Salmonella is a worldwide-distributed pathogen that affects both humans and animals13. This microorganism is usually associated with intensive animal production, such as swine, since the confinement of animals facilitates the transmission and persistence of the pathogen in farms. Salmonellosis may cause significant economic losses due to mortality, morbidity, stunted growth, increased feed conversion rates and costs of non-specific treatments. Some animals, such as pigs and chickens, may be infected without manifesting any clinical signs of the disease and, thus, are relevant in the dissemination of the pathogen within farms and usually serve as sources of contamination of food26. In Argentina, swine production is heterogeneus, comprising large-scale industrial farms and backyard productions6. Intensive farms are usually highly technified with high density of animals, which favors the transmission of infectious diseases30. Furthermore, the use of antimicrobials as therapeutics or growth promoters that may promote the emergence of resistant strains is common. On the other hand, semi-intensive and backyard farms are less technified and pigs may be in close contact with other farm animals such as poultry or wild species1.

S. enterica serovar Choleraesuis (S. Choleraesuis) is the serotype usually associated with the disease in pigs, which can also affect humans10. Likewise, pigs may be infected or carry a large number of serotypes with zoonotic potential, S.enterica serovar Typhimurium (S. Typhimurium) being the most commonly isolated serovar in swine production in Argentina12 and worldwide3. This serotype is also among the most commonly isolated from human samples in Argentina (personal communication ANLIS-Malbran) and worldwide16.

Salmonella can carry different virulence genes, whose expression may determine the course of the infection by favoring the interaction of the microorganism with the host. Thus, in recent years, the analysis of these virulence genes and their involvement in the mechanisms of pathogenicity has been introduced as a tool to characterize Salmonella isolates17. This analysis usually includes the detection of representative genes from the pathogenicity islands 1–5 (avrA, ssaQ, mgtC, siiD and sopB), which are common in all serotypes, prophages (gipA, sodC1 and sopE1), the virulence plasmid (spvC) associated with the systemic dissemination of Salmonella, and the fimbrial cluster (bcfC)22,28. This approach complements the information and knowledge on circulating Salmonella strains and allows to typify them not only by their antigenic properties but also by their pathogenic potential. Furthermore, the virulence of bacteria is influenced not only by the presence of virulence genes but also by their antimicrobial resistance22. Currently, antimicrobials are widely used in animal production as therapeutics or growth promoters. However, the irrational use of these antimicrobials favors the emergence of multiresistant Salmonella strains and their resistance can be spread to other microorganisms in the gut. Many studies have reported high levels of resistance against several antibiotic families in strains isolated from different sources8,9.

The aim of this work was to study the presence of virulence-associated genes and the antimicrobial resistance phenotype in Salmonella isolates obtained from swine intensive and backyard farms in different areas of Argentina during the 2012–2018 period.

In this work, we analyzed 59 strains of Salmonella isolated from intensive and backyard swine production in Argentina. Among them, we included 12 strains of S. Typhimurium, which is the most prevalent serotype in the Argentinean swine industry23, comprising 25% of Salmonella isolates in the human population from Argentina (ANLIS Malbran, personal communication). Strains were characterized by the presence of two relevant indicators for the pathogenicity of Salmonella: virulence genes and susceptibility to antimicrobials, which may help to understand the potential risk of the strains circulating in swine production.

Most of the strains carried the genes from SPIs 1 to 5, supporting previous studies that suggest that virulence determinants located in these SPIs are highly conserved among serotypes5. These genes may be essential for the early infection of the animals since they regulate intestinal epithelial cell invasion, toxin production, intracellular survival, and also control of inflammation induced by the microorganism7. In contrast, the genes located in the prophages, the fimbrial cluster and the virulence plasmid showed variability. Only the strains of S. Typhimurium and S. Choleraesuis analyzed in this work carried the spvC gene, which is strongly associated with strains that cause bacteremia20 and with systemic dissemination of the microorganism21. However, Astolfi-Ferreira et al.2 proposed that the spvC gene may not be essential for virulence, since they found that spvC-negative strains were still able to cause invasive infections in chickens.

Virulence gene gipA, which is related to colonization and invasion of the small intestine31, was found in only 50% of the strains of S. Rissen and in 66.7% of the strains of S. Typhimurium and its monophasic variant; similar percentages were observed by Huehn et al.22,31 The presence of sopE1, which is associated to disruption of tight junctions25 varied among the serotypes. On the other hand, in our work superoxide dismutase-encoding sodC1 gene was detected in all the S. Typhimurium isolates, as also reported by De Toro et al.14, Osman et al.27, and Bertelloni et al.5 To understand its relevance, the association between the presence of virulence genes and the pathogenicity of circulating strains should be considered in a broader gene context. In fact, if a certain gene is essential or not for pathogenicity is still controversial. For example, some authors consider that the presence of the gipA and sopE1 genes in non-typhoidal Salmonella may be irrelevant for invasiveness33.

Overall, in the present study, the strains from intensive farms were resistant to a larger number of antimicrobials than those from backyard farms independently of the serotype. The highest XDR index value was found for a monophasic S. Typhimurium strain isolated from an intensive farm located in the Province of Buenos Aires. This fact may be explained by the extended usage of antibiotics for metaphylaxis or prophylaxis and as growth promoters in industrial production that may favor the emergence of multiresistant strains as proposed by Esperbent and Migliorati15. In fact, the increasing spread of resistance to different drugs through horizontal transfer among the gut microorganisms is an imperative Public Health concern worldwide14.

Susceptibility to other antimicrobials varied among the strains. We found that 44.07% of the strains were resistant to ampicillin; similar levels of resistance were reported by Campos et al.9 in swine farms from Portugal in 2019, but higher levels were reported by Cameron-Veas et al.8 in Spain in 2018 with around 58% of the isolates resistant to this antimicrobial. In contrast, although practices in swine production in Brazil are similar to those in Argentina, Guerra Filho et al.19, reported only 30% of resistance to ampicillin in strains from the former country. In our work, resistance to cefepime to cefalotin was relatively low, which agrees with reports from other countries14,19. With regard to fluoroquinolones, the levels of resistance of Salmonella are diverse worldwide, ranging from 14.4%32 to 90%19. We found that 20.34% percent of the isolates were resistant to ciprofloxacin and 38.98% of the strains were resistant to enrofloxacin. The emergence of mechanisms of resistance to beta-lactams, which are commonly used to treat severe Salmonella infections, may limit their administration in animal production and thus other alternatives should be explored and implemented14. Susceptibility to third and fourth-generation cephalosporins in strains from livestock is critical since these antibiotics are the main therapeutic option for infections caused by enterobacteria in human medicine8.

The presence of the virulence genes analyzed in this work does not seem to be strictly related to antimicrobial resistance as shown by the dissimilar association between carriage of virulence genes and resistance to antimicrobials. Anyhow, it should be considered that the association between the carriage of genes encoding virulence and resistance to antimicrobials should be focused on plasmid genes. Prophage genes gipA and sodC1 showed the highest association with resistance to most of the antimicrobials tested, whereas we found that genes from SPI 1 to 5 had a relatively low association. In contrast, Osman et al.27 reported 100% association between the presence of genes from these SPIs and resistance to ampicillin, chloramphenicol, gentamicin, nalidixic acid, and TMS. Nevertheless these authors only included strains isolated from poultry, most of them being S. Enteritidis, while in our work a broader spectrum of serotypes was analyzed.

In our study strains with extensive antimicrobial resistance profiles and virulence potential were detected, which may represent a serious threat for both animal production and public health. In the context of the One Health approach, continuous surveillance is essential to detect the emergence and circulation of such strains in order to minimize their impact on animal and human medicine. Thus, efforts to control the pathogen throughout the entire production chain and its environment must be done in order to ensure food safety for consumers. Control of Salmonella in swine production requires tailored-made programs adapted to each farm. Together with strict biosecurity measures, vaccination is one of the main tools to control the microorganism in the long-term, although this strategy is still not common in pigs since there is no commercial vaccine available in the Argentinean market. Moreover, the use of alternative products such as probiotics or phytochemicals that promote gut health may be effective24.

The authors declare that they have no conflicts of interest.