Discordant Ertapenem/Imipenem Susceptibilities in Enterobacter Bacteremia: Frequency and Outcomes

Abstract

Background. Enterobacter spp. are an important cause of infections and antibiotic resistance, including carbapenem resistance. Recent studies suggest that infections due to carbapenem-resistant Enterobacteriaceae (CRE) carrying carbapenemase-encoding genes carry a worse prognosis than CRE that do not carry a carbapenemase-encoding gene, but it is unknown whether there are differences by different bacterial genera/species. CRE that are resistant only to one carbapenem -almost always ertapenem -have been reported, but the frequency of discordance between ertapenem and imipenem/ meropenem is not known for Enterobacter spp. Methods. We examined all cases of Enterobacter bacteremia from January 2012 to December 2016 at the Michael E. DeBakey VA Medical Center in Houston, Texas, USA. Clinical and microbiological data were independently extracted by two investigators. Antibiotic suscepitibilty testing results were interpreted according to current CLSI breakpoints. Results. Discordance between ertapenem and imipenem susceptibilities was found in 14/67 (20.9%) isolates; eight isolates were ertapenem susceptible/imipenem non-susceptible, and six isolates were ertapenem non-susceptible/imipenem susceptible. Bacteremia cleared in 94.5% (3/55) of all patients who had follow-up cultures, including infection by all (13/13) isolates with discordant carbapenem susceptibilities. Fourteen-day mortality was not different between the patients with concordant and discordant carbapenem susceptibilities (7.5% vs. 21.4%; P = 0.3408, Fisher's exact test); four patients died within 48 hours of bacteremia and the remaining three deaths had documented clearance of bacteremia. Conclusion. To our knowledge, these are the first systematic data showing bacteremia by Enterobacter spp. with discordant ertapenem and imipenem/meropenem susceptibilities is not infrequent; interestingly, ertapenem susceptible/imipenem non-susceptible was more common than ertapenem non-susceptible/imipenem susceptible isolates. Bacteremia by Enterobacter isolates with discordant carbapenem results were rapidly cleared and no difference in mortality was detected.