Aeromonas spp.

Introduction

Aeromonas are Gram-negative bacilli, widely distributed in nature, especially aquatic environments – freshwater, brackish water, sewage system, hospital water system and even drinking water. It can also be found on food and domestic or farm animals.

Aeromonas consists of more than 30 species based on DNA-DNA hybridisation and 16S rDNA relatedness. Of these 30 species, 17 have been associated with human disease. However, only four species account for more than 95% of cases – Aeromonas caviae, Aeromonas dhakensis, Aeromonas veronii, and Aeromonas hydrophila

Laboratory - identification of the Aeromonas spp.

Gram negative bacilli or coccobacilli.
They are facultative anaerobe and easily grows on blood and MacConkey agar. They may or may not produce haemolysis on blood agar.
Aeromonas are oxidase and catalase-positive, reduces nitrates to nitrites, and ferment glucose.
Can be identified using API20E, MALDI-ToF and other commercial systems.

Differentiation from similar bacteria:

Plesiomonas and vibrio – Aeromonas are negative for ornithine decarboxylase hydrolysis (except some A veronii).
Enterobacterales – Aeromonas are oxidase positive, enterobacterales are negative.

Sensitivity test

Intrinsic resistance:

Amoxicillin, ampicillin and ampicillin-sulbactam – intrinsically resistant.
Co-amoxiclav – EUCAST does not consider Aeromonas intrinsically resistant to co-amoxiclav. (Note – CLSI says Aeromonas is intrinsically resistant to co-amoxiclav)
Aeromonas dhakensis may have chromosomal ampC and resistance to cefoxitin.
Some A caviae could be susceptible to ampicillin.

Aeromonas have multiple inducible chromosomal beta-lactamases and hence may develop resistance to cephalosporins on treatment. Some strains also have chromosomal carbapenemase production (A. jandaei, A. veronii biotype veronii and occasionally in A. hydrophila and A. dhakensis). *

EUCAST and CLSI have breakpoints for these antibiotics

EUCAST

Ceftazidime, cefepime,
Aztreonam
Ciprofloxacin, Levofloxacin
Cotrimoxazole

CLSI

Pip-tazobactam
Cephems – cefepime, cefotaxime, cefoxitin, ceftazidime, ceftriaxone, cefuroxime (IV)
Carbapenems – Doripenem, Ertapenem, Imipenem, Meropenem,
Monobactam – Aztreonam
Aminoglycoside – Amikacin, gentamicin
Tetracycline
Quinolone – Ciprofloxacin, levofloxacin
Cotrimoxazole
Chloramphenicol

Aeromonas infection

Aeromonas infection is often associated with the history of exposure to water (fresh, brackish, sewage, even treated water), leech therapy or consumption of raw or semi-cooked seafood. However, nosocomial infection associated with hospital water sources has been reported. Presenting features are skin and soft tissue infection, gastroenteritis, sepsis – usually in patients with immunocompromised state (malignancy, drugs), hepatobiliary disease, diabetes mellitus.

Common Aeromonas infections are bacteremia, hepatobiliary tract infections, and soft tissue infections.

Bacteraemia

Associated with underlying illnesses such as immunocompromised state (malignancy, immunosuppressive drugs, neutropenia), hepatobiliary disease and diabetes mellitus.
The most common portal of entry into the bloodstream is the gastrointestinal tract.
Case fatality could be 25-60%

Biliary infection

Associated with biliary obstruction due to stone or malignancy.
Spontaneous bacterial peritonitis with high mortality (50%) has also been reported.

Skin and soft tissue infection

Localised infections like furunculosis or cellulitis to
Severe infections like necrotising fasciitis, osteomyelitis and myonecrosis. Necrotising fasciitis could be monomicrobial, aggressive, and life-threatening.

Aeromonas burn infection, associated with a history of water immersion, has also been reported.

The skin and soft tissue infection are associated with a history of exposure to water environments like salt water, fresh water, aquarium, koi pond. It has also been associated with leech therapy.
Usually, the localised infection is seen in immunocompetent patients and more severe disease in immunocompromised patients, but exceptions are not uncommon.

Gastrointestinal infection

The role of Aeromonas in gastrointestinal illness is debatable. It has been isolated from patients with various types of diarrhoeal illness – acute watery diarrhoea, travellers diarrhoea, chronic diarrhoea, dysentery, but whether they are the pathogen, has not been conclusively proven.
The disease is often self-limiting, but complications like haemolytic uraemic syndrome have been reported.

Other infections caused by Aeromonas are – pneumonia, especially associated with malignancy and drowning; eye infection like contact lens associated keratitis; and genitourinary infection.

Virulence

Some Aeromonas are more virulent (A. dhakensis, A. hydrophila, or A. veronii) than others (A. caviae). Virulence factors are haemolysin, aerolysin, type III secretion system, siderophore, endotoxin, haemagglutinin etc.

Treatment

Aeromonas is usually susceptible to 3rd or 4th generation cephalosporins, fluoroquinolone, cotrimoxazole, aminoglycosides, carbapenems, chloramphenicol.

Gastroenteritis is usually self-limiting and may not need treatment.

Cotrimoxazole or quinolones could be used for treatment. 3rd/4th gen cephalosporins are also used.

Serious infection like necrotising fasciitis may need combination of two antibiotics.

Beta-lactamase produced by Aeromonas spp.

Aeromonas produces both chromosomal and plasmid or integron mediated beta-lactamases.

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