Treatment of Disease Caused by Organisms of the Mycobacterium Fortuitum Complex

The rapidly growing mycobacteria include the potentially pathogenic species Mycobacterium fortuitum and M. chelonei, the two species most commonly associated with human infection. Although most mycobacteria cause disease of the lung, M. fortuitum complex organisms do not typically infect the lung. The prototype of disease with these organisms is soft tissue infection after penetrating trauma. They may produce more serious problems, including endocarditis, sternal osteomyelitis following thoracotomy procedures, corneal infections, silicone breast prosthesis infections, peritonitis in peritoneal dialysis patients, and occasionally mycobacterial infections of the lung. 9 At the present time, treatment for M. fortuitum complex disease involves exposure of the patient to potentially toxic drugs and/or some type of surgical procedure. Because of this, it is very important that true infection be differentiated from colonization with these organisms. When these mycobacteria are isolated from body fluids that are ordinarily sterile, or from chronically draining sinuses, the diagnosis of infection may not be difficult. By contrast, M. fortuitum complex organisms are isolated from sputum of healthy individuals, making diagnosis of pulmonary infection extremely difficult. Since M. fortuitum complex lung infections are so unusual and treatment may involve exposure of the patient to significant morbidity, diagnosis should usually be established by the demonstration of parenchymal invasion by the organism. This ordinarily requires the demonstration of acid-fast organisms in lung tissue by special stains and the cultural confirmation of the mycobacterial species as M. fortuitum complex. The resistance of M. fortuitum complex organisms to conventional antimycobacterial drugs has been recognized repeatedly and consistently. This lack of effective chemotherapy has prompted a search for other antimicrobial agents that may be effective against these organisms. Most investigators have found amikacin to have fairly consistent in vitro activity against many strains of these organisms at safely obtainable serum levels. Other drugs that have shown less consistent in vitro activity include doxycycline, erythromycin, sulfonamides, and ethionamide. 1 2 , 1 4 1 9 No standardized method of susceptibility testing with conventional antibacterial agents has been developed for the nontuberculous mycobacteria. When testing the susceptibility for conventional antimycobacterial drugs, methods similar to those used for testing M. tuberculosis may be used. The rapidly growing mycobacteria also grow on standard nutrient media used in the bacteriology laboratory, such as blood agar, MacConkey's agar, or Mueller-Hinton agar. Since the organism may show visible growth within 48 to 72 hours of incubation, other methods have been used to test for susceptibility, including agar dilution and disk diffusion techniques. It is important to recognize, however, that none of these techniques demonstrating in vitro susceptibility has been correlated with the clinical results of treating patients with the same antibiotics.