS effectors uncovers diverse new families and lipid-targeting activities

Abstract

Bacterial warfare is a widespread phenomenon in which bacteria deploy toxins to inhibit or kill competitors. These toxins disrupt essential cellular processes, and their diversification is driven by an evolutionary arms race involving toxin and immunity gene acquisition. Here, we used in-silico approaches to analyze genomes from the 10k Salmonella Project and identify effectors secreted via the Type VI Secretion System (T6SS). We uncovered 128 candidates distributed across diverse Salmonella serovars and other bacterial species. Among them, Tox-Act1 was selected for in-depth characterization. Tox-Act1 contains a permuted NlpC/P60 papain-like catalytic core typical of lipid-targeting enzymes. Evolutionary analysis revealed its relationship with acyltransferases. Biochemical assays and lipidomics of intoxicated cells showed that Tox-Act1 acts as a phospholipase, cleaving phosphatidylglycerol and phosphatidylethanolamine. We further demonstrate that Tox-Act1 is secreted in a T6SS-dependent manner and provides a competitive advantage during mouse gut colonization. This study broadens our understanding of toxin domain diversity and provides the first direct characterization of a lipid-targeting NlpC/P60 toxin domain.