Salmonella

SALMONELLA

Salmonella are not normal inhabitants of the human gut unlike other members of the Enterobacteriaceae. Salmonella are responsible for diarrheal diseases, which may be severe; Salmonella typhi is also invasive and gives rise to systemic infection.





Genus Salmonella:

It has been classified into over 2000 serotypes on the basis of serologic differences. Distinction on the basis of infection is between S. typhi, which causes enteric fevers and S. enteritidis, which causes diarrheal disease.

 

Salmonella Taxonomy:

Kauffman-White classification recognizes each serologically distinct salmonella as a species. These are then arranged in groups. Further serologic distinctions recognized as subtypes.

Kauffman-White classification:

Group

Name Somatic

antigen

Flagella (H) antigen
Phase-I

Phase-II

A

S. paratyphi A 1,2,12

a

B

S. paratyphi B 1,3,5,12 b

1,2

C1

S. typhimurium 1,4,5,12

i

1,2

 

S. paratyphi C 6,7,Vi

c

1,5

D

S. typhi 9,12,Vi d

  S. enteritidis 1,9,12 g, m

 

Characteristics: Gram-negative, motile, non-sporing rods. All except S. typhi are non-capsulate. Capable of aerobic and anaerobic respiration.

Transmission: Widespread in animals; encountered in food chain. Acquired by ingestion of contaminated food or person to person via fecal-oral route. S. typhi human pathogen only. Spread via fecal-oral route, usually via contaminated water or food. Carriers are important source of organisms

Diseases:

  1. S.typhi :   Typhoid fever (enteric fever), Osteomyelitis , Abcesses & meningitis
  2. S.paratyphi A: Paratyphoid fever (enteric fever)
  3. S.paratyphi B: Paratyphoid fever (enteric fever)
  4. S.paratyphi C: Septiceamia
  5. S.typhimurium: Food poisoning




LABORATORY DIAGNOSIS:

Sample collection:

According to site of infection and clinical features: Blood, Faeces & Urine.

In special situation: Bone marrow & Duodenal aspiration to detect carrier

  1. Blood: It should be collected during rising temperature. 5 to 10 ml of blood is collected in 50-100 ml of bile broth or Brain-heart infusion broth. Ratio between blood and broth should be 1:5 to 1:10. After overnight incubation at 37ºC if turbidity seen then the broth is subcultured on solid media.
  1. Faeces: In third week- in 80% of patient, organisms can be detected Selenite F and Tetrathionate broth are used as enrichment media. Subculture is done on Wilson-Blair medium.
  1. Urine: Chances of isolation are high in fourth week of infection.

 

Methods for Identification of Organism:

 Direct methods:

  1. Microscopy: Salmonella are Gram negative, motile, non- capsulated & non-sporing rod
  1. Culture: Nutrient agar, MacConkey agar, Wilson and Blair medium

     → Nutrient agar: Colonies are large, circular, low convex  & smooth.

→ MacConkey agar: Colonies are colorless and nonlactose fermenter (NLF)

→ Wilson and Blair medium (Selective Medium):

S.typhi: Black colonies with metallic sheen

S.paratyphi B: Black colonies with metallic sheen

S.paratyphi A:  Green colonies

  1. Biochemical Reactions:

Species

TSI

Citrate

 

Slant/Butt

Gas

H2S

 

S.typhi

Pink/yellow +

S.paratyphi A

Pink/yellow +

S.paratyphi B

Pink/yellow + +

+

 

Salmonella ferments glucose & mannitol forming acid and gas while lactose and sucrose are not fermented.

  1. Antibiotic sensitivity test

 

Indirect Methods: 

  1. Antibody Detection: Widal test: Antibody – H & O antigen & ELISA
  1. Antigen Detection:
    • This test is performed during first week from blood & urine.
    • Antigen is demonstrated by sensitized staphylococcal agglutination test

 For Carrier Detection:

  1. Bile culture
  2. Vi antibody titer

To trace source of an outbreak of salmonella organisms, salmonella phage typing is recommended.