Antimicrobial susceptibility in gram-negative bacteremia: are nosocomial isolates really more resistant?

Bloodstream isolates of gram-negative aerobic bacilli from nosocomial infections are more likely to be resistant to antimicrobial agents than isolates from community-acquired cases are. It is not clear, however, how much this is due to the markedly different distribution of organisms in the two groups. We compared the susceptibilities of organisms of a given species which caused community-acquired bacteremia with the susceptibilities of isolates from nosocomial cases. Nine antimicrobial agents were tested against 1,077 isolates which were obtained during a 4-year nonepidemic period. Marked differences in crude rates of resistance were noted for all isolates from nosocomial cases versus all isolates from cases acquired in the community. When results were adjusted for the different organism distributions in the two groups, statistically significant differences were found for only six drug-organism pairs; in each of these, resistance rates were higher in nosocomial isolates. However, when results were further adjusted for the effect of multiple analyses, no significant differences were seen. The major factor leading to the greater prevalence of antimicrobial resistance in our hospital organisms was the markedly different distribution of organisms in the nosocomial and community-acquired groups. For individual organisms, greater resistance in nosocomial strains was confined to certain drugs. Factors that influence differences in organism distribution may not be solely the result of antimicrobial use.