Effect of Antibiotic Therapy in Acute Salmonellosis on the Fecal Excretion of Salmonellae

Treatment of patients with acute salmonella gastroenteritis with either chloramphenicol or ampicillin prolonged the period of postconvalescent excretion of salmonellae. Of 185 patients treated with antibiotics, 65.4 per cent were still positive for Salmonella typhimurium 12 days after exposure, and 27.0 per cent were positive at 31 days. In contrast, of 87 who were not treated, only 42.5 per cent and 11.5 per cent were positive at 12 and 31 days, respectively. Therapy also favored the in vivo acquisition of antibiotic resistance by the infecting strain, which was initially susceptible to multiple antibiotics. Salmonella strains resistant to one or more antibiotics were isolated from 9.7 per cent of patients treated with antibiotics, whereas no resistant strains were obtained from untreated patients. Resistance transfer factor was demonstrated in nine of 12 strains with acquired multiple resistances that were studied. Thus, antibiotic therapy increases the opportunity not only for person-to-person spread of infection, but also for dissemination of resistant organisms.