373
There is some evidence that Y. ruckeri may well be capable of surviving in the
environment in a form (the so-called dormant or non-culturable state) that is not
readily culturable on conventional media (Romalde et al. 1994 ). These workers
found that the number of culturable cells increased for the fi rst 15 days after Y. ruckeri was seeded into an experimental system. Thereafter, a decline in numbers was
recorded over a 100-day period. Culturable cells persisted in sediment more so than
water at 6 and 18 °C. Yet, by means of acridine orange staining and fl uorescence
microscopy, a so-called dormant state was considered to develop. There were slight
changes in the LPS of the dormant cells, but not in membrane proteins or plasmid
composition. Interestingly, virulence was maintained during the period of nonculturability. Unlike the situation with Aer. salmonicida, cells were resuscitated to a
culturable state following the addition of nutrients (Romalde et al. 1994 ).
A serotyping scheme, based on O-antigens, has been proposed (Davies 1990 ),
and may have value in epizootiological investigations.
PFGE and MLST were used to study isolates from outbreaks on French fi sh
farms. For the PFGE analysis, two enzymes, NotI and AscI, were used either separately or together, with the latter approach revealing the great homogeneity among
the isolates with 43 pulsotypes defi ned. The dominant pulsotypes have been
described elsewhere, i.e. in Denmark, Finland, Germany, Italy and Spain. MLST
revealed two dominant sequence types, i.e. ST31 and ST36; this is an epidemic
within France (Calvez et al. 2015 ).
Pathogenicity
Uptake from an aqueous suspension is likely to be across the epithelial cells in the
secondary gill lamella, with recovery of the pathogen from blood possible after
1 min from challenge. With the development of septicaemia, the pathogen was
widespread in all organs, including the brain (Ohtani et al. 2014 ). Some interesting
experiments were carried out with rainbow trout, which were immersed in a suspension containing a luminescent culture. This was developed by using the pCS26-Pac
plasmid, which contains the lux operon from Photorhabdus luminescens. The fate
of this culture in the digestive tract was followed with bioluminescence imaging, in
which dissemination and the development of infection were studied. In vivo expression technology reveled that there was expression of promotors, i.e. yhlBA, cdsAB ,
yctCBA and yrp1, which are involved/associated with haemolysin production, cysteine and citrate uptake, and the encoding of serralysin metalloprotease, respectively, during the infection process (Méndez and Guijarro 2013 ). Other studies
using immunohistochemistry and in situ hybridization techniques, followed the fate
of Y. ruckeri cells, in which the lateral lines, dorsal fi n, epidermis and mucosal tissue
in gastro-intestinal tract were determined to be the primary areas of uptake
(Khimmakthong et al. 2013 ).
In terms of pathogenicity to rainbow trout, Type 1 (Hagerman) is the most virulent, followed by Type 2 (O’Leary) and then Type 3 (Australian) (Bullock et al.
Yersinia ruckeri
There is some evidence that Y. ruckeri may well be capable of surviving in the
environment in a form (the so-called dormant or non-culturable state) that is not
readily culturable on conventional media (Romalde et al. 1994 ). These workers
found that the number of culturable cells increased for the fi rst 15 days after Y. ruckeri was seeded into an experimental system. Thereafter, a decline in numbers was
recorded over a 100-day period. Culturable cells persisted in sediment more so than
water at 6 and 18 °C. Yet, by means of acridine orange staining and fl uorescence
microscopy, a so-called dormant state was considered to develop. There were slight
changes in the LPS of the dormant cells, but not in membrane proteins or plasmid
composition. Interestingly, virulence was maintained during the period of nonculturability. Unlike the situation with Aer. salmonicida, cells were resuscitated to a
culturable state following the addition of nutrients (Romalde et al. 1994 ).
A serotyping scheme, based on O-antigens, has been proposed (Davies 1990 ),
and may have value in epizootiological investigations.
PFGE and MLST were used to study isolates from outbreaks on French fi sh
farms. For the PFGE analysis, two enzymes, NotI and AscI, were used either separately or together, with the latter approach revealing the great homogeneity among
the isolates with 43 pulsotypes defi ned. The dominant pulsotypes have been
described elsewhere, i.e. in Denmark, Finland, Germany, Italy and Spain. MLST
revealed two dominant sequence types, i.e. ST31 and ST36; this is an epidemic
within France (Calvez et al. 2015 ).
Pathogenicity
Uptake from an aqueous suspension is likely to be across the epithelial cells in the
secondary gill lamella, with recovery of the pathogen from blood possible after
1 min from challenge. With the development of septicaemia, the pathogen was
widespread in all organs, including the brain (Ohtani et al. 2014 ). Some interesting
experiments were carried out with rainbow trout, which were immersed in a suspension containing a luminescent culture. This was developed by using the pCS26-Pac
plasmid, which contains the lux operon from Photorhabdus luminescens. The fate
of this culture in the digestive tract was followed with bioluminescence imaging, in
which dissemination and the development of infection were studied. In vivo expression technology reveled that there was expression of promotors, i.e. yhlBA, cdsAB ,
yctCBA and yrp1, which are involved/associated with haemolysin production, cysteine and citrate uptake, and the encoding of serralysin metalloprotease, respectively, during the infection process (Méndez and Guijarro 2013 ). Other studies
using immunohistochemistry and in situ hybridization techniques, followed the fate
of Y. ruckeri cells, in which the lateral lines, dorsal fi n, epidermis and mucosal tissue
in gastro-intestinal tract were determined to be the primary areas of uptake
(Khimmakthong et al. 2013 ).
In terms of pathogenicity to rainbow trout, Type 1 (Hagerman) is the most virulent, followed by Type 2 (O’Leary) and then Type 3 (Australian) (Bullock et al.
Yersinia ruckeri