Salmonella is a member of the family Enterobacteriaceae. It is a Gram-negative bacilli, motile and non-lactose fermenter. The genus can be divided into two species (S. enterica and S. bongori), based on their phenotypic profile.
It causes acute gastroenteritis and when Salmonella infections become invasive, they can affect the bloodstream (bacteremia), bone (osteomyelitis), joint (septic arthritis), brain, or nervous system (meningitis). Salmonella bacteria are classified as either “typhoidal” or “nontyphoidal,” based on their serotype.
- Typhoidal Salmonella refers to the specific Salmonella serotypes which cause typhoid fever or paratyphoid fever, including typhi, paratyphi A, paratyphi B, and paratyphi C.
- Nontyphoidal Salmonella refers to all other Salmonella serotypes.
For the recovery of Salmonella from clinical specimens, three general types of media are available.
- Non-selective media for primary isolation (blood agar)
- Selective or differential agar (e.g. MacConkey agar, hektoen enteric agar); and
- Enrichment broths (e.g. selenite broth)
Commonly used culture media for isolation of Salmonella from clinical specimen and their colony characteristics is as follows:
- Bismuth sulfite agar: Salmonellae produce black colonies.
- Blood Agar: S. typhi and S. paratyphi usually produce non-hemolytic smooth white
- MacConkey Agar: Non-lactose fermenting smooth colonies i.e. pale colonies
- Deoxycholate Citrate Agar (DCA): Salmonella appear as pale colonies.
Xylose Lysine Deoxycholate (XLD) Agar: Salmonella appear as pink (lactose non-fermenting colonies) with a black center (due to H2S production)
Salmonella-Shigella (SS) Agar: Used for selective recovery of Salmonella from stool culture (contains five times the concentration of bile salts compared with MacConeky Agar). Salmonellae usually produce lactose non-fermenting colonies with black centres (except S. paratyphi A, whose colonies do not have black centres).
Hektoen Enteric (HE) Agar: On hektoen enteric agar, salmonellae produce transparent green colonies with black centres (except S. paratyphi A, whose colonies do not have black centres).
Eosin Methylene Blue (EMB) Agar: Non-lactose fermenters, including Salmonella, Shigella and Proteus forms colorless or transparent colonies.
Wilson Blair media:
- S.Typhi forms large black colonies with a metallic sheen.
- S.Paratyphi A produces green colonies due to the absence of H2S production.
Selenite broth or Tetrathionate broth: For the enrichment of Salmonellae present in feces. Feces is incubated for 12-18 hours before subculture into plates.
Trypticase soy broth or brain heart infusion broth: Non-selective broth for primary blood culture.
Commonly used Biochemical tests for the identification of Salmonella and their results are as follows;
- Catalase test: positive
- Oxidase test: negative
- Indole test: negative.
- Urea is not hydrolyzed (urea medium, Urease Test)
- Sucrose/salicin is usually not fermented.
- Nitrate is reduced to nitrite (this property is shared by all the members of Enterobacteriaceae family)
- Triple sugar iron agar (TSI) test: Alk/Acid, H2S
- Arginine-dihydrolase: variable
- Lysine-decarboxylase: positive
- Ornithine-decarboxylase: positive
- Voges-Proskauer (VP) test: negative (media; MR/VP broth).
- Methyl-red test: positive
- Citrate utilization test: positive (media: Simmon’s citrate agar).