Evaluation the cefoxitin 30 mG disk diffusion method for detection of methicillin-resistance in selected Staphylococcus aureus isolates

Abstract number: 1733_1097

Lara M., Batista N., Laich F., Méndez-Álvarez S.

Objective: To evaluate the discriminative capacity of the CLSI disk diffusion method with a cefoxitin 30 mg disk on S. aureus isolates with unusual phenotypic characteristics of antimicrobial resistance.

Material and Methods: We studied 53 clinical S. aureus isolates from N.S. Candelaria Hospital, Tenerife, Spain. The antimicrobial susceptibility of all isolates was routinely studied by the VITEK 2 System (bioMèrieux). The methicillin resistance was also studied by CLSI oxacillin method, and confirmed by a previously described multiplex PCR method which permitts S. aureus identification and simultaneous detection of methicillin resistance.

MecA positive isolates presenting a diffuse growth within the zone of inhibition when exposed to oxacillin, were considered heteroresistant; mecA negative, oxacillin intermediate or resistant isolates were considered borderline.

All the isolates were tested with a cefoxitin 30 mg disk, according to the CLSI guidelines (susceptibility: ≥20 mm; resistance: ≤19 mm). Control strains for all assays included MRSA strain ATCC 43300, MSSA ATCC 25923, and MRSA B/3/02 SEIMC. Secondly, we also considered the criterium of Skov et al (J Clin Microbiol, Oct 2006) who proposes a zone diameter for cefoxitin susceptibility (≥22 mm).

Results: The isolates formed four groups. The first one comprised 20 multi-resistant but oxacillin susceptible and mecA negative isolates; the second group

was a series of 16 borderline isolates; the third group comprised a selection of 11 heteroresistant isolates, and the fourth group was a series of six mecA positive isolates with atypical resistance profiles (penicillin and oxacillin, or ciprofloxacin and erythromycin resistance).

The 36 mecA negative isolates included in groups 1 and 2 showed susceptibility to cefoxitin disk. Fifteen mecA positive isolates from groups 3 and 4 showed inhibition zones ≤ 19 mm; two isolates from these groups (one of each), showed 20 and 21 mm respectively. So, sensitivity was 96.3% (51 of 53 isolates). After applying the Skov proposal, the susceptibility was 100%.

Conclusions: The 30 mg cefoxitin disk diffusion method is proposed as an efficient method for the detection of methicillin resistance and permitts a clear determination for S. aureus isolates with atypical antimicrobial characteristics.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
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