Effect of different media on determination of novobiocin resistance for differentiation of coagulase-negative staphylococci.

Species identification of coagulase-negative staphylococci often requires the determination of novobiocin susceptibility. Although previous investigators have recommended the use of P agar for this purpose, most clinical laboratories do not routinely utilize this medium. For this reason, studies were performed to compare novobiocin susceptibility results obtained with 11 different species of staphylococci (10 isolates of each species), using P agar, Trypticase soy agar with 5% sheep blood, and Mueller-Hinton agar. Tests performed on 70 susceptible isolates (minimal inhibitory concentration less than 1.6 micrograms/ml) resulted in zones of inhibition around 5-micrograms novobiocin disks ranging from 19.6 to 33.9, 16.2 to 26.6, and 21.3 to 36.4 mm on P agar, Trypticase soy agar with 5% sheep blood, and Mueller-Hinton agar, respectively. Forty resistant isolates (minimal inhibitory concentration greater than or equal to 1.6 micrograms/ml) exhibited zones of inhibition ranging from 6.0 to 11.3 mm on P agar, 6.0 to 11.6 mm on Trypticase soy agar with 5% sheep blood, and 6.0 to 13.5 mm on Mueller-Hinton agar. Using the established cut off of 16 mm to define novobiocin resistance for the identification of coagulase-negative staphylococci, we correctly identified 100% of the strains tested, regardless of the media utilized.