- Most strains of Salmonella are Lac– (MCQ)
- produce acid and gas during fermentation of glucose (MCQ)
- They also produce H2S from sulfur-containing amino acids. (MCQ)
- motile with peritrichate flagella
- Ingested Salmonella enter small intestinal cells by endocytosis.
- Serovar Typhi is an exclusively human pathogen
- Fecal–oral transmission occurs
- Salmonella serovar Typhi may involve chronic carriers.
- Risk factors to Salmonella infection.
- Young children and older adults
- Individuals in crowded institutions
- organisms are facultative, intracellular parasites
- they survive in phagocytic cells
- Antigenic structure
- H antigen:
- flagellar antigen H
- heat labile protein.
- When mixed with antisera, H suspensions agglutinate rapidly, producing large, loose, fluffy clumps. (MCQ)
- It is strongly immunogenic and induces antibody formation rapidly and in high litres following infection or immunization
- It is of a dual nature, occurring in one of two phases.
- O antigen:
- somatic antigen O
- a phospholipid-protein-polysaccharide complex
- forms an integral part of the cell wall
- It is identical with endotoxin. (MCQ)
- called the Boivin antigen
- It can be extracted from the bacterial cell treatment with trichloracetic acid,
- When mixed with antisera, O antigen suspensions form compact, chalky, granular clumps(MCQ)
- O antigen is less immunogenic than the H antigen
- titre of the O antibody induced after infection or immunisation is generally lower than that of the H antibody.
- Salmonellae are classified into a number of groups based on the presence of characteristic O antigens on the bacterial surface. (MCQ)
- Vi antigen
- surface antigen Vi
- It is analogous lo the K antigens of coliforms
- It is heal labile.
- acls as a virulence factor (MCQ)
- it is poorly immunogenic
- H antigen:
- caused primarily by serovars Enteriditis and Typhimurium
- nausea, vomiting, and diarrhea (usually nonbloody), which develop generally within 48 hours of ingesting contaminated food or water
- Fever and abdominal cramping are common
- It is generally self-limiting (48 to 72 hours), (MCQ)
- More than 95 percent of cases of Salmonella infection are foodborne
- Food handlers or cooks who become carriers are particularly dangerous.
- Mary Mallon (‘Typhoid Mary’), a New York cook
- typhoid carrier
- over a period of 15 years, caused at least seven outbreaks affecting over 200 persons.
- Enteric or typhoid fever:
- characterized by fever and, frequently, abdominal symptoms
- It is caused primarily by serovar Typhi.
- Nonspecific symptoms may include chills, sweats, headache, anorexia, weak- ness, sore throat, cough, myalgia, and either diarrhea or constipation.
- About 30 percent of patients have a faint and evanescent (transient) maculopapular rash on the trunk (rose spots). (MCQ)
- incubation period varies from 5 to 21 days
- symptoms generally resolve in 3 to 4 weeks
- Complications can include
- intestinal hemorrhage and/or perforation
- Infected gallbladders are the main source of chronic carriage(MCQ)
- Other sites of Salmonella infection:
- Sustained bacteremia
- often associated with vascular Salmonella infections
- occur when bacteria seed atherosclerotic plaques
- Salmonella can also cause
- abdominal infections (often of the hepatobiliary tract and spleen); osteomyelitis(MCQ)
- septic arthritis
- Sustained bacteremia
- Laboratory identification
- isolated from stools on MacConkey agar or selective media (MCQ)
- Castaneda’s method of culture(MCQ)
- eliminates the risk of introducing contamination during repeated subcultures
- a double medium is used.
- The bottle of bile broth has an agar slant on one side.
- After inoculation of blood, the bottle is incubated in the upright position
- For subculture, the bottle is merely tilted so that the broth runs over the surface of the agar.
- It is reincubated in the upright position.
- If salmonellae are present, colonies will appear on the slant.
- On MacConkey and deoxycholate citrate media(MCQ)
- colonies are colourless
- due to the absence of lactose fermentation
- On Wilson and Blair bismuth sulphite medium(MCQ)
- jet black colonies with a metallic sheen are formed
- due to production of H2S
- S. paratyphi A and other species that do not form H2S produce green colonies.
- Selenite F and tetrathionate broth (MCQ)
- commonly employed as enrichment media
- Biochemical reactions:
- Salmonellae ferment glucose, mannitol and maltose, forming acid and gas
- An important exception is S. typhi which is anaerogenic.(does not forming gas )
- Lactose, sucrose and salicin are not fermented
- Indole is not produced
- They are MR positive, VP negative and citrate positive
- S. typhi do not grow in Simmons’ citrate medium as they need tryptophan as the growth factor.
- Widal reaction: (MCQ)
- a test for the measurement of H and O agglutinins for typhoid and paratyphoid bacilli in the patient’s sera.
- Dreyer’s agglutination tube(MCQ)
- a narrow tube with a conical bottom
- used for the H agglutination
- Felix tube(MCQ)
- a short round bottomed tube
- used for the O agglutination.
- Procedure :
- Equal volumes (0.4 ml) of serial dilutions of the serum (from 1/10 to 1/640) and the H and O antigens are mixed in Dreyer’ s and Felix agglutination tubes, respectively,
- H agglutination leads to the formation of loose, cotton woolly clumps,
- agglutination is seen as a disc-like pattern at the bottom of the tube..
- Interpretation of results of the Widal test
- The agglutination titre will depend on the stage of the disease.
- Agglutinins usually appear by the end of the first week, (MCQ)
- tlitre increases steadily till the third or the fourth week, after which it declines gradually.
- Demonstration of a rise in titre of antibodies, by testing two or more serum samples, is more meaningful than a single test.
- Titres of 1/100 or more for O agglutinins and 1/200 or more for H agglutinins are significant. (MCQ)
- Agglutinins may be present on account of
- prior disease
- inapparent infection
- H agglutinins persist longer than O agglutinins.
- Serum from an individual immunised with TAB vaccine will generally have antibodies to S. typhi, S. paratyphi A and B, while in case of infection antibodies will be seen only against the infecting species.
- Anamnestic response(MCQ)
- Persons who have had prior infection or immunisation may develop an anamnestic response during an unrelated fever.
- This may be differentiated by repetition of the test after a week.
- The anamnestic response shows only a transient rise, while in enteric fever the rise is sustained.
- Cases treated early with chloramphenicol may show a poor agglutinin response.
- Blood Culture – First week
- Agglutination (Widal) – Second week
- Stool Culture – Third week
- Urine Culture – Fourth week
- Typhoral vaccine(MCQ)
- delivered orally
- consists of live attenuated Salmonella serovar Typhi.
- Killed vaccine
- consists of the Vi capsular polysaccharide
- it is delivered parenterally.
- Typhoral vaccine(MCQ)
- Bacteriophage typing
- Intraspecies classification of S.typhi for epidemiological purposes was made possible by bacteriophage typing (MCQ)
Movie showing salmonella pathogenesis
Salmonella Entering the Intestinal Tract
3D Animation of a salmonella bacterium entering the intenstinal tract.
Created for Eric Stebbins, The Rockefeller University. Created by Joseph Alexander.
Shown for portfolio purposes only. All copyrights reserved by Stebbins/Rockefeller University.
Duplication or public distribution is strictly forbidden.
Salmonella [HD] – SketchyMicro USMLE Microbiology Review
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Narration: Andrew B
Created By: Andrew B, Bryan L and Saud S
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