The cholera bacterium, Vibrio cholerae, flourishes in countries suffering from poor sanitation and urban crowding. Now a new study suggests that the presence of human beings can make the disease more communicable. Findings published today in the journal Nature indicate that passage through the human digestive system produces a version of the bacteria up to 700 times more infectious than the same bacteria originating in water or soil.

Andrew Camilli of Tufts University and his colleagues collected bacterial samples from the stools of cholera sufferers in Bangladesh and compared them with a V. cholerae strain grown in the laboratory. They injected both forms of the bacterium into mice and cultured their intestinal bacteria a few days later. The version extracted from the stools of human beings, the team found, had outcompeted and was far more infectious than its nondigested alter ego. To be medically significant, however, this increased virulence must occur not only in the laboratory but also persist in the natural world. To determine if this is the case, the scientists cultured human-shed V. cholerae in pond water for a few hours and then injected those microbes along with the laboratory version into different mice. The same result materialized: the hyperinfectious state of human-passaged cholera bacteria remained intact.

But why are these bacteria so much more dominant? The researchers attempted to answer this question by establishing which genes are active--and thus, which proteins are made--in V. cholerae that has passed through a human as compared with bacteria that has not. Using this approach, they found 237 differentially regulated genes. After examining the functions of the related proteins, the group proposed that increased shedding of the bacteria from the gastrointestinal tract or a faster bacterial growth rate could contribute to the higher infectiousness they observed.

Cholera produces watery diarrhea and uncontrolled vomiting in victims, which causes severe dehydration and can sometimes lead to death. The new findings could reveal important targets for antimicrobial therapies or vaccines to treat this debilitating illness. The authors conclude that "the fact that the human host not only provides a suitable niche for growth but also prepares V. cholerae for infection of additional humans has interesting implications for the study of human-to-human spread of other virulent microorganisms."

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