112
Epizootiology
Apart from fi sh, the organism was recovered from water and the ‘scum’ forming at
the air-water interface on the tank walls (Baya et al. 1992b ).
Pathogenicity
The fi sh isolate, RB 968 BA, killed rainbow trout and striped bass, with LD 50 doses
calculated as 5.8 × 10
4 and 1.0 × 10
5 , respectively (Baya et al. 1992a ). Experimentally
infected fi sh developed haemorrhaging in the cranial cavity, but did not develop any
external signs of disease. ECP, which contained caseinase and gelatinase activity,
was harmful to fi sh, with an LD 50 dose equivalent to 1.2 μg of protein/g of fi sh.
Disease Control
Use of Antimicrobial Compounds The organism was sensitive to ampicillin,
erythromycin, oxytetracycline and potentiated sulphonamide, one or more of which
may be useful for chemotherapy (Baya et al. 1992b ).
Coryneform Bacteria
Occasional mention has been made of the role of coryneforms as fi sh pathogens.
Ajmal and Hobbs ( 1967 ) referred to Corynebacterium infections in rudd, salmon
and trout. However, there may have been confusion with BKD, the aetiological
agent of which used to be regarded as Corynebacterium but is now classifi ed as Ren.
salmoninarum. Nevertheless during a routine examination of apparently healthy
rainbow trout, Austin et al. ( 1985 ) recovered an organism with some of the salient
features of Ren. salmoninarum. Cultures were subsequently assigned to the coryneform group of bacteria.
Box 3.7: Coryneforms
Growth occurs on BHIA and plate count agar, weakly on CLED, but not on
MacConkey agar or TCBS. Characteristically, isolates are non-motile, nonacid- fast rods of 0.75 × 1.5–3.0 μm in size, which contain darkly stained intracellular granules. Growth occurs at 15 and 30 °C but not at 4 or 37 °C, and in
0 % and weakly in 2 % (w/v) sodium chloride but not at all in 4 % (w/v)
sodium chloride, Catalase is produced, but not arginine dihydrolase,
ß- galactosidase, H 2 S, indole, lysine or ornithine decarboxylase, oxidase, phenylalanine deaminase or phosphatase. The methyl red test and Voges Proskauer
reaction are negative. Nitrates are reduced to nitrites, weakly. Aesculin is
degraded, but not blood, DNA, gelatin, lecithin or urea. Sodium citrate is
utilised slowly. Acid is not produced from glucose.
3 Aerobic Gram-Positive Rods and Cocci
Epizootiology
Apart from fi sh, the organism was recovered from water and the ‘scum’ forming at
the air-water interface on the tank walls (Baya et al. 1992b ).
Pathogenicity
The fi sh isolate, RB 968 BA, killed rainbow trout and striped bass, with LD 50 doses
calculated as 5.8 × 10
4 and 1.0 × 10
5 , respectively (Baya et al. 1992a ). Experimentally
infected fi sh developed haemorrhaging in the cranial cavity, but did not develop any
external signs of disease. ECP, which contained caseinase and gelatinase activity,
was harmful to fi sh, with an LD 50 dose equivalent to 1.2 μg of protein/g of fi sh.
Disease Control
Use of Antimicrobial Compounds The organism was sensitive to ampicillin,
erythromycin, oxytetracycline and potentiated sulphonamide, one or more of which
may be useful for chemotherapy (Baya et al. 1992b ).
Coryneform Bacteria
Occasional mention has been made of the role of coryneforms as fi sh pathogens.
Ajmal and Hobbs ( 1967 ) referred to Corynebacterium infections in rudd, salmon
and trout. However, there may have been confusion with BKD, the aetiological
agent of which used to be regarded as Corynebacterium but is now classifi ed as Ren.
salmoninarum. Nevertheless during a routine examination of apparently healthy
rainbow trout, Austin et al. ( 1985 ) recovered an organism with some of the salient
features of Ren. salmoninarum. Cultures were subsequently assigned to the coryneform group of bacteria.
Box 3.7: Coryneforms
Growth occurs on BHIA and plate count agar, weakly on CLED, but not on
MacConkey agar or TCBS. Characteristically, isolates are non-motile, nonacid- fast rods of 0.75 × 1.5–3.0 μm in size, which contain darkly stained intracellular granules. Growth occurs at 15 and 30 °C but not at 4 or 37 °C, and in
0 % and weakly in 2 % (w/v) sodium chloride but not at all in 4 % (w/v)
sodium chloride, Catalase is produced, but not arginine dihydrolase,
ß- galactosidase, H 2 S, indole, lysine or ornithine decarboxylase, oxidase, phenylalanine deaminase or phosphatase. The methyl red test and Voges Proskauer
reaction are negative. Nitrates are reduced to nitrites, weakly. Aesculin is
degraded, but not blood, DNA, gelatin, lecithin or urea. Sodium citrate is
utilised slowly. Acid is not produced from glucose.
3 Aerobic Gram-Positive Rods and Cocci