Fitness of multi-and extensively drug-resistant mycobacterium tuberculosis clinical strains.
The biological fitness of a pathogen is defined as its ability to reproduce, survive, cause disease and be transmitted. Drug-resistant M. tuberculosis isolates often exhibit reduced competitive ability against susceptible isolates in the absence of drug selection. However, compensatory mutations may, to some extent, offset fitness costs associated with resistance-conferring mutations. Most studies to date have focused on the fitness of isogenic laboratory strains or globally-relevant strains. Fewer studies have addressed the fitness of endemic strains which propagate in multidrug and extensively drug-resistant forms (MDR and XDR, respectively). In this study, the fitness of four genotypes which drive the transmission of tuberculosis (TB) in KwaZulu-Natal (KZN), South Africa was explored. This included F15/LAM4/KZN genotype strains, some of which were involved in the notorious XDR-TB outbreak in Tugela Ferry, KZN as well as Beijing, F11 and F28 genotype strains. Drug-resistant F15/LAM4/KZN and F28 genotype strains demonstrated increased in vitro fitness, whilst Beijing and F11 MDR strains had markedly reduced fitness. These findings correlated with whole-genome and Sanger sequencing data which revealed the presence of low/no-cost resistance-conferring mutations as well as intra- and intergenic compensatory mutations in drug-resistant F15/LAM4/KZN and F28 strains, respectively. In contrast, high cost katG mutations and no accompanying compensatory mutations were identified in Beijing and F11 MDR strains. During co-culture experiments, a novel observation was made whereby susceptible and resistant strains exhibited synergistic growth compared with axenic cultures i.e. in vitro trans-complementation. Drug-resistant F15/LAM4/KZN strains did not undergo fitness costs in THP-1 macrophages and produced increasing levels of TNF-α which may enhance tissue destruction and dissemination to other hosts. Although demonstrating similar intracellular fitness, susceptible and MDR Beijing, F11 and F28 strains induced heterogeneous cytokine responses. Thus, the lack of a direct relationship between bacillary burden and cytokine responses indicates that this diversity results from strain heterogeneity. Relapse isolates, including those from F15/LAM4/KZN and Beijing genotypes, may reactivate without any changes in biological fitness, thereby retaining the potential to re-transmit. Taken together, the enhanced fitness of drug-resistant F15/LAM4/KZN and F28 genotype strains is due to the presence of beneficial mutations, while the reduced fitness of MDR Beijing and F11 strains is associated with high cost mutations.