Vibrio – summary

• Vibrio – derived from characteristic vibrating motility
• V. cholera – causes cholera, first isolated by Koch

Morphology & Taxonomy

• Gram -ve
• Curved bacilli – comma shaped
• Actively motile – single polar flagellum – darting motility
• Fish in stream appearance
• Swarm of gnats appearance – when actively motile
• Classification
  • Heiberg – 6 groups based on fermentation of mannose, sucrose, arabinose
  • Serological classification ( by Gardner & Venkatraman )
    1. Grp A – cholera vibrios,
       • have common flagellar(H) antigen
       • Subdivided into O1 & non-O1 serovars
       • O1 consists of classical & El Tor biotypes, both are classified into 3 serotypes – ogawa, inaba, hikojima
       • Non-O1 or non-agglutinable(NAG) vibrios classified upto 139 serotypes
    2. Grp B – heterogeneous vibrios
  • By requirement of NaCl
    1. Halophilic – V. alginolyticus, V. vulnificus, V. parahaemolyticus
    2. Non halophilic – V. cholera

Culture

• Aerobic
• Optimum temperature 37°C
• pH optimum 8.2
• 0.5-1 % NaCl required for optimal growth
• Ordinary media
  • Nutrient agar – moist, translucent, round discs
  • MacConkey agar – colourless but red coloured on prolonged incubation – late lactose fermentor
  • Blood agar – intially zone of greening, cleared later due to hemodigestion
  • Gelatin stab culture – funnel shaped/ turnip shaped liquefaction in 3 days
  • Peptone water – growth in 6 hrs
• Special media
  • Holding/ Transportation media – VR ( Venkatraman Ramakrishnan ) media, Cary Blair media – also for Salmonella & Shigella , autoclaved sea water
  • Enrichment media – alkaline peptone water, Monsur taurocholate tellurite peptone water
  • Plating media – Alkaline bile salt agar, Monsur Gelatin taurocholate tellurite trypticase agar, Thiosulphate citrate bile salt sucrose ( TCBS ) medium

• ‘String test’ – to identify Vibrio colonies, growth is mixed with Sodium deoxycholate in saline – mucoid string formation when loop is drawn slowly ( +ve Test )

Epidemiology

• No animal reservoir
• Natural habitat – coastal salt water, brackish waters
• Mostly pediatric disease in endemic areas
• Burden greatest during “cholera seasons” – high temperature, high rainfall, flooding
• Blood group O – greatest risk of severe disease if infected, Blood group AB – least risk
• 7 global pandemics occured – El tor biotype displaced endemic classical type in 7th pandemic
• Asymptomatic carriers may be present

Pathogenesis

• Toxin mediated disease
• Virulence factors – Cholera toxin, Toxin coregulated pilus, other factors ( regulated by ToxR protein )
• Environmental factors, bacterial response to density of bacterial population ( quorum sensing ) modulate virulence
• Cholera toxin – A & B subunits
  • A subunit – enzymatic, causes intracellular accumulation of cAMP
  • B subunit – pentameric binding moeity, facilitates attachment of A subunit
• In intestine, cAMP inhibits absorptive Na+ transport system & activates secretary Cl- transport
• Thus NaCl accumulates in lumen & passive movement of water into the lumen results in watery diarrhea
• Other effects of Cholera toxin – increase skin permeability ( permeability factor ), skin bluing test ( when injected intradermally along with pontamine blue I.v, injected site becomes blue ) – used for detection of toxin

Clinical features

• Incubation period 1-2 days
• Asymptomatic / mild diarrhea in some patients
• Sudden onset explosive & life threatening diarrhea – cholera gravis
• Painless watery diarrhea, vomitings
• Hypovolemic shock ( cause of death in Cholera )
• Fever absent
• Muscle cramps due to electrolyte disturbances
• Stool – Rice watery stool
  • Non bilious
  • Gray, slightly cloudy with mucus flecks
  • No blood
  • Somewhat Fishy odour
• Symptoms depend on water volume lost
  • < 5% of body wt lost – thirst
  • 5-10% lost – postural hypotension, weakness, tachycardia, decreased skin turgor
  • > 10% lost – oliguria, weak pulse, sunken eyes, washerwoman/ wrinkled skin, somnolence, coma
• Complications develop from effects of volume & electrolyte disturbances – renal failure, acute tubular necrosis

Lab diagnosis

• Microscopy – identification of V. cholerae in stool
• Culture
• Slide agglutination tests
• Biochemical tests
  • Ferments carbohydrates but no gas
  • Ferment glucose, mannitol, maltose, mannose
  • Indole formed, nitrates reduced – contributes to cholera red reaction ( reddish pink colour on adding conc. H2SO4 to 24 hr peptone water culture )
  • Catalase +ve, oxidase +ve
• Testing of water samples
• Lab data usually indicates
  • Elevated hematocrit
  • Mild neutrophilia
  • Elevated blood urea nitrogen & creatinine – prerenal azotemia
  • Normal Na+, K+, Cl-
  • Reduced bicarbonate
  • Elevated anion gap

Treatment

• Fluid & electrolyte replacement – ORS is effective ( hexose- Na+ cotransport is intact ), ringer lactate with potassium supplements
  • Total fluid deficit in severe dehydration can be replaced in 3-4 hrs safely – half in first hr
• Oral tetracycline, ciprofloxacin

Prevention

• Provision of safe water
• Safe disposal of feces
• Improved nutition
• Safe food preparation & storage practices
• Vaccines
  1. Parenteral – killed suspension of V. cholera, protection <60%, no local immunity
  2. Oral – killed whole cell & live oral

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