Rev Esp Quimioter 2008;21(4):213-216

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

N. Batista Díaz ,  I. Gutiérrez González ,  M. Lara Pérez ,  F. Laich ,  S. Méndez Álvarez 


Oxacillin tests may fail to detect some methicillinresistant S. aureus populations. The objective of this study is to evaluate the discriminative capacity of the Clinical and Laboratory Standards Institute (CLSI) disk diffusion method with a cefoxitin 30 μg disk on S. aureus isolates with unusual phenotypic characteristics of antimicrobial resistance. We studied 53 clinical S. aureus isolates. The antimicrobial susceptibility of all isolates was routinely studied by the VITEK 2 System (bioMérieux). Methicillin resistance was also studied by CLSI oxacillin method and confirmed by a previously described multiplex polymerase chain reaction (PCR) method which permits 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 μg disk, according to the CLSI guidelines (susceptibility: > 22 mm; resistance: < 21 mm). Control strains for all assays included MRSA ATCC 43300 and MSSA ATCC 25923. The isolates formed four groups. Group I: 20 multiresistant, oxacillin susceptible and mecA negative isolates; group II: 16 resistant or with intermediate oxacillin susceptibility and mecA negative isolates; group III: 11 heteroresistant and mecA positive isolates; group IV: six mecA positive isolates with atypical resistance profiles (penicillin and oxacillin, or ciprofloxacin and erythromycin resistance). Thirty-five mecA negative isolates included in groups I and II showed inhibition zones > 22 mm; one isolate from group II showed 20 mm. The 17 mecA positive isolates from groups III and IV showed resistance to cefoxitin disk. The 30 μg cefoxitin disk diffusion method is proposed as an efficient method for the detection of methicillin resistance and permits a clear determination set S. aureus isolates, even those with atypical antimicrobial characteristics.


Key words: Staphylococcus aureus. Cefoxitin. Methicillin-resistance


Rev Esp Quimioter 2008;21(4):213-216 [pdf]