Mycobacterium microti - pulmonary tuberculosis in an immunocompetent patient

Human tuberculosis is caused by members of the Mycobacterium tuberculosis complex, which includes M. tuberculosis, M. bovis, M. africanum and M. bovis BCG. However there are increasing reports of rarely occurring genetic variants such as M. canettii, M. microti and M. pinipedii. The natural reservoir, mode of transmission and potential modification of host interaction of these species is not yet fully elucidated. We report a rare case of extensive cavitary smear-positive tuberculosis of the left lung caused by M. microti in an immunocompetent tuberculin-negative 68-year-old man. Transmission by a raccoon dog or raccoon as a novel M. microti reservoir was suspected. Spoligotyping of the isolate revealed the llama subtype. The strain exhibited no detectable drug resistance. Response to standard tuberculosis treatment, initially comprising isoniacid, rifampicin, pyrazinamide and ethambutol, was excellent. Delayed growth on solid media, specific phenotypic features and contact with animals should raise suspicion for this rare mycobacterial infection.