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Aeromonas

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Aeromonas are Gram negative bacteria common in water and soil. Not commonly pathogenic.

Aeromonas species are associated with rare but serious conditions including wound infections, necrosis, septicaemia and meningitis but the role of Aeromonas in food or waterborne gastroenteritis is a matter of sometimes violent debate (primarily amongst drunken microbiologists at conferences). Usually found in places faaaaaaar warmer than the UK it is part of the normal gut flora of leeches. Which are now used medicinally again. So if you are more likely to get this from having a finger reattached than from food. Unless it’s lightly-sautéed leeches. Members of four Aeromonas groups may cause gastroenteritis: A. hydrophila, A. veronii biovar sobria, A. caviae and A. trota. A. schubertii and A. jandaei have also been isolated from faeces and some reported cases / outbreaks have been associated with consumption of food contaminated with Aeromonas. But association is not necessarily causation. Because (a) there is no animal model for gastrointestinal infection and (b) the taxonomy of this genus is very complex, problems arise with collecting data to demonstrate a causal link between gastroenteritis and consumption of food or water contaminated with Aeromonas. Commercial laboratory test kits are limited in their ability to identify these bacteria and most laboratories cannot routinely culture or type them. Many potential virulence factors have been identified and these may one day assist in the identification of virulent strains. But not today. Or even this week. Aeromonas can grow at refrigeration temperatures and under both aerobic and anaerobic conditions – but they are easily destroyed when food is cooked. They do not form spores. Only the bacilli and the clostridia do that.

Growth and Control Inactivation Reservoirs / Sources Plague and Pestilence Clinical Notes
Significant inter-strain variability is reported in conditions for growth and survival.

Temperature

Optimum 28 to 35°C. Range-2 up to 42-45°C, although often < 40°C depending on the strain

pH

Optimum 7.2, Minimum 4.5, Maximum ≥ 8.7. Unlikely to grow in food below pH 6.0 and stored at low temperatures

Atmosphere

Facultative anaerobe.

Water Activity

Optimum approximately 1-2% NaCl

pH

At pH 4.5, no growth observed at 4 or 28°C

Water Activity

4.5% (~0.975 aw) NaCl inhibits growth for >14 days at 4°C. At 28°C 5% NaCl (~0.97aw) inhibits most strains and 6% NaCl inhibits them all.

Temperature

D45°C = 12-29 minutes. In saline (0.85% NaCl), D48°C = 2.2-6.6 minutes and D51°C = 1.2-2.3 minutes

pH

Inactivated at values <4.5

Water Activity

< 0.96 aw (~ 6 to 7% NaCl) controls all strains tested. Susceptible to disinfectants, including sodium hypochlorite and quaternary ammonium compounds. Also iodophors, 2-chlorophenol and glutaraldehyde.

Human

Prevalence comparisons between symptomatic and asymptomatic individuals show higher values for those with symptoms, but the ranges are broad and overlapping. The organism is, however, not considered to be a normal inhabitant of the gut.

Animal

May colonise aquatic plants and animals e.g. fish, leeches and frogs. Causes disease in animals associated with water, e.g. reptiles, fish, shellfish and snails. Minor flora component of domestic animal faeces (pigs, cows, sheep, poultry). Has been isolated from houseflies, mosquitoes and ticks (Galindo and Chopra, 2007). Recently isolated from faeces of Macaca fascicularis, a primate (Harf-Monteil et al., 2004).

Food

Organism has been isolated from fresh produce (McMahon and Wilson, 2001) and foods of animal origin, such as meat, raw milk, poultry, fish, and shellfish.

Environment

Found in salt, fresh, stagnant, estuarine and brackish water worldwide. Tends more towards freshwater because as salinity increases, recovery of organism decreases. Also isolated from soil, sewage, and even tree bark. Its isolation from water and sediments decreases during cooler months. Typing data have been reported supporting water to human transmission (Khajanchi et al, 2010).

Transmission Routes

Via water and possibly by ingestion of foods including seafood, particularly oysters that receive little or no cooking during their preparation.

Seasonal variation observed with Aeromonas-associated gastroenteritis peaking in warmer months. Collated information on 16 outbreaks/incidences of Aeromonas-associated gastroenteritis implicated a range of suspect foods including fish, land snails, oysters, prawns, shrimp cocktail, dried fish sauce and egg salad. Adults are the largest age group reported among cases. Larger, suspected, outbreaks include:

Oysters

Louisana, USA, 472 adult cases (A. hydrophila)

Dried fish sauce

France, 10 adult cases

Mixed meal including seafood, meat and offals

Sweden, 27 people (A. hydrophila)

Don’t forget to read the disclaimer! It is likely that some strains of Aeromonas cause gastroenteritis, but the role of members of this genus in foodborne illness remains controversial.

Incubation

1-2 days. In an outbreak of A. caviae infections in France, the mean incubation time was 10.6 hours.

Symptoms

Broad spectrum of symptoms ranging from mild, self-limiting watery diarrhoea to dysentery. Abdominal pain, nausea, chills, headache and colitis may also occur. Symptoms last 1-7 days. Chronic diarrhoea has also been reported, usually 7-10 days. A. veronii biovar sobria has been associated with severe gastroenteritis with dysenteric symptoms. Some strains produce aerolysin, which is toxic to vero cells, and a number of haemolytic uremic syndrome cases have been attributed to such strains.

Dose

In one human trial where up to 1,000 cells were given, only two of 57 (healthy) volunteers developed diarrhoeal symptoms.

At Risk Groups

Immunocompromised individuals including adults with disrupted gastrointestinal flora. Symptoms are more severe for children.

Long Term Effects

Older patients more likely to present with chronic enterocolitis.

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