Salmonella

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  • Some serotypes tend to be more species specific, whereas others can affect a wide range of species. For example:
  • Salmonella enteritidis
  • Salmonella typhimurium
    • Widespread in most species.
  • Salmonella dublin
  • Cattle
  • Salmonella cholerae suis
  • Salmonella montevideo
    • Produces outbreaks from contaminated imported meat and bone meal.


Overview

  • Important member of the enterobacteria
  • Cause disease in humans and animals
  • Reservior of infection in poulty, pigs, rodents, cattle, dogs
  • Cause enteritis and systemic infection (septicaemia and abortion)
  • Salmonella may be carried sub-clinically
  • Some human strains cause enteric fever (S. Typhi causes typhoid), also gastroenteritis, septicaemia or bacteraemia

Characteristics

  • Gram negative bacilli
  • Facultative intracellular pathogens
  • Non-lactose fermentor, oxidase negative
  • Do not produce urease or indole from tryptophan
  • Utilise citrate as a carbon source
  • Reduce nitrates to nitrites
  • Grow on MacConkey
  • Ferment glucose to produce acid and gas
  • Usually produce hydrogen sulphide
  • Most motile with flagellae (H antigen)
  • H antigen can be in phase 1 or phase 2, depending on a genetic switch allowing for one of the H antigen genes to be transcribed at any one time

Classification

  • Single species, Salmonella enterica
  • Over 2400 pathogenic serotypes or serovars identified
  • Grouped into 9 groups according to O antigen (lipopolysaccharide) by the Kauffmann-White scheme - determined by slide agglutination of the bacteria with specific antisera
  • Categorised into serovars depending on and H (flagellar) antigen, e.g. Salmonella enterica subspecies enterica serovar Tymphimurium; must also determine phase of H antigen

Pathogenesis

  • Faecal-oral transmission
  • Comparitively large dose required for infection due to gastric acid, normal intestinal flora and local immunity
  • Enterocolitis:
    • Acute enteritis
    • Bacteria multiply in the intestine and damage epithelial cells
    • Cytotoxin may cause epithelial cell damage
    • Enterotoxin may induce fluid secretion into intestinal lumen
    • Degeneration of microvilli
  • Systemic disease:
    • Bacteria internalised by intestinal epithelial cells
    • Stimulate immune response on reaching the lamina propria
    • Inflammatory response with phagocytosis of bacteria by neutrophils and macrophages
    • Bacteria either destroyed by the phagocytic cells or survive and multiply in the cells to cause systemic disease
    • Intracellular carriage if bacteria no completely removed
    • Invasive potential of certain strains e.e Salmonella Dublin associated with carriage of a large plasmid, encoding genes to allow intracellular survival in macrophages and also to allow iron acquisition
    • Salmonellae are facultative intracellular organisms, allowing them to move from the gut in macrophages and cause a bacteraemia and lesions throughout the body
    • Possession of Pathogenicity Islands associated with virulence
  • Carriage:
    • Salmonellae can persist in the gut or gall bladder
    • Excreted in faeces after clinical signs disappeared - active carriage
    • Bacteria can survive intracellularly, avioding the immune system and antimicrobials
    • May have latent carriage and intermittent excretion in faeces
    • Stress promotes excretion in carrier animals
    • Tortoises, terrapins, snakes and other reptiles ofter carry Salmonellae
    • Asymptomatic carriage allows faecal spread of infection


Clinical infections

  • Zoonotic
  • Some serotypes are host-specific, some are not
  • S. Tymphimurium infects many species; causes severe diarrhoea; non-invasive; causes of food poisoning in humans, e.g. from infected poultry
  • S. enteritidis: non species-specific; losses in young birds; causes food poisoning in humans
  • S. Dublin: invasive serovar; infects cattle
  • S. Cholerae-Suis: primarily infects pigs; also causes severe human disease
  • S. Pullorum: infects poultry; egg-transmitted; causes bacillary white diarrhoea, known as pullorum disease
  • S. Gallinarum: infectes older birds; known as howl typhoid
  • S. Pullorum and S. Gallinarum now rare in UK due to eradication programs including the Pullorum test (whole blood slide agglutination to detect antibody to both S. Pullorum and S Gallinarum
  • S. Abortis-ovis: infects sheep
  • S. Abortus-equi: infects horses outside of the UK
  • S. Typhi, S. Paratyphi: infect humans
  • Most human infections contracted from animals, especially poulty and cattle

Diagnosis

Treatment

Control