Vibrio, Aeromonas, and Plesiomonas

Aqua Rods - V.A.P. Intro

• General Features: Gram-negative rods, facultative anaerobes.
• Motility: Most are motile via polar flagella.
• Biochemical Test: Oxidase-positive.
  • 📌 V.A.P. are Oxidase Positive: Very Awesome Pathogens.
• Natural Habitat: Predominantly aquatic environments (freshwater, brackish, marine).
• Clinically Important Genera: Vibrio, Aeromonas, Plesiomonas.

⭐ Most Vibrio, Aeromonas, and Plesiomonas species are oxidase-positive. This is a key biochemical test distinguishing them from the Enterobacteriaceae family, which are typically oxidase-negative.

Vibrio Spotlight - Cholera & Kin

• Vibrio cholerae:
  • Serogroups: O1 (El Tor, Classical biotypes), O139 - cause cholera epidemics.
  • Cholera Toxin (CT): A-B toxin. Mechanism: $G_s\alpha \xrightarrow{\text{ADP-ribosylation}} \text{activated } G_s\alpha \rightarrow \uparrow \text{adenylate cyclase} \rightarrow \uparrow cAMP \rightarrow \text{PKA activation} \rightarrow \text{CFTR phosphorylation} \rightarrow Cl^- \text{secretion}$. Leads to 'rice-water stool'.
  • Lab: TCBS agar (sucrose fermenter $\rightarrow$ yellow colonies), oxidase (+), string test (+).
• Other Pathogenic Vibrios:
  • V. parahaemolyticus: Common cause of seafood gastroenteritis. Kanagawa phenomenon (thermostable direct hemolysin, TDH). TCBS: green (sucrose non-fermenter).
  • V. vulnificus: Severe wound infections, primary septicemia (raw oysters). High mortality. Lactose fermenter. TCBS: variable (often green, some strains ferment sucrose).

📌 Mnemonic - CHOLERA: Comma-shaped, Halophilic (most), Oxidase +, Lactose -, Epidemics, Rice-water stool, A-B toxin. 📌 TCBS: Yellow for Cholerae (Sucrose +), Green for paraHaemolyticus & Vulnificus (Sucrose Non-fermenters, mostly).

Pathogenesis of Cholera:

⭐ The El Tor biotype of Vibrio cholerae O1 produces acetoin (Voges-Proskauer test +), is more resistant to polymyxin B, causes a higher ratio of asymptomatic carriers to cases, and has longer environmental survival compared to the Classical biotype.

Aeromonas & Plesiomonas - The Other Water Warriors

• Aeromonas (📌 'A' for 'Against' O/129 - resistant)
  • Species: A. hydrophila, A. caviae, A. veronii; often β-hemolytic.
  • Diseases: Gastroenteritis, wound infections (contaminated water), septicemia (immunocompromised).
  • Key: Resistant to O/129.

    ⭐ Aeromonas species are characteristically resistant to the vibriostatic compound O/129 (2,4-diamino-6,7-diisopropylpteridine), a key test to differentiate them from Vibrio and Plesiomonas species, both of which are generally susceptible.

• Plesiomonas (📌 'Pleases' by O/129 sensitivity; 'shares' shigelloides symptoms with Shigella but motile)
  • Species: P. shigelloides.
  • Diseases: Gastroenteritis (raw seafood/water), rare extraintestinal infections.
  • Keys: Oxidase-positive (unlike Shigella), O/129 sensitive, ferments inositol.

Lab Detectives - Nailing the Diagnosis

Key lab tests differentiate Vibrio, Aeromonas, Plesiomonas (all GNR, Oxidase +):

⭐ The string test, where a mucoid string is formed when colonies of Vibrio cholerae are emulsified in 0.5% sodium deoxycholate solution, is a simple, rapid, and highly presumptive test for V. cholerae identification in resource-limited settings.

High‑Yield Points - ⚡ Biggest Takeaways

• Vibrio cholerae: Profuse rice-watery stools (cholera toxin ↑cAMP), TCBS agar (yellow colonies), string test positive.
• V. parahaemolyticus: Diarrhea from raw seafood; Kanagawa phenomenon (hemolysis).
• V. vulnificus: Severe wound infections, necrotizing fasciitis, septicemia in liver disease or immunocompromised.
• Aeromonas: Gastroenteritis, cellulitis/wound infections (aquatic exposure), oxidase-positive.
• Plesiomonas shigelloides: Gastroenteritis (watery/dysenteric), oxidase-positive, ferments inositol, shares antigens with Shigella.
• All are Gram-negative rods, motile, oxidase-positive, common in aquatic environments.