Evolution and Population Structure of Salmonella enterica Serovar Newport

Abstract

Salmonellosis caused by Salmonella enterica serovar Newport is a major global public health concern, particularly because S. Newport isolates that are resistant to multiple drugs (MDR), including thirdgeneration cephalosporins (MDR-AmpC phenotype), have been commonly isolated from food animals. We analyzed 384 S. Newport isolates from various sources by a multilocus sequence typing (MLST) scheme to study the evolution and population structure of the serovar. These were compared to the population structure of S. enterica serovars Enteritidis, Kentucky, Paratyphi B, and Typhimurium. Our S. Newport collection fell into three lineages, Newport-I, Newport-II, and Newport-III, each of which contained multiple sequence types (STs). Newport-I has only a few STs, unlike Newport-II or Newport-III, and has possibly emerged recently. Newport-I is more prevalent among humans in Europe than in North America, whereas Newport-II is preferentially associated with animals. Two STs of Newport-II encompassed all MDR-AmpC isolates, suggesting recent global spread after the acquisition of the bla CMY-2 gene. In contrast, most Newport-III isolates were from humans in North America and were pansusceptible to antibiotics. Newport was intermediate in population structure to the other serovars, which varied from a single monophyletic lineage in S. Enteritidis or S. Typhimurium to four discrete lineages within S. Paratyphi B. Both mutation and homologous recombination are responsible for diversification within each of these lineages, but the relative frequencies differed with the lineage. We conclude that serovars of S. enterica provide a variety of different population structures.Salmonellosis is a major global cause of diarrheal and extraintestinal disease in humans and animals (66). Salmonella enterica subspecies enterica (referred to herein as S. enterica) has been subdivided serologically into Ͼ1,500 serovars (35), but we focus on S. enterica serovar Newport (S. Newport) here because over the last decade it has been a very common cause of human salmonellosis in both the United States and Europe (13,16,27). Furthermore, multidrug-resistant S. Newport isolates that are also resistant to extended-spectrum cephalosporins (MDR-AmpC) have now been reported from several countries (3,24,36) and are a serious problem among both food animals and humans (17,25,36,48,67). MDR-AmpC isolates are resistant to ␤-lactams, including third-generation cephalosporins, aminoglycosides, tetracyclines, sulfonamides and chloramphenicol (12, 36). Resistance to ␤-lactams is caused by plasmids carrying the ampC gene bla CMY-2 , which encodes the CMY-2 ␤-lactamase (11,65).Most of our current understanding of the population structure of S. enterica relies on a series of seminal publications from R. K. Selander's group in the 1990s. These publications showed that some serovars consisted of monophyletic groups-so-called clonal groupings-but many other serovars confounded isolates from multiple lineages and were therefore polyphyletic (5,51,55,56). More recent studie...

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Scientific Opinion on the public health risks of bacterial strains producing extended-spectrum β-lactamases and/or AmpC β-lactamases in food and food-producing animals

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