KPC-Producing Klebsiella pneumoniae Strains That Harbor AAC(6′)-Ib Exhibit Intermediate Resistance to Amikacin

Abstract

The aminoglycoside-modifying enzyme AAC(6=)-Ib is common among carbapenem-resistant Klebsiella pneumoniae (CR-Kp) strains. We investigated amikacin (AMK) activity against 20 AAC(6=)-Ib-producing CR-Kp strains. MICs clustered at 16 to 32 g/ml. By the time-kill study, AMK (1؋ and 4؋ the MIC) was bactericidal against 30% and 85% of the strains, respectively. At achievable human serum concentrations, however, the majority of strains showed regrowth, suggesting that AAC(6=)-Ib confers intermediate AMK resistance. AMK and trimethoprim-sulfamethoxazole (TMP-SMX) were synergistic against 90% of the strains, indicating that the combination may overcome resistance.A minoglycosides are important options for the treatment of infections caused by carbapenem-resistant Klebsiella pneumoniae (CR-Kp) strains. These agents retain potent bactericidal activity against some but not all CR-Kp strains (1). Inactivation by enzymatic modification is the most prevalent mechanism of resistance to the class (2). Indeed, 98% of CR-Kp strains studied at our center possessed at least one aminoglycoside-modifying enzyme (AME), which resulted in variable susceptibility among the aminoglycosides (3).Amikacin (AMK) is uniquely stable against most AMEs. Of the AMEs known to affect AMK, APH(3=)-VI and ANT(4=) confer high-level resistance and are infrequently encountered (4, 5). On the other hand, AAC(6=)-Ib is common among Enterobacteriaceae (2), but the clinical relevance is largely unknown. AAC(6=)-Ib encodes an N-acetyltransferase that catalyzes acetyl coenzyme A (acetyl-CoA)-dependent acetylation of the 6= amino group of AMK. The impact of AAC(6=)-Ib modification on the susceptibility of Gram-negative bacteria to AMK is uncertain. Published reports have identified AMK MICs above (6, 7) and below (7-9) susceptibility breakpoints. Time-kill studies of AMK against CR-Kp strains or other Enterobacteriaceae that produce AAC(6=)-Ib have not been published. The objective of this study was to characterize AMK MICs, time-kill responses, and interactions with other antimicrobials among AAC(6=)-Ib-producing CR-Kp strains at our center.We selected 20 CR-Kp strains from unique patients for analysis. All strains were sequence type (ST) 258, Klebsiella pneumoniae carbapenemase (KPC) producers (18 producers of KPC-2 and 2 of KPC-3), and harbored the aac(6=)-Ib gene by PCR (3). Each strain also harbored SHV-12 and TEM-1 ␤-lactamase genes, but none carried NDM, IMP, VIM, or OXA-48 genes. All carried mutant ompK35 porin genes (AA89 STOP), and 60% (12/20) had mutant ompK36 genes. Sequence analysis revealed three ompK36 mutant genotypes, the most common of which were an insertion at amino acid 134 to 135GD (n ϭ 6) and IS5 promoter insertions (n ϭ 5). The remaining ompK36 mutant isolate had a frameshift at nucleotide position 382. MICs were determined by standard broth microdilution methods. Median MICs of AMK, gentamicin (GEN), and tobramycin (TOB) were 32 (range, 4 to 32 g/ml), 2 (range, 0.5 to Ͼ64 g/ml), and 32 (range, 8 to Ͼ64 g/ml) g/ml, respectively. Susceptib...

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