Improvement in detection of enterotoxigenic Escherichia coli in patients with travelers' diarrhea by increasing the number of E. coli colonies tested.

No cause of one-third of travelers' diarrhea (TD) cases can be detected despite microbiologic assessment. We propose that these pathogen-negative TD cases include undetected enterotoxigenic Escherichia coli (ETEC). As a standard in diagnostic microbiology, samples of five E. coli colonies were tested to detect ETEC from stool cultures. We compared the sensitivities and the number of ETEC detected by 20 colonies with those of 5, 10, and 15 colonies. The percent detection of ETEC with five E. coli colonies was significantly different from that using 20 E. coli colonies. Of the 116 subjects studied, the number positive for ETEC for 5, 10, 15 and 20 colonies tested were 22 (19.0%), 37 (31.9%), 45 (38.8%) and 46 (39.7%), respectively. Based on our results, we recommend the testing of at least 10 E. coli colonies for optimum detection of ETEC in patients with TD.