Formula feeding is an alternative method to prevent mother-to-child infection with human immunodeficiency virus through breast-feeding in developing countries. Growth of bacterial pathogens in reconstituted infant formula has become a health hazard when contaminated water is used for rehydration. This study was conducted to assess bacterial safety risk of using contaminated water to reconstitute infant formula. Survival and growth characteristics were determined for three bacterial pathogens, Vibrio cholerae O1, Shigella flexneri, and Salmonella enterica serovar Enteritidis, inoculated into sterile tap water (3.2-3.4 log10 colony-forming units [CFU]/ml) and infant formula (1.5-1.7 and 3.2-3.4 log10 CFU/ml) and incubated at 4 degrees C or 30 degrees C for up to 24 hours. Vibrio cholerae O1 was the most sensitive of the three pathogens when inoculated into water, with no viable cells detected within 2 hours at 4 degrees C or 30 degrees C. The rate of inactivation in water was greater at 30 degrees C than at 4 degrees C. Vibrio cholerae O1, Shigella flexneri, and Salmonella enterica serovar Enteritidis grew rapidly in infant formula at 30 degrees C, reaching populations of 9.2, 8.7, and 9.2 log10 CFU/ml, respectively, at 24 hours. Populations of all three pathogens did not change significantly after incubating infant formula for 24 hours at 4 degrees C, but continuously decreased in water throughout incubation for 24 hours, regardless of temperature. Results suggest that unless refrigerated, reconstituted infant formula should be consumed soon after preparation to avoid increased risk of illness associated with increases in populations of pathogenic bacteria that may be introduced by contaminated water.