Diverse distribution of Toxin-Antitoxin II systems in Salmonella enterica serovars.

Andrea Di Cesare,Daniele Conficoni,Giulia Sarasini,Carmen Losasso,Antonia Ricci,Lisa Barco,Ester M Eckert,Gianluca Corno

doi:10.1038/srep28759

Abstract

Type II Toxin-Antitoxin systems (TAs), known for their presence in virulent and antibiotic resistant bacterial strains, were recently identified in Salmonella enterica isolates. However, the relationships between the presence of TAs (ccdAB and vapBC) and the epidemiological and genetic features of different non-typhoidal Salmonella serovars are largely unknown, reducing our understanding of the ecological success of different serovars. Salmonella enterica isolates from different sources, belonging to different serovars and epidemiologically unrelated according to ERIC profiles, were investigated for the presence of type II TAs, plasmid content, and antibiotic resistance. The results showed the ubiquitous presence of the vapBC gene in all the investigated Salmonella isolates, but a diverse distribution of ccdAB, which was detected in the most widespread Salmonella serovars, only. Analysis of the plasmid toxin ccdB translated sequence of four selected Salmonella isolates showed the presence of the amino acid substitution R99W, known to impede in vitro the lethal effect of CcdB toxin in the absence of its cognate antitoxin CcdA. These findings suggest a direct role of the TAs in promoting adaptability and persistence of the most prevalent Salmonella serovars, thus implying a wider eco-physiological role for these type II TAs.

Highlights

Salmonella spp. are the second most frequent zoonotic agent in the European Union (EU)1 and represent a major challenge for animal health and food safety because of their high endemicity and morbidity rate, and of the difficulty in controlling the pathogen2
This study aimed to further our understanding of the eco-physiological role for these type II Toxin-Antitoxin systems (TAs), on a variety of isolates belonging to different serovars, including serovars which are commonly associated with human infections
The spread of type II TAs, in relation to source, specific antibiotic resistance, and plasmid profile was studied through analysis of 45 Salmonella isolates belonging to highly pathogenic serovars, both for humans and animals, as well as serovars which have been rarely associated with infection episodes

Summary

Introduction

Salmonella spp. are the second most frequent zoonotic agent in the European Union (EU) and represent a major challenge for animal health and food safety because of their high endemicity and morbidity rate, and of the difficulty in controlling the pathogen. In the EU, in 2013, 22.5% of all food-borne outbreaks were related to Salmonella, and 82,694 confirmed cases of human salmonellosis were reported. There is much evidence suggesting that only a few Salmonella serovars are responsible for the vast majority of human infection cases, but, at the same time, it is unclear why these common serovars might have greater ecological success. Virulence of Salmonella serovars are evolutionarily related to gene acquisition by horizontal transfer of mobile elements and by the loss of genes or gene functionality11 Following these considerations, great interest exists in reducing the Salmonella impact on human and animal health by lowering its prevalence along the food chain through the implementation of control measures from farm to fork

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