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Identification of cephalosporin-resistant Staphylococcus aureus with the disc diffusion method
Antimicrobial Agents and Chemotherapy
|May 1, 1972
Summary
Detecting heteroresistant Staphylococcus aureus is crucial. Cephalexin discs accurately identify these methicillin-resistant strains, unlike cephalothin discs which can lead to misclassification in susceptibility testing.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Antimicrobial Resistance
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) poses a significant public health threat.
- Heteroresistance in MRSA complicates accurate susceptibility testing.
- Standardized methods are needed for reliable detection of cephalosporin resistance.
Purpose of the Study:
- To evaluate the efficacy of different disc diffusion methods for detecting heteroresistant MRSA.
- To identify optimal conditions for susceptibility testing of S. aureus to cephalosporins.
Main Methods:
- Analysis of 84 MRSA strains from 16 laboratories across 8 countries.
- Disc diffusion testing using cephalothin and cephalexin discs.
- Standardized inoculum preparation, incubation temperature (30°C), and time (24 hr) on Mueller-Hinton agar.
Main Results:
- All tested MRSA strains exhibited resistance to 32 µg/ml cephalothin with a heteroresistant pattern.
- Cephalexin discs effectively detected heteroresistance.
- Cephalothin discs resulted in 26-49% false classifications (Group I or II) after 24 hours.
Conclusions:
- Cephalexin disc diffusion is recommended for reliable detection of heteroresistant MRSA.
- Specific testing parameters (30-µg cephalexin discs, Mueller-Hinton agar, 10⁶ CFU/ml inoculum, 30°C, 24 hr) are proposed.
- A zone diameter <10 mm with cephalexin discs suggests presumptive heteroresistance, aligning with MIC breakpoints.