Morbidity of infections caused by antimicrobial-resistant bacteria.

Antimicrobial resistance can have 2 effects on the outcome of infection: there can be an accompanying change in the virulence of the organism, and there can be a poorer response to treatment because of the empiric choice of an antimicrobial to which the organism is resistant. We have reviewed published studies relating antimicrobial resistance to the outcomes of infection caused by enteric pathogens. The data for Salmonella and Campylobacter infections suggest that antimicrobial-resistant strains are somewhat more virulent than susceptible strains-that is, they cause more prolonged or more severe illness than do antimicrobial-susceptible strains. However, not all studies corrected for possible differences in age and underlying diseases between patients infected by antimicrobial-resistant and -susceptible strains of Salmonella. Two studies of Campylobacter infection suggest that poorer outcomes with antimicrobial-resistant pathogens could be related to the initial choice of an ineffective antimicrobial for treatment. Estimates from various sources indicate that fluoroquinolone resistance, likely acquired from the administration of antimicrobials to food animals, leads to >400,000 excess days of diarrhea in the United States per year compared with the duration that would occur if all of the isolates were susceptible. Antimicrobial resistance also could account for an extra 8677 days of hospitalization for nontyphoidal salmonellosis, mainly arising from food animals.