Salmonella: Introduction, Pathogenesis, Lab Diagnosis and treatment


History of Salmonella

Salmonella Typhi is also known as Eberth-Gaffkey or Eberth Typhi. Eberth  (1880) observed Salmonella into the mesenteric lymph node and spleen of typhoid patients. Gaffney isolated this organism in 1884.

Introduction of Salmonella

Family: Enterobacteriaceae

Gram-negative bacilli, non-sporing, non-encapsulated, aerobic or facultative anaerobes, 2 to 4 × 0.6 µm, most strains are motile except Salmonella gallinarum and pullorum.

Salmonella Typhi and Paratyphi 


2 to 3 µm Χ 0.6  µm

Gram-negative bacilli

Motile due to peritrichous flagella

Non- encapsulated


Colony characteristics

Aerobic or facultative anaerobes

Optimal temperature 37°C

Optimal pH

Nutrient broth: Uniform turbidity

Blood agar: Colonies 2 to 3 mm, circular, low convex, smooth, translucent, and non-hemolytic

MacConkey agar: Non- lactose fermenter ( colorless colonies)

Deoxycholate Citrate Agar (DCA): Non-lactose fermenter colonies

Wilson and Blair bismuth sulfite medium: Jet black colonies with a sheen

Enrichment media like selenite F broth or tetrathionate broth: Uniform turbidity

Biochemical reactions

Methyl red positive, Voges-Proskauer test negative, non-lactose fermenter, urea hydrolysis test negative, glucose fermenter, and citrate utilization test variable. Lysine decarboxylase  test positive, motile, and hydrogen sulfide (H2S) variable

Serotyping: With the use of polyvalent and monovalent antisera

Kauffmann and White Scheme of serological classification of Salmonella works on the use of these antisera.

Antigens of Salmonella

O antigen (somatic):  Phospholipid protein-polysaccharide  complex, heat stable, alcohol stable, acid-stable and less immunogenic

H antigen (flagellar): Heat labile protein, alcohol labile, withstand formaldehyde and strongly immunogenic

Vi antigen and K antigen are capsular antigens. Vi antigen: glycolipid, surface antigen, heat-labile, virulent to mice, persistent antibodies against it indicate carrier state.

F antigen (fimbrial): Preserved in 0.1 or 0.2 formaldehyde

M antigen: Loose extracellular polysaccharide slime consisting of colonic acid

R antigen : In S R mutation due to the loss of  O antigens

Mode of transmission

Faeco-oral route

Infective dose

10 bacilli through ingestion


Enteric fever/typhoid fever

Food poisoning /gastroenteritis

Pathogenesis of Salmonella

Ingestion of Salmonella

Enter into the body through the lymphoid of the pharynx.

In the gut, organisms attach with epithelial cells of intestinal villi and penetrate lamina propria, submucosa

Bacilli phagocytosed by macrophages or polymorphonuclear cells.

Enter mesenteric lymph nodes and multiply there.

Enter thoracic duct and then bloodstream.

It may affect the liver, spleen, bone marrow, lung, kidney, and heart.

Special features of Salmonella

Ability to withstand phagocytosis  ( intra-cellular multiplication)

Resistance to bile

Produces endotoxin

Clinical Symptoms

Symptoms due to endotoxin

step ladder fever



Congestion of mucus membrane

Palpable spleen

Rose spots

Laboratory Diagnosis

In the first week

Rose spot

Blood culture

Bone marrow

Second week

Serology ( Widal test)

Third week

Stool culture

In four week

Urine culture


Media available for Salmonella are-

XLD gar


Salmonella-Shigella (SS) agar

Rambach’s agar

Enrichment media are-

Tetra thionate broth

Selenite F broth

Specimens are




Discharge from lesion


Blood culture

Transfer patient blood into the blood culture medium (i.e. liquid medium like BHI broth, tryptone soy broth, bile broth, etc.).

Incubate medium into an incubator at 37°C for 5 days. ( If you are using Bactec, no need for blind sub-culturing because the positive vial is indicated by this instrument via siren).

Check turbidity and subculture on solid media like blood agar and MacConkey agar.

After overnight incubation at 37°C, check the growth of the organisms and colony morphology.

Non -lactose fermenter colonies on MacConkey agar.

Feces culture

Inoculate stool in selenite F broth or tetrathionate broth and incubate.

Later, subculture in XLD agar or deoxycholate agar or MacConkey agar or Wilson and Blair medium.

Non-lactose fermenter colonies on MacConkey agar or XLD (For Salmonella Typhi -black colonies) or DCA  or Wilson and Blair medium ( For Salmonella Typhi -black colonies with sheen against a green background).

Urine culture ( positive in second and third week)

Non-lactose fermenter colonies on MacConkey agar.

Culture using pus, rose spots, lymph nodes, CSF, and bone marrow

Slide agglutination test

using polyvalent and grouping cum factor antisera

Serological tests

Widal test: The Widal test is an agglutination test employed in the serological diagnosis of enteric fever. It is also applicable in febrile agglutinins tests. The test is named after Georges Fernand Isidore Widal, a French physician, and bacteriologist. Salmonella antibody starts appearing in the serum at the end of the first week and rises sharply during the third week of enteric fever. This test measures agglutinating antibody levels against O and H antigens.

Diagnosis of carrier

Feces culture

Bile culture

Urine culture

Vi agglutination test









TAB vaccine

It contains Salmonella Typhi 1000million/ ml, Salmonella Paratyphi  A 750 million/ ml and Salmonella Paratyphi B 750 million/ ml.

Streptomycin dependent strain vaccine

Killed vaccine

Ty21a (oral vaccine)

Further Readings

  • Topley and Wilson’s microbiology and microbial infection Topley and Wilson’s microbiology and microbial infection – Bacteriology-2-10th Edn.
  • Manual of Clinical Microbiology   -Patrick R. Murray -8th Edn.
  • Bailey and Scott’s  Diagnostic Microbiology -13th Edn.
  •  Mackie & Mc Cartney  Practical Medical Microbiology- 14th  Edn.
  • Diagnostic Microbiology -Connie R. Mahon & George Manuselis
  • Cowan and Steel’s, manual for the identification of medical bacteria
  • Koneman Color Atlas and Textbook of Diagnostic Microbiology-6th  Edn.
  • Jawetz Melnick and Adelberg’s Medical Microbiology- 25th Edn.
  • Lippincott’s –Illustrated- review-Microbiology-3rd Edn.
  •  Mandell’s Infectious Disease-7th Edn.
  • Bergey’s Manual of Systemic Bacteriology – 2nd  Edn.
  • Medical Microbiology-The Practice of Medical Microbiology Vol-2-12th Edn. –Robert Cruickshank
  • District Laboratory Practice in  Tropical Countries  –  Part-2-   Monica Cheesebrough-   2nd Edn Update
  • Text Book of Microbiology -Anantanarayan & Paniker-9th Edn.
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