Antimicrobial resistance, extended-spectrum β-lactamase determinants, and virulence gene profiles of Escherichia coli along the pork production chain in central Thailand

Background and Aim: Antimicrobial resistance (AMR) in foodborne bacteria presents a significant threat to public health, especially in countries with intensive livestock production systems. Pig farming is a major source of animal protein in Thailand and is recognized as an important reservoir of antimicrobial-resistant bacteria. Escherichia coli is commonly used as an indicator organism for monitoring AMR, including extended-spectrum β-lactamase (ESBL) production and pathogenic potential. This study aimed to assess the frequency of AMR, multidrug-resistant (MDR), ESBL determinants, and virulence genes in E. coli isolates collected from slaughterhouses and fresh markets in central Thailand. Materials and Methods: A total of 498 archived E. coli isolates were analyzed, including 236 isolates from slaughterhouses (feces and carcasses) and 262 isolates from fresh markets (pork and cutting boards). Antimicrobial susceptibility testing was performed against 18 antimicrobial agents using the disk diffusion method. MDR was defined as resistance to three or more antimicrobial classes. ESBL production was identified through phenotypic confirmatory tests, and ESBL-producing isolates were screened for blaTEM, blaCTX-M, and blaSHV genes by multiplex polymerase chain reaction. All isolates were further examined for select virulence genes linked to major E. coli pathotypes. Results: Overall, 97.4% of E. coli isolates showed resistance to at least one antimicrobial agent, and 87.3% were classified as MDR. ESBL-producing E. coli made up 23.5% of all isolates, with a significantly higher prevalence in slaughterhouses compared to fresh markets (p < 0.05). Among ESBL producers, 97.4% exhibited MDR phenotypes. Most (89.7%) of the ESBL-producing isolates carried at least one bla gene, with blaTEM being the most common, followed by blaCTX-M. Virulence genes were detected at a low frequency (3.2%), mainly involving eaeA, lt, and stp. Conclusion: The high prevalence of AMR, MDR, and ESBL-producing E. coli throughout the pork production chain highlights slaughterhouses and fresh markets as key points for the spread of resistant bacteria. These findings emphasize the need for stronger antimicrobial stewardship, better hygiene practices, and ongoing AMR surveillance within the One Health approach to reduce public health risks linked to pork consumption. Keywords: antimicrobial resistance, extended-spectrum beta-lactamase, Escherichia coli, food safety, multidrug resistance, pork production chain, slaughterhouse contamination, Thailand, virulence genes.