Effect of gamma irradiation on antimicrobial resistant bacteria and resistance genes in chicken meat

Abstract

Antimicrobial resistance is an important problem for human health affecting many aspects such as medical problems and economics loss. Previous studies showed antimicrobial resistant bacteria (ARB) , antimicrobial resistance genes (ARGs) and mobile genetic element (MGEs) could contaminate several sources including environment, soil, water, and animal farms. This contamination could be transferred to food chain system through vegetables, animal products, especially chicken meat. ARB and ARGs were detected in this type of food even though it should be at a low level of contamination due to exposure to the low level of antibiotic use in the production step. Gamma (γ) irradiation is one method used for chicken meat sterilization. Thus, the effect of gamma rays on antimicrobial resistance should be studied. This experiment aims to determine effects of gamma irradiation on antimicrobial resistant bacteria Salmonella enterica serovar Kedougou (Sal162), 10 target genes composing of resistance genes, MGEs gene and reference genes ( aac3IVa, aac3IId, aadA, tetA, tetM, blaTEM, intI1, 16S rRNA, sdiA1 and SseG gene or specific gene for Sal162 strain) , MIC values, and transformation efficiency of plasmid spiked into chicken meat mimicking the resistant bacteria contamination. Five types of samples: i) resistant bacterial suspension, ii) DNA of resistant bacteria, iii) chicken meat spiked with resistant bacteria, iv) resistance plasmid, and v) chicken meat inoculated with bacteria carrying resistance plasmid, were irradiated by gamma rays at 0.25, 1, 2, 5, and 10 kGy and tested by bacterial CFU count, MIC test, PCR, qPCR, plasmid profiling and transformation efficiency testing. The results of the irradiated bacterial suspension, at increasing doses of gamma rays showed that all bacterial cells were killed at doses above 1 kGy. The MICs values of survived bacteria did not change from the original values. Irradiated DNAs showed that all target genes still gave positive results for PCR although gene quantitations were significantly affected at 10 kGy for all resistance genes except for aac3IVa, aadA, and tetA. Irradiated chicken meat contaminated with resistant bacteria showed that 5 kGy could kill all bacteria, and all target genes were positive for PCR with no significant difference as tested by qPCR. Irradiated resistance plasmid showed that transformation efficiencies decreased in all tested concentration (2, 20, 100 ng/μL) of plasmid DNA at increasing gamma doses with no transfer activities at 10 kGy. However, irradiated chicken meat spiked with resistant bacteria carrying resistance plasmid showed that transformation efficiency did not reduce. From our results, gamma irradiation could reduce contamination of resistant bacteria in chicken meat and some amount of resistance genes. For genetic transferring, this ionizing radiation could affect extracellular plasmids that were directly irradiated more than the plasmids in bacterial cells. Further studies should focus on the mechanism(s) of gamma irradiation on the changes in resistance properties including the physical changes of chicken meat after irradiation.