A Novel Pathogenic Taxon of the Mycobacterium tuberculosis Complex, Canetti: Characterization of an Exceptional Isolate from Africa

Abstract

In an attempt to characterize an unusual mycobacterial strain isolated from a 2-year-old Somali patient with lymphadenitis, we applied various molecular methods not previously used for the taxonomic classification of mycobacteria. This isolate, designated So93, did not differ from Mycobacteriurn tuberculosis in the biochemical tests and in its 16s rRNA sequence, but produced smooth and glossy colonies, which is highly exceptional for this species. This smooth phenotype was unstable and switched nonreversibly to a rough colony morphology with a low frequency. The two colony types were equally virulent for the guinea pig, exhibiting characteristic tuberculous disease. Both morphotypes had shorter generation times than the M. tuberculosis reference laboratory strain H37Rv and clinical isolates of M. tuberculosis and Mycobacterium bovis. Furthermore, the So93 isolate differed from all M. tuberculosis complex strains described thus far by having only a single copy of insertion sequence IS1081, an unusual composition of the direct repeat cluster, and a characteristic phenolic glycolipid and lipooligosaccharide. This glycolipid had previously been observed only in a smooth isolate of M. tuberculosis obtained in 1969 by Canetti in France. Analysis of the Canetti strain showed that it shared virtually all genetic properties characteristic of So93, distinguishing these two strains from the known M. tuberculosis complex taxa, M. tuberculosis, Mycobacterium afiicanurn, M. bovis, and Mycobacteriurn microti. The natural reservoir, host range, and mode of transmission of the group of bacteria described in this paper are presently unknown. This study, partly based on not previously used molecular criteria, supports the idea that the established members within the M. tuberculosis complex and the newly described Canetti grouping should be regarded as a single species, which likely will be designated "M. tuberculosis".The Mycobactenum tuberculosis complex constitutes a genetically closely related group, and its members, M. tuberculosis, Mycobactenum afncanum, Mycobactenurn bovis, and Mycobacterium microti, may be considered as subspecies of M. tuberculosis (39, 47). The close relatedness between M. tuberculosis complex bacteria has been established by DNA-DNA hybridization (>95%) (l), multiple-locus enzyme electrophoresis (15), and Sequencing of 16s ribosomal RNA and housekeeping genes (15). Furthermore, repetitive DNA elements, such as the insertion sequence IS6110 (43) and the direct repeat (DR) (18), have been found restricted to the M. tuberculosis complex, Nevertheless, the host range and pathogenicity of the M. tuberculosis complex species vary enormously. The natural reservoir of M. tuberculosis and M. afncanum is limited to humans (47) and that of M. microti is limited to voles (48). In contrast, the host range of M. bovis is very broad, and this species causes disease among a wide range of wild and domestic mammals as well as in humans (38).Colonies of primary M. tuberculasis cultures almost invariably have a characterist...

Use of the hupB Gene Encoding a Histone-Like Protein of Mycobacterium tuberculosis as a Target for Detection and Differentiation of M. tuberculosis and M. bovis

“…This variation in immunizing efficiency has been attributed to the observed deletions resulting in genetic differences in BCG vaccine strains used for human immunization (4). However, this variation may well be due to vaccine strains being effective against particular human pathogenic mycobacteria and ineffective against others (48,49). Identification of human mycobacterial pathogens in clinical specimens has been limited by the currently available techniques for precise and early identification of mycobacteria.…”

“…The criteria used for the differentiation of Mycobacterium tuberculosis and M. bovis have been colony morphology, nitrate reduction, niacin test, and sensitivity or resistance to pyrazinamide. Deviations from standard patterns in all of the above tests have been reported, making it virtually impossible to differentiate between M. bovis, M. tuberculosis, and M. africanum (12,34,38,48). The high degree of variability in the phenotypic characteristics has made it important to develop reliable techniques to distinguish between members of the Mycobacterium tuberculosis and M. bovis (MTB) complex (22,41).…”

Tuberculosis in seals caused by a novel member of the Mycobacterium tuberculosis complex: Mycobacterium pinnipedii sp. nov.

“…Previous logical arguments have suggested that all members of the M. tuberculosis complex are sufficiently similar to belong to the species M. tuberculosis (Wayne, 1984;Tsukamura et al, 1985;van Soolingen et al, 1997). Acceptance of such an approach would cause currently accepted species (M. bovis, M. microti and M. africanum) to warrant classification at the subspecies level.…”

“…Although all of these strains effectively share the same 16S rRNA gene sequence (Rogall et al, 1990b) and high DNA-DNA homology (from hybridization studies), they can be separated by some phenotypic characteristics (Grange & Yates, 1994) and, as they have different primary hosts, they have been regarded as separate species (Collins et al, 1982). More recently, two novel strains have been described: 'Mycobacterium canettii ', a novel smooth variant of M. tuberculosis that was first isolated from a Somali-born patient (van Soolingen et al, 1997) and subsequently from a Swiss patient exposed in Africa (Pfyffer et al, 1998); and 'Mycobacterium caprae' (basonym: M. tuberculosis subsp. caprae), a strain that occurs primarily in Spanish goats and has recently been elevated to species level (Aranaz et al, 1999;Niemann et al, 2002;Aranaz et al, 2003).…”

Pulmonary Tuberculosis and Mycobacterium Tuberculosis: Modern Molecular Epidemiology and Perspectives

“…Daley [40] found that several molecular epidemiological studies have reported that patients who have drug-resistant strains were less likely to cluster, suggesting that drug-resistant strains might be less prone to being transmitted or to causing active disease [69,78,210]. Burgos and Pym [21] have also recently reported that isoniazidresistant strains confer a significantly lower number of secondary cases than drug-susceptible strains.…”