Longitudinal characterization of monophasic Salmonella Typhimurium throughout the pig's life cycle

- The usage of antimicrobials influenced the development of resistance in a setting where monophasic S. Typhimurium was circulating.
- A common antimicrobial resistance pattern to ampicillin, kanamycin, neomycin, streptomycin, tetracycline and sulphonamides was detected.
- Isolates obtained during the finishing phase showed additional resistance to chloramphenicol, florfenicol, gentamicin and netilmicin.
- S. 4,[5],12:i:- PFGE cluster A predominance in the farrowing phase and in the early finishing phase suggested a carryover from birth to grower.
- PFGE analysis showed that sow infection status does not necessarily predict the offspring's Salmonella infection status.

Swine have been described as an important reservoir of multidrug resistant monophasic Salmonella Typhimurium, though information on its ecology is scarce. A longitudinal study was performed in order to elucidate the Salmonella 4,[5],12:i:- dynamics throughout the pig's production cycle. A total of 209 faecal samples were collected from 10 sows and in six sampling times during the life of 70 pigs from a Portuguese industrial farm, and 43 isolates of S. 4,[5],12:i:- were identified and characterized regarding clonality and antimicrobial resistance phenotype and genotype. Most isolates (n = 42) exhibited resistance to at least ampicillin, kanamycin, neomycin, streptomycin, tetracycline and sulfonamides (encoded by blaTEM, aphAI-IAB, strA, strB, tetB and sul2, respectively). Isolates obtained during the finishing phase showed additional resistance to chloramphenicol and florfenicol (floR), gentamicin and netilmicin (aac(3′)-IV). To our knowledge, this study is the first description of aphAI-IAB in S. 4,[5],12:i:-. PFGE analysis showed uneven distribution of isolates into three clusters, A (n = 34), B (n = 8) and C (n = 1). PFGE cluster A was predominant in sows (n = 5) and piglets in the farrowing phase (n = 17) and in pigs in the early finishing phase (n = 11) suggesting a carryover from birth to adult age. The introduction of PFGE cluster B isolates in adulthood could have had an external source, reinforcing the relevance of environmental transmission in the farm ecosystem. This study reveals a dynamic interaction between monophasic S. Typhimurium and the pressures exerted under an intensive swine production setting.