Détection simple des staphylocoques résistants à la méticilline grâce à un disque de céfoxitine ou de latamoxef

Abstract

Very-low level methicillin-resistant Staphylococcus aureus (MRSA), or class 1 MRSA, is often misdiagnosed as methicillin-susceptible S. aureus (MSSA). We aimed at detecting the low-level MRSA by the disk diffusion method on a NaCl-free Mueller-Hinton agar plate flooded with 10 6 UFC/ml bacterial inoculum using the cephamycin antibiotics cefoxitin and latamoxef for 18 hours' incubation at 37°C. Detection of the mecA gene by PCR was the ⪡gold standard⪢. The cefoxitin and latamoxef disk diffusion tests detected 100% of all the MRSA classes: cefoxitin inhibition zone diameters were <27 mm and latamoxef inhibition zone diameters <24 mm. Consequently, we evaluated the cefoxitin and latamoxef disk diffusion method to detect very low-level coagulase-negative staphylococci (MRCNS) or class 1 MRCNS. The cefoxitin and latamoxef disk diffusion detected 94% and 97% of all MRCNS respectively. Cefoxitin inhibition diameters were <27 mm and latamoxef diameters <22 mm. We studied the Slidex MRSA detection® (bioMérieux) to test the PLP2a in all S. aureus and coagulase negative staphylococci included in our trials with cephamycins. This test detected 98% MRSA, and 98% MRCNS so long as the CNS sample was grown on the edge of an oxacillin or a cephamycin disk. Therefore, we recommend the use of latamoxef disk diffusion method for routine detection of all classes of MRSA and MRCNS, and the subsequent PLP2a detection in cases of inhibition zone diameters in the range of 22–24 mm. Taken together, both tests allow one to detect all class 1 methicillin-resistant staphylococci within 24 hours.