Chapter 41

Vibrio

■ Name the important species of vibrio.

The important species of Vibrio are:

• V. cholerae is the most important species, first isolated by Koch (1883).

• V. parahaemolyticus

• V. vulnificus

■ Mention the morphological characteristics of vibrio cholerae.

Morphological characteristics

■ Write the types of media used and colony characteristics of V. cholerae?

Cultural characters

• Aerobe and facultative anaerobe

• Grow on ordinary media

• Optimum temperature 37°C (range 16°–40°C) and optimum pH 8.2 (range 8.2–9.5)

Media and colony characters

• Nutrient agar: Large, round colonies, 1–2 mm in diameter, translucent with bluish tinge in transmitted light

• Blood agar: Greenish zone of discolouration around colony, which later on becomes clear due to haemodigestion

• MacConkey’s agar: Nonlactose fermenting colonies (colourless), become pink on prolonged incubation (late lactose fermenting)

Special media for Vibriocholerae are:

Selective media or plating media

• Thiosulphate, citrate, bile salt, sucrose (TCBS) agar (Fig. 41.2) – Forms yellow colonies because of sucrose fermentation and indicator bromothymol blue and turn green on continued incubation

• Alkaline bile salt agar (BSA) – Colonies are similar to that on nutrient agar

• Mansur’s gelatin taurocholate trypticase tellurite agar (GTTTA) – Small, translucent with gray-black centres and turbid halo

Transport media or holding media

Enrichment media

• Alkaline peptone water

– High pH suppresses growth of commensals and allows growth of vibrio

– Can also be used as a transport medium

• Taurocholate tellurite peptone water – Incubation of 6–8 hours in enrichment media is sufficient

■ Describe the reactions that are used for identifying V. cholerae.

Biochemical reactions

Cholera red reaction

When 24-hour old liquid culture is mixed with few drops of concentrated sulphuric acid, red-pink colour develops due to formation of nitroso-indole. The test is positive in V. cholerae

String test

When growth is mixed with 0.5% sodium deoxycholate in saline, suspension loses its turbidity and becomes mucoid to form string when loop withdrawn from suspension. Test is positive in V. cholerae

Haemolytic reaction

• Equal volumes of broth culture and 1% sheep erythrocytes are mixed and incubated for 2 hours at 37°C then kept in refrigerator for overnight at 4°C and examined for haemolysis

• Classical Vibrio—nonhaemolytic

• El Tor Vibrio—haemolytic

■ Mention the factors governing survival of V. cholerae.

Vibrio cholerae are:

■ Which antigens are present in V. cholerae?

Antigens present in V. cholerae are:

1. Somatic O Ag: On the basis of O Ag, there are 139 serogroups.

2. Flagellar H Ag

■ Comment on bacteriophage typing of V. cholerae.

• Classical strains are divided into I to V phage groups by using Mukherjee’s four phages

• Later on one more phage I is employed and now 6 phage-groups have been identified

■ Classify vibrio according to gardner and venkataraman’s system of classification.

Classification of Vibrio according to Gardner and Venkataraman’s system of classification is given in Flowchart 41.1.

Table 41.1

Differences between biotypes of V. cholerae

	*Classical*	*El Tor*
1. Voges–Proskauer test	−	+
2. Agglutination of chick RBCs	−	+
3. Haemolysis of sheep RBCs	−	+
4. Sensitivity to polymyxin B	+	−
5. Phage V susceptibility	−	+
6. Phage IV susceptibility	+	−

+ = positive, − = negative.

■ Diagrammatically depict the course of disease development in V. cholerae infection. also write the clinical features of the disease.

Development of disease in V. cholerae infection is given in Flowchart 41.2.

Flowchart 41.2Course of disease development in V. cholerae infection.

Clinical features

■ Describe the laboratory procedures and examinations employed in diagnosing V. cholerae infection.

Collection of specimens

• Stool: It is an ideal specimen, collected by inserting sterile rubber catheter and allowing stool to flow into sterile container

• Rectal swab: Cotton swabs moistened with transport media are inserted in rectum, left at the place for few seconds to allow absorption of fluid. They are used for patients in convalescence or carriers

Transport

Microscopic examination

For rapid diagnosis, the methods are:

• Demonstration of motility by hanging drop preparation and its inhibition by antisera and distilled water

• Direct immunofluorescence test

Culture

Biochemical tests

Serology

• Bacterial suspension is tested by adding O subgroup I serum

• If positive, tested with Ogawa and Inaba sera to detect serotype

• If the colony is not agglutinated by O subgroup I serum, it is then tested with antisera to antigen, if it shows agglutination, reported as non-O₁Vibrio cholerae

• Specific antisera to O_2–139 are available and can be used for its detection

Antibiotic sensitivity testing

It is performed by Kirby–Bauer disc diffusion method

■ How are carriers of vibrio detected?

Detection of carriers is done by:

• Repeated stool culture

• Stool collected after purgative

• Bile culture

• Serological tests

■ What treatment measures are recommended in case of V. cholerae infection?

Treatment recommended includes:

• Fluid and electrolyte replacement—to correct severe dehydration by oral rehydration therapy either alone or supplemented by intravenous fluids

• Antibiotics—usually reduce Vibrio excretion–tetracycline is used

■ What are halophilic vibrios?

Vibrios that need high concentration of sodium chloride and cannot grow in absence of it are called halophilic vibrios, e.g.

1. V. parahaemolyticus

2. V. alginolyticus

3. V. vulnificus

Naturally, they exist in seawater and have a marine life.

■ Mention the salient features of V. parahaemolyticus, V. alginolyticus and V. vulnificus.

V. parahaemolyticus

V. alginolyticus

V. vulnificus