Abstract

Salmonella enterica serovar Napoli (serovar Napoli) is an emerging cause of human salmonellosis in Northern Italy. No specific reservoirs of serovar Napoli have been identified in Italy, so far. However, the environment, especially surface waters, has been hypothesized as an important source of infection based on the observation that genotypically different clusters of serovar Napoli are detected in different geographical macro-areas. To further support the hypothesis of a spatially-restricted pattern of serovar Napoli diffusion, a spatial segregation of serovar Napoli lineages should be observed also at smaller geographical scale. However, classical genotyping techniques used for Salmonella, such as pulsed-field gel electrophoresis (PFGE), did not possess enough discriminatory power to highlight spatial clustering of serovar Napoli within the macro-areas. To this purpose, we performed phylogeographical analyses based on genome-wide single nucleotide polymorphisms to test whether spatio-temporal evolution patterns of serovar Napoli in Northern Italy could be recognized with high geographical resolution, i.e. at local level. Specifically, we analyzed the local spread of the main PFGE clonal group, responsible for more than 60% of human infections in the study area, that did not show any geographical differentiation by PFGE within Northern Italy, i.e. the macro-area considered in the study. Both discrete and continuous phylogeography highlighted the existence of two main geographically-restricted clades: a Southern clade corresponding to the Po Valley and a Northern clade corresponding to the Pre-Alps area. Furthermore, the phylogeographical analyses suggested that the most probable site of origin of the clone was in an area of the Po Valley at the confluence of the Po and Ticino rivers, one of the most important Italian wetlands. These findings provide further support to the hypothesis that environmental transmission may play an important role in the ecology of serovar Napoli.

Highlights

  • Salmonellosis, caused by Salmonella enterica, represents one of the major foodborne diseases in developed and developing countries [1,2]

  • Most serovars associated with disease in humans and other warm-blooded animals belong to subspecies I (Salmonella enterica subspecies enterica) and can be divided in two main groups, typhoidal and non-typhoidal serovars

  • The logistic regression analysis on sub-clusters within A showed that the geographical areas of origin are not significantly associated to the genotypes corresponding to the different sub-clusters (n = 69, likelihood ratio tests (LRT), p = 0.47)

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Summary

Introduction

Salmonellosis, caused by Salmonella enterica, represents one of the major foodborne diseases in developed and developing countries [1,2]. Salmonella enterica is subdivided into 6 subspecies and includes more than 2600 serovars which cause clinical syndromes ranging from asymptomatic carriage to invasive systemic disease. Typhoidal serovars (Typhi and Paratyphi A, B, C) are responsible for typhoid and paratyphoid fevers, while non-typhoidal serotypes typically cause self-limited gastroenteric disease [3,4,5]. Salmonella enterica serovar Napoli (serovar Napoli), an emerging serovar in Europe, has been observed to carry SPI-18 and cdtB islet and to be phylogenetically very close to typhoidal serovars [8]

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