HilD-regulated chemotaxis proteins contribute to Salmonella Typhimurium colonization in the gut.

In the enteric pathogen Salmonella Typhimurium, invasion and motility are coordinated by HilD, a master regulator that activates expression of genes encoding the type III secretion system 1 and some motility genes, including the chemotaxis gene mcpC. Previously, we have shown that McpC induces smooth swimming, which is important for type III secretion system 1-dependent invasion of epithelial cells. Here, we have studied another Salmonella-specific chemotaxis gene, mcpA, and demonstrate that it is also HilD regulated. Whereas HilD induction of mcpC occurs by direct derepression of H-NS, mcpA induction requires neither H-NS derepression nor the flagellar-specific sigma factor fliA; instead it occurs through a HilD-SprB regulatory cascade, providing experimental confirmation of previous transcriptional regulatory mapping. McpA and McpC contain methyl-accepting domains characteristic of bacterial chemoreceptors, and McpA also contains a chemoreceptor zinc-binding (CZB) protein domain found in a variety of bacterial proteins, many of which are involved in signaling or regulatory roles. Here, we show that, in a mouse model for acute Salmonella colitis, both mcpA and mcpC deletion mutants are outcompeted by wild-type Salmonella Typhimurium in the gut lumen. CZB domains bind Zn²⁺ through a conserved cysteine residue and are thought to perform redox-sensing through redox-initiated alterations in zinc homeostasis. We found that the conserved cysteine is required for McpA function in the mouse gut, thus demonstrating a virulence role for the CZB Zn²⁺-binding site during infection.

Importance: The gut-adapted bacterium Salmonella Typhimurium causes inflammatory diarrhea via a process that involves active invasion of intestinal epithelial cells, secretion of inflammatory molecules, and recruitment of immune cells. Although bacterial motility and invasion of host cells are coordinated, how directed movement facilitates luminal survival and growth or invasion at the mucosal surface is not understood. Chemotaxis is the process by which bacteria control movement toward attractants and away from repellents. Previously, we identified a Salmonella-specific chemoreceptor, McpC, that is co-expressed with the invasion machinery and promotes smooth swimming for optimal host cell invasion. Here, we investigated another chemoreceptor, McpA, also regulated with invasion-associated genes and show it contributes to luminal expansion rather than invasion of epithelial cells. McpA activity requires a conserved Zn²⁺-binding domain, thought to be involved in sensing inflammation. This work demonstrates that coordination of invasion and chemotaxis plays a significant role in the gut.