Diffusion Techniques
Quantitative methods of susceptibility testing that require measurement of zone diameters or minimal inhibitory concentrations
(MIC’s) give the most precise estimates of antibiotic susceptibility. One such procedure has been recommended for use with discs to
test susceptibility to this class of drugs. Interpretations correlate diameters on the disc test with MIC values. A penicillinase-resistant
class disc may be used to determine microbial susceptibility to cloxacillin, dicloxacillin, methicillin, nafcillin, and oxacillin. With this
procedure, employing a 5 microgram methicillin sodium disc, a report from the laboratory of “susceptible” (zone of at least 14 mm)
indicates that the infecting organism is likely to respond to therapy. A report of “resistant” (zone of less than 10 mm) indicates that the
infecting organism is not likely to respond to therapy. A report of “intermediate susceptibility” (zone of 10 to 13 mm) suggests that the
organism might be susceptible if high doses of the antibiotic are used, or if the infection is confined to tissues and fluids (eg. urine), in
which high antibiotic levels are attained.
In general, all staphylococci should be tested against the penicillin G disc and against the methicillin disc. Routine methods of
antibiotic susceptibility testing may fail to detect strains of organisms resistant to the penicillinase-resistant penicillins. For this
reason, the use of large inocula and 48-hour incubation periods may be necessary to obtain accurate susceptibility studies with these
antibiotics. Bacterial strains which are resistant to one of the penicillinase-resistant penicillins should be considered resistant to all of
the drugs in the class.