Salmonellosis remains one of the most frequent foodborne zoonosis, constituting a worldwide major public health concern. The most frequent sources of human infections are food products of animal origin, being pork meat one of the most relevant. Currently, particular pig food production well-adapted and persistent Salmonella enterica serotypes (e.g., Salmonella Typhimurium, Salmonella 1,4,[5],12:i:-, Salmonella Derby and Salmonella Rissen) are frequently reported associated with human infections in diverse industrialized countries. The dissemination of those clinically-relevant Salmonella serotypes/clones has been related to the intensification of pig production chain and to an increase in the international trade of pigs and pork meat. Those changes that occurred over the years along the food chain may act as food chain drivers leading to new problems and challenges, compromising the successful control of Salmonella. Among those, the emergence of antibiotic resistance in non-typhoidal Salmonella associated with antimicrobials use in the pig production chain is of special concern for public health. The transmission of pig-related multidrug-resistant Salmonella serotypes, clones and/or genetic elements carrying clinically-relevant antibiotic resistance genes, frequently associated with metal tolerance genes, from pigs and pork meat to humans, has been reported and highlights the contribution of different drivers to the antibiotic resistance burden. Gathered data strengthen the need for global mandatory interventions and strategies for effective Salmonella control and surveillance across the pig production chain. The purpose of this review was to provide an overview of the role of pig and pork meat in human salmonellosis at a global scale, highlighting the main factors contributing to the persistence and dissemination of clinically-relevant pig-related Salmonella serotypes and clones.

Objectives To assess the presence of multidrug-resistant (MDR) Salmonella with human clinical relevance in pig farms from different regions of Portugal and to analyse their mobile genetic elements associated with antibiotic resistance. Methods Seventy-nine samples were collected from six piggeries and analysed for the presence of Salmonella. All isolates were examined for susceptibility to antimicrobial agents and representative isolates for resistance genes and class 1 integrons (PCR/restriction fragment length polymorphism). Clonality was determined by PFGE and multilocus sequence typing (MLST). Plasmid analysis included determination of size, content and characterization of the incompatibility group (rep-PCR and I-CeuI/S1-hybridization). Results Sixty Salmonella isolates were recovered from five samples (two manure, two waste lagoons and one animal feed) in half of the piggeries studied. All isolates were resistant to at least one antibiotic (tetracycline) and 97% to at least four antibiotics from different families. In 10 isolates representing different serogroup and resistance phenotype combinations a diversity of resistance genes and integrons was detected. These isolates belonged to the internationally widespread Salmonella Rissen (ST469) and Salmonella Typhimurium DT104 (ST19) clones, as well as to the emerging Salmonella Typhimurium monophasic variant with examples of Spanish (carrying a sul3-atypical integron within IncA/C plasmids, here assigned to ST19) and European (ASSuT phenotype, assigned to ST34) clones. Conclusions This is one of the few studies reporting emerging MDR Salmonella clones and the first one detecting Salmonella Typhimurium monophasic variant in the pig production setting. The survival of these strains in manure and waste lagoons is of concern, since these environments might allow spread of MDR bacteria beyond pig farms’ boundaries.