Smooth Tubercle Bacilli: Neglected Opportunistic Tropical Pathogens

Abstract

Smooth tubercle bacilli (STB) including "Mycobacterium canettii" are members of the Mycobacterium tuberculosis complex (MTBC), which cause non-contagious tuberculosis in human. This group comprises <100 isolates characterized by smooth colonies and cordless organisms. Most STB isolates have been obtained from patients exposed to the Republic of Djibouti but seven isolates, including the three seminal ones obtained by Georges Canetti between 1968 and 1970, were recovered from patients in France, Madagascar, Sub-Sahara East Africa, and French Polynesia. STB form a genetically heterogeneous group of MTBC organisms with large 4.48 ± 0.05 Mb genomes, which may link Mycobacterium kansasii to MTBC organisms. Lack of inter-human transmission suggested a yet unknown environmental reservoir. Clinical data indicate a respiratory tract route of contamination and the digestive tract as an alternative route of contamination. Further epidemiological and clinical studies are warranted to elucidate areas of uncertainty regarding these unusual mycobacteria and the tuberculosis they cause.

Keywords: Djibouti; Horn of Africa; Mycobacterium tuberculosis complex; amoebas; cellulases; smooth tubercle bacilli; “Mycobacterium canettii”.

Figure 1

(A) Geographical sources for STB infection in 93 patients. (B) Aspects of STB and M. tuberculosis H37Rv colonies on 7H10 solid Middlebrook medium and Ziehl–Neelsen staining of mycobacteria. STB present smooth colonies and distribution of bacilli in singlets or aggregated small clumps instead of the cord-like aggregates usually seen with the rough H37Rv strains. (C) MALDI-TOF spectrum for “M. canettii” (a) and M. tuberculosis H37Rv (b).

Figure 2

STB tuberculosis anatomical sites of infection and potential environmental sources and routes of contamination. Number of STB cases per site is indicated in brackets. Blue is for digestive tract, red for other anatomical sites. Question marks indicate hypothetical routes of contamination.