Affiliation:
1. Department of Pathology, University of Texas Health Science Center, and Microbiology Laboratory, University Hospital, San Antonio, Texas 78229
Abstract
ABSTRACT
Resistance to macrolides in staphylococci may be due to active efflux (encoded by
msrA
) or ribosomal target modification (macrolide-lincosamide-streptogramin B [MLS
B
] resistance; usually encoded by
ermA
or
ermC
). MLS
B
resistance is either constitutive or inducible following exposure to a macrolide. Induction tests utilize closely approximated erythromycin and clindamycin disks; the flattening of the clindamycin zone adjacent to the erythromycin disk indicates inducible MLS
B
resistance. The present study reassessed the reliability of placing erythromycin and clindamycin disks in adjacent positions (26 to 28 mm apart) in a standard disk dispenser, compared to distances of 15 or 20 mm. A group of 130 clinical isolates of
Staphylococcus aureus
and 100 isolates of erythromycin-resistant coagulase-negative staphylococci (CNS) were examined by disk approximation; all CNS isolates and a subset of
S. aureus
isolates were examined by PCR for
ermA
,
ermC
, and
msrA
. Of 114 erythromycin-resistant
S. aureus
isolates, 39 demonstrated constitutive resistance to clindamycin, while 33 showed inducible resistance by disk approximation at all three distances. Only one isolate failed to clearly demonstrate induction at 26 mm. Of 82 erythromycin-resistant CNS isolates that contained
ermA
or
ermC
, 57 demonstrated constitutive clindamycin resistance, and 25 demonstrated inducible resistance, at 20 and 26 mm. None of the 42
S. aureus
isolates or 18 CNS isolates containing only
msrA
and none of the erythromycin-susceptible isolates yielded positive disk approximation tests. Simple placement of erythromycin and clindamycin disks at a distance achieved with a standard disk dispenser allowed detection of 97% of
S. aureus
strains and 100% of CNS strains with inducible MLS
B
resistance in this study.
Publisher
American Society for Microbiology