274. Eight broad-spectrum -lactam agents tested against a large collection of KPC-producing Enterobacteriaceae: Evaluation of current CLSI breakpoints
Session: Poster Abstract Session: Antimicrobial Susceptibility Testing
Friday, October 21, 2011: 1:15 PM
Room: Poster Hall B1

Background: MIC values for the β-lactam agents can vary among strains producing β-lactamases, including KPC serine-carbapenemases, complicating detection by clinical microbiology laboratories. We evaluated the MIC results and current CLSI breakpoints for eight broad spectrum β-lactams tested against a large collection of KPC-producing Enterobacteriaceae.

Methods: 374 KPC-producing Enterobacteriaceae were susceptibility (S) tested by CLSI broth microdilution method (BMD; M07-A8, 2009). Isolates carrying the KPC gene were screened based on imipenem (IMI) or meropenem (MER) MIC values at ≥2 g/mL (non-S). Strains showing MIC values below breakpoints for any of the 8 β-lactams were retested by BMD and PCR targeting KPC-encoding genes.

Results: All 374 KPC-producers (283 K. pneumoniae [KPN], 41 E. cloacae and 7 other species [<20/species]) isolates were categorized using CLSI breakpoints as non-S to aztreonam (MICs, 8-≥32 g/ml), ceftriaxone (MIC, ≥4 g/ml), ertapenem (MIC, ≥0.5 g/ml), IMI (MIC, >2 g/ml) and piperacillin/tazobactam (MIC, ≥32 g/ml). 52 strains (13.9%; 20 KPN 11 K. oxytoca, 7 E. coli, 7 C. freundii, 5 E. cloacae, 2 Raoultella planticola) were categorized as S to cefepime (CEP), with MIC values as low as 1 g/ml. Eight (2.1%) strains were S to ceftazidime (CAZ; including 4 E. coli, 2 K. oxytoca, 2 KPN) and 2 KPN were MER-S. All CAZ-S strains were also S to CEP and one was S to MER.

Conclusion: An elevated percentage of KPC-producing isolates were CEP-S by reference BMD (>13%). This compound demonstrated poor prediction of the presence of KPC and revising the current breakpoints might be considered. Infectious disease and clinical microbiology professionals should be aware that depending on the genetic location of blaKPC, number of copies and presence of other β-lactamases, variations in the β-lactams MICs to S levels may be observed.

Subject Category: A. Antimicrobial agents and Resistance

Mariana Castanheira, PhD, Lalitagauri M. Deshpande, Ph.D., Rodrigo E. Mendes, Ph.D., Helio S. Sader, M.D., Ph.D. and Ronald Jones, MD, Microbiology, JMI Laboratories, North Liberty, IA


M. Castanheira, None

L. M. Deshpande, None

R. E. Mendes, None

H. S. Sader, None

R. Jones, None

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.