Aeromonas and Plesiomonas

Etiology

Aeromonas is a member of the Aeromonadaceae family and includes 2 major groups of isolates: the nonmotile psychrophilic organisms that infect cold-blooded animals, most often fish, and the motile mesophilic organisms that infect humans and other warm-blooded animals. Aeromonas species are oxidase- and catalase-positive, facultatively anaerobic, gram-negative bacilli that ferment glucose. Aeromonas is a diverse genus with difficult taxonomy and species differentiation because of high nucleotide variability and has undergone multiple reclassifications of species and taxa in recent years. Eleven species are recognized as clinically significant human pathogens, with Aeromonas hydrophila, Aeromonas veronii biotype sobria, and Aeromonas caviae most frequently associated with human infection . Aeromonas dhakensis , which was first isolated from children with diarrhea in Dhaka, Bangladesh and initially classified as a subspecies of A. hydrophila , has been recognized as a distinct species and an important cause of human infection.

Epidemiology

Aeromonas organisms are found in fresh and brackish aquatic sources, including rivers and streams, well water, both treated and bottled drinking water, and sewage. These organisms are most often detected in aquatic sources during warm-weather months, when they reach greater population densities. Rates of human infection may also exhibit seasonality depending on local conditions. For example, Aeromonas is isolated with increased frequency from May to October in the Northern hemisphere. Some species resist chlorination of water and exhibit tolerance to high salt concentrations. Aeromonas has been isolated from meats, milk, seafood, seaweed, and vegetables consumed by humans. Asymptomatic colonization occurs in humans and is more common in inhabitants of tropical regions. Most human infections with Aeromonas are associated with exposure to contaminated water but may also be contracted via other routes, including ingestion of contaminated food. A systematic review of cases of traveler's diarrhea worldwide implicated Aeromonas in 0.8–3.3% of infections, with highest frequencies in travelers to Southeast Asia and Africa. A study in Bangladesh of >56,000 stool samples from patients with diarrhea found that approximately 25% had a bacterial etiology detected, 13% of which were Aeromonas . Aeromonas infections have also been acquired at various sites of natural disasters. For example, following the 2004 Thailand tsunami, Aeromonas was the leading cause of skin and soft tissue infection among survivors.

Pathogenesis

Clinical and epidemiologic data seem to support that Aeromonas organisms are enteric pathogens, although this point is not universally accepted. Reasons for uncertainty include a lack of outbreaks with clonally distinct isolates, infrequent person-to-person transmission, absence of a robust animal model, and overlapping prevalence in symptomatic and asymptomatic individuals. In addition, there are conflicting data when comparing the human challenge model with characteristics of suspected outbreaks of Aeromonas enteritis, further complicating interpretation.

Aeromonas isolates possess a variety of potential virulence factors, including constitutive polar and inducible lateral flagella, fimbriae, outer membrane proteins, endotoxin (lipopolysaccharide), capsules, extracellular hydrolytic enzymes, enterotoxins, hemolysins, and multiple secretion systems. The mechanistic role of many of these factors in human pathogenicity remains unclear. Polar flagella provide motility in liquid media, and lateral flagella may act as adhesins. There are numerous hemolysins and heat-labile and heat-stable enterotoxins. Aeromonas cytotoxic enterotoxin (Act/aerolysin) is secreted by a type II secretion system and is able to lyse erythrocytes, inhibit phagocytosis, and induce cytotoxicity in eukaryotic cells. Aeromonas also has a type III secretion system with an effector protein that causes actin reorganization and eventual apoptosis in vitro. A type VI secretion system has been described and functions analogously to a phage tail, with antimicrobial activity.

Aeromonas sobria is the most enterotoxic among clinical isolates, and cytotoxic activity with cytopathic and intracellular effects is found in 89% of isolates. A few strains produce Shiga toxin. Some clinically important species have also been shown to harbor a cholera-like toxin ( Asao toxin ). Aeromonas has serine proteases that can cause a cascade of inflammatory mediators, leading to vascular leakage, and in vitro studies show induction of apoptosis in murine macrophages by human isolates of Aeromonas . There are limited data on quorum-sensing molecules, which coordinate gene expression according to local density and may be involved in biofilm production or population control.

Clinical Manifestations

Aeromonas may colonize humans asymptomatically or cause illness, including enteritis, focal invasive infections, and septicemia. Although apparently immunologically normal individuals may present with any of these manifestations, invasive disease is more common among immunocompromised persons.