Vibrio Cholerae

Vibrio Cholerae

Vibrio is one of the curved rod bacteria, prominent in the Medical Bacteriology. They are present in marine environment and surface waters worldwide. Vibrio is a member of the family Vibrionaceae. The most important member of this genus is Vibrio cholerae, the causative agent of cholera. The term Vibrio is derived from Vibrare (Latin word) which means “to shake or vibrate” and the word Cholera is derived from Chole (Greek word) which means, “to bile” (Figure 7.16).

Morphology

Vibrio cholerae is gram negative, curved or comma shaped, (1.5um x 0.2–0.4um in size) non – capsulated. The organism is very actively motile with a single polar flagellum and the characteristic movement is called as darting motility. In stained smears of mucus flakes from acute cholera patients, the Vibriois seen arranged in parallel rows. This was described by Robert Koch as “fish in stream” appearance. 

Culture Characteristics

Vibrio cholera is strongly aerobic. It grows best in alkaline media with the optimum temperature 37°C and pH 8.2. It is non-halophilic, therefore, cannot grow in media with a concentration of sodium chloride more than 7% (Figure 7.17). Some of the media in which Vibrio cholerae are cultivated are tabulated below in Table 7.16.

Enterotoxin

Vibrios multiplying on the intestinal epithelium produce an enterotoxin called Cholera toxin . It is also known as Choleragen (or CT). This toxin molecule is approximately 84,000 Dalton and consists of two major subunits namely A and B There is only one subunit in A (1A) whereas there are five subunits in B (5B) (Figure 7.18).

Mode of Action

• The B (binding) units of enterotoxin get attached to the GM₁ (Ganglioside membrane receptors I) on the surface of jejunal epithelial cells (target cells).

• The A (active) subunits then enters the target cell and dissociates into 2 fragments, A₁ & A₂. The A₂ fragment links biologically active A₁ fragment to the B – subunit.

• The A₁ fragment causes prolonged activation of cellular adenylate cyclase which in turn accumulates cAMP in the target cell. This leads to outpouring of large quantities of water and electrolytes into small intestinal lumen. Thus, resulting in profuse watery diarrhea.

Natural infection of Vibrio cholera occurs only in human beings

Pathogenesis

The pathogenic mechanism of Vibrio cholerae is discussed below in flowchart 7.7.

Source of Infection – contaminated water or food

Route of entry – fecal – oral route

Site of infection – small intestine

Incubation period – few hours to 5 days (usually 2–3 days)

The loss of water during cholera is about 20–30 litres per day

HOTS: Why is cholera the most severe form of gastroenteritis?

Clinical Feature

Dehydration, anuria (absence of urine excretion), muscle cramps, hypokalemia (low blood potassium) & metabolic acidosis (low serum concentration of bicarbonates).

Laboratory Diagnosis

Specimen: Stool

Direct microscopy: It is not a reliable method for rapid diagnosis, the characteristic darting motility of the vibrio can be observed under dark – field microscope.

Culture: Stool sample is directly inoculated on MacConkey agar and TCBS agar. The plates are examined after overnight incubation at 37°C for typical colonies of Vibrio cholera , and the colonies are identified by gram staining and oxidasetest

Prophylaxis

1. General Measures

• Purification of water supplies Improvement of environment sanitation

• Infected patients should be isolated, and their excreta must be disinfected

2. Vaccines: Two types of oral vaccines have been tried recently:

• Killed oral whole cell vaccines

• Live oral vaccines

Treatment

• Oral Rehydrationtherapy: The severe dehydration & salt depletion can be treated by oral rehydration therapy (as recommended by WHO).

• Antibiotics: It is of secondary importance, oral tetracycline was recommended for reducing the period of Vibrio excretion.

An ideal cholera vaccine is yet to be found