Emergence of resistance to imipenem in Enterobacter isolates masquerading as Klebsiella pneumoniae during therapy with imipenem/cilastatin.

Clinical isolates identified as Klebsiella pneumoniae by the Vitek, Enterotube II, and API 20E systems were recovered from a patient undergoing therapy with imipenem/cilastatin. These isolates were resistant to multiple beta-lactam agents, and some were even resistant to imipenem. Analysis revealed a Bush group 1 beta-lactamase, and imipenem resistance corresponded to the loss of outer-membrane proteins in strains expressing high levels of this beta-lactamase. Further characterization efforts yielded abnormal but positive results of tests for ornithine decarboxylase production and motility, and chromosomal homology to an Enterobacter cloacae ampR, ampC probe was detected. These results suggested that the organisms were actually of an Enterobacter species, perhaps Enterobacter aerogenes. Cefoxitin resistance may be a useful marker for preventing this misidentification in the future; misidentification of such organisms poses a hazard, as it may lead to inappropriate beta-lactam therapy for infections caused by organisms that have the potential for resistance due to inducible group 1 cephalosporinases.