Susceptibility of gram-negative aerobic bacilli from intra-abdominal pathogens to antimicrobial agents collected in the United States during 2011

SummaryObjectivesDuring 2011, a total of 1442 gram-negative pathogens from intra-abdominal infections were collected as part of the Study for Monitoring Antimicrobial Resistance Trends (SMART) from 19 hospital sites within the United States. Susceptibility to ertapenem and comparators and molecular analysis of ertapenem resistant isolates was performed.MethodsExtended-spectrum beta-lactamase ESBL (ESBL) isolates were determined using the Clinical and Laboratory Standards Institute's recommended phenotypic test. Isolates were identified to the species level, and tested for antimicrobial susceptibility using custom MicroScan dehydrated broth microdilution panels ESBLs and carbapenemases were characterized using the Check-Points microarray. Strain typing of Klebsiella pneumoniae was performed by rep-PCR on the DiversiLab System.ResultsThe majority of isolates were Escherichia coli (36%), K. pneumoniae (18.6%), Pseudomonas aeruginosa (12.1%) and Enterobacter cloacae (8.4%). Incidence of ESBL-positive isolates was 12.7%, 9.7%, 3.6% and 3.1% for K. pneumoniae, E. coli, Proteus mirabilis and Klebsiella oxytoca, respectively. Against the majority of isolates and species tested, the most active antibiotics were amikacin, ertapenem, and imipenem, with the carbapenems being the most active in vitro, including against ESBL-positive isolates of E. coli. All other antibiotics exhibited diminished activity. Against K. pneumoniae, the carbapenems were notably less active against ESBL-positive isolates though their activity against this sub-population was still the highest of all antibiotics tested; however, 41.1% (14 of 34) of the phenotypically ESBL-positive K. pneumoniae co-produced a carbapenemase (KPC2 or KPC3), and >90% of the isolates producing only an ESBL remained susceptible to ertapenem.ConclusionsFurther monitoring of susceptibility of intra-abdominal isolates is warranted due to limited therapeutic options available to physicians.