Salmonella: Introduction, Pathogenesis, Lab Diagnosis and treatment


History OF Salmonella

Salmonella Typhi is also knowan as Eberth-Gaffkey or Eberth Typhi. Eberth  (1880) observed Salmonella  into mesenteric lymph node and spleen of typhoid patient. Gaffkey isolated this organisms 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 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 sulphide (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 theses 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 antigen. 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 colinic 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 /gasteroenteritis

Pathogenesis of Salmonella

Ingestion of Salmonella

Enter into body through lymphoid of pharynx.

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

Bacilli phagocytosed by macrophages or polymorphonulcear cells.

Enter mesesnteric lymph nodes and multiply there.

Enter thoracic duct and then blood stream.

It may affect 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 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 blood culture medium (i.e. liquid medium like BHI broth, tryptone soy broth, bile broth etc.).

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

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

After over night incubation at 37°C , check 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 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 testWidal test is an agglutination test employed in the serological diagnosis of enteric fever. It 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 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

Faeces culture

Bile culture

Urine culture

Vi agglutination test





Trimethoprim sulfamethoxazole




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)


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