Fish Diseases and Management

Samples for bacteriology

Once external examination or sampling has been carried out, the body surface is cut open to expose the internal organs. In the event of any external lesions,bacteriological samples could be collected from the lesions after surface disinfection before opening the body cavity. Care must be taken not to puncture any part of the intestinal tract. In the absence of any visible internal lesions, a sample of kidney is taken and inoculated onto suitable agar medium.If no preliminary diagnosis could be made of the infection, use TSA (trypticase soya agar) for isolation of the bacteria. Material from other organs e.g. liver,spleen, heart can be taken if any abnormality is evident. The surface of any internal organs to be sampled should be seared with a hot scalpel blade before insertion of a sterile loop but in the case of very small fish, this may not always be possible. Agar plates containing the streaked out samples should be incubated and examined daily for any evidence of growth. The majority of fish pathogens will grow on TSA within 7 days.

Petri dishes containing solid medium (agar) are used to provide a large surface area for the cultivation of microorganisms. Inoculation of an agar plate is carried out using  streak plate technique. This involves diluting the culture or other sample, e.g. kidney material, by smearing it across the surface of the agar. Organisms present in the samples will be separated and after suitable incubation, each organism present will give rise to a colony. Although this colony contains many millions of organisms they will have all originated from one, and therefore all organisms in one colony should be identical.

By using this method, organisms can be cultured in the laboratory.  However, if as mixed culture is present, it will become apparent on plating out. This is essential before starting identification procedures, as methods are only valid when carried out on pure cultures i.e. cultures containing only one type of organism. Care should be taken to ensure that plates used for this purpose are dry. Avoid unnecessary exposure of the agar surface to potential contamination from the environment throughout the procedure. Biochemical characterisation of bacteria is dealt separately)