Most bacterial infections are caused by Gram-negative organisms including the genera Aeromonas, Citrobacter, Edwardsiella, Flavobacterium, Pseudomonas, and Vibrio.  Aeromonas is more commonly a pathogen in freshwater fish, whereas Vibrio usually affects marine fish.   Streptococcus is a gram-positive genus that causes disease in ornamental fish. 

Many of these bacterial pathogens are ubiquitous in the aquatic environment.  Stress can compromise the fish's immune system, and cause the fish to become more susceptible to infection with the bacteria.  Causes of stress include poor water quality, overcrowding, excessive handling and shipping, poor nutrition, and other pathogens.

Clinical signs of systemic bacterial disease include lethargy, anorexia, abnormal swimming patterns/spinning, hemorrhagic lesions on the skin, abdominal distension, exophthalmia ("pop-eye"), and external ulcerative lesions.  Diagnosis is based on clinical signs and bacterial culture.  The organ of choice for bacterial culture is the posterior kidney.  Other organs that are cultured include the brain, liver, spleen and anterior kidney.  Treatment is with antibiotics, supportive care and removal of underlying stressors.  Ideally, sensitivity data is collected in order to determine which antibiotics the will work to treat the infection. 

Methods of antibiotic administration include bath (in the water), antibiotics in the food, and injections.  Bath antibiotics are usually limited to cases of external infections (such as Columnaris disease and "fin rot") and in fish that are anorexic. 

Contact a fish health specialist if you are concerned about bacterial disease in your fish.

Click on the below links for information on other bacterial diseases:

Mycobacteriosis

Columnaris Disease

Koi Ulcer Disease