Purity plate disk induction testing for clindamycin resistance compared to standard disk induction testing on Mueller-Hinton agar and results of VITEK 2 susceptibility testing

Species (no. of isolates)PhenotypeaNo. of strainsNo. (%) of strains detected by purity plate induction testbNo. (%) of strains detected by VITEK 2 aloneb
S. aureus (75)CS2800
CR-Ind22c22 (100)0
CR-Const2525d (100)25 (100)
Coagulase-negative staphylococci (75)CS1200e
CR-Ind2626 (100)0e
CR-Const3737 (100)37f (100)
  • a Determined by standard disk diffusion test and disk induction test with a disk spacing of 20 mm on Mueller-Hinton agar. CS, clindamycin susceptible, CR-Ind, inducible MLSB resistance to clindamycin; CR-Const, constitutive MLSB resistance to clindamycin.

  • b Detection of clindamycin resistance, reported as either intermediate or resistant for VITEK 2 and either induced (CR-Ind isolates, showing blunting of zone), or constitutive (CR-Const isolates, showing growth to the edge of the clindamycin disk) on the purity plate.

  • c Includes two isolates with intermediate MICs of erythromycin by VITEK 2 (1 and 4 μg/ml); both of these isolates were purity plate screen and disk induction test positive.

  • d One S. epidermidis and seven S. aureus isolates showed blunting of an outer zone and heavy inner growth to the edge of the disk. All of these isolates were reported as resistant by VITEK 2 and contained erm(A).

  • e Of the 75 coagulase-negative staphylococcal isolates, four had insufficient growth for testing on VITEK 2 (three CS and one CR-Ind).

  • f A very major error was reported for one isolate (VITEK 2 erythromycin MIC, 0.5 μg/ml; resistant by the standard disk diffusion test).