Abstract

Yellow pigmented bacteria associated with fish have been divided into 4 major groups. The 3rd and 4th groups contain most fish pathogens while group 3 probably represents the vast majority of fish pathogens. Within the 5 subgroups of this group, 2 divisions contain 90% of the disease producing organisms. Group 4 contains organisms similar to Flavobacterium branchophila. Studies with this group are not complete. Overview comments regarding possible pathogenesis mechanisms including fish invasion are included.

Introduction

YPB (yellow pigmented bacteria) have been reported as a source of bacterial disease in fish since 1922 when they were reported in warmwater fish, later in 1945 in cold-water fish and still later in 1967 in aquarium fish (1-3). This disease has been interchangeably called columnaris disease, Myxobacteriosis, peduncle disease, cotton wool disease and the agent reported as the etiology has been assigned to at least 4 genera since its isolation in 1944 (1-5). In the past 10-15 years there had been speculation that this once thought homogeneous group of organisms were phylogenetically heterogeneous (5-8). To study this heterogenicity and define the scope of pathogenecity this study was instituted.

Materials and Methods

To carry out this study approximately 200 isolates of YPB were made from normal and diseased fish, waters and environment. Initially, blood agar, trypticase soy and Ordal's agar were used (4). Later specific medias were developed for these isolations including Tryptone-yeast-extract-gelatin agar (TYG) and Hsu-Shotts (HS) isolation agar (10). Characterization of these isolates was done using modifications of procedures previously reported with subsequent statistical analysis (11-14). Selected isolates were studied to determine their mechanisms of attachment and invasion of fish (15).

Results

It was found that the YPB were indeed heterogeneous. They are comprised of 4 major groups:

Group 1 3.3%) - Gram positive rods and cocci

Group 2 4.6%) - Gram negative flagellated rods

Group 3 (51.4%) - Gram negative nonflagellated rods which spread on agar

Group 4 (40.7%) - Gram negative nonflagellated rods which do not spread on agar

Of these 4 major groups it was determined that members of the 2 latter groups are responsible for 95% of the fish disease attributed to "Columnaris" disease of fish. Specifically, major group 3 alone causes approximately 90% of the clinical columnaris problems.

Major group 3 is further divided into at least 5 subgroups using phenotypic characters. These were found to relate to some of the known Flexibacter.

subgroup 1 -Flexibacter columnaris grp

subgroup 2 -Flexibacter aurantracus (Cytophaga psychrophila)

subgroup 3 -undefined

subgroup 4 - undefined

subgroup 5 - undefined

Preliminary characterization of major group 4 indicates that most Flavobacteria belong in this group. One subgroup currently under study contains organisms similar to Flavobacterium branchophila.

Studies on attachment indicate that the galactosaminoglycan slime layer plays an important role in the attachment and disease process associated with major group 3 organisms (6). Further studies indicate that only subgroup 1 of this group is capable of instituting disease.

Discussion

It is obvious from these and previous work in our laboratories that the Columnaris disease complex is caused by at least 2-4 genera of organisms belonging to the nonflagellated Gram negative bacteria and may represent as many as 15-20 distinct species. It is also obvious that these organisms are not related to the Myxobacteria and that they do not produce fruiting bodies. The role of these various YPB in disease varies with their enzyme systems and a vast majority of these organisms are opportunistic pathogens requiring damaged fish tissue to institute disease.

Acknowledgements

This work was supported by funds obtained from USDA grants (10-21-RR211-038 and 10-21-RR211-042) and the Veterinary Medical Experiment Station, University of Georgia.

References

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