Ertapenem: No effect on aerobic gram-negative susceptibilities to imipenem

SummaryObjectiveTo describe any potential effect on in vitro susceptibility to imipenem for aerobic gram-negative bacteria following the addition of ertapenem to the formulary of a large teaching hospital.MethodsChanges in imipenem susceptibilities for aerobic gram-negative bacteria were compared among clinical isolates from 2002 to 2007 using a Poisson model. Changes in the susceptibility of imipenem to P. aeruginosa, K. pneumoniae, K. oxytoxa, E. coli, and S. marcescens were compared over time using a chi-squared test for trend. Carbapenem use was measured using a defined daily dose per 1000 patient days. Change in utilization was compared for all years. The prevalence of ESBL-producing K. pneumoniae, K. oxytoxa, and E. coli over time was compared.ResultsSusceptibility to imipenem did not change after the addition of ertapenem and this most notably includes P. aeruginosa (p = 0.43). Additionally, an increase in the incidence of ESBL K. pneumoniae from 2002 (4%) to 2007 (18%) p < 0.0001 occurred with significant increases in both imipenem and ertapenem use in that time frame (p < 0.001).ConclusionThe continued use of ertapenem over 5 years with predominant use of imipenem did not select for P. aeruginosa resistance to imipenem. The rising rate of community and healthcare associated ESBL K. pneumoniae increased total carbapenem use.