Changes in antibiotic usage and susceptibility in nosocomial Enterobacteriaceae and Pseudomonas isolates following the introduction of ertapenem to hospital formulary

There is concern that widespread usage of ertapenem may promote cross-resistance to other carbapenems. To analyse the impact that adding ertapenem to our hospital formulary had on usage of other broad-spectrum agents and on susceptibilities of nosocomial Enterobacteriaceae and Pseudomonas isolates, we performed interrupted time-series analyses to determine the change in linear trend in antibiotic usage and change in mean proportion and linear trend of susceptibility pre-(March 2004-June 2005) and post-(July 2005-December 2008) ertapenem introduction. Usage of piperacillin-tazobactam (P = 0.0013) and ampicillin-sulbactam (P = 0.035) declined post-ertapenem introduction. For Enterobacteriaceae, the mean proportion susceptible to ciprofloxacin (P = 0.016) and piperacillin-tazobactam (P = 0.038) increased, while the linear trend in susceptibility significantly increased for cefepime (P = 0.012) but declined for ceftriaxone (P = 0.0032). For Pseudomonas, the mean proportion susceptible to cefepime (P = 0.011) and piperacillin-tazobactam (P = 0.028) increased, as did the linear trend in susceptibility to ciprofloxacin (P = 0.028). Notably, no significant changes in carbapenem susceptibility were observed.