Page 243 - The Veterinary Laboratory and Field Manual 3rd Edition

212 Susan C. Cork and Roy Halliwell

General types of media and terminology nutrient agar which is often used as a base agar
used to make media such as blood agar.
Enriched media contain ingredients which
Solid and liquid media can be used to culture will facilitate bacterial growth (see Figure 4.5).
bacteria. Most are commercially available in a Many pathogenic bacteria require additional
dehydrated compound form that can be stored. nutrients such as blood or specific salts, sugars
Liquid media are especially useful for propagat- or proteins and would grow very slowly or not
ing anaerobes and allow motility. Liquid media grow at all in basal media. Blood and serum are
are also useful for maintaining cultures and for usually added at 5 to 10% and glucose at 1%.
biochemical tests. They have the disadvantage of Selective media contain ingredients which
not allowing assessment of colony characteris- inhibit the growth of certain microorgan-
tics which are important for identification. isms and allow others to grow. For example,
Solid media are used for most primary culture MacConkey agar contains bile salts which inhibit
work. The consistency of the medium is modi- the growth of most Gram-positive bacteria but
fied by addition of agar, gelatine or albumin in allow enteric (gut) organisms to grow. Selective
order to change it into a solid or semi-solid media may also contain a colour indicator and
state. Agar (derived from a seaweed) is com- specific sugars. For example, when a lactose fer-
monly used because it is relatively inert. For menting organism grows in media containing
solid media agar is used at a concentration of lactose there is acid production (fermentation)
1%, it melts at about 95°C and sets at 42°C. Heat and consequently pH is lowered. This can be
sensitive nutrients such as blood and some car- detected by a change in the colour of an indicator
bohydrates can be added at temperatures below in the media (for example, from colourless to red
50°C before the agar sets. on MacConkey agar). Growth of a non-lactose
Basal media are media or compounds which fermenting organism will not result in acid pro-
can be used alone or as a basis for other media, duction and there is therefore no colour change.
for example, nutrient broths (peptone and/or Media such as these are sometimes referred to
meat infusion) or digest broths (digested with as ‘differential media’ (Table 4.2b).
proteolytic enzymes to release protein). The
addition of agar to a broth at 1% will produce

Table 4.2b Cultural characteristics of some common bacteria found in veterinary
medicine.

Bacterial species Blood agar** MacConkey agar* Comments
Beta haemolytic Translucent round No growth The size of the zone of
Streptococci colonies, 0.5–1 mm haemolysis varies with the
diameter, glistening. Clear Lancefield group type and
area around colonies species. Catalase –ve
(haemolysis)
Alpha haemolytic White, smooth and round No growth Greenish or partial zone of
Streptococci colonies, 0.5–1 mm haemolysis. Rarely pathogenic.
diameter Catalase –ve
Staphylococcus White or yellow, smooth No growth Some strains do grow on
aureus or round colonies, 2–3 mm (depends on strain) MacConkey agar, Catalase +ve.
S. intermedius diameter, may have a Some strains of S. aureus show a
double zone of haemolysis characteristic yellow pigment

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