A patient with diarrhea has a fecal smear that is negative for leukocytes. The patient&;s diarrhea is most likely caused by

The enterotoxigenic strain of Escherichia coli secretes a heat-labile toxin that stimulates adenylate cyclase, producing a cholera-like syndrome that is responsible for the majority of cases of traveler's diarrhea. Because the bowel mucosa is not invaded and there is no toxin-induced damage, the fecal smear is negative for leukocytes. Campylobacter enteritis, shigellosis, typhoid fever, and amebiasis are all invasive, and infected individuals have stools with inflammatory cells. Typhoid fever is unique because the inflammatory cells are not neutrophils, but mononuclear cells. Ref - Harrison's internal medicine 20e pg 1150t,1152-1153