Page 245 - The Veterinary Laboratory and Field Manual 3rd Edition

214 Susan C. Cork and Roy Halliwell

the desired organism from a sample while a flame. This is repeated for each step so that
‘inhibiting’ the growth of others (acid, pH < 7.0; the inoculum is diluted to a point when indi-
alkali, pH > 7.0). vidual and separate colonies will develop (Figure
4.8). The idea is to spread the bacterial inocu-
lum across the largest distance possible on the
temperature surface area available. For anticipated light loads
of organisms, for example from a milk sample,
The temperature range suitable for the growth the inoculum can be plated out without flam-
of most microorganisms lies between 15ºC and ing the loop between each stage while streaking
43ºC. Some specialized microorganisms can out. Alternatively, a culture swab or plastic loop
grow at 0ºC (and below) and others can grow could be used. On well prepared plates individ-
at temperatures of over 80ºC. However, most of ual bacteria will grow into discrete colonies and
the organisms pathogenic to animals have a nar- the growth characteristics of these colonies can
row preferred temperature range based around then be described. If streaking is not well done,
28–37ºC. non-pathogenic contaminants may overgrow the
more fastidious pathogenic organisms which
may then not be seen.
routine approach to samples The plating out method for solid media is
submitted for bacteriological illustrated in Figure 4.8. This method is designed
examination to ensure that isolated bacterial colonies will be
present to facilitate subculture and bacterial
Goals of primary inoculation
identification.
1 To cultivate the causative (pathogenic)
agent(s) but minimize the growth of con-
taminants. Identification of bacteria
2 To obtain discrete colonies of organisms to
allow selection of the pathogen(s) for sub- After primary culture and incubation, the plates
culture and identification. are examined and colonies can be described (see
characteristics to record below). If, after 24 h,
To culture bacteria from a specimen inoculate bacterial growth is not readily observed the
the edge of an agar plate with the sample mate- plates should be replaced in the incubator and
rial using a swab or sterile wire loop. Wire loops read again after a further 24 h. The number of
can be bought or made and are useful making colonies of each type of bacteria may not directly
smears and for ‘streaking’ culture plates. Most relate to the number of bacteria present in the
loops are made of platinum alloy or nicrome sample nor do they indicate the relative signifi-
wire which withstands heating/cooling. The cance of each type of bacteria isolated. Pure or
loop should be 1.5 × 3 mm in diameter and the predominant growth of one type of organism is
standard thickness of the wire is standard gauge usually a significant result.
26 or 27. Streaking out, thereafter, depends on
the anticipated load of bacteria in the inoculum. Characteristics to record
For example, for a faecal sample that will have
a heavy load of bacteria, a loop is used to allow 1 Is there heavy, light, pure or mixed growth?
dilution of the sample. After each stage, the loop Is there a predominant colony type?
must be re-sterilized by passing the tip through 2 What is the size and shape of the colonies

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