Salmonella

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    • Salmonella
      • 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
      • Gastroenteritis:
        • 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
          • endocarditis.
          • 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
      • 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
            • immunisation.
          • 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.
          • BASU(MCQ)
            • Blood Culture – First week
            • Agglutination (Widal) – Second week
            • Stool Culture – Third week
            • Urine Culture – Fourth week
        • Vaccines
          • Typhoral vaccine(MCQ)
            • delivered orally
            • consists of live attenuated Salmonella serovar Typhi.
          • Killed vaccine
            • consists of the Vi capsular polysaccharide
            • it is delivered parenterally.
      • Bacteriophage typing
          • Intraspecies classification of S.typhi for epidemiological purposes was made possible by bacteriophage typing (MCQ)


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