Antimicrobial Susceptibility of Clinical and Environmental Mycobacterium chimaera Isolates

Mycobacterium chimaera is a slow-growing nontuberculous Mycobacterium species belonging to the Mycobacterium avium complex (MAC). It has been identified globally as the cause of a large outbreak of cardiovascular infections following open heart surgery, but it can also cause respiratory infections in individuals with underlying structural pulmonary disease. Invasive M. chimaera infections are associated with poor clinical responses, and the optimal antibiotic treatment regimen for these infections is not known. In this study, the drug susceptibility profiles of clinical and environmental M. chimaera isolates for antimicrobial agents that are commonly considered for treatment of MAC infections were determined. All M. chimaera isolates were susceptible to clarithromycin, with a median MIC of 2 μg/ml, while 98% (85/87 isolates) were susceptible to amikacin. Twenty-five percent of isolates (22/87 isolates) had intermediate susceptibility and 52% (46/87 isolates) were resistant to moxifloxacin. Similarly, 39% of isolates (34/87 isolates) had intermediate susceptibility and 39% (34/87 isolates) were resistant to linezolid. MIC breakpoints derived from the literature were used to determine resistance to rifampin (16/87 isolates [18%]), ethambutol (10/87 isolates [11%]), rifabutin (2/87 isolates [2%]), and streptomycin (1/87 isolates [1%]). In conclusion, our results showed that clarithromycin, amikacin, rifabutin, and streptomycin had the best activity against M. chimaera isolates, while susceptibility rates were lower for rifampin and ethambutol. In contrast, there was a high prevalence of isolates that were not susceptible to moxifloxacin or linezolid. While factors in addition to antibiotic susceptibility may determine the outcomes of treatment of M. chimaera infections, our results should inform the selection of antimicrobials as part of the overall therapeutic strategy.