Diagnosis of bacterial fish diseases is not possible purely on the basis of the clinical signs and symptoms observed in diseased fish, because many of them can be caused by more than one etiological agent. Presumptive diagnosis is usually based on previous epidemiological data and a rough bacteriological analysis of cultured organisms from affected animals. In the present study, isolates were recovered from diseased trout suspected of F. psychrophilum
infection because the fish farm had previous records of infections with this fish pathogen and the macroscopic characteristics of the colonies obtained on Anacker and Ordal agar were also compatible with that presumptive diagnosis. However, cell morphology of the trout isolates following Gram staining was different of the filamentous rods exhibited by F. psychrophilum
], they were non-motile and none gave a positive reaction with a F. psychrophilum
species-specific PCR assay [13
]. This lack of amplification is consistent with the low similarity observed (81.8-81.9
%) between the 16
S rRNA gene sequences of trout isolates and F. psychrophilum
. The trout isolates exhibited the highest 16
S rRNA sequences similarities with C. shigense
%), percentages higher than the 99
% sequence similarity used as the criterion for species identification [19
]. In addition, most of the phenotypic characteristics of the C. shigense
trout isolates were consistent with the current description of this species based solely in the strain DSM 17126T
], which support the identification based on 16
S rRNA gene sequencing. Despite the overall phenotypic similitude, some differences were found between the C. shigense
trout isolates and the type strain C. shigense
. Thus, trout isolates reduced nitrate, assimilated citrate but not mannitol and produce the enzyme valine arylamidase but not the enzymes ester lipase C8 and β-glucosidase, while the type strain of C. shigense
gave opposite reactions for these traits.
Widely spread microorganisms are usually genetically heterogeneous [21
]. Members of the genus Chryseobacterium
are isolated from a wide range of environments [1
] and therefore, it could be expected to be genetically diverse. However, trout isolates exhibited undistinguishable RAPD fingerprints indicating that they represent a single strain. This fact, together with their isolation in pure culture from internal organs might suggest a clinical significance; however the role of C. shigense
as the causative agent of the disease episodes in trout cannot be undoubtedly established in absence of experimental infections trials.
Some members of the family Flavobacteriaceae,
usually are opportunistic pathogens [4
], because they require the existence of different predisposing factors such as coinfections with other bacteria or virus [7
] or other environmental or husbandry sources of stress. No infestations or viral infections were detected previously to the disease episodes, but these usually occurred after transportation of rainbow trout fry from the hatchery to fish farm, or in tanks with elevated stock densities. These circumstances represent stressful conditions for fish [22
] and might have increased the susceptibility of fry trout to infection.
Several species of Chryseobacterium,
such as C. piscicola
], C. chaponense
], and C. shigense
in this study, have been isolated from diseased fish in which F. psychrophilum
infections were initially suspected. Although there are no clear evidences for considering these species as consistent pathogens for fish, they should be considered for a differential diagnosis in those cases with a suspicious of F. psychrophilum
infection. Table shows some phenotypic characteristics that can be useful for their differentiation.
Phenotypic characteristicsathat can be useful to differentiate the speciesC. shigense,C. piscicola,C.chaponenseandF. psychrophilum
Since the initial description of C. shigense
from a fresh lactic acid beverage in Japan [20
], no further isolations of this species have been reported. Consequently, the isolation of C. shigense
from trout shows that it can also occur in a very different habitat. To our knowledge, this is the first description of the isolation of C. shigense
from clinical specimens.