Clinical characteristics of the smooth tubercle bacilli 'Mycobacterium canettii' infection suggest the existence of an environmental reservoir

Over a 3-year follow-up, 30 out of the 318 unique Mycobacterium tuberculosis complex isolates recovered in the Republic of Djibouti had a smooth-type morphology and were Niacine-negative, the characteristics of 'Mycobacterium canettii' strains. Unlike M. tuberculosis, 'M. canettii' grew on nutrient-poor media at 30 degrees C, and possessed characteristic lipids. They were isolated from respiratory and extra-respiratory sites from patients with typical forms of tuberculosis. Most cases resolved with antibiotic therapy but in two human immunodeficiency virus-positive patients 'M. canettii' infection led to septicaemia and death. No cases of human-to-human transmission were observed. The proportion of tuberculosis cases caused by 'M. canettii' was higher among French patients than among Djiboutian patients. Patients with 'M. canettii' were significantly younger than those with tuberculosis caused by other M. tuberculosis complex strains. Smooth tubercle bacilli could be misidentified as non-tuberculous mycobacteria and appear to be limited to the Horn of Africa. Their characteristics are consistent with the existence of non-human sources of infection.