New bacterial fish pathogens and their implications for fish farming

A diverse array of bacterial taxa has been implicated as pathogens of farmed and wild fish. Recent trends indicate the emergence of many Gram-negative bacterial pathogens, particularly those classified in the families Aeromonadaceae and Vibrionaceae (namely Aeromonas salmonicida subsp. smithia, A. sobria, Shewanella putrefaciens, Vibriofischeri, V. harveyi, V. parahaemolyticus, V. salmonicida and V. splendidus), Enterobacteriaceae (i.e. Citrobacter freundii, Salmonella arizonae, Serratia liquefaciens and Ser. plymuthica) and Pseudomonadaceae (Pseudomonas putida), the yellow/orange-pigmented rods, which are difficult to identify, i.e. Cytophaga-like bacteria, Flavobacterium branchiophilum and Flexibacter maritimus, and miscellaneous taxa including Alcaligenes cupidus and groups loosely associated with Brucella and Moraxella. Comparatively fewer Gram-positive taxa (Lactococcus piscium, Nocardia seriolae, Planococcus sp. and Vagococcus salmoninarum) have been implicated as new fish pathogens. In addition, a role for cell wall defective/deficient bacteria, encompassing Mycoplasma mobile and L-forms, is becoming increasingly apparent. Of the emerging pathogens, Cit. freundii, Cytophagalike bacteria, Fla. branchiophilum, Fle. maritimus, Ser. liquefaciens, Ser. plymuthica, She. putrefaciens, V. parahaemolyticus, V. salmonicida and V. splendidus are likely to be troublesome for aquaculture.