ANTIMICROBIAL RESISTANCE IN LIVESTOCK PATHOGENS WITH MOLECULAR MECHANISMS HOST IMMUNE RESPONSES AND PATHOLOGICAL CONSEQUENCES

Abstract

Antimicrobial resistance (AMR) has become a major challenge to the effective treatment of bacterial infections in livestock, with consequences for animal health, productivity, welfare, and the sustainability of veterinary antimicrobial therapy. Although AMR is commonly discussed in relation to antimicrobial use, prevalence, transmission, and surveillance, comparatively less attention has been given to its biological consequences within the infected host. This review integrates molecular mechanisms of resistance with bacterial fitness, virulence-associated traits, host immune responses, pathological injury, and therapeutic outcome in livestock infections. Resistance may arise through chromosomal mutations, horizontal acquisition of resistance determinants, antimicrobial-inactivating enzymes, target modification, altered permeability, active efflux, metabolic bypass, and biofilm-associated tolerance or persistence. These mechanisms can interact with bacterial physiology and adaptation, although antimicrobial resistance should not be interpreted as synonymous with increased virulence or greater pathological severity. Resistant pathogens may persist during ineffective therapy, prolonging host–pathogen interactions and sustaining inflammatory responses that contribute indirectly to chronic infection, tissue damage, recurrence, and treatment failure. Innate and adaptive immune responses remain central determinants of infection outcome, while biofilm formation, intracellular survival, and immune-evasion strategies may further complicate bacterial clearance. Advances in whole-genome sequencing, metagenomics, transcriptomics, and integrated pathology offer new opportunities to connect resistance genotype with expressed phenotype and disease progression. Future veterinary AMR research should therefore integrate antimicrobial susceptibility, pathogen genomics, virulence, host immunity, lesion characterization, and clinical outcome to better understand resistant infections as complex biological disease processes.