Screening for penicillin tolerance in viridans streptococci by a simple disk method.

A disk method was developed for demonstrating penicillin tolerance in viridans streptococci. This was achieved by the substitution of the penicillin disk used for susceptibility testing by a disk containing penicillinase. After reincubation, penicillin-tolerant strains exhibited new growth in the area adjacent to this disk, providing a rapid screening test.