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.

Ready Experimental Translocation of Mycobacterium canettii Yields Pulmonary Tuberculosis

“…This supports the existence of an unknown environmental or animal reservoir and suggests that M. canettii is poorly adapted to any specific host, behaving as an opportunist rather than an obligate pathogen (9). Published cases indicate that in Djibouti, cervical lymphadenitis is significantly more prevalent during M. canettii infection than during M. tuberculosis infection (1). Also, M. canettii was reported to cause esophageal tuberculosis (14), mesenteric lymph node tuberculosis (15), and ascites (16), and several isolates have been cultured in gastric fluid (14).…”

“…ycobacterium canettii is a rare member of the Mycobacterium tuberculosis complex (MTBC) that is responsible for fewer than 100 reported cases of tuberculosis in patients with exposure to the Horn of Africa, with a few exceptions (1). In the MTBC, M. canettii is most closely related to the vanished ancestor (1)(2)(3)(4)(5).…”

“…In the MTBC, M. canettii is most closely related to the vanished ancestor (1)(2)(3)(4)(5). M. canettii is uniquely characterized by a large (4.48 Ϯ 0.05 Mb) and mosaic genome with traces of intraspecies horizontal gene transfer (HGT) (3,5,6), including M. tuberculosis-specific deletion 1 (TbD1), which is deleted in "modern" M. tuberculosis lineages (7).…”

“…Since higher hydrophobicity was linked to enhanced aerosol performance for nontuberculous mycobacteria (13), it was suggested that the relatively low hydrophobicity of M. canettii could be responsible for its lack of aerosol transmission (11). M. canettii-infected patients present with pulmonary and lymphatic forms of tuberculosis (1). The absence of properly designed studies limits interpretation of the current lack of evidence of interhuman transmission (9).…”

“…Also, M. canettii was reported to cause esophageal tuberculosis (14), mesenteric lymph node tuberculosis (15), and ascites (16), and several isolates have been cultured in gastric fluid (14). These clinical observations led to our hypothesis that the digestive tract is a possible portal of entry of M. canettii (1). We developed a murine model to explore this hypothesis from an etiological perspective to contribute to a better understanding of M. canettii infections.…”