Theresa.J.Ochoa@uth.tmc.edu / Article / BACKGROUND: Diarrheagenic Escherichia coli strains are being recognized as important pediatric enteropathogens worldwide. However, it is unclear whether there are differences in age-related susceptibility to specific strains, especially among infants. METHODS: We conducted a passive surveillance cohort study of diarrhea that involved 1034 children aged 2-12 months in Lima, Peru. Control stool samples were collected from randomly selected children without diarrhea. All samples were analyzed for common enteric pathogens and for diarrheagenic E. coli with use of multiplex real-time polymerase chain reaction. RESULTS: The most frequently isolated pathogens in 1065 diarrheal episodes were diarrheagenic E. coli strains (31%), including enteroaggregative (15.1%) and enteropathogenic E. coli (7.6%). Diarrheagenic E. coli, Campylobacter species, and rotavirus were more frequently isolated from infants aged >or=6 months. Among older infants, diffusely adherent E. coli and enterotoxigenic E. coli were more frequently isolated from diarrheal samples than from control samples (P <.05). Children aged >or=6 months who were infected with enterotoxigenic E. coli had a 4.56-fold increased risk of diarrhea (95% confidence interval, 1.20-17.28), compared with younger children. Persistent diarrhea was more common in infants aged <6 months (13.5% vs 3.6%; P <.001). Among children with diarrheagenic E. coli-positive samples, coinfections with other pathogens were more common in children with diarrhea than in control children (40.1% vs 15.6%; P <.001). CONCLUSIONS: Diarrheagenic E. coli strains were more frequently isolated in samples from older infants. In this setting with high frequency of pathogen exposure and high frequency of breastfeeding, we hypothesize that the major age-related differences result from decreased exposure to milk-related protective factors and from increased exposure to contaminated food and water.