Epidemiologic evidence that prior antimicrobial exposure decreases resistance to infection by antimicrobial-sensitive Salmonella

Exposure to antimicrobials is known to increase the risk of infection with antimicrobial-resistant strains of Salmonella, but this effect has not been previously demonstrated with antimicrobialsensitive strains. The effect of prior antimicrobial exposure was studied during a multistate outbreak of salmonellosis caused by an antimicrobial-sensitive strain of Salmonella havana: 19 (26%) of 72 persons infected had taken an antimicrobial within 30 days before onset of symptoms. Antimicrobial therapy was begun a mean of 15.9 days before and stopped a mean of 79 days before onset. In a case-control study, 11 (31%) of 35 case-patients had taken antimicrobials within 30 days of onset compared with 9 (13%) of 70 ageand neighborhood-matched controls (matched odds ratio, 4.3; 95% confidence interval, 1.3, 13.5). The association remained when controlled for the presence of underlying illness or immunosuppression. Prior antimicrobial exposure appears to increase the risk of infection with antimicrobial-sensitive and resistant strains of Salmonella. The delay between antimicrobial use and onset of symptoms suggests that the effect may be due to prolonged alteration of the colonic bacterial flora, resulting in decreased resistance to colonization.