A rapid slide test for penicillinase.

The authors developed a rapid slide test modification of the iodometric method for detection of penicillinase produced by organisms growing on routine plating media. A loopful of colonies is scraped from the agar surface and emulsified in one drop of an iodine-penicillin solution on a glass slide. Addition of a drop of 0.4% starch solution results in a purple color when penicillinase is not present; a colorless reaction denotes a positive test. The slide test yielded positive results identical to those of a starch agar-plate method with 26 Staphylococcus aureus isolates; a further seven showed comparable negative tests. Penicillinase production was associated with a S. aureus penicillin MIC of greater than or equal to 0.5 micron/ml. All 15 Staphylococcus epidermidis isolates gave negative test results, as did 22 Bacteroides fragilis (MIC greater than or equal to 3.1). Twenty ampicillin-susceptible Haemophilus influenzae were negative by both the slide test and a Levinthal broth method; an additional five resistant (MIC greater than or equal to 10) isolates were positive by both methods. Twenty-eight (penicillin MIC greater than or equal to 0.8) of 50 Bacteroides melaninogenicus were slide test-positive for penicillinase. Two penicillinase-producing strains of Neisseria gonorrhoeae gave positive slide tests, while eight other non-penicillinase-producers were negative.