Resistance to widely-used disinfectants and heavy metals and cross resistance to antibiotics in Escherichia coli isolated from pigs, pork and pig carcass

A total of Escherichia coli isolated from pigs (n = 643), pork (n = 111) and pig carcasses (n = 110) in Thailand during 2008–2018 were included. Susceptibilities to antibiotics, disinfectants (i.e. triclosan (TCS), chlorhexidine digluconate (CHX), benzalkonium chloride (BKC), glutaraldehyde (GLU), formaldehyde (FOR)) and heavy metals (i.e. zinc chloride and copper sulfate) were examined. The E. coli isolates susceptible to all antibiotics (n = 24) were included for in vitro exposure experiment. The effect of phenylalanine arginine β-naphthylamide (PAβN) on the susceptibilities was determined. The results showed that the majority of E. coli (89%) from pigs, pig carcass and pork were multidrug resistant strains. The MICs of all disinfectants and heavy metals, except triclosan were clustered under 1 or 2 concentrations, indicating that there was no development or development at limited degree of resistance to these substances. TCS MICs for E. coli isolated in 2011–2014 were significantly higher than that of the strains in 2008–2010. The weak correlation between biocide MICs (BKC, GLU, TCS, CHX and CuSO4) and antibiotic resistance was observed (p < 0.05). Exposure to triclosan, benzalkonium chloride and chlorhexidine digluconate selected for spontaneous-resistant mutants exhibited cross resistance to at least one antibiotic. The cross resistance was observed between TCS and all eight antimicrobial drugs; BKC and chloramphenicol/ciprofloxacin/sulfamethoxazole/tetracycline; and CHX and ciprofloxacin/gentamicin/streptomycin. The presence of PAβN restored MICs of chloramphenicol and trimethoprim in some BKC- and TCS-spontaneous resistant mutants, highlighting the important role of multidrug efflux system as cross-resistance mediated mechanism. The presence of PAβN, carbonyl cyanide m-chlorophenylhydrazone (CCCP) and reserpine could not restore ciprofloxacin MIC in some ciprofloxacin-resistant mutant derivatives with no mutation in neither gyrA nor parC, suggesting the existence of proton motive force-independent mechanisms. In conclusions, the widely-used disinfectants and heavy metals play an important role as non-antibiotic selective pressure for emergence and spread of AMR. Therefore, susceptibilities to disinfectants/heavy metals should be routinely monitored.