42
Isolation
LG agar was developed as a selective medium for Lactococcus garvieae that differentiated the virulent capsulated from the non-virulent uncapsulated cultures. The
medium exploited the ability of the pathogen to grow in high concentrations of bile
salts and potassium tellurite, and to reduce tetrazolium dye to coloured formazans.
Thus, noncapsulated cultured produced white colonies without red halos whereas
capsulated Lactococcus garvieae cultures comprised black colonies with red halos
(Chang et al. 2014 ).
Characteristics of the Pathogen
The fi rst attempt at clarifying the taxonomic status of the causal agents of streptococcicosis/streptococcosis was the landmark publication of Kusuda et al. ( 1991 ),
who described a new species, i.e. Enterococcus seriolicida, to accommodate 12
isolates recovered from eels and yellowtail in Japan.
Interestingly in the original publication describing En. seriolicida , only low
DNA homology values were obtained with reference species of Enterococcus.
Indeed, the greatest DNA homology, i.e. 24 %, was with En. hirae (Kusuda et al.
1991 ). Perhaps, it was inevitable that the association of these fi sh pathogens with
Enterococcus would be challenged. Thus, it was determined that En. seriolicida was
really identical (77 % DNA:DNA homology) with a previously described lactococcus, namely Lactococcus garvieae (Tiexeira et al. 1996 ). This view has been reinforced by others. For example, Pot et al. ( 1996 ) detailed research by SDS-PAGE of
Box 2.9: Lactococcus garvieae
Cultures comprise non-motile facultatively anaerobic Gram-positive cocci in
short chains, which do not produce catalase, H 2 S, indole or oxidase. Blood is
degraded (α-haemolysis). Aesculin and arginine are hydrolysed, but not so
casein, gelatin or sodium hippurate. Acid is produced from a wide range of
carbohydrates, namely aesculin, cellobiose, D-fructose, galactose, D-glucose,
maltose, mannitol, D-mannose, salicin, sorbitol and trehalose, but not from
adonitol, D-arabinose, glycerol, glycogen, inositol, lactose, melezitose, melibiose, raffi nose, L-rhamnose, starch, sucrose or D-xylose. The methyl red and
tetrazolium reduction tests and the Voges Proskauer reaction are positive, but
not nitrate reduction. Growth occurs at 10–45 °C but not 50 °C, in 0–6.5 %
(w/v) sodium chloride, and at pH 4.5–9.6. The G + C ratio of the DNA is 44
mols % [G + C ratio was quoted as 38 % for strain 8831 in a publication
describing the genome sequence; Aguado-Urda et al. 2011 ]. The organisms
do not belong to Lancefi eld groups, A, B, C, D, E, F, G, H, K, L, M, N or O.
2 Gram-Positive Bacteria (Anaerobes and ‘Lactic Acid’ Bacteria)
Isolation
LG agar was developed as a selective medium for Lactococcus garvieae that differentiated the virulent capsulated from the non-virulent uncapsulated cultures. The
medium exploited the ability of the pathogen to grow in high concentrations of bile
salts and potassium tellurite, and to reduce tetrazolium dye to coloured formazans.
Thus, noncapsulated cultured produced white colonies without red halos whereas
capsulated Lactococcus garvieae cultures comprised black colonies with red halos
(Chang et al. 2014 ).
Characteristics of the Pathogen
The fi rst attempt at clarifying the taxonomic status of the causal agents of streptococcicosis/streptococcosis was the landmark publication of Kusuda et al. ( 1991 ),
who described a new species, i.e. Enterococcus seriolicida, to accommodate 12
isolates recovered from eels and yellowtail in Japan.
Interestingly in the original publication describing En. seriolicida , only low
DNA homology values were obtained with reference species of Enterococcus.
Indeed, the greatest DNA homology, i.e. 24 %, was with En. hirae (Kusuda et al.
1991 ). Perhaps, it was inevitable that the association of these fi sh pathogens with
Enterococcus would be challenged. Thus, it was determined that En. seriolicida was
really identical (77 % DNA:DNA homology) with a previously described lactococcus, namely Lactococcus garvieae (Tiexeira et al. 1996 ). This view has been reinforced by others. For example, Pot et al. ( 1996 ) detailed research by SDS-PAGE of
Box 2.9: Lactococcus garvieae
Cultures comprise non-motile facultatively anaerobic Gram-positive cocci in
short chains, which do not produce catalase, H 2 S, indole or oxidase. Blood is
degraded (α-haemolysis). Aesculin and arginine are hydrolysed, but not so
casein, gelatin or sodium hippurate. Acid is produced from a wide range of
carbohydrates, namely aesculin, cellobiose, D-fructose, galactose, D-glucose,
maltose, mannitol, D-mannose, salicin, sorbitol and trehalose, but not from
adonitol, D-arabinose, glycerol, glycogen, inositol, lactose, melezitose, melibiose, raffi nose, L-rhamnose, starch, sucrose or D-xylose. The methyl red and
tetrazolium reduction tests and the Voges Proskauer reaction are positive, but
not nitrate reduction. Growth occurs at 10–45 °C but not 50 °C, in 0–6.5 %
(w/v) sodium chloride, and at pH 4.5–9.6. The G + C ratio of the DNA is 44
mols % [G + C ratio was quoted as 38 % for strain 8831 in a publication
describing the genome sequence; Aguado-Urda et al. 2011 ]. The organisms
do not belong to Lancefi eld groups, A, B, C, D, E, F, G, H, K, L, M, N or O.
2 Gram-Positive Bacteria (Anaerobes and ‘Lactic Acid’ Bacteria)