Aeromonas and Plesiomonas

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Chapter 196 Aeromonas and Plesiomonas

Aeromonas and Plesiomonas are pathogenic gram-negative bacilli that commonly cause enteritis and less frequently cause skin and soft tissue infections and septicemia. They are common in fresh and brackish water and colonize animals and plants in these niches.

196.1 Aeromonas


Clinical and epidemiologic data support that Aeromonas organisms are enteric pathogens, although adult volunteers can ingest 104-1010 colony-forming units without developing diarrhea or becoming colonized. Aeromonas isolates possess a variety of potential virulence factors, including constitutive polar and inducible lateral flagella, fimbriae, outer membrane proteins, an S-layer, endotoxin (lipopolysaccharide), capsules, collagenase, elastase, nuclease, gelatinase, lipase, chitinase, enterotoxins, hemolysins, and multiple secretion systems. Polar flagella provide motility in liquid media, and lateral flagella act as adhesins. There are various hemolysins and heat labile and heat stable enterotoxins. Aeromonas cytotoxic enterotoxin (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 was identified in a clinical isolate. A. sombria 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. 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. Aeromonas also has enzyme systems and efflux pumps that enable it to develop resistance to antibiotics.

Human serum generally promotes phagocytosis and intracellular killing of Aeromonas. Absence of this serum action has been associated with a poor prognosis.

Clinical Manifestations

Colonization with Aeromonas may be asymptomatic or cause illness, including enteritis, focal invasive infection, and septicemia. Apparently immunologically normal individuals may present with each manifestation, but invasive disease is more common among immunocompromised persons.