Integron Carrying a Novel Metallo-β-Lactamase Gene, bla IMP-16 , and a Fused Form of Aminoglycoside-Resistant Gene aac(6′)-30/aac(6′)-Ib′ : Report from the SENTRY Antimicrobial Surveillance Program

Abstract

Since January 2002 Pseudomonas sp. strains resistant to carbapenems and ceftazidime have been routinely screened as part of the SENTRY Antimicrobial Surveillance Program for metallo-β-lactamase production, and their resistance determinants have been analyzed. Pseudomonas aeruginosa index strain 101-4704, which harbors a novel bla_IMP variant, bla_IMP-16, was isolated in April 2002 from a 60-year-old man in Brasília, Brazil. bla_IMP-16 was found on the chromosome of the P. aeruginosa index strain, and the deduced amino acid sequence (IMP-16) showed the greatest identities to IMP-11 (90.3%) and IMP-8 (89.5%). Sequence analysis revealed that bla_IMP-16 was associated with a class 1 integron, which also encoded aminoglycoside-modifying enzymes. Downstream of bla_IMP-16 resided an open reading frame, which consisted of a new aminoglycoside-modifying gene, namely, aac(6′)-30, which was fused with aac(6′)-Ib′. The amino acid sequence of the aac(6′)-30 putative protein showed the most identity (52.7%) to the sequence of AAC(6′)-29b described previously. The fourth gene cassette constituted aadA1. The steady-state kinetics of IMP-16 demonstrated that the enzyme preferred cephalosporins and carbapenems to penicillins. The main functional difference observed among the kinetic values for IMP-16 compared to those for other IMPs was a lack of cefoxitin hydrolysis and a lower k_cat/K_m value for imipenem (0.36 μM⁻¹ · s⁻¹). This report further emphasizes the spread of metallo-β-lactamase genes and their close association with various aminoglycoside resistance genes.