Salmonella Typhi

MORPHOLOGY:
  • Gram -ve, motile (except S.gallinorum pullorum which is nonmotile)
  • Non lactose fermenters
  • Salmonellae non-spore-forming, facultatively anaerobic bacilli.
  • An increased frequency of infections due to salmonella has been reported among patients with achlorhydria
  • S.typhi need tryptophan as a growth factor
  • Salmonella can enter through the shell if eggs are left on contaminated chicken feed or feces and grow inside.
PATHOGENESIS:
  • Typhoid fever is transmitted via the faecal-oral route or urine-oral routes. This may take place directly through soiled hands contaminated with faeces or urine of cases or carriers, or indirectly by the ingestion of contaminated water, milk and/or food, or through files.
  • S paratyphi B also produces H2S; S paratyphi A does not produce H2S
  • The infectious dose of Salmonella typhi causing typhoid fever is 103 - 10 colony forming units.
  • Any conditions which decrease either the stomach acidity or intestinal integrity increases the susceptibility to Salmonella infection.
  • They reduce nitrates, and do not produce cytochrome oxidase.
  • Salmonella infection begins with the ingestion of contaminated water
  • Salmonellae ferment glucose, mannitol and maltose, forming acid and gas. An important exception is S.typhi which does not form gas.
  • S.typhi is anaerogenic (other Salmonella produce acid and gas)
  • Enteric Fever-
  1. Typhoid fever- S. typhi
  2. Paratyphoid fever – S.paratyphi A,B,C
ANTIGEN:
Flagellar antigen (H) Somatic antigen (O) Vi antigen
Heat labile protein Heat stable Heat labile polysaccharide
With antisera produce Phospholipid-Protein- Virulence factor
large, loose, fluffy clumps Polysaccharide complex Poorly immunogenic
Strongly immunogenic Boivin antigen Total absence of Vi antibody in a
First antibody to appear Granular clumps typhoid case - poor prognosis
High titre following Less immunogenic Disappears early during convalescence;
infection and immunization Classification of persistence - carrier state
Salmonella Used in epidemiological typing
Not detected in WIDAL
CULTURE & MEDIUM:
  • Incubation period of salmonella typhi is 3-21 days
  • Wilson and Blair medium - S.typhi produce jet black colonies with metallic sheen due to H2S production
  • Enrichment media: Selenite F broth and Tetrathionate broth
  • Selective media: Salmonella-Shigella agar, Deoxycholate citrate agar, Hektoen enteric agar, XLD agar
  • Blood culture is + ve in 3-7 days
  • Deoxycholate citrate agar (DCA) medium is a selective medium for Salmonella and Shigella.
  • It is also used as a differential medium to differentiate lactose fermenter from non-lactose fermenting enteric pathogens.
  • Salmonella and Shigella (except Shigella Sonnei) are non-lactose fermenters.
Exam Question
  • An increased frequency of infections due to salmonella has been reported among patients with achlorhydria
  • The infectious dose of Salmonella typhi causing typhoid fever is 103 - 106 colony forming units.
  • Selenite F broth is an enrichment media for Salmonella & shigella
  • S.typhi do not produce gas
  • Incubation period of salmonella typhi is 3-21 days
  • Blood culture in S.typhi is + ve in 3-7 days
  • Vi antigen found in Salmonella Typhi & paratyphi
  • Salmonella can enter through the shell if eggs are left on contaminated chicken feed or feces and grow inside.
  • S.typhi need tryptophan as a growth factor
  • Salmonella infection is most commonly caused by contaminated water, milk and/or food
  • Deoxycholate citrate agar (DCA) medium is used in differentiation of Salmonella from lactose fermenter
  • Salmonella is non-lactose fermenting bacteria
  • Salmonella have human reservoir
Don't Forget to Solve all the previous Year Question asked on Salmonella Typhi