Aeromonas, Plesiomonas and Vibrio species

Chapter 19


Aeromonas, Plesiomonas and Vibrio species




Genus Characteristics


The genus Aeromonas is a member of the family Aeromonadaceae, in the class Proteobacteria. The genus has undergone a number of nomenclatural revisions in recent years and there are now 30 recognized species in the genus Aeromonas. Both A. hydrophila and A. salmonicida, two of the main pathogens of veterinary interest in the genus, have five subspecies. Plesiomonas shigelloides is the only member of the genus. There are numerous members of the genus Vibrio, which is in the family Vibronaaceae, including many recently described marine vibrios. Members of the Aeromonas, Plesiomonas and Vibrio genera are Gram-negative rods (0.5–0.8 × 3.0–4.0 µm) which are either straight or curved. They are facultative anaerobes, catalase-positive and most are motile by polar flagella. Aeromonas salmonicida and other psychrophilic aeromonads are non-motile. All ferment glucose with acid production and a few Aeromonas spp. also produce gas. Many of the Vibrio spp. require sodium chloride for growth. Plesiomonas lacks exoenzymes whereas Vibrio and Aeromonas species produce diastase, lipase, DNase and various proteinases. Most of the species in the three genera will grow on common laboratory media at 35–37°C, although many of the saprophytic Vibrio and Aeromonas species have an optimum temperature for growth lower than 35°C.



Natural Habitat


Many of the species in the three genera are free-living saprophytes although some are associated with reptiles, fish and animals.


Aeromonas spp. are widespread in freshwater, sewage and soil. Their numbers rise with the amount of organic matter present. Aeromonas hydrophila is part of the normal flora of freshwater fish and is commonly present in fish ponds and tanks. Animals can be faecal carriers of Aeromonas spp.


Plesiomonas shigelloides is present in freshwater but its distribution is limited by its 8°C minimum temperature for growth and lack of halophilism. It has been isolated from a wide host range that includes freshwater fish, shellfish, oysters, toads, snakes, monkeys, dogs, cats, goats, pigs, cattle and poultry.


Vibrio spp. can be present in both fresh- and seawater as well as in the alimentary tracts of animals and man.



Pathogenesis and Pathogenicity


Aeromonas hydrophila is an opportunistic pathogen causing disease in fish and reptiles with rare reports of infections in mammals. The organism is important in fish suffering from stress or weakened by the presence of other diseases. As it is ubiquitous in aquatic environments, its importance as a secondary invader is not surprising when husbandry conditions in aquaculture are suboptimal. Aeromonas hydrophila produces a range of putative virulence factors including adhesins, exoenzymes, haemolysins, enterotoxins and an acyltransferase toxin secreted by a type III secretion system (Yu et al. 2004, Li et al. 2011). Aeromonas hydrophila can occasionally cause infections in humans that range from wound infections to septicaemia to self-limiting diarrhoea in children, as well as food poisoning. There are apparent differences between environmental and human clinical isolates but environmental isolates possess a wide range of virulence factors also and further studies are required to clarify their roles (Aguilera-Arreola et al. 2005). Aeromonas salmonicida is an obligate parasite of salmonid fish, causing furunculosis. As with A. hydrophila infections, acute or chronic stress is important and may trigger inapparent infections to become clinical. Virulence factors of A. salmonicida include its outer protein coat, the S-layer, which functions as an adhesin, Type I and Type IV pili and a Type III secretion system (Dacanay et al. 2006, 2010).


Plesiomonas shigelloides has been reported as a cause of gastroenteritis in man with cases occurring mainly in tropical and subtropical regions. Virulent strains have been found to produce heat-stable and heat-labile enterotoxins. Its role in animal disease is uncertain but it can be isolated from diagnostic specimens.


At least five Vibrio species are human pathogens including V. cholerae, the cholera bacillus, and V. parahaemolyticus which causes food poisoning. Only V. metschnikovii is associated with disease in domestic animals. It causes a cholera-like disease in chickens and other birds but its geographical distribution is very limited. There are a large number of marine vibrios, including V. anguillarum (Listonella anguillarum) which cause infections in many species of fish. The pathogenicity of many of the newly identified species of marine vibrios was reported by Austin et al. (2005).


A summary of the diseases of veterinary importance caused by Aeromonas, Plesiomonas and Vibrio species is given in Table 19.1. Some of the principal virulence factors which have been identified in these pathogens are listed in Table 19.2.





Laboratory Diagnosis







Identification



Colonial morphology



• Aeromonas hydrophila: colonies are large (2–3 mm), flat, greyish and surrounded by a large zone of beta-haemolysis (Fig. 19.1). As some other species of Aeromonas are haemolytic, particularly strains of A caviae, haemolytic activity may not be as useful as previously thought as a means of differentiating A. hydrophila (Abbott et al. 2003). Newly isolated strains have a pungent, foul odour. It grows well on MacConkey agar (Fig. 19.2) often with pale colonies (non-lactose-fermenting), but a minority of strains yield lactose-fermenting colonies. In addition, most Aeromonas strains produce tan to buff-coloured colonies on Trypticase soy agar (Abbott et al. 2003).


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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Aeromonas, Plesiomonas and Vibrio species

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