The planktonic V. cholerae that are shed from humans in rice-water stool are central to the life cycle. These highly motile bacteria can be ingested by a new human host after shedding, or can associate with abiotic surfaces (possibly forming biofilms), copepods, algae and Chironomid egg sacs in the environment. These environmental V. cholerae can, presumably, disassociate from these hosts and be ingested by humans or form associations with a new environmental host. Alternatively, or in addition, environmental host-associated V. cholerae can be ingested by humans, causing infection and resulting in shedding of planktonic V. cholerae, therefore completing the life cycle. Chironomid egg mass panel reproduced with permission from REF. 126 © (2003) American Society for Microbiology.

Vibrio cholerae colonizes the small intestine in infant and adult humans and in experimentally infected infant mammals. Although several animal models of V. cholerae infection have been developed, the most widely used is colonization of the infant-mouse small intestine following oral inoculation92. Important factors for human colonization have been identified using this model, lending credence to its use as a viable model of infection. Furthermore, virulence genes that are crucial for infection have been shown to be expressed in infant mice93. A schematic of the small intestine is shown. a | In the scanning electron micrograph (SEM) of an opened section of infant-mouse small intestine, the numerous and closely packed villi are seen. The villi surround a central lumen (not shown), and intervillous spaces are present between villi. At the base of the villi are the crypts of Lieberkühn. b,c | The villi are composed of absorptive epithelial cells (enterocytes) as well as Goblet cells, which produce mucus. This mucus forms a gel covering the villi, which concentrates at the tips. As V. cholerae is known to colonize epithelial surfaces on the villi and crypts, it must therefore penetrate not only the mucus layer present at the tips of the villi, but also any mucus gel covering the intestinal epithelium.

There are many schemes for flagellation in bacteria, of which peritrichous flagella and a single polar (monotrichous) flagellum are two types. a | In the case of peritrichous flagella, such as those found in Escherichia coli, counter-clockwise (CCW) flagellar rotation results in the formation of a helical bundle that propels the cell forward in one direction in a smooth-swimming motion (a ‘run’). By contrast, the presence of clockwise (CW) rotation causes unbundling of the helical bundle, allowing the bacterium to randomly reorient its direction (a ‘tumble’). b | In the case of a single polar flagellum, CCW rotation propels the cell forward in a run, whereas CW rotation propels the cell backward with a concomitant random reorientation.

A gradient of chemoattractant is present in the lumen of the upper small intestine, oriented away from the villi (shown by arrows, panels a,c). Using chemotaxis, wild-type V. cholerae responds to this gradient by concentrating within the lumen. However, this chemoattractant gradient is absent from the lumen of the lower small intestine (panels b,d), allowing wild-type V. cholerae to respond to a different chemoattractant gradient that directs them into the intervillous spaces and onto epithelial surfaces (see scanning electron micrograph (SEM) panels). By contrast, non-chemotactic counter-clockwise (CCW)-biased V. cholerae are blind to these gradients and therefore colonize both the upper and lower small intestine, presumably on the luminal side of the mucus gel and villi (panels c,d). The V. cholerae that get stuck in mucus can also multiply, as evidenced by the presence of dividing bacteria in the SEM of mucus. SEM lower panel reproduced with permission from REF. 94 © (2002) American Association for the Advancement of Science.