Page 261 - The Veterinary Laboratory and Field Manual 3rd Edition

230 Susan C. Cork and Roy Halliwell

Figure 4.13b Antibiotic sensitivity
testing. Staphylococcus aureus growing
on test agar incubated with discs
containing gentamicin (GN), enrofloxacin
(ENO), chloramphenicol (C) and
tetracycline (TT). This isolate appears to
be ‘resistant’ to tetracycline. (a) zone
of inhibition. If a > 5 mm it is likely that
the bacterial strain tested is sensitive to
the antibiotic(s) tested.

with the control strain bacterium. It is important Moderately sensitive
to assay the control strain at the same time An organism is considered to be moderately
as the sample strain to reduce variability. The ‘sensitive’ if the zone diameter is at least 12 mm
method for determination of sensitivity using but is reduced by more than 4 mm compared to
this method is explained below.
a ‘sensitive’ control culture.

Sensitive Resistant
An organism is considered to be ‘sensitive’ if the An organism is considered to be ‘resistant’ if the
zone of inhibition around the antibiotic disc is zone of growth (or the diameter of the zone) is
greater than, equal to, or not more than 4 mm not more than 10 mm (that is, 2 mm on each
less than that of a ‘sensitive’ control strain (see side of a 6 mm disc).
figures 4.13c and d).

(c) (d)

Figure 4.13c and d Antibiotic sensitivity testing plates. This is an example of the Kirby-Bauer disc diffu-
sion assay for antibiotic sensitivity. The clear zones in the solid ‘lawn’ of bacteria around discs indicate
bacterial growth has been inhibited by the antibiotic. Where bacteria have grown to the disc edge indicates
resistance to that antibiotic.

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