Of all known Salmonella enterica serovars, S. Infantis is one of the most widespread and has recently been emerging worldwide.
Recently, we demonstrated that a highly prevalent S. Infantis strain that has been emerging in Israel since 2007 had acquired a conjugative megaplasmid (pESI) of about 280kb, which provides a multidrug resistance and hyper-virulence phenotype.
Here we show the fixation of pESI in the current (2014) S. Infantis population and demonstrate different stages of its evolution via integration of mobile genetic elements into an identified ancestor that contains the pESI backbone. Regulation studies have shown that microaerophilic and high osmolarity conditions upregulate the transcription of the pESI conjugation system (pil and tra operons).
Conjugation experiments between S. Infantis and E. coli ORN172 showed that the conjugation frequency is significantly increased under microaerophilic conditions, but reduced in the presence of sodium chloride or bile salts. Furthermore, we found that the conjugation is tightly temperature-dependent, as 27ºC and 41ºC repress conjugation, while physiological temperature of 37ºC induces horizontal transfer of pESI. In-vivo conjugation using C57BL/6 mice that were orally infected with S. Infantis harboring pESI showed that pESI can be transferred inside the mouse to various bacteria composing the mouse microbiome, including Escherichia coli and even Gram positive bacteria such as Lactobacillus Reuteri. Collectively our results suggest that pESI has been evolved and adapted to facilitate horizontal transfer in the mammalian intestine.
