Laboratory investigation of diarrhea in travelers to Mexico: evaluation of methods for detecting enterotoxigenic Echerichia coli

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Abstract
A laboratory investigation was conducted on cultures collected from travelers before, during and after a trip to Mexico to characterize the etiology of traveler''s diarrhea. Four laboratory methods for detecting enterotoxigenicity of E. coli were evaluated: the infant mouse assay, the Chinese hamster ovary (CHO) cell assay, the Y1 [mouse] adrenal [tumor] cell assay and the rabbit ileal loop. Although a number of common enteric pathogens were identified as a cause of traveler''s diarrhea, including 6 serotypes of Salmonella, 2 serotypes of Shigella, Vibrio parahaemolyticus, Giardia lamblia and Entamoeba histolytica, enterotoxigenic E. coli was most commonly isolated. Strains were identified that produced only heat-labile enterotoxin (LT), only heat-stable enterotoxin (ST), or both LT and ST. The infant mouse assay yielded results falling into 2 distinct groups, providing a clear separation of positive and negative cultures. The CHO assay also formed 2 groups, with positive cultures producing 11% or more of the elongated cells. There was good agreement between the CHO and the Y1 adrenal cell assays for detection of LT. The adrenal cell system for detection of LT was more suitable than the CHO assay for processing large numbers of specimens because of the miniculture modification of this method utilized. The infant mouse method was a simple and reliable method for detecting ST.