370
share some of the characteristics of Haf. alvei, was confi rmed as Y. ruckeri by 16S
rRNA sequencing (homology = 100 %), and was regarded as a new biotype,
i.e. biotype 2. In contrast to the species description, cultures often appeared to be
non- motile (although fl gA and fl hA fl agellar genes have been found in German
isolates; Huang et al. 2014 ), lacked phospholipase activity, were positive for the
Voges Proskauer reaction and displayed a unique LPS profi le (Tinsley et al. 2011a ).
Similar non-motile variants were also recovered from previously vaccinated rainbow
trout in Spain (Fouz et al. 2006 ), Denmark, Finland (Ström-Bestor et al. 2010 ),
France (Calvez et al. 2014 ), Germany (Huang et al. 2014 ) and the USA (Welch et al.
2011 ). In the case of Germany, during June 2011 to June 2012, 48 Y. ruckeri isolates
were collected from 12 different rainbow trout hatcheries in Germany. Of these
isolates, the majority (44 = 91.7 %) were nonmotile (Huang et al. 2015 ). Indeed, one
study pointed to four independent natural mutations in fl iR , fl hA or fl hB genes that
led to lack of motility due to their effect on the fl agellae secretion system (Evenhuis
et al. 2009 ). These authors pointed to the similarity between UK and US strains in
terms of the same mutant allele, suggesting a common origin of biotype 2. However,
evidence has been presented which suggests that isolates from Denmark, Spain and
the UK emerged separately because of the difference in pulsotypes (Wheeler et al.
2009 ). Notwithstanding, the non-motile isolates were more active at lower temperatures than their motile counterparts. This may explain why non-motile cultures were
more common I Germany during winter and were responsible for more outbreaks
then than motile forms (Huang et al. 2014 ).
Diagnosis
Phenotypic Methods Diagnosis of Y. ruckeri may be achieved by isolation of the
pathogen, such as on the selective media of Waltman and Shotts ( 1984 ) or Rodgers
( 1992 ), and thence identifi cation. According to Waltman and Shotts ( 1984 ), 53/60
isolates hydrolysed Tween 80 but none fermented sucrose. Therefore, typically on
the selective medium, Y. ruckeri colonies were green with a zone of hydrolysis
(indicated by the presence of insoluble calcium salts) around them. Unfortunately in
our experience with this medium, UK isolates rarely hydrolysed Tween 80.
Therefore, interpretations should be made carefully.
Bacteriophage Typing Development of a bacteriophage typing scheme may be of
considerable value for diagnosis in the future. Reference is made here to a collection
of tailed icosahedral bacteriophages which are specifi c to Y. ruckeri (Stevenson and
Airdrie 1984b ).
Serology iFAT has a proven track record with the diagnosis of ERM (Johnson et al.
1974 ). An indirect ELISA has been effective at detection (Cossarini-Dunier 1985 ).
6 Enterobacteriaceae Representatives
share some of the characteristics of Haf. alvei, was confi rmed as Y. ruckeri by 16S
rRNA sequencing (homology = 100 %), and was regarded as a new biotype,
i.e. biotype 2. In contrast to the species description, cultures often appeared to be
non- motile (although fl gA and fl hA fl agellar genes have been found in German
isolates; Huang et al. 2014 ), lacked phospholipase activity, were positive for the
Voges Proskauer reaction and displayed a unique LPS profi le (Tinsley et al. 2011a ).
Similar non-motile variants were also recovered from previously vaccinated rainbow
trout in Spain (Fouz et al. 2006 ), Denmark, Finland (Ström-Bestor et al. 2010 ),
France (Calvez et al. 2014 ), Germany (Huang et al. 2014 ) and the USA (Welch et al.
2011 ). In the case of Germany, during June 2011 to June 2012, 48 Y. ruckeri isolates
were collected from 12 different rainbow trout hatcheries in Germany. Of these
isolates, the majority (44 = 91.7 %) were nonmotile (Huang et al. 2015 ). Indeed, one
study pointed to four independent natural mutations in fl iR , fl hA or fl hB genes that
led to lack of motility due to their effect on the fl agellae secretion system (Evenhuis
et al. 2009 ). These authors pointed to the similarity between UK and US strains in
terms of the same mutant allele, suggesting a common origin of biotype 2. However,
evidence has been presented which suggests that isolates from Denmark, Spain and
the UK emerged separately because of the difference in pulsotypes (Wheeler et al.
2009 ). Notwithstanding, the non-motile isolates were more active at lower temperatures than their motile counterparts. This may explain why non-motile cultures were
more common I Germany during winter and were responsible for more outbreaks
then than motile forms (Huang et al. 2014 ).
Diagnosis
Phenotypic Methods Diagnosis of Y. ruckeri may be achieved by isolation of the
pathogen, such as on the selective media of Waltman and Shotts ( 1984 ) or Rodgers
( 1992 ), and thence identifi cation. According to Waltman and Shotts ( 1984 ), 53/60
isolates hydrolysed Tween 80 but none fermented sucrose. Therefore, typically on
the selective medium, Y. ruckeri colonies were green with a zone of hydrolysis
(indicated by the presence of insoluble calcium salts) around them. Unfortunately in
our experience with this medium, UK isolates rarely hydrolysed Tween 80.
Therefore, interpretations should be made carefully.
Bacteriophage Typing Development of a bacteriophage typing scheme may be of
considerable value for diagnosis in the future. Reference is made here to a collection
of tailed icosahedral bacteriophages which are specifi c to Y. ruckeri (Stevenson and
Airdrie 1984b ).
Serology iFAT has a proven track record with the diagnosis of ERM (Johnson et al.
1974 ). An indirect ELISA has been effective at detection (Cossarini-Dunier 1985 ).
6 Enterobacteriaceae Representatives