In vitro evaluation of novel bi-or tri-antibiotic combination against clinical isolates of Mycobacterium abscessus

Mycobacterium abscessus is an emerging pathogen, intrinsically resistance to many antimycobacterial drugs. The recommended treatment is limited to combination of intravenous amikacin (AMK), and cefoxitin (FOX) with oral clarithromycin (CLR). However, recent reports demonstrate intrinsic resistance to CLR in M. abscessus clinical isolates. Fluoroquinolones, rifamycins, linezolid (LZD) or clofazimine (CLO) can be added when standard therapy is ineffective. This study aims to evaluate the in vitro efficacy of several combinations against clinical isolates of M. abscessus including FOX and AMK and replacing CLR to avoid the induced resistance. Minimum inhibitory concentration (MICs) were determined for ciprofloxacin (CIP), moxifloxacin (MXF), rifampicin (RIF), rifabutin (RFB), CLO, LZD and CLR according to CLSI guidelines. Then, these antibiotics were investigated in tri-combinations with FOX plus AMK to compare their efficacy against M. abscessus reference strain CIP104536, and two clinical isolates Ma1611 and T28 using time-kill kinetic assays. Efficacy of several bi-combinations was evaluated against T28. Clinical isolate T28 was resistant to all antibiotics, CIP104536 and Ma1611 were susceptible to intermediate against all tested antibiotics. Tri-combinations including FOX plus AMK in presence of LZD, MXF, RIF or RFB were active against CIP104536 and Ma1611. Tri-combinations including CLO or CIP were also active against CIP104536 but inactive against Ma1611. All tested triple combinations were inactive against T28. Since T28 was highly resistant to AMK and FOX alone demonstrated only initial killing followed by regrowth, FOX was used in bi-combinations. Hereafter, bi-combinations of FOX with LZD, RIF and RFB were effective and prevented the regrowth observed with FOX alone. Bi-combination FOX with RFB was the most active against strain T28. Tri-combinations were highly efficient against M. abscessus reference strain and intermediate to susceptible clinical isolate Ma1611 but not against multidrug-resistant isolate T28. The synergy between FOX and rifamycins suggests a potent role of this combinations that may warrant further optimization of treatment regimen for the treatment of M. abscessus pulmonary infections