Vibrionaceae
General characteristics:Gram –ve ,curved or straight rods .Motile by means of single polar sheathed flagellum .Glucose is fermented by all species without the production of gas .Oxidase test positive .About one third of the more than 35 species are pathogenic for humans.The primary habitat is marine and fresh water , transmission to humans is by ingestion of contaminated water or seafood.Vibrio cholerae causes pandemic and epidemics, during outbreaks the organism is transmitted by feco-oral spread .Genus Vibrionaceae
Important species are V. cholerae V. parahaemolyticus V. alginolyticusAntigenic structure of Vibrio cholerae
Antigenic variation play an important role in the epidemiology and virulence of cholera. Flageller Ag H-Ags are shared with other vibrios and therefore are of no use in differentiating between strains causing epidemic cholera. O-LPS(somatic Ag ) represent serological specificity . 139 O-Ags groups (serotypes), almost all these serogroups are non virulent , only serotype O1 and serotype O 139 cause classic cholera. These two strains almost always produce ctx, whereas non O1 and O139 do not produce the ctx and hence do not produce cholera . The V.C. serogroup O1 Ag has determinants that make further possible typing , the main serotypes are Inaba(A,C) ,Ogawa(A,B) and Hikojima(A,B,C) .Cont,
Two biotypes of epidemic V.C. had been identified , Classic and El Tor. The El Tor biotype characterized by the following : 1. produce hemolysis 2. V.P. test is (+) 3. resistant to polymyxin.4. agglutinate chicken erythrocytes .5. Susceptible to bacteriophage IV ElTor biotype produce less ctx but appear to colonize intestinal epithelium better than vibrios of classic variety , El Tor strains have a higher tendency to become endemic .Pathogenesis
V. cholerae is very sensitive to acids, therefore gastric acidity is a powerful natural resistance mechanism . Cholera is a disease of small intestine. To establish residence and multiply in the small intestine, V. cholerae has one or more adherence factors that enable them to adhere to microvilli.Cont,
The motility of V.C. may affect virulence by enabling them to penetrate the mucus layer . It produce mucinolytic enzymes (protease and neuraminidase). Specific adherence to the intestinal mucosa is probably mediated by long filamentous fimbria that form bundles at the poles of the cells.Cont,
V.cholerae produce cholera toxin which is responsible for the manifestation of the disease . The genes that encode the synthesis of CT reside on the V. cholerae chromosome . Cholera toxin consist of two subunits A and five subunits B. It acts on intestinal mucosa leading to increase level of intracellular cAMP and the secretion of chloride,H2O, Na+ and K+ into the lumen of intestine
Clinical picture
60% of infection with classic V.C. are asymptomatic or is mild illness. severe infection occur only in 5% of cases. I.P. :1-4 days, there is sudden onset of nausea, vomiting ,profuse diarrhea with abdominal cramps. May lose 20-30 L of fluid within a day or two, this lead to dehydration, anuria, metabolic acidosis, hypovolumic shock. Watery diarrhea contain mucus, epithelial cells and enormous number of v.c .(rice-water stool). Untreated cholera frequently result in a high mortality rate ( 25- 50% )V. Cholerae ,non- O1/O139, NAG
Cholerae strains that do not produce ctx with rare exception and they do not cause epidemic cholera . Other toxins that have been identified from NAG : 1.Hemolysin similar to that produced by El Tor V. cholerae O1. 2.Heat stable enterotoxin very similar to that produced by E. coli.( NAG- ST ) . 4. NAG may contain a capsule which block the bactericidal effect of serum. NAG strains cause self limiting gastroenteritis, wound infection and bacteremia in patients with liver disease, bacteremia may be fatalLab identification
Clinical specimens are stool and rectal swab .Direct detection method :Direct fluorescent – Ab staining of smears from liquid stool.Coagglutination test with monoclonal antibodies against the O1 Ag.Latex agglutination for detection of ctx. PCR assays.Cultivation
Pathogenic vibrio species can grow on most several types of culture media: Alkaline peptone water contain ( pH 8.5) , suppress the growth of other bacteria , is an excellent transport medium incubated for 4-8 hrs then subculture on TCBS ( selective medium) .On TCBS ( Thiosulfate citrate bile salt sucrose agar)
Sucrose fermenting appear yellow (V. cholerae and V. alginolyticus ). Sucrose non- fermenting colonies appear green ( V. parahaemolyticus).Cont,
MacConkey agar containing lactose, vibrio species are non fermenters ( colorless colonies).Biochemical reactions of V. cholerae
Oxidase (+) Ferment glucose,sucrose, mannos and mannitol but not lactose and arabinose with the production of acid with no gas . Lysine and ornithine decarboxylase test (+), Arginine (-) Not reduce nitrate to nitrite. Indole test (+) ONPG , positive. KIA: K/A String test , positive