Antimicrobial resistance is a complex and widespread problem threatening human and animal health. In poultry farms, a wide distribution of resistant bacteria and their relative genes is described worldwide, including in Italy. In this paper, a comparison of resistance gene distribution in litter samples, recovered from four conventional and four antibiotic-free broiler flocks, was performed to highlight any influence of farming systems on the spreading and maintenance of resistance determinants. Conventional PCR tests, targeting the resistance genes related to the most used antibiotics in poultry farming, along with some critically important antibiotics for human medicine, were applied. In conventional farms, n. 10 out of n. 30 investigated genes were present in at least one sample, the most abundant fragments being the tet genes specific for tetracyclines, followed by those for aminoglycosides and chloramphenicol. All conventional samples resulted negative for colistin, carbapenems, and vancomycin resistance genes. A similar trend was observed for antibiotic-free herds, with n. 13 out of n. 30 amplified genes, while a positivity for the mcr-1 gene, specific for colistin, was observed in one antibiotic-free flock. The statistical analysis revealed a significant difference for the tetM gene, which was found more frequently in the antibiotic-free category. The analysis carried out in this study allowed us to obtain new data about the distribution of resistance patterns in the poultry industry in relation to farming types. The PCR test is a quick and non-expensive laboratory tool for the environmental monitoring of resistance determinants identifying potential indicators of AMR dissemination.
In this study a multidisciplinary approach was applied in order to determine the diffusion of resistant bacteria and selected antibiotic resistance genes in antibiotic-free and conventional broiler farms. Litter samples coming from the two farming types and surface sponges obtained from carcasses at slaughterhouse level were screened by end-point PCR targeting specific resistance for tetracycline, ampicillin, sulfonamide, aminoglycoside, carbapenem, nitrofurantoin, vancomycin, quinupristin-dalfopristin, lincomycin, linezolid, chloramphenicol molecules. Microbiological investigations were conducted from the carcasses to determine phenotypical and genetic resistance patterns from pathogenic and commensal Gram-negative and Gram-positive strains. At farm level, catA1, sul2, blaTEM and aadA2 genes were amplified in all samples, while from carcasses the most representative genes were sul2, blaTEM, along with the vatD, relative to quinupristin-dalfopristin resistance. Gram-negative isolates included Aeromonas, Salmonella, Proteus spp. And Escherichia coli, while the Gram-positive were represented by Enterococcus strains. Phenotypical and genetic analysis revealed multidrug resistance patterns in Salmonella, E. coli and Serratia isolates, followed by the Enterococcus species. The comparison between antibiotic-free and conventional farming systems showed some difference regarding the distribution of resistance genes at farm level but no significance was obtained comparing the phenotypical resistance profiles of bacterial strains from both groups of samples, suggesting a poor influence of farming model on the diffusion of antibiotic resistance in poultry meat production chain.
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