Genome variation and molecular epidemiology of Salmonella enterica serovar Typhimurium pathovariants

Salmonella enterica serovar Typhimurium is one of approximately 2,500 distinct serovars of the genus Salmonella but is exceptional in its wide distribution in the environment, livestock, and wild animals. S. Typhimurium causes a large proportion of nontyphoidal Salmonella (NTS) infections, accounting for a quarter of infections, second only to S. enterica serovar Enteritidis in incidence. S. Typhimurium was once considered the archetypal broad-host-range Salmonella serovar due to its wide distribution in livestock and wild animals, and much of what we know of the interaction of Salmonella with the host comes from research using a small number of laboratory strains of the serovar (LT2, SL1344, and ATCC 14028). But it has become clear that these strains do not reflect the genotypic or phenotypic diversity of S. Typhimurium. Here, we review the epidemiological record of S. Typhimurium and studies of the host-pathogen interactions of diverse strains of S. Typhimurium. We present the concept of distinct pathovariants of S. Typhimurium that exhibit diversity of host range, distribution in the environment, pathogenicity, and risk to food safety. We review recent evidence from whole-genome sequencing that has revealed the extent of genomic diversity of S. Typhimurium pathovariants, the genomic basis of differences in the level of risk to human and animal health, and the molecular epidemiology of prominent strains. An improved understanding of the impact of genome variation of bacterial pathogens on pathogen-host and pathogen-environment interactions has the potential to improve quantitative risk assessment and reveal how new pathogens evolve

The biological base of Salmonella phage typing

The complexity of Salmonella transmission routes and the observation that Salmonella serovars diversify during evolution have led to the development of special methods for strain discrimination. Phage typing systems of Salmonella have been developed since the 30s (Marcuse, 1934), and still they are very helpful tools for epidemiological work (Rabsch, 1996). Nevertheless, their biological basis is not well understood. We have studied the Anderson typing system (1959) which has a long tradition, to answer the following questions: • To which extent do host controlled modification/ restriction systems interfere? • What is the significance of different phage receptor sites for typing? • How important are prophage controlled super-infection exclusion systems

Differences in the outer membrane protein pattern of Salmonella typhimurium DT8, DT10 and DT104 strains

The most practicable methods for epidemiological classification of S. Typhimurium are serotyping (determination of serovar) and phage typing in order to disclose their infection routes

Prevalence and molecular-basis of fluoroquinolone resistance of Salmonella enterica subsp. enterica field isolates

Nowadays fluoroquinolones are used in human and veterinary medicine. In the last decade Salmonella isolates with reduced susceptibility to quinolones have been isolated with increasing prevalence. Among these isolates high-level resistant strains were detected. The present study aimed at investigating Salmonella field isolates (n=56) with reduced quinolone susceptibility with respect to comparative susceptibilities to various fluoroquinolones used in human and veterinary medicine and the genetic basis of quinolone resistance

The dual role of wild phages for the horizontal gene transfer among Salmonella strains

Horizontal gene transfer (e.g. transduction, transformation, conjugation) has a great impact on the evolution of bacteria. Several transducing phages related P22 have been reported, transfering chromosomal as well as plasmid born determinants (Schicklmaier & Schmieger, 1995)

Competitive exclusion of Salmonella enteritidis by Salmonella gallinarum in poultry.

Salmonella Enteritidis emerged as a major egg-associated pathogen in the late 20th century. Epidemiologic data from England, Wales, and the United States indicate that S. Enteritidis filled the ecologic niche vacated by eradication of S. Gallinarum from poultry, leading to an epidemic increase in human infections. We tested this hypothesis by retrospective analysis of epidemiologic surveys in Germany and demonstrated that the number of human S. Enteritidis cases is inversely related to the prevalence of S. Gallinarum in poultry. Mathematical models combining epidemiology with population biology suggest that S. Gallinarum competitively excluded S. Enteritidis from poultry flocks early in the 20th century

Pilot study on the prevalence of salmonella in slaughter pigs in Germany: III. Detection of salmonellae by PCR, serovar distribution and population analysis of isolates

In a survey on the prevalence of Salmonellae in German slaughter pigs, almost 12,000 pig carcasses were investigated by standard cultural, immunological and PCR technique\u27s. Seven abattoirs distributed all over Germany were included into the study. Each abattoir was investigated on ten weekly sampling occasions. In total 6% of all slaughter pigs were positive in faecal or lymphnode material. This number increased to I 0% when surface swabs were evaluated as well. From all sources nearly 1400 bacterial isolates were obtained and characterised by traditional and molecular typing methods. Typing at the National Reference Laboratory for Salmonella (NRL) follows a hierarchic protocol. At the beginning the serotype is determined by using poly- and monoclonal antisera for standard slide agglutination. In total 28 serotypes could be detected

Epidemiological and molecular analysis of an outbreak of Salmonella Typhimurium DT12 associated with consumption of pork in Thuringia

In the course of a sentinel-study the National Reference Centre (NRC) observed in February 1999 a considerable increase of human infections due to S. Typhimurium. These strains ofS. Typhimurium typed as phage type DT12 in the NRC were isolated in four counties in the north of Thuringia (Nordhausen, Eichsfeld, Kyffhiiuser, Unstrut-Hainich)

Rapid Discrimination of Salmonella enterica Serovar Typhi from Other Serovars by MALDI-TOF Mass Spectrometry

Systemic infections caused by Salmonella enterica are an ongoing public health problem especially in Sub-Saharan Africa. Essentially typhoid fever is associated with high mortality particularly because of the increasing prevalence of multidrug-resistant strains. Thus, a rapid blood-culture based bacterial species diagnosis including an immediate sub-differentiation of the various serovars is mandatory. At present, MALDI-TOF based intact cell mass spectrometry (ICMS) advances to a widely used routine identification tool for bacteria and fungi. In this study, we investigated the appropriateness of ICMS to identify pathogenic bacteria derived from Sub-Saharan Africa and tested the potential of this technology to discriminate S. enterica subsp. enterica serovar Typhi (S. Typhi) from other serovars. Among blood culture isolates obtained from a study population suffering from febrile illness in Ghana, no major misidentifications were observed for the species identification process, but serovars of Salmonella enterica could not be distinguished using the commercially available Biotyper database. However, a detailed analysis of the mass spectra revealed several serovar-specific biomarker ions, allowing the discrimination of S. Typhi from others. In conclusion, ICMS is able to identify isolates from a sub-Saharan context and may facilitate the rapid discrimination of the clinically and epidemiologically important serovar S. Typhi and other non-S. Typhi serovars in future implementations

Phage typing and clonal analysis of Salmonella Heidelberg strains isolated from animals and other sources from Minnesota (USA) and Germany

Salmonella Heidelberg isolates has become an emerging pathogen during the 80s in the United States (Martin et al., 1989). Approximately 60% of human cases reported to the CDC in 1995 were caused by only four serovars, including S. Enteridis (24,7%), S. Typhimurium (23,5%), S. Newport (6,2%) and S. Heidelberg (5,1%), (CDC, Salmonella surveillance) and were frequently isolated from chicken and pork (Sawari et al., 2001)