368
ern areas of the USA than any other disease (Hester 1973 ). Onset of an epizootic is
often gradual with resulting heavy losses (Newman and Majnarich 1982 ). It is relevant to note that a second disease, known as salmonid blood spot, in Australia
(Llewellyn 1980 ) is attributable to the same aetiological agent (Green and Austin
1982 ).
Isolation
The pathogen may be readily recovered from kidney on routine bacteriological
media, e.g. BHIA or TSA, following incubation at 20–25 °C for 48 h whereupon
round, raised, entire, shiny, off-white colonies of 2–3 mm diameter develop (e.g.
Ross et al. 1966 ). Three selective media have been devised (Appendix in Chap. 12 ;
Waltman and Shotts 1984 ; Rodgers 1992 ; Furones et al. 1993 ). Tween 80 hydrolysis, which occurred commonly among isolates from the USA, resulted in the precipitation of insoluble calcium salts around colonies on Waltman-Shotts medium.
However, according to Hastings and Bruno ( 1985 ), there were some limitations
insofar as there is some growth variation among bona fi de strains of Y. ruckeri.
Moreover, non- motile isolates, which did not degrade Tween 80 have been recovered in Europe, e.g. Germany (Klein et al. 1994 ). Ribose ornithine deoxycholate
medium of Rodgers ( 1992 ) appears to have overcome the problems of the earlier
formulation, and laboratory use points to its value for the recovery of Y. ruckeri.
Furones et al. ( 1993 ) supplemented TSA with 1 % (w/v) SDS, 100 μg/ml of coomassie brilliant blue and 100 μg/ml of Congo red. On this medium, Y. ruckeri produced a creamy deposit around the colonies.
Characteristics of the Pathogen
Box 6.18: Yersinia ruckeri
The organisms comprise a homogeneous group of fermentative Gramnegative, slightly curved rods of 1.0 × 2.0–3.0 μm in size, which are motile
usually by means of 7 or 8 peritrichously arranged fl agella, or nonmotile.
Catalase, ß-galactosidase, and lysine and ornithine decarboxylase are produced, but not H 2 S, indole, oxidase, phenylalanine deaminase or phosphatase.
The methyl red test is positive, but not the Voges Proskauer reaction. Nitrates
are reduced. Gelatin, and Tween 20, 40 and 60 are degraded, but not aesculin,
chitin, DNA, elastin, pectin, tributyrin, Tween 80 or urea. Growth occurs in
0–3 % (w/v) sodium chloride. Sodium citrate is utilised. Acid is produced
from fructose, glucose, maltose, mannitol and trehalose, but not from inositol,
lactose, raffi nose, salicin, sorbitol or sucrose. The G + C content of the DNA
is 47.5–48.5 moles % (Ewing et al. 1978 ).
6 Enterobacteriaceae Representatives
ern areas of the USA than any other disease (Hester 1973 ). Onset of an epizootic is
often gradual with resulting heavy losses (Newman and Majnarich 1982 ). It is relevant to note that a second disease, known as salmonid blood spot, in Australia
(Llewellyn 1980 ) is attributable to the same aetiological agent (Green and Austin
1982 ).
Isolation
The pathogen may be readily recovered from kidney on routine bacteriological
media, e.g. BHIA or TSA, following incubation at 20–25 °C for 48 h whereupon
round, raised, entire, shiny, off-white colonies of 2–3 mm diameter develop (e.g.
Ross et al. 1966 ). Three selective media have been devised (Appendix in Chap. 12 ;
Waltman and Shotts 1984 ; Rodgers 1992 ; Furones et al. 1993 ). Tween 80 hydrolysis, which occurred commonly among isolates from the USA, resulted in the precipitation of insoluble calcium salts around colonies on Waltman-Shotts medium.
However, according to Hastings and Bruno ( 1985 ), there were some limitations
insofar as there is some growth variation among bona fi de strains of Y. ruckeri.
Moreover, non- motile isolates, which did not degrade Tween 80 have been recovered in Europe, e.g. Germany (Klein et al. 1994 ). Ribose ornithine deoxycholate
medium of Rodgers ( 1992 ) appears to have overcome the problems of the earlier
formulation, and laboratory use points to its value for the recovery of Y. ruckeri.
Furones et al. ( 1993 ) supplemented TSA with 1 % (w/v) SDS, 100 μg/ml of coomassie brilliant blue and 100 μg/ml of Congo red. On this medium, Y. ruckeri produced a creamy deposit around the colonies.
Characteristics of the Pathogen
Box 6.18: Yersinia ruckeri
The organisms comprise a homogeneous group of fermentative Gramnegative, slightly curved rods of 1.0 × 2.0–3.0 μm in size, which are motile
usually by means of 7 or 8 peritrichously arranged fl agella, or nonmotile.
Catalase, ß-galactosidase, and lysine and ornithine decarboxylase are produced, but not H 2 S, indole, oxidase, phenylalanine deaminase or phosphatase.
The methyl red test is positive, but not the Voges Proskauer reaction. Nitrates
are reduced. Gelatin, and Tween 20, 40 and 60 are degraded, but not aesculin,
chitin, DNA, elastin, pectin, tributyrin, Tween 80 or urea. Growth occurs in
0–3 % (w/v) sodium chloride. Sodium citrate is utilised. Acid is produced
from fructose, glucose, maltose, mannitol and trehalose, but not from inositol,
lactose, raffi nose, salicin, sorbitol or sucrose. The G + C content of the DNA
is 47.5–48.5 moles % (Ewing et al. 1978 ).
6 Enterobacteriaceae Representatives