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
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.
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
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
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
Symptoms due to endotoxin
step ladder fever
Congestion of mucus membrane
In the first week
Serology ( Widal test)
In four week
Media available for Salmonella are-
Salmonella-Shigella (SS) agar
Enrichment media are-
Tetra thionate broth
Selenite F broth
Discharge from lesion
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.
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
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
Vi agglutination test
It contains Salmonella Typhi 1000million/ ml, Salmonella Paratyphi A 750 million/ ml and Salmonella Paratyphi B 750 million/ ml.
Streptomycin dependent strain vaccine
Ty21a (oral vaccine)
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.