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Practical disk diffusion method for detection of inducible clindamycin resistance in Staphylococcus aureus and
K R Fiebelkorn1, S A Crawford, M L McElmeel
1Department of Pathology, University of Texas Health Science Center, San Antonio, Texas 78229-3750, USA. fiebelkorn@uthscsa.edu
Abstract:
Resistance to macrolides in staphylococci may be due to active efflux (encoded by msrA) or ribosomal target modification (macrolide-lincosamide-streptogramin B [MLSB] resistance; usually encoded by ermA or ermC). MLSB 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 MLSB 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 MLSB resistance in this study.
Insights
Standard disk placement reliably detects inducible macrolide-lincosamide-streptogramin B (MLSB) resistance in Staphylococcus strains. This method accurately identifies inducible MLSB resistance in Staphylococcus aureus and coagulase-negative staphylococci (CNS).
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
Background:
- Staphylococcal resistance to macrolides arises from active efflux (msrA) or ribosomal target modification (macrolide-lincosamide-streptogramin B [MLSB] resistance; ermA, ermC).
- MLSB resistance can be constitutive or inducible, with induction detected by observing the flattening of the clindamycin zone adjacent to an erythromycin disk.
- The reliability of standard disk dispenser distances (26-28 mm) for detecting inducible MLSB resistance requires reassessment.
Purpose of the Study:
- To evaluate the effectiveness of standard disk approximation (26-28 mm) for detecting inducible MLSB resistance in Staphylococcus aureus and coagulase-negative staphylococci (CNS).
- To compare the standard disk distance with closer approximations (15 or 20 mm) in identifying inducible resistance.
Main Methods:
- Disk approximation tests were performed on 130 Staphylococcus aureus and 100 erythromycin-resistant coagulase-negative staphylococci (CNS) isolates.
- PCR was used to detect ermA, ermC, and msrA genes in CNS isolates and a subset of S. aureus isolates.
- Inducible resistance was assessed using erythromycin and clindamycin disks placed at standard (26-28 mm) and closer (15, 20 mm) distances.
Main Results:
- Standard disk placement detected inducible MLSB resistance in 97% of S. aureus and 100% of CNS strains.
- Only one S. aureus isolate showed unclear induction at the 26 mm distance.
- No inducible resistance was detected in isolates with only the msrA gene or in erythromycin-susceptible strains.
Conclusions:
- Simple placement of erythromycin and clindamycin disks using a standard disk dispenser is a reliable method for detecting inducible MLSB resistance in staphylococci.
- This standard method effectively identifies inducible MLSB resistance in both S. aureus and CNS, simplifying routine laboratory diagnostics.