From farm to fork: epidemiological study, genetic characterization and plasmid identification of antibiotic resistant Salmonella strains isolated along the food chain in Marche Region

Antimicrobial Resistance (AMR) is a serious public health problem. Genes involved in AMR can be localized on mobile genetic elements (e.i. transposons or plasmids)and this allows them to be easily disseminated. Salmonellosis is the second most commonly reported gastrointestinal infection in humans and an important cause of food-borne outbreaks in the EU. Among the different serovars of Salmonella enterica, S. Infantis is one of the five main causes of human salmonellosis in the European Union (EU). In Italy, a MDR and ESBL producing S. Infantis clone, harboring a blaCTX-M-1 pESI-like megaplasmid (~280–320 kb), has increasingly spread. The antibiotic resistance profiles of Salmonella spp. strains isolated in Marche Region from veterinary and food-related environments, food animals, foods and human clinical samples was investigated in order to possibly trace the spread of AMR and plasmid distribution in the food chain. Moreover, the presence of the pESI-like plasmid-carrying S. Infantis clone in AMR S. Infantis strains, isolated from various sources in Marche Region, was investigated. A total of 101 AMR Salmonella strains, of serovars S. Derby (n. 20), S. Typhimurium (n. 18), MVST (n. 28) and S. Infantis (n. 35), were collected and analyzed for this study. Resistance to sulfisoxazole (86%) and tetracycline (81%) and ampicillin (76%) were the most common. FIIS was the most predominant replicon (17%), along with FII (11%) and FIB (11%), all belonging to IncF incompatibility group. Concerning the characterization of tet genes, tetB was the most frequently detected (27/86). Most strains (n. 27 MVST, n. 6 S. Derby, n. 4 S. Typhimurium and n. 3 S. Infantis) were positive for blaTEM gene. Other strains (n. 23 S. Infantis and n. 1 S. Typhimurium) were positive for blaCTX-A/B. Only one MVST strain was positive for blaCMY. In light of the importance that the third generation cephalosporins represent in human therapy, all S. Infantis strains of this study were sequenced. Two different ESBL genetic profiles comparable to S. Infantis pESI-like clone were observed. 81% carried the blaCTX-M-1 gene and 8% carried the blaCTX-M-65. Moreover, the close genetic relationship of S. Infantis blaCTX-M-1 harboring IncFIB(K)_1_Kpn3 megaplasmids from diverse sources confirmed the presence and spread of S. Infantis pESI-like blaCTX-M-1 clone in Marche Region along the food chain. The implementation of a One Health approach, integrating surveillance of MDR strain spread from farm to fork, contributes to the monitoring of MDR zoonotic pathogens, such as S. Infantis.