Testing for induction of clindamycin resistance in erythromycin-resistant isolates of Staphylococcus aureus

Disk diffusion and broth microdilution (BMD) were used to perform clindamycin (CLI) induction testing on 128 selected nonduplicate isolates of Staphylococcus aureus. Disk diffusion testing involved placing CLI and erythromycin (ERY) disks approximately 12 mm apart (measured edge to edge) on a Mueller-Hinton agar plate that had been inoculated with an S. aureus isolate; the plate was then incubated for 16 to 18 h. Two distinct induction phenotypes (labeled D and D(+)) and four noninduction phenotypes (designated as negative [Neg], hazy D zone [HD], resistant [R], and susceptible [S]) were observed in disk diffusion results. A clear, D-shaped zone of inhibition around the CLI disk was designated as the D phenotype and was observed for 21 isolates while a D-shaped zone containing inner colonies growing up to the CLI disk was designated as D(+) (17 isolates). In addition, 10 isolates were CLI susceptible and ERY resistant but were not inducible and showed no blunting of the CLI zone (Neg phenotype). Isolates that were CLI and ERY resistant (constitutive macrolide-lincosamide-streptogramin B resistance) demonstrated either a double zone of inhibition with an inner ring of reduced growth up to the edge of the disks (HD phenotype; 33 isolates) or solid growth around the CLI and ERY disks (R phenotype; 16 isolates). Finally, 31 isolates were susceptible by disk testing to both CLI and ERY (S phenotype). PCR results showed that isolates with a D phenotype harbored ermA, isolates with a D(+) phenotype contained either ermC (16 isolates) or ermA and ermC (one isolate), and all 10 isolates with a Neg phenotype contained msrA. All isolates with an HD or R phenotype harbored at least one erm gene. Isolates showing the D(+) phenotype by disk diffusion were also detected by BMD using a variety of CLI and ERY concentrations; however, isolates with the D phenotype were more difficult to detect by BMD and will likely require optimization of ERY and CLI concentrations in multilaboratory studies to ensure adequate sensitivity. Thus, at present, disk diffusion is the preferred method for testing S. aureus isolates for inducible CLI resistance.