introduction

Bacteria belonging to Aeromonaceae, Shewanellaceae and Vibrionaceae and are gram-negative curved to straight rods, motile (some exceptions), fermentative and oxidase positive. Bacteria belonging to these genera are non-fastidious and grow rapidly in a wide range of artificial nutrient media. They are usually resistant to amoxicillin. Like the Enterobacterales, they belong to the Gammaproteobacteria. The natural habitat of bacteria in these bacteria is aquatic, either in freshwater (most Aeromonas species) or in marine environments (most Vibrio species). Many are important aquatic vertebrate and invertebrate pathogens.

Where they fit in phylogenetically is shown in the Table below.

Important bacterial diseases of aquatic animals in Australia

Bacterial diseases of many aquatic animals tend to be associated with the natural aquatic habitat of the bacteria. Halophilic bacteria are found in marine environments and those that don’t have a salt tolerance are found in freshwater. Some bacteria thrive in the warm tropical waters (greater than 24°C)  of North Queensland and others will thrive in the cold waters that are found surrounding Tasmania. Fishes, amphibians, reptiles and crustaceans in tropical waters  suffer from bacterial disease caused by Aeromonas, Vibrio, Flavobacterium columnare and Streptococcus iniae. Those in cold water environments suffer from diseases caused by Photobacterium, Aeromonas salmonicida (cold water vibriosis of fish – not taught), Tenacibaculum maritimus and Renibacterium species

Aeromonas species are associated with the intestinal tract and skin of animals living in freshwater. Its numbers go up when there is a lot of organic material present in water. Most Vibrio species are found in marine animals and their environs. However, there are species and even strains of Aeromonas that will grow in seawater and Vibrio that are fresh water bacteria i.e. Vibrio cholerae, the agent of cholera in humans is found in sewage contaminated freshwater.

Aquaculture is a growing agricultural industry that has the manage infectious agents that have the potential to cause mass mortalities. This is due to the high animal animals population densities of farming and that the animals are immersed in a watery environment  can be teeming with potential pathogens.

Some of the important diseases are listed below.

Bacteria plaguing aquaculture are opportunistic pathogens entering the body via injured skin or gills. Cannibalism will also allow some agents to transmit. Thus, most diseases will have lesions in the skin and/or gills. Skin injuries are caused by rough handling, parasitic infestations and viral infections. Poor water quality such as excessive water ammonium levels can also damage the slime layer of fish and sensitive skins of frogs. Overcrowding and excessive organic matter allow excessive bacterial loads that can overwhelm the immune systems of stressed animals. Higher water temperatures will also encourage bacterial overgrowth in water.

​​​​​​​Young animals with immature immune systems are more susceptible to bacterial disease.

Both Aeromonas and Vibrio species form biofilms and have developed a means to communicate with each other known as quorum sensing. In some species, such as V. fischeri and V. harveyi this is observed as bioluminescence.​​​​​​​

​​​​​​​(The Enterobacteriaceae – Edwardsiella ictaluri causing enteric septicaemia of catfish and Yersinia ruckeri causing enteric red mouth disease are exotic to Australia and notifiable. They will not be taught.)

specific diseases

Vibrionaceae including Vibrio and Photobacterium  (excluding V. cholerae, a human pathogen)

Vibrio species are water-associated, gram-negative, comma-shaped, facultative anaerobes which are oxidase-positive, motile and fermentative. Large coccoid forms are also observed. They are divided into freshwater and marine species, with most being in the marine environment. They are common commensals in the intestinal tracts and body surfaces of all marine animals including fish and invertebrates. It is, therefore, not surprising that this genus is responsible for opportunistic infections of marine animals. These bacteria have a preference for warm waters with the highest prevalence of disease in tropical environments.

Bioluminescent V. harveyi is a pathogen of several marine animals, and is notable as a cause of luminous vibriosis in shrimps and prawns. A related bioluminescent species is V. owensii. Other species of Vibrio have also caused sepsis in larvae and juvenile farmed prawns. Typically these bacteria either infect locally causing infections of the cuticle, eyes or legs, or via the gastrointestinal tract where the hepatopancreas is affected. They produce two major exotoxins, a haemolysin and a protease. Other Vibrio species have also been implicated in vibriosis of crustacea where they are named after the pathological appearance of the diseases i.e. white gut disease, tail necrosis, red disease and loose shell syndrome. Vibrio spp. infections can result in sepsis.

Bacterial disease in fish has a similar clinical appearance with the fish not eating and becoming lethargic. The skin form develops with discoloured skin and red blotches which ulcerate to exude a reddish pus. This skin is reddish around the mouth and fins. The eyes may appear swollen = exopthalmia or “pop-eye”. Some fish may die without any evidence of illness.

Photobacterium damselae subsp. damselae unlike Vibrio species is non-motile. It causes skin ulcerations and septicaemia in cold water marine fish, molluscs, crustaceans, reptiles and sea mammals. Pathogenic strains have a large virulence plasmid that encodes for a phospholipase D toxin, damsenolysin, and a porin, phobalysin. Bacteria with these virulence plasmids show a wide zone of beta-haemolysis when grown on blood agar. Rising water temperatures (>24°C) increase the virulence of these bacteria.

Note on Public health (No Need to learn)

In humans mainly V. parahaemolyticus and occasionally Vibrio vulnificus causes a food-borne gastroenteritis known as seafood poisoning. Vibrio species can infect wounds resulting in ulcers. Septicaemia can result in individuals with a comorbidity such as liver disease. Up to 70% of persons with septicaemia also have bullous skin lesions and 50% of untreated person die.    Aeromonas hydrophila and Streptococcus iniae can cause similar lesions in people. Therefore, any wound sustained in a marine or freshwater environments should be thoroughly cleaned, disinfected and treated as soon as possible.

Plesiomonas shigelloides an unclassified gram-negative member of the Enterobacteriaceae and a commensal in aquatic animals, is a cause of bacterial diarrhoea in people who eat contaminated seafood.  Since it is oxidase positive, it has to be distinguished from Aeromonas and non-halophilic (salt-loving) Vibrio species.

Aeromonas species

This rapidly growing and non-fastidious genus produces beta-haemolysis on blood agar and will on prolonged incubation produce a brown pigment. It is the most common genus isolated from freshwater and even from seawater. Most often isolated from samples is Aeromonas hydrophila. It and other mesophilic Aeromonas species can be found in the gastrointestinal tract of water inhabiting animals and are the occasional cause of opportunistic infections i.e. skin ulcers and haemorrhagic septicaemia in fish, gastroenteritis and sepsis in all animals, red leg in frogs and ulcerative stomatitis in snakes. A cold water species known as A. salmonicida is the cause of furunculosis, a skin disease of cold water fish. It is one of the most important diseases of farmed salmon.

Septicaemic disease in fish has a similar appearance irrespective of the cause. Other than Vibrio and Aeromonas species, Streptococcus iniae and Streptococcus agalactiae can cause outbreaks of fatalities in fish. Fish with clinical signs of septicaemia may show breathing difficulties – open mouth and increased opening and closing of the gills; erratic swimming, spinning, spiraling and flashing. Pin-point haemorrhages are found in the skin, gills and body organs. The fish appear bloated (ascites). Often the fish also exhibit skin ulceration and may show exopthalmia (pop eye).

Flavobacterium columnare infections

Tenacibaculum maritimum infection

Diagnosis of bacterial diseases

• Cytology. Fresh lesions can be scraped and wet mounted to check for parasites. Scraping of lesions with Flavobacterium columnare show typical stacking appearance and will distinguish from fungal infections. Acid-fast staining for Mycobacterium species.
• Histopathology. Fixed in 10% buffered formalin or
• Culture and Antimicrobial susceptibility testing. Samples should be collected as fresh as possible as aquatic animals decompose very rapidly and many opportunistic pathogens such as Vibrio and Aeromonas will rapidly invade a dead animal assisting in putrefaction. Submit samples to laboratories that can cater for aquatic animals as they have to use cooler culture conditions and specialised growth media.
• Molecular diagnosis. Species-specific qPCR for important viral pathogens and to diagnose some cultured bacteria to species level i.e. Vibrio species

Treatment and Control of bacterial infections

• Make sure water quality is optimum. Replace ¼ of the tanks’ water especially from the bottom of the tank – this will reduce bacterial load.
• Increase the salinity – will cause unfavourable conditions for some bacteria. For marine microorganisms decrease the salinity.
• Decrease the water temperature by aerating or shade over the tank/pond.
• Antibiotics such as tetracyclines and potentiated sulphonamides are used in outbreaks, however, risk of environmental contamination & antibiotic resistance. Can result in destruction of biofilter and probiotic bacteria.
• Bacteriophage therapy. This is more environmentally friendly than antibiotic therapy.
• Bacteriocin therapy – breeding resistant fish that produce bacteriocins
• Destocking, cleaning tank etc and restock with disease-free animals – ideal if economically feasible
• Vaccination i.e. Streptococcus iniae

END OF CHAPTER