Hidden Mycobacteria in Cattle Carry Genes Linked to Drug Resistance

Non-tuberculous mycobacteria, a sprawling group of environmental bacteria long dismissed as mere bystanders in veterinary medicine, are now commanding attention for their ability to mimic tuberculosis in cattle and to harbor genetic elements associated with antimicrobial resistance. A new study from researchers at Guru Angad Dev Veterinary and Animal Sciences University in Ludhiana, India, published in Molecular Biology Reports, has taken a close molecular look at these organisms in bovine clinical samples, and the results offer both reassurance and caution. The team, led by Bandita Panigrahi and corresponding author Deepti Narang, isolated and identified non-tuberculous mycobacteria from a broad panel of samples collected from cattle and buffaloes, then probed the isolates for genes that in related mycobacteria are known to underpin resistance to frontline anti-tuberculosis drugs. The work addresses a conspicuous gap: molecular data on resistance-associated genes in bovine non-tuberculous mycobacteria from India have been scarce, even as these opportunistic pathogens increasingly complicate diagnosis and surveillance of bovine tuberculosis.

The scale of the investigation reflects the difficulty of working with organisms that are ubiquitous in soil and water. The researchers analyzed 193 clinical samples drawn from five distinct sources: 101 fecal samples, 21 nasal swabs, 13 trans-tracheal washes, 30 tissue specimens, and 28 blood samples. Each sample was processed through two parallel routes, direct polymerase chain reaction and culture-based isolation, a dual strategy designed to capture both organisms present in the sample at the time of collection and those capable of growing under laboratory conditions. Decontamination steps, essential when dealing with samples teeming with faster-growing environmental flora, were carefully applied to give the slow-growing mycobacteria a chance to emerge on culture media. The approach mirrors established diagnostic protocols for mycobacterial recovery from clinical material, where the balance between eliminating contaminants and preserving viable target organisms is notoriously delicate.

Identification proceeded in a tiered molecular fashion. Genus-level confirmation relied on PCR targeting the hsp65 gene, a workhorse marker in mycobacterial taxonomy that encodes a heat shock protein and allows discrimination of mycobacteria from other acid-fast organisms. Once the isolates were confirmed as members of the genus Mycobacterium and distinguished from the Mycobacterium tuberculosis complex, species-specific PCR assays were deployed to pin down exactly which non-tuberculous species had been recovered. This hierarchical approach, from broad genus screening to species-level resolution, is critical because the clinical and epidemiological significance of non-tuberculous mycobacteria varies enormously from one species to the next. In total, the workflow yielded eighteen confirmed non-tuberculous mycobacterial isolates, a modest but workable collection for downstream genetic analysis.

The species picture that emerged was strikingly lopsided. Mycobacterium vaccae accounted for two-thirds of the isolates, representing 66.7 percent of the confirmed collection. This organism, first described decades ago and long studied for its immunomodulatory properties, is a common inhabitant of soil, water, and dust, and its dominance in bovine samples underscores how readily environmental mycobacteria colonize or transit through livestock. The remaining isolates comprised other non-tuberculous species, though M. vaccae clearly set the tone of the collection. For veterinary diagnosticians, this predominance matters: when a bovine sample yields acid-fast organisms or tuberculosis-like lesions, the probability that the culprit is an environmental species rather than Mycobacterium bovis has direct consequences for how the finding is interpreted within national tuberculosis control frameworks.

With the isolates in hand, the team turned to the central question of the study: what resistance-associated genes do these bovine non-tuberculous mycobacteria carry? Four genes were selected for screening, each with a well-characterized role in drug resistance in Mycobacterium tuberculosis. The rpoB gene, encoding the beta subunit of RNA polymerase, is the principal target of rifampicin, and mutations within its resistance-determining region are the classic molecular signature of rifampicin resistance. The katG gene encodes catalase-peroxidase, the enzyme that activates the prodrug isoniazid, and disruptions in katG are strongly associated with high-level isoniazid resistance. The rpsL gene, encoding ribosomal protein S12, is implicated in streptomycin resistance, while gyrA, encoding the A subunit of DNA gyrase, harbors mutations in its quinolone resistance-determining region that confer fluoroquinolone resistance. Screening these four targets across the eighteen isolates provided a snapshot of the genetic resistance landscape in bovine non-tuberculous mycobacteria from the region.

The screening results revealed a clear hierarchy. The rpoB gene was the most frequently detected, present in 88.9 percent of the isolates, and its detection was mainly associated with Mycobacterium vaccae. The katG gene turned up in 16.7 percent of isolates, and the gyrA gene in just 5.6 percent, and notably both of these were found exclusively in M. vaccae. The rpsL gene, by contrast, was not amplified in any isolate, suggesting that streptomycin-associated genetic determinants, at least as targeted by the primers used, are absent from this collection. The pattern indicates that within this bovine sample set, genetic elements associated with resistance cluster heavily in one environmental species, raising questions about whether M. vaccae’s ecological lifestyle, including prolonged exposure to antimicrobial residues in agricultural environments, might shape its genetic repertoire.

Sequencing of selected amplicons added an important layer of nuance. When the researchers examined the hotspots of rpoB and katG, the regions where resistance-conferring mutations most commonly occur in Mycobacterium tuberculosis, they found no mutations at all. This is a meaningful negative result: the mere presence of a gene does not imply the presence of resistance-conferring variants within it. However, the story differed for gyrA. Sequence analysis of the quinolone resistance-determining region of gyrA revealed non-synonymous substitutions, meaning nucleotide changes that alter the encoded amino acid sequence of DNA gyrase. Whether these substitutions affect fluoroquinolone susceptibility in these isolates remains an open question, but their presence in the canonical resistance region of the enzyme is exactly the kind of finding that warrants closer phenotypic scrutiny.

The authors are careful, and rightly so, about what these results do and do not demonstrate. The detection of resistance-associated genes and even of amino acid substitutions reflects genotypic variation only. Because drug susceptibility testing was not performed on the isolates, the study cannot confirm phenotypic resistance to rifampicin, isoniazid, streptomycin, or fluoroquinolones. This distinction is fundamental in mycobacteriology. Genes such as rpoB and katG are essential housekeeping and metabolic genes present in virtually all mycobacteria; their detection by PCR says nothing about function unless the specific mutational hotspots are altered. Moreover, resistance mechanisms in non-tuberculous mycobacteria differ in important ways from those in the tuberculosis complex, involving intrinsic factors such as impermeable cell walls, efflux pumps, and species-specific enzymatic machinery, which means that extrapolating interpretive criteria from M. tuberculosis to environmental species is fraught with uncertainty.

Nevertheless, the study carries practical implications for animal health and public health alike. Non-tuberculous mycobacteria can produce tuberculosis-like lesions in bovines, and such lesions can trigger false-positive reactions in tuberculin skin testing, the backbone of bovine tuberculosis surveillance programs. Every misattributed lesion represents a potential misdiagnosis, with consequences ranging from unnecessary slaughter to missed transmission of true Mycobacterium bovis infection. By documenting which non-tuberculous species circulate in Indian cattle and buffaloes, and by beginning to map their resistance gene content, the researchers are building the species-level baseline that surveillance programs need to distinguish environmental noise from genuine tuberculosis signals. The finding that M. vaccae dominates the isolates gives diagnosticians a concrete expectation about what they are most likely to encounter when non-tuberculous organisms appear in bovine material.

The road forward, as the authors indicate, runs through phenotypic and species-specific analyses. Establishing the functional significance of the gyrA substitutions observed in M. vaccae will require correlating genotypes with measured minimum inhibitory concentrations of fluoroquinolones, and ideally with functional studies of the mutant gyrase enzymes. Broader panels of resistance genes, and ultimately whole-genome sequencing, could reveal the full complement of intrinsic and acquired resistance determinants in these organisms, an approach increasingly advocated in the literature on non-tuberculous mycobacterial susceptibility prediction. For now, the study stands as a careful first census: eighteen isolates, one dominant species, one frequently detected resistance-associated gene without hotspot mutations, and a handful of amino acid changes in a quinolone target that demand follow-up. In a world where antimicrobial resistance knows no boundaries between human clinics, farms, and soil, even the humblest environmental mycobacterium from a cow’s gut deserves that kind of attention.

Subject of Research: Molecular identification of non-tuberculous mycobacteria from bovine clinical samples and profiling of resistance-associated genes

Article Title: Molecular identification and resistance-associated gene profiling of non-tuberculous mycobacteria isolated from bovine clinical samples

Article References: Panigrahi, B., Narang, D., Chandra, M., Narang, A., Gupta, K., & Singh, S. T. (2026). Molecular identification and resistance-associated gene profiling of non-tuberculous mycobacteria isolated from bovine clinical samples. Molecular Biology Reports, 53(1), Article 1640. https://doi.org/10.1007/s11033-026-12814-7

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12814-7

Keywords: non-tuberculous mycobacteria, Mycobacterium vaccae, bovine tuberculosis, rpoB, katG, gyrA, antimicrobial resistance, PCR, cattle, diagnostics, genotypic variation, India