Gram negative, rigid, curved rods, actively motile by polar flagellum, nonsporing, noncapsulated, present in marine environments and surface waters. The most important member of the genus is V. cholerae, the causative agent of cholera / Asiatic cholera. It was first isolated by Robert Koch.
Morphology & Cultural characteristics - short, curved, cylidrical rod, 1.5µm X 0.2-0.4µm, rounded or slightly pointed ends, typically comma-shaped. Pleomorphism is frequent in old cultures. It is actively motile, show vigorous darting motility.
It is strongly aerobic, temperature range 16-400C, growth better in alkaline medium, range of pH 6.4-9.6. NaCl (1%) required for optimal growth, but high concentration are inhibitory. It grows well on ordinary media. On NA it shows moist, translucent round colonies with a bluish tinge in transmitted light. It has a distinctive odour. On Mac Conkey agar, the colonies are colorless first, but become reddish on prolonged incubation due to the late lactose fermentation. In a gelatin stab, at first a white line of growth appears along the track of the inoculating wire, liquefaction of the gelatin begins at the top which spreads downwards in a funnel- shaped form in 3 days at 220C. In peptone water , this organism grows as a surface pellicle because of its affinity for oxygen, which becomes visible in 6-9 hours.
Special media – A number of special media have been employed for the cultivation of the bacteria:
1. Transport media – V. cholerae is sensitive to drying, exposure to sunlight and extreme changes in pH, also inhibited by normal intestinal flora. If the cultures cannot be processed immediately, stool samples should be transported in transport media.
a. Venkataraman – Ramakrishnan (VR) medium – about 10-15ml of the medium, 1-3ml of the stool is added, V. cholerae do not multiply but remain viable for several weeks, also prevents overgrowth by other organisms.
b. Cary – Blair medium – in this the pH is adjusted at 8.4, suitable for Salmonella, Shigella and V. cholerae.
2. Enrichment media
a. Alkaline peptone water (APW) – this medium contains 1% peptone and sodium chloride at pH 8.6. High pH of the medium suppresses the growth of many commensal intestinal bacteria while permitting uninhibited growth of V. cholerae.
b. Taurocholate peptone transport and enrichment medium – pH at 9.0, to make selective for vibrios, sterile potassium tellurite solution added .
3. Plating media
a. Alkaline bile salt agar (BSA) – modified nutrient agar medium with 0.5% sodium taurocholate, colonies similar to those in NA.
b. Thiosulphate-citrate-bile-sucrose agar (TCBS) – selective plating medium for vibrios. Constituents are sodium thiosulphate, sodium citrate, ox bile, sucrose, yeast extract, peptone, sodium chloride, ferric citrate, thymol blue, bromothymol blue and water. Sucrose fermenters form yellow colonies and non fermenters produces blue green ones.
Vibrio colonies can be identified by’string test’. A loopful of the growth is mixed with a drop of 0.5% sodium deoxycholate in saline on a slide. If the test is positive, suspension becomes mucoid and forms a string when the loop is slowly drawn away from the suspension.
Biochemical characteristics- They ferment glucose,maltose, sucrose and lactose very slowly. Indole is formed, nitrates are reduced, contributes to the ‘cholere-red reaction’, catalase and oxidase positive. It possess many enzymes-lipase, neuraminidase, chitinase, elastase etc. They are killed by heating at 560C for 30 minutes, sensitive to drying, acids, affected by vibriophages, survive in clean tap water for 30 days, survive in untreated night soil, sensitive to disinfectants, survive on fruits at room temperature.
Pathogenicity - Cholera is an acute diarrheal disease caused by V. cholerae. It affects people anywhere sanitation is poor and faecal contamination of water occurs. Some transmitted to humans from contaminated shell fish. When ingested it invades the intestinal mucosa, multiplies, releases a potent enterotoxin, choleragen, which binds to epithelial cells of the small intestine and makes plasma membrane highly permeable to water. This results in a significant secretion of fluids and chloride ions and the inhibition of sodium absorption. At this time the intestinal lining will be shredded causing numerous small, white flecks, resembling rice grains to be passed in the faeces. Infected individuals experience severe nausea, vomiting, abdominal pain, diarrhea. Stools rapidly become clear, containing mucus, therefore “rice-water stools”. About 22 litres of fluids lost per day and therefore dehydration. Most deaths by shock due to reduced blood volume and dehydration.
Epidemiology : sporadic, epidemic, endemic, pandemic, infection through faecally contaminated water and food.
Diagnosis & Prophylaxis - Stool collected in the acute stage, transported to the lab in VR medium, plated on TCBS, then string test and other biochemical tests. Prevention by protecting water supply and environmental sanitation. Vaccination is widely used method. The treatment of cholera consists , replacement of lost fluid and electrolytes. Cereal based preparation more acceptable. Antibacterial therapy, oral tetracycline is recommended.
V. parahemolyticus is an enteropathogenic, caausative agent of sea fish food poisoning.