Genetic and Biochemical Characterization of the First Extended-Spectrum CARB-Type ß-Lactamase, RTG-4, from Acinetobacter baumannii

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Abstract
Acinetobacter baumannii isolate KAR was uncommonly more resistant to cefepime and cefpirome than to ceftazidime and cefotaxime. Cloning and expression of the β-lactamase gene content of this isolate into Escherichia coli TOP10 identified ß-lactamase RTG-4 (or CARB-10), which corresponds to the first reported extended-spectrum CARB-type enzyme. RTG-4 is a plasmid-encoded Ambler class A β-lactamase whose sequence differs by 4 amino acid substitutions from the narrow-spectrum β-lactamase RTG-3. RTG-4 hydrolyzes cefepime and cefpirome and weakly hydrolyzes ceftazidime due to the single Ser-to-Thr substitution at Ambler position 69. RTG-4 is less susceptible to inhibition by tazobactam and sulbactam than RTG-3. Expression of β-lactamase RTG-4 in a wild-type A. baumannii reference strain showed that it conferred resistance to cefepime and cefpirome. The genetic environment of the bla RTG-4 gene was made of a peculiar transposon located on a ca. 50-kb plasmid. IS Aba9 , located upstream of bla RTG-4 , may be responsible for its acquisition by recognizing a secondary right inverted repeat sequence, thus acting by a one-ended transposition process.

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