Mycobacterial Factors, Strain Differences, and Interactions with Sources, Environment, and Host

Mycobacterial Factors, Strain Differences, and Interactions with Sources, Environment, and Host - As mycobacteria multiply during the course of infection, they acquire point mutations, frame shift mutations, deletions, and genetic sequence insertions as a result of chance DNA transcriptional errors and recombinations. As discussed earlier, there may be as many as 108 or 109 organisms within a lung cavity, and, within this population, genetic mutations may occur at a rate of 1 in 10 7 to 1 in 10 10 per bacterium per generation.60 These genetic mutations can be associated with a variety of phenotypic outcomes, including alterations in drug resistance, nutrient uptake, metabolism, or interaction with host immune cells, that is, altered virulence.

Mycobacterium Tuberculosis and the desease tuberculosis

Multidrug resistant Mycobacterium tuberculosis

How often do these mutations result in reduced mycobacterial fitness, how often do compensatory mutations occur that restore fitness to the wild-type state, and how important are these microbial determinants of infection or disease amid the many other factors depicted in Fig. 2.2? These are among the most important questions being addressed by molecular epidemiological and laboratory researchers.

Anecdotal observations, combined with molecular typing data, suggest that some strains propagate more readily or are more ‘fit’ than others. In the natural history of TB infection, there are several stages that might be altered by genetic mutations that could influence the propagation of TB, In 1998, Valway et al.61 reported on the results of a contact investigation in the setting of an unusual outbreak of TB. Interestingly, while the rate of skin test conversion was unusually high and the reactions unusually large among exposed individuals, the rate of disease development was lower than expected, suggesting that this strain of TB was highly infectious, a strong stimulator of DTH, but not particularly virulent. Conversely, the strong induced host response itself might explain the lower rate of disease progression.

Several decades earlier, Middlebrook and Cohn62 described transmission patterns for isoniazid (INH) susceptible and resistant strains of MTB among guinea pigs exposed via the aerosol route. INH resistant strains were less virulent for the guinea pigs than the susceptible strains, as evidenced by smaller and less numerous foci of infection on autopsy, fewer organs involved, a milder clinical course, and prolonged survival. Early on, researchers attributed the loss of virulence of INH resistant strains to a loss of catalase activity, the enzyme responsible for metabolizing INH from a prodrug to its metabolically active form.63–65 Subsequent work has found associations between the type of KatG mutation responsible for catalase activity and the apparent virulence of the organism. 

As an example, it was shown that, although both point mutations and deletions in the gene sequence disrupt the production of catalase, the deletions confer a more profound reduction in catalase levels, a higher degree of resistance to INH, and a more marked attenuation in virulence than point mutations.66 In contrast to these observations for INH mutations, mutations which lead to pyrazinamide resistance alone tend not to alter the fitness of MTB.67 Whether mutations which contribute to drug resistance necessarily reduce the reproductive fitness of the MTB has recently been the subject of considerable debate.

Ordway et al.68 examined 15 different clinical isolates of MTB, some of which were fully drug sensitive, INH resistant, or multidrug resistant. They found that, although growth rates in experimentally infected mice varied for each of the 15 isolates, these growth rates did not correlate with the extent of drug sensitivity or resistance.68 Likewise, Cohen and colleagues69 recently evaluated epidemiological data available on transmission of MDR-TB using mathematical models to determine whether drug resistance mutations also result in a loss of reproductive fitness. In their analysis they conclude that the fitness of drug-resistant strains is quite heterogeneous and that attempting to discover an average degree of reproductive fitness for MDR-TB upon which to base programmatic and policy decisions may be misleading.69 Instead, they argue, it is important to determine the distribution of highly resistant and less resistant strains, so that outbreaks can be averted and handled appropriately. 

As a real-life example of this heterogeneity, we might consider another contact investigation conducted by Texeira et al.70 in Brazil, who evaluated rates of TST conversion among household contacts of index cases of MDR-TB. They found no association between the degree of drug resistance and rates of TST conversion or disease development among the household contacts, even though the drug-resistant index cases were infectious for a longer period of time than the drug-sensitive cases in their study. Using IS6100 pattern molecular typing techniques, they also confirmed that the secondary cases of TB among household contacts were from the same strain as the index case

for any given household, thereby eliminating any doubt that secondary cases in a household were from a non-household exposure.70

As discussed elsewhere in this volume, developments in the molecular biology and genetics of MTB have permitted closer analysis of the interplay of genes, proteins, and bacterial enzymes in the life cycle of this organism. We mention this here only in relation to transmission. Mycobacterial growth in the face of exogenous stressors has been shown to be dependent upon the elaboration of gene products called sigma factors, which regulate defence regulons within the mycobacterial genome. aboratory studies in which several sigma factors were mutated demonstrated that mutation of the sigma factors had no impact on the ability of the H37Rv strain of mycobacterium to grow in culture dishes or within in vitro macrophage systems, but had a substantial impact on the in vivo growth and replication of H37Rv within a guinea pig infection model. Among the factors studied, mutations in SigC affected the adaptive survival of H37Rv in guinea pigs more than mutations in SigF.