MORPHOLOGY OF SALMONELLA TYPHI (S. TYPHI)
Shape – Salmonella typhi is a rod shape (bacillus) bacterium.
Size – The size of Salmonella typhi is about 1–3 µm × 0.5–0.6 µm (micrometer).
Arrangement Of Cells – Salmonella typhi is arranged singly or in pairs.
Motility – Salmonella typhi is a motile bacterium.
Flagella – Salmonella typhi is a flagellated bacterium with peritrichous flagella arrangement.
Spores – The Salmonella typhi is a non–sporing bacterium.
Capsule – S. typhi is a non–capsulated bacterium.
Gram Staining Reaction – Salmonella typhi is a Gram -ve (Negative) bacterium.
CULTURE REQUIREMENTS OF SALMONELLA TYPHI (S. TYPHI)
⇒ Special requirements – Salmonella typhi or S. typhi have no complex nutritional requirements and readily grow in an ordinary media like Nutrient Agar medium (NAM). Commonly the NAM & MacConkey Agar medium is used for the cultivation of Salmonella typhi in Laboratory. For the isolation from feces, selenite F broth & XLD medium are commonly used.
⇒ Optimum temperature –The optimum temperature for the cultivation of Salmonella typhi in the laboratory is 37°C.
⇒ Optimum pH – S. typhi can survive at 4.1– 9.0 pH but the maximum growth observed around 6.5-7.5 i.e. slightly acidic to slightly alkaline pH. Also, the pH requirements vary as per the strain of Salmonella typhi.
⇒ Oxygen requirements – Salmonella typhi (S. typhi) is an aerobic bacterium i.e. grow best in the presence of oxygen and it is also a Facultative anaerobic organism i.e. can grow in the low oxygen environment.
⇒ There are various culture media used for the cultivation of Salmonella typhi (S. typhi) in the laboratory and most commonly the MacConkey Agar medium, XLD medium, and Selenite F broth medium is used which may vary as per the SOPs of the laboratory, the other media are as follows –
- Columbia Horse Blood Agar medium.
- Sheep Blood Agar medium.
- Eosin Methylene blue Agar (EMB) medium
- Deoxycholate Citrate Agar (DCA) medium (Selective medium for Salmonella & Shigella).
- Salmonella – Shigella Agar medium (Selective medium for Salmonella & Shigella).
- Wilson & Blair bismuth sulfite medium (Selective medium for Salmonella).
- Xylose Lysine Deoxycholate (XLD) medium (Selective medium for Salmonella & Shigella).
- Tetrathionate broth (Selective medium).
- Selenite F Broth (Selective medium for Salmonella & Shigella).
- The Liquid medium (Trypticase Soy Broth, Nutrient broth etc.)
⇒ The Deoxycholate Citrate Agar (DCA) medium which is a Selective medium for Salmonella typhi & other Salmonella Species as well as Shigella Spp. contains Deoxycholate and Citrate salts in a concentration that inhibits the growth of many gram-positive bacteria & most of the intestinal flora and supports the growth of Salmonella typhi.
⇒ Similarly, the Salmonella – Shigella Agar medium which is also a selective medium for Salmonella typhi & other species of salmonella as well as Shigella spp. contains 4 key components that make it selective for Salmonella & Shigella i.e. Bile salts, Brilliant green dye, Sodium citrate & Sodium thiosulfate which selectively inhibits the growth of many gram-positive bacteria & coliform bacteria and supports the growth of Salmonella & Shigella.
⇒ The Wilson & Blair bismuth sulfite medium which is an excellent selective medium for the S. typhi contains a Brilliant green dye which is inhibitory to many Gram-positive organisms and Bismuth sulfite which is inhibitory to many gram-negative enteric organisms and supports the growth of Salmonella typhi.
CULTURE CHARACTERISTICS OF SALMONELLA TYPHI (S. TYPHI)
|Cultural Characteristics||Nutrient Agar Medium (NAM)||Wilson & Blair bismuth sulfite Medium||MacConkey Agar medium||Blood Agar Medium||Xylose Lysine Deoxycholate Agar (XLD) Medium|
|Size||2-3 mm||2-3 mm||2-3 mm||2-3 mm||2-3 mm|
|Elevation||Low Convex||Convex||Low Convex||Low Convex||Convex|
|Color||Greyish white||Jet Black with metallic sheen||Colorless||Greyish white||Red with black center|
|Hemolysis||-----||-----||-----||γ-Hemolysis (Non - Hemolytic)||-----|
In liquid culture media like Trypticase soy broth or Nutrient broth, the growth of the bacterium occurs as turbidity in the broth medium but the chances of contamination are high especially when the isolation is carried out from Feces specimen, commonly the Selenite F broth and Tetrathionate broth is used for the isolation of Salmonella typhi.
In Blood Agar medium, the Salmonella typhi shows gamma hemolysis i.e. no hemolysis occur on blood agar medium, the colonies are grayish white and grows well.
In MacConkey Agar medium, the colonies of Salmonella typhi are colorless due to the lack of lactose fermentation which is of great importance in differentiating S. typhi from other Bacteria present in the specimen, especially from Gram-positive bacteria and Escherichia coli which are lactose fermentors and gives Pink color colonies on MacConkey agar medium.
In Eosin Methylene Blue Agar (EMB) medium, the colonies of Salmonella typhi are colorless due to lack of lactose fermentation.
In Xylose Lysine Dextrose Agar (XLD) medium, the colonies of Salmonella appears red due to the fermentation of xylose which lowers the pH of medium and the presence of Phenol Red (indicator) imparts a pink or red color to the salmonella typhi colonies with a black center which is due to the production of H2S (hydrogen sulfide).
In Deoxycholate Citrate Agar medium, the colonies of Salmonella typhi are colorless due to Non-Lactose Fermentation (NLF).
In Salmonella Shigella Agar medium, the colonies of Salmonella typhi are Colorless with black center, due to the production of H2S (Hydrogen sulfide).
Selenite F broth & Tetrathionate broth are the enrichment medium for Salmonella species and commonly used for the isolation of the organism from the fecal sample where the density of Salmonella typhi may be too low in early stages of infection. The fecal specimen inoculated in Selenite F broth are incubated for 6-12 hours generally (not more than 24 hours) and then subcultured on Wilson & Blair bismuth sulfide medium or XLD medium or any other selective medium.
That’s All about the Morphology & Culture Characteristics of Salmonella typhi (S. typhi)