## 4 Discussion
It should be pointed out that in livestock systems and, particularly, in intensive farming, the prophylactic use of certain antibiotics is common practice. The administration of small doses of antibiotics to healthy animals causes the microbiota in the guts of animals to become familiar with the drugs and gives them a chance to develop resistance to the given substances [5; 32]. The bacteria then share the antibiotic resistance determinants via mobile genetic elements such as plasmids, transposons, integrins, and phages [33; 34]. This renders healthy animals carriers of antibiotic-resistant bacteria [35]. Since antibiotic-resistant bacteria are found in the gut, fecal contamination is the main route for the transmission of these pathogens to humans. The environments in which the animals are reared and indirect contact with animals are possible sources of the transmission of resistant zoonotic pathogens to humans [36; 37]. A previous epidemiological study from 1999 to 2002 carried out on Sicilian territory reported that _Salmonella_ Enteritidis (39%) was the most prevalent serovar, followed by _S._ Typhimuriumium (16%), _S._ _newport_ (6%), _S._ _salamae_ (5%), and others. The highest rate of antibiotic resistance was observed in _S._ Typhimuriumium [38]. In the last decade, antimicrobial resistance to five antibiotic classes, including penicillins, tetracyclines, sulphonamides, ciprofloxacin, and third-generation cephalospororus, has increased from 20% to 80% in Abruzzo and in Molise (Italy) [39]. Among the numerous serotypes of _Salmonella_, _S._ Typhimurium has always represented the ubiquitous type most frequently isolated both in humans and in the animal sector, surpassed only in some periods by emerging serotypes. The interest in the _S._ Typhimurium serotype is linked not only to its widespread diffusion in nature and the high frequency of infection in humans but also to the appearance of conjug antibiotic-resistant characteristics, as already highlighted in the 1960s. The mechanisms encoded by antimicrobial resistance determinants include antimicrobial modification and inactivation, the alteration of the antimicrobial target site, efflux pumps, and membrane impermeability [40]. These protect the bacteria from being attacked by antibiotics [40]. In particular, _Salmonella_ spp. uses the well-studied AcrAB-TolC efflux pump to extrude antibiotics such as tetracycline, chloramphenicol, and quinolones [41; 42]. Bacteria resist antibiotic entry into the cell by reducing or modifying porn channels in the outer membrane, which are used by antibiotic molecules to enter the bacterial cell to reach their targets [43; 44]. In addition to their high pathogenic potential, bacteria of the _Salmonella_ genus are of particular interest for their contribution to the spread of antibiotic resistance, as they are able to accumulate and spread antibiotic-resistance genes [45]. Genes that encode antibiotic resistance are either located on the chromosome or plasmids within a bacterial cell and are mobilized by transposons and integrins during conjugation or phages through transduction [34; 46]. The genetic plasticity of _Salmonella_ bacteria allows them to accumulate and disseminate antibiotic-resistance genes that often are located in plasmids that also carry other virulence genes [47; 48]. Thanks to this characteristic, the genus _Salmonella_ spp. can easily transfer resistance genes horizontally to other bacteria [49]. The molecular analysis of data for the presence of resistance genes highlighted the prevalent presence of blaTEM, whereas blaCTX-M I was found only in a _Salmonella_ Infantis isolated from a chicken meat sample. Bacteria that are resistant to the beta-lactam class of antibiotics produce the beta-lactamase enzyme, which destroys the beta-lactam ring, thus deactivating the antibiotic [50,51]. The blaTEM gene is the most frequently reported worldwide, especially in Gram-negative bacteria.