4 Bacterial isolation

4.1 Different specimens require different treatments

Specimens received by the microbiology laboratory must be inoculated without delay into an appropriate bacteriological medium. This allows optimal growth of the bacteria present in the specimen and ultimately obtaining a ‘clinical isolate’, which is a pure culture of the presumed pathogen. Methods for inoculation into primary media differ between fluid and solid specimens.

Fluid and semi-fluid specimens are taken directly from the specimen and inoculated into liquid growth medium or spread on the surface of solid medium using bacteriological loops or pipetting.

A common method of dealing with solid specimens consists of:

1. an initial sterilisation of the specimen’s surface by searing it with a hot spatula
2. an incision of the seared surface using a sterile scalpel to expose the core of the specimen
3. extraction of material from the core and transfer to the primary media using a bacteriological loop or a Pasteur pipette.

For small specimens (for example, an avian spleen), the external surface can be sterilised by passing it through a Bunsen flame several times and then cutting it with sterile tools to expose the core. The whole or part of the specimen is then inoculated into the media. The surface of very small specimens is difficult to sterilise and the laboratory depends upon strict aseptic specimen collection techniques. This highlights the need to submit large pieces of tissue.