Correlation of TEM, SHV and CTX-M extended-spectrum beta lactamases among Enterobacteriaceae with their in vitro antimicrobial susceptibility

Purpose: The present study was carried out to characterize the ESBL types and evaluated their in vitro activity against a collection of Gram negative bacteria (GNB) from a multicentric Indian surveillance study.
Material and Methods: During January 2005 to June 2006, six tertiary care centres in India forwarded 778 non-duplicate GNB to our reference laboratory. Three hundred GNB from this collection were selected based on clinical significance and were used in the present study. Tested isolates included Escherichia coli (167), Klebsiella spp. (122) and Enterobacter spp. (11). ESBL screening and confirmation was performed for all the isolates. Minimum inhibitory concentration of imipenem, meropenem, ertapenem, levofloxacin, amikacin, piperacillin/tazobactam and ceftriaxone was determined by the E-test method. Molecular typing of the ESBLs was performed by polymerase chain reaction among the 121 selected isolates.
Results: The study showed excellent susceptibility among the strains to imipenem (100%), meropenem (100%) and ertapenem (98.7%); good susceptibility to amikacin (89.7%) and piperacillin/tazobactam (85.3%) was observed. TEM and CTX-M were predominantly found in E. coli (39.2%) while, among the Klebsiella spp., TEM, SHV and CTX-M occurred together in 42.6% of the isolates.
Conclusion: More than one ESBL was produced by many strains, and this was correlated with increased resistance levels. Carbapenems continue to show good in vitro activity and ertapenem is a potential alternative to imipenem and meropenem. Continued antimicrobial resistance surveillance is warranted in light of these findings.

Keywords
Carbapenems, extended-spectrum beta lactamase, resistance, susceptible