Effect of β-lactamase production and β-lactam instability on MIC testing results for Mycobacterium abscessus.

Objectives: Limited treatment options available for Mycobacterium abscessus infections include the parenteral β-lactam antibiotics cefoxitin and imipenem, which show moderate in vitro activity. Other β-lactam antibiotics (except meropenem) have no considerable in vitro activity, due to their rapid hydrolysis by a broad-spectrum β-lactamase (Bla_Mab). We here addressed the impact of β-lactamase production and β-lactam in vitro stability on M. abscessus MIC results and determined the epidemiological cut-off (ECOFF) values of cefoxitin, imipenem and meropenem.
Methods: By LC high-resolution MS (LC-HRMS), we assessed the in vitro stability of cefoxitin, imipenem and meropenem. M. abscessus ATCC 19977 strain and its isogenic blaMab deletion mutant were used for MIC testing. Based on MIC distributions for M. abscessus clinical strains, we determined ECOFFs of cefoxitin, imipenem and meropenem.
Results: A functional Bla_Mab increased MICs of penicillins, ceftriaxone and meropenem. LC-HRMS data showed significant degradation of cefoxitin, imipenem and meropenem during standard antibiotic susceptibility testing procedures. MIC, MIC50 and ECOFF values of cefoxitin, imipenem and meropenem are influenced by incubation time.
Conclusions: The results of our study support administration of imipenem, meropenem and cefoxitin, for treatment of patients infected with M. abscessus. Our findings on in vitro instability of imipenem, meropenem and cefoxitin explain the problematic correlation between in vitro susceptibility and in vivo activity of these antibiotics and question the clinical utility of susceptibility testing of these chemotherapeutic agents.
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