Salmonella are Gram-negative, facultatively anaerobic bacteria that can cause diseases like typhoid fever and gastroenteritis. There are typhoidal strains like S. Typhi that only infect humans and cause enteric fever, as well as non-typhoidal strains that typically infect animals but can also infect humans and cause food poisoning. Salmonella are classified based on their O and H surface antigens and different culture media can be used to identify Salmonella colonies based on their appearance. The three main antigenic structures of Salmonella are the somatic O antigen, flagellar H antigen, and capsular Vi antigen.

1. Salmonella
For NEET PG

2. General
● Gram negative, aerobic or facultatively anaerobic,non sporing,non capsulated
gram negative bacilli.
● Parasitise small intestine of humens leading to enteric fever, gastroenteritis,
septicemia.
● Motile with perithrichate flegella( except S. gallinarum, S. pullorum.
● Transmission occurs through feco-oral, uro-oral route, contaminated water is
a major source of transmission.
● Boiling or chlorination of water and pasteurisation of milk destroys bacilli.

3. Classification
● Clinical classification into
(1) Typhoidal ( enteric fever group consisting of S typhi, S. paratyphi A,B and C
which are exclusively human pathogen)
Non Typhoidal: food poisoning group,essentially animal parasites but can also
infect humans.
● Kauffman and White scheme: Based of characterisation of O and H antigen
● DNA Hybridization study: 7 groups have been identified.
● Nearly all salmonella that infect humans belongs to DNA hybridization group 1

4. Culture
Differential media: MacConkey agar-non lactose fermenting colony.
Selective media:
Deoxycholate citrate Agar(DCA agar): NLF pale colony with black centre.
Xylose lysin deoxycholate agar(XLD agar): red colony with black centre.
Salmonella-Shigella agar(SS agar): colourless with black centre.
Hektoic enteric agar: blue green with black centre
Enrichment media : tetrathionate broth and selinite F broth both of them
inhibits replication of bacteria other then salmonella.

5. Culture media. Cont.
Enrichment media : tetrathionate broth and selinite F broth both of them
inhibs replication of bacteria other then salmonella.

6. Antigenic Structure

7. Somatic(O) antigen
● Heat stable, alcohol stable,phospholipid polysaccharide complex identicle
with endotoxin.
● Located in outer membrane anchored to cell wall.
● When mixed with antisera agglutination occurs slowly resulting in formation of
chalky, granular clumps.
● Less immunogenic then H antigen.
● O antibody appears early, disappears early indicating recent infection.

8. Flagellar (H) antigen
● Heat labile,alcohal labile protein well preserved in formaldehyde.
● Made up of protein flagellin.
● More immunogenic
● Appears late disappears late; indicates convalescence.
● Exists in two phases I and II
● Most of the salmonella are biphasic except S. typhi
● When mixed with antisera agglutination occurs rapidly resulting in formation of
large loose fluffy clumps.

9. Vi (capsular) antigen
● Heat labile,surface polysaccharide Ag enveloping o antigen.
● Named Vi antigen as it is related with virulence .
● Inhibits phagocytosis.
● When fully developed renders the bacteria inagglutinable with O antisera
however the strain becomes reagglutinable after boiling or heating at 100
degree C for 1 hour which removes Vi antigen as it is heat labile.
● Disappears early in convalescence,persistence indicates development of
carrier state.
● Absence of Vi antibody in typhoid patient indicates poor prognosis.