Salmonella
Salmonella are Gram-negative bacteria which cause intestinal infections. The taxonomy of Salmonella species is complicated. Formally, there are only two species within this genus: S. bongori and S. enterica (formerly called S. choleraesuis), which are divided into six subspecies:
I – enterica
II – salamae
IIIa – arizonae
IIIb – diarizonae
IV – houtenae
V – bongori
VI – indica
However, there are numerous (over 2,500) serovars within both of these species. For the sake of simplicity, the CDC recommends that Salmonella species be referred to only by their genus and serovar, e.g. Salmonella Typhi instead of: Salmonella enterica subspecies enterica serovar Typhi. The Kauffman-White classification divides Salmonella isolates according to their somatic O antigens and flagellar H antigens. H antigens are further divided into phase 1 and phase 2, but in practice, most labs leave this level of detail to specialized reference laboratories.
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Prevention of Salmonella as a cause of food poisoning mostly involves effective sanitary precautions. Salmonellosis can also be caught from pet animals. All the disease-causing Salmonella species are now classified as a single species, Salmonella enterica, with numerous serovars. Most Salmonella isolates cause gastroenteritis which is usually self-limiting and does not require treatment with antibiotics. However, these infections can be life-threatening in the very young, very old and the seriously ill. Salmonella Typhi causes typhoid fever, a more serious systemic infection which does require medical intervention.
Investigation of the mechanisms that underlie the interactions of Salmonellae with their hosts has advanced greatly over the past decade, mainly through the study of Salmonella enterica serovar Typhimurium in tissue culture and animal models of infection. Knowledge of the bacterial processes and host responses has painted a dynamic and complex picture of the interactions between salmonellae and animal cells (Salmonellae interplay with host cells. Nature Reviews Microbiology 2008 6: 53-66).
Most Salmonella infections are acquired by ingestion of contaminated food or water. Salmonellae have an adaptive acid-tolerance response that promotes their survival in the low pH of the stomach. After entering the small intestine, they enter the intestinal mucous layer in order to gain access to the underlying epithelium and so evade being killed by digestive enzymes, bile salts, secretory IgA, antimicrobial peptides and other innate immune defences. The presence of Salmonella in the gut results also in production of pro-inflammatory cytokines such as IL-8, which stimulate inflammatory response leading to diarrhoea.
Salmonellae invade non-phagocytic enterocytes of the intestinal epithelium by bacterial-mediated endocytosis. After adherence to the apical surface of the cell, the bacteria disrupt the epithelial brush border and induce membrane ruffles that engulf the organism. Alternatively, they can translocate through the intestinal epithelia after uptake by CD18-expressing phagocytes. In vitro, Salmonellae are able to disrupt tight junctions, which seal the epithelial cell layer and restrict the passage of ions, water and immune cells. This, in addition to intestinal inflammatory responses, probably contributes to the induction of diarrhoea. The fact that there are multiple mechanisms for crossing the intestinal barrier indicates the importance of this strategy to the lifestyle of these bacteria. At least five different virulence proteins are required for efficient invasion of cultured epithelial cells, and optimal invasion of animal tissues might be even more complex and diverse.
Emerging technologies such as whole-genome sequencing will allow us to investigate the diversity of effectors that are used by the many pathogenic Salmonella isolates. The study of Salmonella pathogenesis should yield a rich treasure trove of information for those who are interested in microbial pathogenesis, innate immunity, cell biology and genomics, and will remain an important model system of host pathogen interactions for many years to come.
Tags: Antibiotics, Bacteria, Biology, Food, Health, Medicine, Microbiology, Podcast, Science
