Diclofenac-cefoxitin double-disk method is a novel tool for phenotypic detection of methicillin resistant Staphylococcus aureus: a preliminary report


Background: the detection of methicillin resisatance is essential for both the institution of appropriate antimicrobial therapy and infection control measures. The development of largely divergent mecA genes by the new methicillin resistant Staphylococcus aureus (MRSA) made the present ‘gold-standard’ tests unable to determine whether they are MRSA or not. Therefore, there is an essential need to develop a new MRSA testing method. Aim of Study: to demonstrate diclofenac (Dc) to be a very strong inducer of low expression methicillin resistance in MRSA. Materials and Methods: standard disk diffusion method was performed for 26 Staphylococcus aureus isolates obtained from blood culture and wound swabs from patients admitted to Ramadi Teaching Hospital (from October, 2010 to March, 2011), against cefoxitin (FOX) 30µg disk, and MRSA screen test (Denka Seiken, Japan), as indicators to detect the presence of MRSA, and against selected β-lactam and non-β-lactam antimicrobial agents. Double disk tests (D-test) were performed to determine MLSB-inducible resistance mechanism and to assess the activity of Dc 50µg disk in approximation with selected antimicrobial agents. Results: Both in complete agreement, FOX disk diffusion test and MRSA screen test revealed that 19/26 (73.1%) of the isolates appeared as MRSA, and 7/26 (26.9%) as methicillin susceptible Staphylococcus aureus (MSSA). In MRSA isolates only, Dc produced a paradoxical antagonistic effect upon combination with β-lactam drugs only, and this effect appeared in all the 19 MRSA isolates; while with non-β-lactam drugs, it potentiated their antistaphylococcal activity including inducible-MLSB isolates.Conclusion: Dc is suggested to be a strong inducer of the expression of methicillin resistance. The use of Dc in detecting MRSA could be considered as a backup test with FOX to improve the accuracy of phenotypic detection of MRSA