IS 6110 , a Mycobacterium tuberculosis Complex-Specific Insertion Sequence, Is Also Present in the Genome of Mycobacterium smegmatis , Suggestive of Lateral Gene Transfer among Mycobacterial Species

Abstract

IS6110 is an insertion element found exclusively within the members of the Mycobacterium tuberculosis complex (MTBC), and because of this exclusivity, it has become an important diagnostic tool in the identification of MTBC species. The restriction of IS6110 to the MTBC is hypothesized to arise from the inability of these bacteria to exchange DNA. We have identified an IS6110-related element in a strain of Mycobacterium smegmatis. The presence of IS6110 indicates that lateral gene transfer has occurred among mycobacterial species, suggesting that the mycobacterial gene pool is larger than previously suspected.Genetic exchange is thought to be a driving force behind the ability of bacterial species to evolve and adjust to environmental challenges. Lateral gene transfer (LGT) in bacteria is mediated by one of three processes, namely, conjugation, transformation, or transduction; examples of these processes have been described for almost all bacterial species (9,19). By contrast, the Mycobacterium tuberculosis complex (MTBC) species, comprising M. tuberculosis, M. africanum, M. bovis, M. cannetti, M. caprae, M. microti, and M. pinnipedi (10, 15), are clonal populations evolved from a single progenitor species that has diversified by the acquisition of spontaneous mutations rather than by LGT. Genome comparisons between these seven species show that they have almost identical 16S rRNA sequences and highly similar genome sequences, and there is no strong evidence for genetic exchange (Ͼ99% identity [2,3,16]). The lack of clearly documented LGT among members of the MTBC is thought to be a consequence of the organisms' solitary lifestyles within their hosts, preventing their contact with other mycobacterial species, or perhaps even other bacteria. Thus, it has become generally accepted within the scientific community that the MTBC species do not undergo genetic exchange (10,15,16).IS6110 is an insertion element that is found exclusively within the MTBC; the assumption has been that this restriction is a result of the lack of genetic exchange with other mycobacterial species. A benefit of this exclusivity is that IS6110 has become an important diagnostic tool in the differentiation of MTBC species from other mycobacteria. Moreover, the element's presence in multiple copies, and at differing locations in the genome, has provided an excellent method by which strains can be genotyped; because of these characteristics, IS6110 has been used extensively for epidemiological studies (12,18,20).Our studies have focused on DNA transfer between strains of M. smegmatis. This work has shown that DNA transfer occurs by a process most similar to conjugation: distinct donor and recipient strains exist and transconjugants are detected only after prolonged cell-cell contact (14,21,22). The transfer process is chromosomally encoded and can occur only from a donor to a recipient. The donor and recipient strains are independent isolates of M. smegmatis with distinct colony morphologies (13). The genetic basis for donor and recipient abilit...

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“…Since different M. tuberculosis strains have different copy numbers of IS6110 and in different regions, this test can be used to distinguish between members of the Mycobacterium tuberculosis complex (MTBC). However, recent studies have demonstrated that this insertion sequence is not exclusive to MTBC, as a similar sequence has been found in the non-pathogenic Mycobacterium smegmatis [6]. PCR of the virulence determining gene devR is also used to identify pathogenic M. tuberculosis, and has been found to be equivalent in sensitivity to IS6110 PCR [7].…”

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