Is meropenem MIC increase against KPC-producing Klebsiella pneumoniae correlated with increased resistance rates against other antimicrobials with Gram-negative activity?

Objectives: The aim of this study was to assess the minimum inhibitory concentration (MIC) distribution for meropenem and other antimicrobials with Gram-negative activity against Klebsiella pneumoniae carbapenemase-producing K. pneumoniae (KPC-Kp) clinical isolates collected at a tertiary hospital in Italy between 2013–2016. Methods: The antimicrobial susceptibility of KPC-Kp strains was tested by the broth microdilution method using customised 96-well plates and the results were interpreted according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) recommendations. Results: Among 169 consecutive KPC-Kp clinical isolates, 45 (26.6%) were susceptible to meropenem (MIC ≤ 2 mg/L). Among the 124 meropenem-resistant isolates, 73 (58.9%) had a meropenem MIC between 16–64 mg/L. The overall resistance rate for the other antimicrobials tested was very high both for ciprofloxacin and levofloxacin (99.0%), was moderate for amikacin (37.4%) and was low for gentamicin (11.2%), colistin (8.2%) and tigecycline (7.7%). Aminoglycosides had a dichotomous behaviour in relation to meropenem MIC increase. The resistance rate for gentamicin remained <20% across all meropenem MICs; conversely, that for amikacin increased from <20% in the presence of meropenem MIC ≤ 8 mg/L up to ca. 80% in the presence of meropenem MIC ≥ 64 mg/L. Resistance rates for tigecycline and colistin remained <20% in the presence of meropenem MICs up to 64 mg/L. Conclusion: The overall susceptibility rates of antimicrobials with Gram-negative activity may vary greatly among KPC-Kp clinical isolates. A tight relationship between meropenem MIC increase and the resistance rate for amikacin was documented.