Although the initial phases of intestinal infection with Vibrio cholerae have been intensively scrutinized, the later stages of infection, which are characterized by mass detachment of bacteria from epithelial surfaces and movement into the intestinal lumen, have been largely overlooked. Alex Nielsen and Nadia Dolganov, working in the Schoolnik laboratory, now redress this balance by providing insights into this luminal migration, which takes place about 12 hours after infection and which they call the 'mucosal escape response'. They reveal that this phenomenon is accompanied by the induction of a specific genetic programme in V. cholerae that activates a range of motility and chemotaxis genes.

The authors used a combination of confocal microscopy, expression profiling and mutant analysis to study the mucosal escape response in a rabbit model of V. cholerae infection, the ligated ileal-loop model. They first showed that 12 hours after inoculation in this model, at the time of bacterial detachment, V. cholerae replication slows to a rate similar to that of organisms entering stationary phasein culture. The stationary phase alternative sigma factor RpoS is required for the mucosal escape response, evidenced by the fact that an rpoS mutant strain did not detach from ileal-loop epithelial cells. HapR, an RpoS-controlled factor that downregulates virulence determinant expression and biofilm formation, is also required for this stage of cholera infection.

Expression profiling of stationary phase cultures and bacteria collected from the luminal fluid at the time of the bacterial detachment revealed the upregulation of genes involved in chemotaxis and bacterial motility, including those involved in flagella biosynthesis. Importantly, 81% of these upregulated genes were dependent on RpoS for their induction during stationary phase. The assembly of flagella and the activation of chemotactic function during the mucosal escape response would facilitate efficient movement of bacteria into the intestinal lumen in preparation for exit from the host. Schoolnik and colleagues point out that these data conflict with previous studies showing that chemotaxis genes are downregulated in cholera patient rice-water stools, and they discuss the possible reasons for these contrasting results.

The effect of RpoS on virulence gene expression was also addressed in a series of experiments. The research team reasoned that the continued production of virulence factors after bacterial detachment would be ineffectual and so RpoS might downregulate virulence gene expression. In accordance, virulence assays confirmed that RpoS markedly downregulated cholera toxin production in vitro.

The authors conclude by proposing a model for the role of RpoS in V. cholerae infection, detailing its part in the genetic programme that integrates the mucosal escape response and the control of virulence and chemotaxis.