Summary
A bioRxiv preprint reports that insertions and deletions in several classes of repeated DNA sequences can change antibiotic susceptibility in Mycobacterium tuberculosis. Laboratory reconstruction identified a ppe53 repeat deletion that produced intermediate resistance to isoniazid, rifampicin and streptomycin.
A bioRxiv preprint reports that changes in repeated DNA sequences can alter how Mycobacterium tuberculosis responds to antibiotics. The researchers combined genomic analysis of clinical tuberculosis strains from Vietnam and Peru with laboratory experiments using engineered bacteria.
The study focused on insertions and deletions—known as INDELs—in simple sequence repeats (SSRs). It identified repeat changes associated with clinical antibiotic resistance and experimentally reconstructed several of them in otherwise matched bacterial backgrounds. Four reconstructed variants directly reduced drug potency, including a deletion in the ppe53 gene that produced intermediate resistance to isoniazid, rifampicin and streptomycin.
How repeated DNA can change bacterial traits
Simple sequence repeats are short DNA patterns copied multiple times in a row. The number of copies can change when DNA is inserted or deleted, creating genetic variation that can arise relatively frequently and can also reverse. In bacteria, this provides a mechanism for adapting to changing conditions.
Some repeats consist of a single nucleotide repeated in a homopolymer tract. Insertions or deletions in these regions can shift the gene's reading frame, changing how the downstream sequence is translated into a protein. The preprint identified such frameshifting changes in ppe13, glpK, Rv2081c and ppsA.
Other repeats are made from three-nucleotide units, or triplets. Insertions or deletions in triplet repeats can preserve the reading frame while adding or removing amino acids, producing more subtle changes in protein structure. The study found variable triplet-repeat INDELs in ponA1, ppe53 and ppe59.
This distinction matters because a mutation does not need to destroy a gene completely to affect antibiotic response. A smaller structural change may alter how a protein is processed, transported or functions inside the bacterium.
The ppe53 variant and three antibiotics
The researchers reconstructed repeat changes in engineered M. tuberculosis strains to test whether the variants themselves altered drug susceptibility. This approach allowed the genetic changes to be examined in an otherwise isogenic background—bacteria with the same genetic background apart from the variant being tested.
One clinically prevalent ppe53 mutation, described as ppe53 CGCdel, deletes part of a triplet repeat. The preprint reports that this change shortens a polyalanine stretch next to the protein's conserved WxG domain and impairs processing and secretion of the full-length protein. In the reconstructed strain, the deletion conferred intermediate resistance to isoniazid, rifampicin and streptomycin.
The genomic analyses also found that many SSRs show evidence of diversifying selection, meaning that different versions of these repeat regions are maintained or favoured as bacterial populations adapt. The results add triplet-repeat changes to the better-known homopolymer changes that can influence M. tuberculosis fitness during host or antibiotic stress.
The work is presented as a preprint rather than a completed peer-reviewed publication. Its evidence combines associations observed in clinical strain data with causal testing of selected variants in laboratory-engineered bacteria. The findings therefore clarify a possible genetic route to altered drug susceptibility, while the clinical effect of each variant depends on the bacterial and treatment context in which it occurs.