The genus Aeromonas comprises Gram-negative, oxidase-positive, facultatively anaerobic rods. These are insensitive to the vibriostat O/129, have no requirement for Na+ and further differ from vibrios in producing gas during sugar fermentation (most strains). There is considerable phenotypic variation among Aeromonas. Gas production is variable and may be temperature-dependent, while there can also be marked differences in cellular morphology. Two distinct groups of Aeromonas exist, the psychrophilic group represented by A. salmonicida, a fish pathogen, rarely isolated in a free-living state and the mesophilic (more correctly psychrotrophic), genera, which are usually motile. The genus has undergone tax-onomic reassessment in recent years and now comprises 10 phenotypic species, of which 8 have been associated with human enteric disease (Table 14.4).
The taxonomy of the genus has been further refined by DNA-DNA hybridisation techniques and comprises at least 14 genospecies (DNA hybridisation groups: HGs). Some species of Aeromonas are pathogens of fish and reptiles, while others are associated with human disease (Altwegg et al., 1991; Deodhar et al., 1991). Although epidemiological links are relatively easily established, these have not been confirmed with feeding trials and direct causal links exist in only a few cases. Despite a probable involvement in foodborne disease, the status of Aeromonas remains as a putative enteropathogen.
Plesiomonas, which comprises a single species P. shigelloides, is a member of the family Vibrionaceae and shares characteristics with both Vibrio and Aeromonas, although it contains the Enterobacteriaceae common antigen. Like Aeromonas, it has been regarded as a putative enteropathogen, although a direct causal relationship has been established following outbreaks of illness associated with water and oysters (Wadstrom and Ljungh, 1991).
The putative nature of Aeromonas as a pathogen means that data concerning incidence are not readily available. In the UK, 261 isolations were made from faecal samples during 2000, of which 144 were identified with A. hydrophila and 75 with unidentified species. In the same year 31 isolations of Plesiomonas from faeces were made.
14.18.2 Aeromonas and Plesiomonas as human pathogens
Aeromonas has been associated with both gastroenteritis and extraintestinal disease, usually in immunocompromised hosts. Traditionally, Aeromonas hy-drophila has been most commonly involved, although descriptions of outbreaks have not always distinguished between this species, A. caviae, A. sobria and A. veronii. Most human pathogenic strains are now recognised as grouping in three
Table 14.4 Phenotypic species of Aeromonas
A. caviae A. eucrenophilaa
A. hydrophila A. jandei
A. media A. salmonicidaa
A. schubertii A. sobria
genomic species, A. hydrophila HG 1, A. caviae HG 4 and A. veronii biovar sobria HG 8. As the role as an enteropathogen is not definitely established, descriptions of symptoms must be treated with care. Strains vary in pathogenicity, Kirov et al. (1994) postulating that this results from differences in the degree to which virulence factors can be expressed. Gastroenteritis associated with Aeromonas varies in severity from mild diarrhoea to a life-threatening, cholera-like illness (ICMSF, 1996). Two syndromes have been described. The first, which accounts for ca 75% of cases is characterised by watery diarrhoea with mild, or absent, fever, possibly accompanied by abdominal pain and nausea. The second form is dysenterylike and characterised by bloody, mucoid stools. The incubation period is not known. Mild infections are self-limiting and recovery is complete within 1 to 7 days. In severe cases, symptoms are prolonged and may persist for a month or more, often requiring antibiotic therapy.
Aeromonas is associated with a range of extraintestinal disease, of which septicaemia and meningitis are most common. Other symptoms include endocarditis, eye infections and osteomyelitis. Portal of entry may be the intestinal tract and mortality can exceed 60%.
Aeromonas is able to colonise the intestine and invade epithelial cells. Significant systemic spread, however, usually occurs only in the immunocom-promised. The bacterium produces a number of putative toxins, although their relationship with illness in man is not fully resolved. Both a cytotonic enterotoxin, with genetic relatedness to cholera toxin, and a cytotoxic enterotoxin, with b-haemolysin activity, have been described (Varnam and Evans, 1996). The relative importance of the two enterotoxins is disputed, but it has been suggested that the toxins of Aeromonas act by facilitating colonisation of the intestine and/or invasion rather than directly inducing enteritis (Todd et al., 1989).
Symptoms of Pl. shigelloides infection appear within 24 to 48 hours and are predominantly diarrhoeal (94%). Diarrhoea is accompanied by abdominal pain (74%), nausea (72%), chills (49%), fever (37%), headache (34%) and vomiting (33%). Recovery is usually within 1 to 9 days, but may be prolonged. Three types of diarrhoeal symptoms have been described, secretory, shigella-like and choleralike. Secretory diarrhoea is most common and can vary greatly in severity. The shigella-like type is also potentially severe, with symptoms of prolonged duration, while the cholera-type illness is very rare.
Plesiomonas shigelloides is also a relatively uncommon cause of extraintestinal infection, usually where predisposing conditions are present. The intestine is the portal of entry in at least some cases and there may be prodromal enteritis. Meningitis is most common and has a mortality rate of ca 80%.
Although Pl. shigelloides has a number of putative virulence factors, including ability to adhere to epithelial cells, invasive ability (possibly only in certain strains) and toxin production, direct relationships with pathogenicity have been difficult to demonstrate. Two distinct enterotoxins are produced and there is also evidence for endotoxin, protease, elastase and haemolysin production.
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