Significance: Gram-negative bacteria use the type VI secretion system (T6SS) to deliver effectors into adjacent cells. Salmonella Typhimurium is an enteric pathogen that causes disease in millions of individuals each year. Its ability to infect the mammalian gut is a key factor that contributes to its virulence and transmission to new hosts. However, many of the details on how Salmonella successfully colonizes the gut and persists among members of the gut microbiota remain to be deciphered. In this work, we provide evidence that Salmonella uses an antibacterial weapon, the type VI secretion system, to establish infection in the gut. In addition, our results suggest that S. Typhimurium selectively targets specific members of the microbiota to invade the gastrointestinal tract.

Size determination represents a fundamental requirement for multicomponent biological structures. Some pathogenic bacteria possess a weapon derived from the flagellum. Like the flagellum, this type-III secretion apparatus, called the injectisome, has a transmembrane basal body, but the external component is a needle-like structure instead of a hook and a filament. Here, we provide evidence that the length of this needle is determined by the size of a protein, YscP, acting as a molecular ruler.