Geographic, species and lineage driven antimicrobial resistance patterns in poultry-associated Campylobacter across South and Southeast Asia

Antimicrobial-resistant Campylobacter jejuni and Campylobacter coli are increasingly reported, particularly in regions experiencing rapid poultry intensification. Whole-genome sequencing, combined with antimicrobial susceptibility testing of 342 representative isolates collected along poultry production and distribution networks (PDNs) in Bangladesh, India, Sri Lanka and Vietnam, revealed pronounced geographic, species and lineage-specific resistance patterns. Quinolone resistance was near universal (>90%), irrespective of country or Campylobacter species, indicating region-wide loss of clinical utility for this class of antibiotic. C. coli consistently displayed a broader resistome and higher macrolide non-susceptibility than C. jejuni (67–80% vs 0–27%). Vietnamese C. coli formed a distinct antimicrobial resistance (AMR) ecotype, carrying the bifunctional aac(6′)-Ie-aph(2″)-Ia (50%), phenicol resistance genes cat , fexA and optrA , and mosaic tet(O/M/O ) variants, while phenicol resistance genes were not detected in South Asian isolates. In contrast, Bangladeshi and Indian cohorts were dominated by classical tet(O ), with the aminoglycoside cassette aad (6)-sat-4-aph(3′)-IIIa being particularly prevalent in Bangladesh. Lineage analysis identified discrete resistance reservoirs: in Bangladesh, ST-3236 and ST-10345 accounted for 57 and 20% of aad(6) -positive isolates, respectively, whereas in Vietnam multiple C. coli lineages carried distinct combinations of macrolide, aminoglycoside, phenicol and tetracycline-associated determinants. Phenotypically, 94% of macrolide-resistant C. coli were non-susceptible to ≥4 antibiotic classes, with 70% prevalence in Vietnamese C. coli non-susceptible to ≥5 classes. Resistance profiles did not differ between farm, market and slaughter nodes within countries, suggesting that resistance profiles were broadly distributed across the studied PDNs rather than confined to specific segments of the network. These findings demonstrate that AMR in poultry-associated Campylobacter is shaped by interacting geographic, species and clonal factors. The near-fixed quinolone resistance, emergence of highly multidrug-resistant C. coli particularly in Vietnam and spread of mobile macrolide determinants ( ermB ) collectively narrow therapeutic options, with florfenicol susceptibility retained in only 38% of Vietnamese C. coli isolates resistant to five or six drug classes, indicating limited residual in vitro phenicol activity in the tested panel. Region-specific stewardship programmes and integrated genotypic/phenotypic surveillance across One Health interfaces are needed to monitor remaining antimicrobial susceptibility and curb further dissemination of resistant lineages in South and Southeast Asian food systems.