Improvement of the Detection Sensitivity for Staphylococcus aureus in Spices and Herbs

Abstract

Various variants of ISO 6888-1:1999 and ISO 6888-3:2003 methods for the detection of Staphylococcus aureus were evaluated and improved for their application to conduct analysis in spices and herbs. Improvement substantiated in washing of the sample to remove compounds interfering with the analysis and in the use of PCR for final detection, instead of plating on Baird-Parker agar, to improve sensitivity at high backgrounds and to save time. The evaluation of the method variants was based on determination of the detection limit (LOD) using series of artificially contaminated spices (allspice, black pepper, cinnamon, nutmeg, paprika, vanilla) and herbs (basil, oregano, parsley, thyme). The method without enrichment, ISO 6888-1:1999, produced LODs of 103–105 CFU/g with no positive effect of washing the sample or use of PCR for final detection. The method with enrichment, ISO 6888-3:2003, had LOD of 100 CFU/g for basil, black pepper, paprika and parsley. If the washing step was added and PCR was used for final detection, LOD of 100 CFU/g was determined also for cinnamon, nutmeg and vanilla, and LOD of 101 CFU/g was determined for allspice. For oregano and thyme, which strongly inhibit the growth of S. aureus, an alternative enrichment-independent method based on direct DNA extraction coupled to real-time PCR may be advantageous.