Aeromonas Species

Description of the Pathogen

The genus Aeromonas (from the Greek “Aer”: gas; and “Monas”: unit; meaning “gas producing units”) consists of environmental, facultatively anaerobic, gram-negative bacilli that are predominantly motile as a result of a single polar flagellum. Although at least 36 species have been described in this genus, only A. hydrophila, A. dhakensis, A. caviae, A. veronii biovar sobria, and A. trota are relevant human pathogens. Aeromonas spp. produce oxidase, catalase, nitrate reductase, and an array of exoenzymes. They can display a variety of virulence factors, including heat-stable and heat-labile enterotoxins, lipopolysaccharides, hemolysins, proteases, leukocidin, elastase, fibrinolysin, DNase, lecithinase, lipases, amylase, adhesins, agglutinins, pili, and an array of outer membrane proteins. ^{, ,} The motility of these species is essential for survival in liquid environments and, in addition to the capability to produce biofilm, for colonization of surfaces. Subsequently, other outer membrane proteins, including fimbrial systems, mediate human cell adherence. Some strains are also capable of cell invasion and immune system evasion, thereby favoring invasive disease. ^,

Epidemiology

Aeromonas spp. are ubiquitous and can be isolated from almost any environment. Most environmental isolates come from fresh or brackish water sources; contamination of chlorinated tap water also has been reported. ^{, ,} Aeromonas spp. colonize and can cause disease in cold-blooded animals, including fish, amphibians, and reptiles, which can act as reservoirs. ^,

This genus has also been isolated from food including fruits, vegetables, dairy products, meats and sausages, fish, and shellfish. Aeromonas spp. can survive under conditions of food preservation that inhibit the growth of other bacteria, such as low temperatures (2°C–10°C), high salt concentration (up to 4% NaCl) and low pH (up to 5). These properties make this agent an issue for ready-to-eat food management. ^,

The first report of human Aeromonas infection occurred in 1954 in a woman with myositis. In 1961, Aeromonas spp. were first isolated from human feces. Since then, Aeromonas spp. have been incriminated as agents of gastroenteritis, skin and soft tissue infections, sepsis and other invasive infections; the last two, mainly in patients with underlying disease or immune suppression. ^, Infection is most common during warmer months and is acquired through ingestion of contaminated water or food, or by swimming or bathing in contaminated water. ^, Skin and soft tissue infections caused by wild animal bites and leech therapy also have been described.

Based on molecular taxonomy of 1852 strains obtained from patients included in different studies around the world, 95.4% of the isolates correspond to four species, A. caviae (37,3%), A. veronii (23.5%), A. dhakensis (21.5%), and A. hydrophila (13.1%).

Disease burden is difficult to determine as a result of underreporting and lack of routine diagnosis. Specific diagnostic tests are required to differentiate pathogenic species from normal flora and other pathogenic gram-negative bacilli such as Vibrio and Plesiomonas spp. ^,

Although asymptomatic individuals from tropical areas with low sanitation may carry Aeromonas spp. in stools, this finding is infrequent in developed countries. ^, Acute gastroenteritis is the most frequent clinical manifestation, with a demonstrated causal role. Aeromonas spp. have been detected in 1%–32% of cases of endemic diarrhea in children, and in 2%–7% of cases of diarrhea in adults, more commonly from developing countries. ^, These pathogens have also been associated with approximately 2% of dysenteric episodes in developed countries and in 7%–22% of these cases in developing countries. Among travelers, 1%–9% of cases of diarrhea have been attributed to Aeromonas species, with a higher incidence in travelers to Southeast Asia. The second most prevalent manifestation is skin and soft tissue infection, that present preferentially in injured skin after exposure to water. These agents are the predominant pathogen isolated from infected wounds after natural disasters. Bacteremia and sepsis are infrequent, representing 0.1%–3.3% of cases in adults, but mortality may reach 30%. ^,

Clinical Manifestations