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Disk Diffusion Testing for Detection of Methicillin-Resistant Staphylococci: Does Moxalactam Improve upon Cefoxitin?
Marie Bonjean1, Elisabeth Hodille1,2,3, Oana Dumitrescu1,2,3
1IAI, Hospices Civils de Lyon, Lyon, France.
Journal of Clinical Microbiology
|September 16, 2016
Summary
Moxalactam does not improve the detection of methicillin resistance in staphylococci using high-inoculum disk diffusion testing when combined with cefoxitin. Cefoxitin alone is sufficient for accurate identification of mecA- and mecC-positive isolates.
Area of Science:
- Clinical Microbiology
- Antimicrobial Resistance
- Diagnostic Testing
Background:
- Disk diffusion testing is a standard method for detecting methicillin resistance in staphylococci.
- Cefoxitin is the preferred marker for mecA-mediated resistance.
- Moxalactam's utility in high-inoculum testing for methicillin-heteroresistant staphylococci is uncertain as it's not in current guidelines.
Purpose of the Study:
- To evaluate the effectiveness of moxalactam, alone and with cefoxitin, in high-inoculum disk diffusion testing for detecting methicillin resistance.
- To compare moxalactam's performance against cefoxitin alone for identifying mecA and mecC genes in Staphylococcus isolates.
Main Methods:
- Evaluated inhibition zone diameters of cefoxitin and moxalactam (alone and combined) against a large collection of clinical Staphylococcus isolates.
- Utilized high-inoculum disk diffusion testing (10^8 CFU/ml).
- Performed receiver operating characteristic, sensitivity, and specificity analyses to assess diagnostic accuracy.
Main Results:
- Moxalactam, alone or combined with cefoxitin, did not enhance the detection of mecA- or mecC-positive isolates compared to cefoxitin alone.
- These findings were consistent across Staphylococcus aureus, Staphylococcus lugdunensis, Staphylococcus saprophyticus, and other coagulase-negative staphylococci groups.
- No improvement in identifying methicillin-resistant staphylococci was observed with the addition of moxalactam.
Conclusions:
- Moxalactam is not recommended as an additional marker for methicillin resistance detection in high-inoculum disk diffusion testing.
- Cefoxitin alone provides adequate sensitivity and specificity for identifying methicillin resistance in staphylococci under these testing conditions.
- Current EUCAST and CLSI guidelines using high-inoculum testing with cefoxitin are sufficient for detecting methicillin resistance.
