Susceptibility of Mycobacterium malmoense to antibacterial drugs and drug combinations.

Mycobacterium malmoense is an opportunistic pathogen with increasingly recognized clinical importance. It is mainly isolated in northern Europe and Great Britain, most often from patients with pulmonary infections. Conventional therapy of M. malmoense infections with antituberculosis drugs is often of limited value, and there is thus a need for improved drug regimens. The potential efficacies of new alternative drugs, such as quinolones, macrolides, amikacin, and rifabutin, are still unknown, and so is the pathogen's in vitro susceptibility to most of these drugs. In this study, we used the BACTEC system for determining the pattern of resistance of clinical M. malmoense isolates to a number of antibacterial drugs as well as their possible synergistic interactions when each of them was combined with ethambutol. The majority of the strains were resistant or moderately resistant to the drug when it was tested alone at selected concentrations. However, pronounced in vitro synergism was demonstrated for combinations of ethambutol with ciprofloxacin, amikacin, and rifampin, rendering most isolates susceptible to the combined drugs. Thus, for in vitro susceptibility testing of M. malmoense, examination of the possible synergistic effects of combined drugs also can be recommended.