Influence of extended-spectrum β-lactams on gram-negative bacterial resistance

Purpose. The influence of extended-spectrum β-lactams on gram-negative bacterial resistance was studied. Methods. Hospital pharmacists were asked to provide data on antimicrobial use and bacterial susceptibilities. Defined daily doses per 1000 patient-days of cefepime, ceftazidime, ceftriaxone, and piperacillin-tazobactam were assessed for significant associations with gram-negative susceptibility. To account for midyear changes in usage patterns and the lag between changes in usage and resistance, susceptibility over two-year periods was evaluated. Results. Susceptibility data of more than 300,000 gram-negative isolates, representing 10 species of interest, were provided by 82 U.S. hospitals. Two-year periods (n = 45) were evaluable for 25 hospitals, containing 159 hospital-years of data and 204,513 clinical isolates. Use of cefepime increased, while use of ceftazidime, ceftriaxone, and piperacillin-tazobactam decreased. Excluding Pseudomonas aeruginosa, bacteria were most susceptible to cefepime, followed by piperacillin-tazobactam, ceftriaxone, and ceftazidime. Significantly decreased susceptibilities of gram-negative bacteria to the antibiotics themselves were observed with ceftazidime (Enterobacter aerogenes) and ceftriaxone (Escherichia coli). Extended-spectrum β-lactams associated with significantly decreased susceptibilities of gram-negative bacteria to other β-lactams included cefepime (E. aerogenes and E. coli susceptibility to ceftazidime), ceftazidime (Enterobacter cloacae susceptibility to cefepime), ceftriaxone (E. cloacae susceptibility to cefepime), piperacillin-tazobactam (E. cloacae, Klebsiella pneumoniae, and P. aeruginosa susceptibility to cefepime). There were insufficient susceptibility data to allow for impact analysis for piperacillin-tazobactam. Conclusion. Use of cefepime, ceftazidime, ceftriaxone, and piperacillin-tazobactam was associated with variably changing susceptibilities of several key gram-negative bacteria, either to themselves or to each other. Despite the increased use of cefepime, gram-negative bacterial susceptibility to cefepime remained high.