Page 248 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 217

outlines the cultural characteristics of some
bacteria of veterinary importance and Figure 4.9
outlines the staining characteristics that can be
used to help identify bacteria to the family or
genus level. However, because several species
of bacteria have a similar appearance on agar,
and share similar staining characteristics, addi-
tional tests are required. It is usually necessary
to perform a series of six or more preliminary
biochemical tests to allow confirmation of the
genus and further tests to allow identification Figure 4.10 Sink in a small microbiology laboratory
to species level. at a district veterinary diagnostic facility. This illus-
As outlined earlier, the number of colonies trates the location of staining racks and commonly
of each type of bacteria grown on a culture plate used reagents. Each laboratory technician will have
does not directly relate to the number of bacte- his/her own preferred reagents which are usually
ria present in the sample submitted nor does it placed in an area where they can be readily used.
indicate the relative importance of each type of However, it is important to make sure that standard
bacteria grown from the sample. Until a techni- operating practices are followed. Photo: Regional
cian is experienced in deciding which colonies Veterinary Laboratory, Khaling, Bhutan.
to follow up it is important to note down every-
thing that is seen on the plate and to seek advice
from a senior technician. Owing to the time This is believed to be associated with the ‘poros-
and expense involved in the culture and iden- ity’ of the cell wall and cytoplasmic membrane.
tification of bacteria it is also helpful to discuss
the case with the submitter so that a targeted MEtHod
approach can be taken to ensure all likely patho- A small colony of the bacteria to be examined is
gens are considered. mixed with a drop of water on a microscope slide
and heat fixed using the cool part of the Bunsen
flame (or air dried). The staining procedure is
routine stains used in microbiology as follows:

Step 1. Apply methyl violet
Gram staining of bacterial colonies grown on
culture media is part of the routine bacterial All bacteria pick up the dye
identification procedure. See Figure 4.10 (stain- Step 2. Fix (or mordant) the dye with iodine
ing station in a microbiology laboratory). A change takes place in the dye and/or the
bacteria
The Gram stain Step 3. Apply decolouring agent (acetone/
alcohol)
As outlined earlier, the Gram stain is an
extremely important stain and is routinely used Dye retained by some bacteria, not others
in bacteriology tests (see Figures 4.11a and b). Step 4. Apply counter-stain of different
The Gram staining characteristic of bacteria is colour
based on the fact that some bacteria will retain a Counter-stain picked up by bacteria not
specific dye (methyl violet) and others will not. retaining methyl violet

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