Antimicrobial sensitivity testing of stored bacterial isolates from 2000 to 2021 in the agricultural sector

Abstract

Antimicrobial resistance is a critical global concern that has a detrimental effects on both human and animal health as well as food security, development, and the economy. This study investigated the antimicrobial sensitivity of stored bacterial isolates from 2000 to 2021 in the agricultural sector across eight provinces in South Africa. A total of 216 bacterial isolates which represented four distinct bacterial pathogens: Salmonella enterica (n = 88), Escherichia coli (n = 30), Staphylococcus aureus (n = 64), and Listeria monocytogenes (n = 34) were revived and analyzed. Phenotypic antimicrobial profiling was determined against 12 antibiotics for S. enterica and E. coli isolates and 13 antibiotics for S. aureus and L. monocytogenes, using Kirby-Bauer disk diffusion and minimum inhibitory concentration methods. Conventional PCR was performed for confirmation of isolate species, serotypes, and screening of antimicrobial resistance genes (bla, tet, Dfr, qnr, sul, erm, cat1, flo, cm1A and mecA). The isolates that were lyophilized yielded a poor viability rate (31%), compared to 69% of the isolates preserved in tryptose broth-glycerol. Salmonella enterica and E. coli isolates were generally sensitive against all tested antibiotics except for tetracyclines. S. aureus showed high resistance (90-100%) to 11 antibiotics (penicillin, gentamycin, nitrofurantoin, chloramphenicol, linezolid, quinupristin-dalfopristin, tetracycline, vancomycin, erythromycin, kanamycin, and ciprofloxacin). All the tested L. monocytogenes showed a complete (100%) resistance against a wide range of antibiotics, including penicillin, gentamycin, kanamycin, vancomycin, ciprofloxacin, quinupristin-dalfopristin, tetracycline, and erythromycin. The tetA and blaPSE genes were predominantly detected in S. enterica and E. coli while S. aureus isolates harbored a wide variety of antibiotic resistance genes namely mecA (44%), blaZ (39%), aap-apl (31%), and tetM (22%). Among the four serogroups of L. monocytogenes tested in this study, 71% of serogroup IIc harbored ermB and 42% of serogroup IVa carried the tetA gene. This retrospective study provides a glimpse into the antimicrobial sensitivity profile of major animal and foodborne pathogens over the years in the country. These data can be used in monitoring existing anti-microbial resistance (AMR) policies and strategies in South Africa.