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A genus of Gram-negative bacteria of the Vibrionaceae. Cells variable: straight, round-ended rods or coccobacilli (0.3-1.0 x 1.0-3.5 µm) - occurring singly, in pairs or in short chains - or short filaments. The genus is divided into two groups: non-motile psychrotrophs (A. salmonicida ) and motile mesophiles iA. hydrophila group). Motile cells usually have a single, polar, unsheathed flagellum (wavelength ca. 1.7 µm). Aeromonas spp are oxidase +ve. The taxonomy of the motile strains is disputed, though three species are currently recognised:A. hydrophila, A. caviae and A. sobria. Motile strains can grow on many media. Optimal growth temperatures ca 280C,with the maximum usually 38-410C, though some strains can grow at 50C. Species occur in fresh water, sewage, and in association with aquatic animals; certain strains can cause disease in fish, for example red pest and red mouth. A. salmonicida is haemolytic on blood agar. This species is strictly parasitic and often pathogenic in fish, causing for example furunculosis and secondary infections associated with other diseases.