Survival of staphylococci and transmissibility of their antimicrobial resistance genes in milk after heat treatments

Abstract

There is growing concern that the milk heat treatments, which are the main antibacterial safeguard in the dairy industry, are not sufficient in inactivating the antimicrobial resistance (AMR) genes of staphylococci and may induce a viable but non-culturable (VBNC) state of these microorganisms. This study investigated the persistence and quantification of blaZ, mecC and tetK plasmid-mediated AMR genes copy numbers of two staphylococcal strains in both milk and Tris-EDTA (TE) buffer. During subsequent storage after pasteurization, all tested genes showed increased copy numbers. By electroporation of these genes to the Staphylococcus aureus RN42200 electro-competent strain, both mecC and tetK genes were still expressive and transferable. The formation of VBNC cells was estimated with viability staining and quantitative PCR of 16S rDNA copy numbers of both staphylococcal strains. The copy numbers increased over the storage period, which suggests that the pasteurization process was a stress factor and likely induces VBNC state in tested staphylococci. This explains the continual increase in the copy numbers of AMR genes. In contrast, after the sterilization treatment all the genes showed decreases in copy numbers. Further studies are required to evaluate the efficiency of currently practiced pasteurization and to assess the hazard level of VBNC staphylococci.