TEM-168, a Heretofore Laboratory-Derived TEM β-Lactamase Variant Found in an <i>Escherichia coli</i> Clinical Isolate

There have been over 160 variants of the TEM-1 and TEM-2 penicillinases described, most of which also exhibit activity against extended-spectrum cephalosporins (www.lahey.org/Studies/). It was noted in 1994 that a T265M mutation (Ambler numbering) was often associated with active-site mutations E104K, R164S, G238S, and E240K in some TEM variants (4). Comparison of MICs and kinetic studies of TEM-1 variants with the amino acid substitutions T265M, G238S, G238S:T265M, G238S:E240K, and G238S:G240K:T265M constructed by in vitro mutagenesis showed that the T265M mutation had no measurable effect on the TEM-1 wild type or on the G238S or G238S:240K variant. To date, no naturally occurring T265M variant of TEM-1 has been reported, though currently, 22 TEM variants contain an M265 residue along with other residue changes (www.lahey.org/Studies/). Escherichia coli N08-1503 obtained by the National Microbiology Laboratory for advanced testing was resistant to ampicillin, cefazolin, and ampicillin-sulbactam, as determined by the submitting laboratory. Antimicrobial testing was carried out by disk diffusion and broth microdilution according to CLSI guidelines and Etests (AB Biodisk) according to manufacturer's instructions (Table ​(Table1)1) (1). The CLSI methods indicated that E. coli N08-1503 did not harbor an extended-spectrum β-lactamase (ESBL), though a reduced susceptibility to ceftazidime was noted. Analysis using the ESBL Etest strips yielded similar results. Interestingly, the cefepime ESBL Etest strip yielded a positive result. This strip is recommended outside of the United States when testing a strain with an inducible ampC gene (e.g., Enterobacter) or when a nondeterminable result is obtained. Broth microdilution confirmed the reduced susceptibility to cefepime and synergy with clavulanic acid, though the cefepime MICs were one or two doubling dilutions higher than the Etest results were. Resistance to amoxicillin-clavulanic acid and piperacillin-tazobactam and intermediate susceptibility to cefoxitin were also noted for E. coli N08-1503 and DH10B transformed with a plasmid (pT168) harboring blaTEM-168 (see below). TABLE 1. Antimicrobial data for E. coli N08-1503 and the transformants harboring pT168a PCR analysis results for β-lactamase genes were positive for TEM and negative for SHV, CTX-M, OXA-1, and CMY-2 types. Sequence analysis of the TEM amplicon revealed a translation product with a single amino acid change from TEM-1, a T261M change (T265M by Ambler numbering) caused by a C782T transition. This variant has been assigned the name TEM-168 (www.lahey.org/Studies). In the blaTEM-168 promoter region, we detected the G162T change that defines the strong P4 promoter (3, 5). Plasmid analysis showed that an ∼13-kb plasmid, pT168, harboring blaTEM-168 could be transformed into E. coli DH10B. Isoelectric focusing of crude extracts from E. coli N08-1503 and the DH10B transformant identified a single band of β-lactamase activity with a pI of 5.4. The resistance to β-lactam inhibitor combinations was likely due to the hyperproduction of plasmidic TEM-168 expressed from the P4 promoter, similar to what has been shown for TEM-1 (5, 6). Since the TEM-168 did not test positive with standard ESBL tests, it is easy to understand why this variant has not been described previously in clinical isolates. It was the unusual result with the cefepime ESBL Etest strip that made us continue our investigation to further characterize the TEM harbored by E. coli N08-1503.

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