Ertapenem resistance among extended-spectrum-beta-lactamase-producing Klebsiella pneumoniae isolates

Abstract

Ertapenem resistance in Klebsiella pneumoniae is rare. We report on an ertapenem-nonsusceptible phenotype among 25 out of 663 (3.77%) extended-spectrum-beta-lactamase (ESBL)-producing K. pneumoniae isolates in a multicenter Israeli study. These isolates originated from six different hospitals and were multiclonal, belonging to 12 different genetic clones. Repeat testing using Etest and agar dilution confirmed ertapenem nonsusceptibility in only 15/663 (2.3%) of the isolates. The molecular mechanisms of ertapenem resistance in seven single-clone resistant isolates was due to the presence of ESBL genes (CTX-M-2 in four isolates, CTX-M-10 and OXA-4 in one isolate, SHV-12 in one isolate, and SHV-28 in one isolate) combined with the absence of OMPK36. Seven of 10 isolates initially reported as ertapenem nonsusceptible and subsequently classified as susceptible showed an inoculum effect with ertapenem but not with imipenem or meropenem. Population analysis detected the presence of an ertapenem-resistant subpopulation at a frequency of 10(-6). These rare resistant subpopulations carried multiple ESBL genes, including TEM-30, SHV-44, CTX-M-2, and CTX-M-10, and they lacked OMPK36. The clinical and diagnostic significance of the results should be further studied.

FIG. 1.

Population analysis of four ESBL-producing Klebsiella pneumoniae strains with and without an inoculum effect. The distribution of the resistant subpopulation among the population of two ertapenem-sensitive strains that possess an inoculum effect (strains 5 and 7) differed from the population distribution of a stable ertapenem-resistant strain (strain 490) and that of an ertapenem-sensitive strain (strain 8). Strains 5 and 7 showed the preexistence of a rare ertapenem-resistant subpopulation absent in strain 8 that may lead to discrepancies in ertapenem susceptibility test results for the inoculum effect strains.

FIG. 2.

(A) OMP profiles of ertapenem-sensitive K. pneumoniae isolates and ertapenem-resistant isolates (ertapenem-S and ertapenem-R, respectively). OMPK36 was present in ertapenem-sensitive isolates (K. pneumoniae ATCC strain 13883 [ATCC] and isolates 5 and 7, the two sensitive isolates with a positive inoculum effect). M, molecular size marker. (B) The OMP profile of isolate 7 (possessing an inoculum effect) with ertapenem (7+ERT) and without ertapenem (7) in growth medium demonstrated a lack of OMPK36 in the presence of the antibiotic similarly to the OMP pattern of resistant strains, supporting the preexistence of a resistant subpopulation that remains viable in the presence of ertapenem.