Salmonella Infantis (SI) became endemic in Hungary where the PFGE cluster B, characterized by a large multiresistance (MDR) plasmid emerged among broilers leading to an increased occurrence in humans. We hypothesized that this plasmid (pSI54/04) assisted dissemination of SI. Indeed, Nal-Sul-Tet phenotypes carrying pSI54/04 occurred increasingly between 2011 and 2013 among SI isolates from broilers and humans. Characterization of pSI54/04 based on genome sequence data of the MDR strain SI54/04 indicated a size of ∼277 kb and a high sequence similarity with the megaplasmid pESI of SI predominant in Israel. Molecular characterization of 78 representative broiler and human isolates detected the prototype plasmid pSI54/04 and its variants together with novel plasmid associations within the emerging cluster B. To test in vitro and in vivo pathogenicity of pSI54/04 we produced plasmidic transconjugant of the plasmid-free pre-emergent strain SI69/94. This parental strain and its transconjugant have been tested on chicken embryo fibroblasts (CEFs) and in orally infected day old chicks. The uptake of pSI54/04 did not increase the pathogenicity of the strain SI69/94 in these systems. Thus, dissemination of SI in poultry could be assisted by antimicrobial resistance rather than by virulence modules of the endemic plasmid pSI54/04 in Hungary.
Keywords: Broilers; Molecular typing; PFGE clone; Plasmid; S. Infantis.
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