Prior antimicrobial therapy and resistance of Enterobacter Citrobacter and Serratia to third generation cephalosporins.

During a 6-month period all inpatients from whom Enterobacter, Citrobacter or Serratia. had been isolated were reviewed and information on selected variables recorded. Two groups, one including 19 patients with organisms resistant to third generation cephalosporins and the other 111 patients with susceptible organisms were compared. In the initial analysis, the mean number of antimicrobials received in the prior 2 months was the variable most strongly associated with isolation of resistant organisms (2.6 +/- 1.5 vs 1.5 +/- 1.6; P = 0.002). When patients who had received no antimicrobials were omitted from the analysis, the mean number of antimicrobials was similar (2.6 +/- 1.5 vs 2.3 +/- 1.5; P = 0.19). Comparisons of antimicrobials received in the prior 2 months showed only cefoxitin (9/70 vs 7/19; P = 0.016) and cefotaxime (4/70 vs 5/19; P = 0.008) to be associated with isolation of resistant organisms. These data suggest that, at our institution, antimicrobial therapy with an extended spectrum cephalosporin is an important risk factor for subsequent acquisition of an organism resistant to third generation cephalosporins.