Correlative interpretation of staphylococcal resistance to penicillinase-resistant penicillins by ceftizoxime disk susceptibility test using Showa disks.

The Showa disk susceptibility test (Showa Yakuhin Kako Company, Japan) was evaluated to discriminate between the strains of Staphylococcus aureus resistant to penicillinase-resistant penicillins (PRPs) and those susceptible. When we tested 129 PRP-resistant and 112 susceptible strains, many PRP-resistant strains were interpreted to be false susceptible to methicillin and oxacillin, especially when incubated at 37 degrees C. Growth at 30 degrees C and when NaCl was added to the medium improved the test reliability for detection of PRP resistance. Ceftizoxime disk (30 micrograms) susceptibility test results highly correlated with reference PRP resistance when incubated at 35 degrees C. All the PRP-resistant strains produced no zone of inhibition, whereas all of the PRP-susceptible strains were categorized as susceptible to ceftizoxime (greater than or equal to 22 mm, less than or equal to 3.13 micrograms/ml), for example, 100% correlation. It was concluded that the Showa ceftizoxime disk, in replacement of PRPs, will provide a reliable, alternative method to predict PRP resistance in S. aureus pending reevaluation of the Show PRP disk tests.