General Characteristics and Morphology
The genus Vibrio belongs to the family Vibrionaceae. These are Gram-negative rods with a characteristic curved shape, measuring 1.4–5.0 × 0.3–1.3 µm. They do not form spores or capsules, but exhibit high motility due to a polar flagellum (monotrichous).
Nutritionally, vibrios are chemoorganotrophs with oxidative and fermentative metabolism. Most representatives are oxidase-positive. An important biochemical marker of the genus is the ability to ferment carbohydrates (maltose, mannose, trehalose) exclusively to acid, without gas production. They inhabit freshwater and marine environments globally, as well as aquatic organisms.
Cultural Properties and Biochemistry
Vibrios are facultative anaerobes, but prefer aerobic conditions. They are not overly fastidious regarding nutrient media, but a high pH (optimum 8.5–9.0) and the presence of salts are critical for their growth.
- Enrichment medium: 1% alkaline peptone water. Vibrios grow very rapidly in this medium, forming a characteristic surface pellicle within 6–8 hours.
- Differential diagnostic medium: TCBS agar (thiosulfate-citrate-bile salts-sucrose agar). On this medium, pathogenic vibrios form yellow colonies due to sucrose fermentation.
Biochemically, vibrios are highly active: they produce indole and lysine decarboxylase, liquefy gelatin (forming a "funnel-shaped" zone), and exhibit nitrate reductase activity, but do not produce hydrogen sulfide.
Causative Agent of Cholera (Vibrio cholerae)
Cholera is an acute, highly dangerous quarantine infection affecting the small intestine, leading to severe water-electrolyte imbalance and dehydration. The causative agents are serogroups O1 and O139.
Within the O1 serogroup, two main biovars are distinguished:
- Classical (V. cholerae classical) — lysed by group IV bacteriophages.
- El Tor (V. cholerae El Tor) — resistant to polymyxin B, Voges-Proskauer positive, hemolyzes sheep erythrocytes, and is lysed by group V bacteriophages.
The main virulence factor is cholera enterotoxin (CT), consisting of A and B subunits. Subunit B binds to the monosialoganglioside GM1 on intestinal epithelial cells. Toxin synthesis is encoded by the CTX bacteriophage genome. The phage enters the bacterium via toxin-coregulated pili (TCP) of type 4, which are also responsible for the colonization of intestinal microvilli and biofilm formation.
Halophilic and Non-Halophilic Vibrios
In addition to Vibrio cholerae, other members of the genus have medical significance:
- Halophilic vibrios (e.g., V. parahaemolyticus). Inhabit seawater (optimal salt concentration 2–3%). Transmitted via contaminated raw or undercooked seafood, causing severe gastroenteritis.
- Non-halophilic vibrios (V. cholerae non-O1/O139). Inhabit freshwater bodies. They are not agglutinated by O1 and O139 antisera, but can secrete heat-labile and heat-stable toxins, causing sporadic diarrhea and even systemic disease up to septicemia.