In an attempt to overcome some of the limitations
of the existing procedures, a new susceptibility
test method is described in this report which
combines the simplicity of the disc test with the
quantitative results obtained by serial dilution. In
this method, organisms are tested for susceptibility
to known concentrations of antibiotic, thus
avoiding completely the question of interpretation
of a zone of inhibition. The antibiotic concentrations
are achieved by placing papers impregnated
with antibiotic onto the surface of small volumes
of agar contained in a plastic tray and allowing
the antibiotic to diffuse from the paper into the
agar. The experimental work has shown that the
antibiotic diffuses almost completely into the agar
and becomes almost uniformly distributed
throughout the agar in a few hours. After the diffusion
period, the papers are removed, and the
agar surface is inoculated with the test organism.
After overnight incubation, bacterial growth or
inhibition of growth can be observed. In this way,
it is possible to test directly for susceptibility to
particular concentrations of antibiotic by a procedure
which is not significantly more time-consuming
than the paper-disc method.