Salmonella Typhi: Gram Stain, Colony Characteristics, Pathogenesis, Lab Diagnosis and Treatment

Salmonella Typhi in Gram stain

Salmonella Typhi in Gram stain 

Salmonella Typhi in Gram is Gram-negative bacilli, non-sporing, non-encapsulated having sizes of  2 to 4 × 0.6 µm as shown above image.

Colony Characteristics of Salmonella

  • 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

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
  • Headache
  • Anorexia
  • 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
  • DCA
  • Salmonella-Shigella (SS) agar
  • Rambach’s agar
  • Enrichment media are-
  • Tetra thionate broth
  • Selenite F broth

Specimens are

  • blood
  • feces
  • Urine
  • Discharge from lesion
  • CSF

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


  • Ciprofloxacin
  • Ampicillin
  • Chloramphenicol
  • Trimethoprim-sulfamethoxazole
  • Azithromycin
  • Ceftriaxone

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.
  •  Mandell’s Infectious Disease-7th Edn.
  • Bergey’s Manual of Systemic Bacteriology – 2nd  Edn.
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