Genetic mutations occur gradually in in vivo populations of Mycobacterium tuberculosis bacteria.

Benjamin et al. (1) reported that two Mycobacterium tuberculosis isolates (isolated from the same bronchoscope 2 days apart) demonstrated distinct, but similar IS6110 restriction fragment length polymorphism (RFLP) patterns, as well as slightly different spoligopatterns. They interpreted this finding as the result of a single transposition event of IS6110 and mentioned as its possible explanation (i) that the transposed strain was a rare constituent of the M. tuberculosis population or (ii) that adverse bacterial conditions stimulated the transpositional event. We think these explanations represent different angles of the same phenomenon and may be explained by our previous observations (2). We showed with single-colony cultures that M. tuberculosis populations can consist of subpopulations with different IS6110 RFLP patterns (2). The occurrence of these mixed bacterial populations in M. tuberculosis isolates was associated with increased patient age. Therefore, it seems likely that, in the human body, bacterial populations change gradually over time. That specific growing conditions play a role in the occurrence of transpositions of IS6110 was confirmed in our earlier study of 544 patients with serial isolates, for whom we found that the change of IS6110 RFLP patterns of M. tuberculosis isolates was associated with extrapulmonary disease (3). In conclusion, mutations in populations of M. tuberculosis bacteria, promoted by specific growing conditions, occur gradually, resulting in a different DNA fingerprinting pattern for part of a given M. tuberculosis population compared to the predominant pattern of the larger part of that population. If a part of a population of M. tuberculosis bacteria is typed, only the DNA fingerprinting pattern of that part of the population is revealed. This could explain why Benjamin et al. found slightly different RFLP and spoligotype patterns for the patient isolate and the isolate of the bronchoscope contaminant. Further experimental research is needed to determine in how far adverse bacterial conditions, i.e., conditions comparable to those in the bronchoscope, can stimulate transpositional events.