Aeromonas

Morphology and cultural characteristics

The genus Aeromonas comprises 12 species, of which three are clinically significant in humans: A. hydrophila, A. caviae, and A. veronii (subspecies sobria).

These are Gram-negative, motile rods with a capsule and a single polar flagellum. An interesting feature is that when cultured on solid nutrient media, some species are capable of forming entire tufts of flagella.

By type of respiration, the bacteria are facultative anaerobes. They are fastidious-free (undemanding) and grow very well on simple nutrient media as well as in alkaline peptone water.

Features of growth on differential media:

• On blood agar, they form colonies with a zone of clear hemolysis.
• On lactose-containing media, they manifest as lactose-nonfermenting microorganisms.

Aeromonads possess natural resistance to beta-lactams, therefore, blood agar supplemented with ampicillin (15 mg/L) is used for their selective isolation.

Pathogenicity factors

The pathogenic potential of aeromonads is due to a complex of structures and secreted substances. Attachment to host cells (carbohydrate receptors of erythrocytes and enterocytes) is provided by adhesins.

Toxic substances and aggression enzymes include:

1. Toxins:
2. Cytotoxins: aerolysin and cytolysin.
3. Enterotoxins: heat-labile and heat-stable. Notably, the heat-stable enterotoxin shares antigenic similarity with cholera toxin.
4. Hemolysins.
5. Aggression enzymes:
6. Heat-stable serine protease.
7. Metalloproteases (including phospholipase and chitinase).

Microbiological diagnostics

The main diagnostic method for infections caused by Aeromonas is bacteriological. The inoculation algorithm depends on the specimen tested:

• Feces: the material is first inoculated into an enrichment broth (alkaline peptone water). After 24 hours, subculturing is performed onto a selective medium — blood agar with ampicillin.
• Wound discharge: parallel inoculation of the test material is performed onto blood agar with ampicillin and into alkaline peptone water.

For further identification, colonies surrounded by a zone of hemolysis are selected. The bacteria are confirmed by catalase and oxidase tests (aeromonads are positive for both). Final biochemical identification is carried out using standard test systems designed for enterobacteria (Enterobacteriaceae).

Epidemiology and treatment

Infections caused by aeromonads belong to the group of emerging infections. The reservoir and natural environment for the bacteria is water.

In humans, the infection can occur in several forms:

• Gastroenteritis (when the pathogen enters via the gastrointestinal tract).
• Wound infections (upon contact of damaged skin with contaminated water).
• Septicemia (a severe generalized form, characteristic primarily of immunocompromised individuals).

The basis of treatment is etiotropic therapy with antibacterial drugs (taking into account their natural resistance to beta-lactams).

Briefly: answers to topical questions

What else to study on this topic

• Structure and mechanisms of action of aerolysin and cytolysin.
• Antigenic similarity of the Aeromonas heat-stable enterotoxin to cholera toxin.
• Application of test systems for Enterobacteriaceae in the diagnostics of non-fermenting and other Gram-negative bacteria.
• The role of chitinase and phospholipase as bacterial aggression factors.
• Mechanisms of natural resistance of aeromonads to beta-lactam antibiotics.

Other topics in the section "Gram-negative facultatively anaerobic rods (Enterobacteriaceae, Vibrio, Haemophilus, Pasteurella)"

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