Whole genome sequencing reveals genomic heterogeneity and antibiotic purification in Mycobacterium tuberculosis isolates

BACKGROUND

Whole genome sequencing has revolutionised the interrogation of mycobacterial genomes. Recent studies have reported conflicting findings on the genomic stability of Mycobacterium tuberculosis during the evolution of drug resistance. In an age where whole genome sequencing is increasingly relied upon for defining the structure of bacterial genomes, it is important to investigate the reliability of next generation sequencing to identify clonal variants present in a minor percentage of the population. This study aimed to define a reliable cut-off for identification of low frequency sequence variants and to subsequently investigate genetic heterogeneity and the evolution of drug resistance in M. tuberculosis.

METHODS

Genomic DNA was isolated from single colonies from 14 rifampicin mono-resistant M. tuberculosis isolates, as well as the primary cultures and follow up MDR cultures from two of these patients. The whole genomes of the M. tuberculosis isolates were sequenced using either the Illumina MiSeq or Illumina HiSeq platforms. Sequences were analysed with an in-house pipeline.

RESULTS

Using next-generation sequencing in combination with Sanger sequencing and statistical analysis we defined a read frequency cut-off of 30% to identify low frequency M. tuberculosis variants with high confidence. Using this cut-off we demonstrated a high rate of genetic diversity between single colonies isolated from one population, showing that by using the current sequencing technology, single colonies are not a true reflection of the genetic diversity within a whole population and vice versa. We further showed that numerous heterogeneous variants emerge and then disappear during the evolution of isoniazid resistance within individual patients. Our findings allowed us to formulate a model for the selective bottleneck which occurs during the course of infection, acting as a genomic purification event.

CONCLUSIONS

Our study demonstrated true levels of genetic diversity within an M. tuberculosis population and showed that genetic diversity may be re-defined when a selective pressure, such as drug exposure, is imposed on M. tuberculosis populations during the course of infection. This suggests that the genome of M. tuberculosis is more dynamic than previously thought, suggesting preparedness to respond to a changing environment.

Frequency and implications of pyrazinamide resistance in managing previously treated tuberculosis patients

OBJECTIVE

To determine the extent of pyrazinamide (PZA) resistance in isolates from previously treated patients from the Western Cape, South Africa.

DESIGN

Drug-resistant isolates, isolates resistant to one or more drugs other than PZA (PZA resistance is not routinely determined) (n = 127), and drug-susceptible (n = 47) clinical isolates of Mycobacterium tuberculosis from previously treated patients from the Western Cape were phenotypically (BACTEC MGIT 960) and genotypically (pncA gene sequencing) analysed for PZA resistance.

RESULTS

MGIT analysis found that 68 of the 127 drug-resistant isolates were PZA-resistant. Nearly all (63/68) PZA-resistant isolates had diverse nucleotide changes scattered throughout the pncA gene, and five PZA-resistant isolates had no pncA mutations. Of the 47 phenotypically susceptible isolates, 46 were susceptible to PZA, while one isolate was PZA-monoresistant (OR = 53.0, 95% CI = 7.1-396.5). A pncA polymorphism (Thr114Met) that did not confer PZA resistance was also identified. PZA resistance was strongly associated with multidrug-resistant tuberculosis (MDR-TB).

CONCLUSION

An alarmingly high proportion of South African drug-resistant M. tuberculosis isolates are PZA-resistant, indicating that PZA should not be relied upon in managing patients with MDR-TB in the Western Cape. A method for the rapid detection of PZA resistance would be beneficial in managing patients with suspected drug resistance.