To 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.

Changes 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.

Susceptibility 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).

The continued use of ertapenem over 5years 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.