Salmonella Typhi : Gram stain, Colony Characteristics, Pathogenesis, Lab

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

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

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

Treatment

Ciprofloxacin
Ampicillin
Chloramphenicol
Trimethoprim-sulfamethoxazole
Azithromycin
Ceftriaxone

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