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Chapter 196 Aeromonas and Plesiomonas
Guenet H. Degaffe, Gloria P. Heresi, James R. Murphy
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
Aeromonas is a member of the family Aeromonadaceae. These organisms are oxidase-positive, facultatively anaerobic, gram-negative bacilli that ferment glucose. At least 17 phenotypic species are known, including 8 that are recognized as human pathogens. A. hydrophila, A. veronii biotype sobria, and A. caviae are the species most often associated with human infection. A. trota is being isolated with increasing frequency from human stool.
Aeromonas infects many cold- and warm-blooded animals. There are 2 major groups of Aeromonas isolates: the nonmotile psychrophilic organisms that infect cold-blooded animals, including fish, and the motile mesophilic organisms that infect humans and other warm-blooded animals.
Aeromonas organisms are ubiquitous and are found in fresh and brackish aquatic sources, including rivers and streams, well water, and sewage. They are most often cultivated from aquatic sources during warm weather months when they can attain large populations. The prevalence of human infection may or may not exhibit seasonality, depending on local conditions. Some species can resist chlorination of water and show tolerance to high salt. Aeromonas has been isolated from meats, milk, seafood, seaweed, and vegetables consumed by humans. Most human infections with Aeromonas are associated with exposure to contaminated water. Asymptomatic colonization occurs in humans and is more common in inhabitants of tropical regions. 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. Aeromonas infections have been acquired at sites of natural disasters. A. hydrophila has been isolated from ticks and cockroaches. Prophylaxis against A. hydrophila should be used in conjunction with therapy with medicinal leeches, which carry symbiotic A. hydrophila.
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.
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.
The most common clinical manifestation of infection with Aeromonas is enteritis, which occurs primarily among children <3 yr of age. Aeromonas is the 3rd or 4th most common cause of childhood bacterial diarrhea and has been isolated from 2-10% of patients with diarrhea and 1-5% of asymptomatic control subjects. One study showed isolation from hospitalized neonates with diarrhea at rates of 0-19% depending on season. Diarrhea is often watery and self-limited, although a dysentery-like syndrome with blood and mucus in the stool has also been described. Fever, abdominal pain, and vomiting are common in children. Enteritis caused by A. hydrophila and A. sobria tends to be acute and self-limited, whereas 30% of the patients with A. caviae enteritis have chronic or intermittent diarrhea that may last 4-6 wk. A. sobria and A. caviae are most frequently associated with traveler’s diarrhea. Complications of Aeromonas enteritis include intussusception, failure to thrive, hemolytic-uremic syndrome, bacteremia, and strangulated intestinal hernia. A. caviae infection may mimic inflammatory bowel disease.
A. hydrophila is the predominant species associated with skin and soft tissue infections, with peak incidence during the summer months. Skin and soft tissue infection is the 2nd most common presentation of Aeromonas. Predisposing factors include local trauma and exposure to contaminated fresh water. Aeromonas
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