Low prevalence of bacteria harbouring the main transferable genes coding for β-lactams resistance in ovine bulk tank milk produced in Central Italy

Abstract

Sheep farming for milk production is an important sector of the Italian agricultural industry. Italy has a significant annual sheep milk production, which amounted to 477.000 ton in 2021 and almost exclusively employed for cheeses production. The aim of this work was to evaluate the prevalence of beta-lactams-resistant bacteria (methicillin-resistant staphylococci and Gram-negative bacteria) in bulk tank ovine milk collected from 97 farms located in central Italy (Tuscany and Lazio). Samples were screened for the presence of resistant microorganisms employing selective media, bacteria were identified by 16 S rRNA gene sequencing and tested for susceptibility for different beta-lactams (penicillins, carbapenems, cephalosporins, as well as beta lactams association with beta-lactamase inhibitors). Resistant microorganisms were subjected to PCR for the detection of the main transferable gene coding for beta-lactams resistance mechanisms spread among Gram-positive and Gram-negative bacteria. A low prevalence of methicillin resistant staphylococci (4%) and methicillin resistant Staphylococcus aureus (1%) in bulk tank milk from sheep herds raised in Central Italy was highlighted. This resistance was related to the presence of mecA gene in S. aureus and coagulase negative staphylococci. All the tested samples resulted negative for the presence of bacteria able to produce carbapenemases or ESBLs due to the presence of some of the main mobile genetic determinants in their genomes and targeted in this study. Most of the phenotypic resistance observed were instead related to the common presence of psychrotrophic bacteria in bulk tank milk belonging to Stenothrophomonas spp. which harboured L1 and L2 genes coding for inducible β-lactamases located on the bacterial chromosome. In conclusion, a low prevalence of bacteria resistant to β-lactams due to the presence of horizontally transferable genes was detected, suggesting a low selective pressure for resistant microrganisms whitin this farming system.