At the International Centre for Diarrhoeal Disease Research, Bangladesh, one-half of the rice-water stool samples that were culturepositive for Vibrio cholerae did not contain motile V. cholerae by standard darkfield microscopy and were defined as darkfieldnegative (DF ؊ ). We evaluated the host and microbial factors associated with DF status, as well as the impact of DF status on transmission. Viable counts of V. cholerae in DF ؊ stools were three logs lower than in DF ؉ stools, although DF ؊ and DF ؉ stools had similar direct counts of V. cholerae by microscopy. In DF ؊ samples, non-V. cholerae bacteria outnumbered V. cholerae 10:1. Lytic V. cholerae bacteriophage were present in 90% of DF ؊ samples compared with 35% of DF ؉ samples, suggesting that bacteriophage may limit culture-positive patients from producing DF ؉ stools. V. cholerae in DF ؊ and DF ؉ samples were found both planktonically and in distinct nonplanktonic populations; the distribution of organisms between these compartments did not differ appreciably between DF ؊ and DF ؉ stools. This biology may impact transmission because epidemiological data suggested that household contacts of a DF ؉ index case were at greater risk of infection with V. cholerae. We propose a model in which V. cholerae multiply in the small intestine to produce a fluid niche that is dominated by V. cholerae. If lytic phage are present, viable counts of V. cholerae drop, stools become DF ؊ , other microorganisms bloom, and cholera transmission is reduced.bacteriophage ͉ biofilm ͉ darkfield ͉ mucin D espite more than a century of investigation, much remains to be discovered about how pathogenic strains of Vibrio cholerae interact with the human host and how the biology of disseminating stool V. cholerae drives devastating cholera outbreaks. Cholera is an ancient secretory diarrheal disease caused by the O1 and O139 serogroups of V. cholerae. Today the burden of disease is estimated to reach as much as several million cases a year in both Asia and Africa, with fewer cases in Latin America (1). Despite the dramatic reduction of mortality rates due to the development of oral rehydration solution (2), the emergence of multiple drug-resistant V. cholerae (3, 4) may reduce the efficacy of antimicrobial treatment and alter the dynamics of outbreaks.V. cholerae is a Gram-negative bacterial pathogen of the gastrointestinal tract. Cholera toxin is largely responsible for the massive fluid loss that may reach between 0.5 and 1.0 liter per hour (2). V. cholerae from human stool is hyperinfectious, compared with laboratory-grown bacteria, with a lower infectious dose in a murine model of infection (5-7). Mathematical models have quantified the importance of hyperinfectivity to the explosive nature of the biannual outbreaks of cholera seen so commonly in areas such as Bangladesh (8), but proof of direct person-to-person transmission of hyperinfectious V. cholerae is still lacking. One force driving the subsequent collapse of a cholera outbreak has been proposed to be the presence of bacter...
