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At the International Centre for Diarrhoeal Disease Research. Bangladesh, one-half of the rice-water stool samples that were culture-positive for Vibrio cholerae did not contain motile V. cholerae by standard darkfield microscopy and were defined as darkfield-negative (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 bacterio-phage 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 DF4 stools. This biology may impact transmission because epidemiological data suggested that house-hold 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. [ABSTRACT FROM AUTHOR]