239
Haemophilus piscium
What about Haemophilus piscium, the causal agent of ulcer disease? The name was
coined by Snieszko et al. ( 1950 ). However, the detailed taxonomic study of Kilian
( 1976 ) showed that the organism did not belong in the genus Haemophilus. In particular, the strains did not exhibit requirements for haemin or NAD, which contrasted with the genus description. H. piscium differed from the type species, H.
infl uenzae, in the inability to reduce nitrate or alkaline phosphatase and to grow at
37 °C, together with a relatively high G + C ratio of the DNA. Unfortunately, Kilian
did not establish the most appropriate taxonomic position of the pathogen. The
validity of the taxonomic position was similarly questioned by Broom and Sneath
( 1981 ), as a result of a detailed numerical taxonomic study. The low similarity of H.
piscium with other Haemophilus spp., i.e. only 65 %, suggested that the organism
should be excluded from the genus. From examination of DNA, biochemical, serological and bacteriophage sensitivity data, it is apparent that H. piscium represents
an atypical, achromogenic variant of Aer. salmonicida (Paterson et al. 1980 ).
Evidence for this conclusion consists of the G + C ratio of the DNA (55.1 moles %),
which is well within the range reported for Aer. salmonicida by McCarthy ( 1978 ).
Moreover, Paterson et al. ( 1980 ) regarded H. piscium to be serologically indistinguishable from Aer. salmonicida. Also, the pathogen was sensitive to several Aer.
salmonicida bacteriophages, and exhibited biochemical reactions similar to those
expected for some achromogenic variants of Aer. salmonicida. Trust et al. ( 1980a )
also concluded, on the basis of bacteriophage sensitivity, that H. piscium is, in fact,
atypical Aer. salmonicida. Thus, a virus that produced lysogeny in Aer. salmonicida
but displayed no such activity in Aer. hydrophila, caused plaque formation in several
isolates of H. piscium. On the basis of one strain, Austin et al. ( 1998 ) concurred with
the view that H. piscium should probably be classifi ed with Aer. salmonicida .
However its precise relationship to the four subspecies refl ected the nature of the
phenotypic and molecular methods used.
Diagnosis
Culturing and Phenotypic Characters Diagnosis is readily achieved by culturing
techniques, usually on TSA or BHIA (the preferred medium in the view of many
scientists) in which case ‘typical’ isolates produce a characteristic brown, diffusible
pigment. Also, CBB may be employed as a differential medium (Markwardt et al.
1989 ). However as a primary isolation medium, CBB appears to be less sensitive
and gives a poor recovery of the pathogen compared to BHIA (B. Austin, unpublished data).
Aer. salmonicida may be distinguished from other fi sh pathogens on the basis of
a small number of phenotypic tests, notably the Gram-staining reaction (small
Gram-negative rods), motility (usually appears to be non-motile), growth at 37 °C
Aeromonas salmonicida
Haemophilus piscium
What about Haemophilus piscium, the causal agent of ulcer disease? The name was
coined by Snieszko et al. ( 1950 ). However, the detailed taxonomic study of Kilian
( 1976 ) showed that the organism did not belong in the genus Haemophilus. In particular, the strains did not exhibit requirements for haemin or NAD, which contrasted with the genus description. H. piscium differed from the type species, H.
infl uenzae, in the inability to reduce nitrate or alkaline phosphatase and to grow at
37 °C, together with a relatively high G + C ratio of the DNA. Unfortunately, Kilian
did not establish the most appropriate taxonomic position of the pathogen. The
validity of the taxonomic position was similarly questioned by Broom and Sneath
( 1981 ), as a result of a detailed numerical taxonomic study. The low similarity of H.
piscium with other Haemophilus spp., i.e. only 65 %, suggested that the organism
should be excluded from the genus. From examination of DNA, biochemical, serological and bacteriophage sensitivity data, it is apparent that H. piscium represents
an atypical, achromogenic variant of Aer. salmonicida (Paterson et al. 1980 ).
Evidence for this conclusion consists of the G + C ratio of the DNA (55.1 moles %),
which is well within the range reported for Aer. salmonicida by McCarthy ( 1978 ).
Moreover, Paterson et al. ( 1980 ) regarded H. piscium to be serologically indistinguishable from Aer. salmonicida. Also, the pathogen was sensitive to several Aer.
salmonicida bacteriophages, and exhibited biochemical reactions similar to those
expected for some achromogenic variants of Aer. salmonicida. Trust et al. ( 1980a )
also concluded, on the basis of bacteriophage sensitivity, that H. piscium is, in fact,
atypical Aer. salmonicida. Thus, a virus that produced lysogeny in Aer. salmonicida
but displayed no such activity in Aer. hydrophila, caused plaque formation in several
isolates of H. piscium. On the basis of one strain, Austin et al. ( 1998 ) concurred with
the view that H. piscium should probably be classifi ed with Aer. salmonicida .
However its precise relationship to the four subspecies refl ected the nature of the
phenotypic and molecular methods used.
Diagnosis
Culturing and Phenotypic Characters Diagnosis is readily achieved by culturing
techniques, usually on TSA or BHIA (the preferred medium in the view of many
scientists) in which case ‘typical’ isolates produce a characteristic brown, diffusible
pigment. Also, CBB may be employed as a differential medium (Markwardt et al.
1989 ). However as a primary isolation medium, CBB appears to be less sensitive
and gives a poor recovery of the pathogen compared to BHIA (B. Austin, unpublished data).
Aer. salmonicida may be distinguished from other fi sh pathogens on the basis of
a small number of phenotypic tests, notably the Gram-staining reaction (small
Gram-negative rods), motility (usually appears to be non-motile), growth at 37 °C
Aeromonas salmonicida