Lysostaphin disk test for routine presumptive identification of staphylococci.

The sensitivity of the lysostaphin disk test was evaluated for routine differentiation of staphylococci from micrococci. Lyophilized paper disks impregnated with 10 micrograms of lysostaphin were placed on a Mueller-Hinton agar plate inoculated with 10 species of staphylococci and 7 species of micrococci. After 16 h of incubation at 37 degrees C, a circular zone of growth inhibition was noted for all staphylococci species tested. By contrast, all the micrococci species were resistant. The activity of the impregnated disks was not diminished by storing them at 4 degrees C for 3 months. With disks impregnated by 10, 6.6, 5, and 2.5 micrograms of lysostaphin, diameters of the growth inhibition zone for S. aureus Cowan 1 NCTC 8530 strain were proportional to the logarithm of the quantity of lysostaphin present in each disk.