Investigation of inducible clindamycin and telithromycin resistance in isolates of β-hemolytic streptococci

Abstract

We evaluated the accuracy of an erythromycin–clindamycin double-disk test (D-zone test) and an erythromycin–telithromycin D-zone test for detection of inducible resistance in isolates of β-hemolytic streptococci with erythromycin resistance. The results of these tests were compared to results of a broth microdilution (BMD) induction test using combinations of erythromycin and either clindamycin or telithromycin. Of 29 erythromycin-resistant, clindamycin-susceptible isolates, 16 were positive by the erythromycin–clindamycin D-zone test; all of these demonstrated inducible clindamycin resistance by BMD. Twelve isolates were D-zone test–negative, did not demonstrate inducible resistance by BMD, and were positive for a mef determinant. Of 39 erythromycin-resistant, telithromycin-susceptible isolates, 13 were erythromycin–telithromycin D-zone test–positive, 19 questionably positive (unclear blunting of the zone), and 7 were D-zone test–negative. The erythromycin–telithromycin D-zone test result did not correlate with inducible resistance by BMD or the presence of an erm or mef gene. These results demonstrate that the erythromycin–clindamycin D-zone and BMD induction tests accurately detect inducible clindamycin resistance, but the erythromycin–telithromycin D-zone test is not reliable for detecting inducible telithromycin resistance.