Evaluation of Enumeration and MPN Prediction Methods for Staphylococcus aureus

Introduction: Staphylococcal food poisoning is one of the most prevalent causes of foodborne intoxication worldwide. As such, the use of enumeration procedures for Staphylococcus aureus continues to be essential in order to monitor the safety and quality of food products. The most probable number (MPN) method is utilized for routine surveillance of products in which small numbers of S. aureus are expected or in foods predicted to contain a large population of competing species.

Purpose: The research objective was to evaluate culture methods for the detection and isolation of S. aureus, including testing different pre-enrichment broths, plated media, and the automated TEMPO instrument for the enumeration of S. aureus from assorted foods.

Methods: Seven different food commodities, including two naturally contaminated foods, were evaluated at levels ranging from 10-10⁵CFU/g of inoculated target and non-target organisms. In total, 80 food samples were analyzed. Duplicate test portions of 25g each were homogenized with 225mL of Butterfield’s phosphate buffer (1/10). Five tube MPN predictions comparing two pre-enrichment broths, TSB containing 10% NaCl and 1% sodium pyruvate (PTSBS), and Giolitti and Cantoni (GC) were evaluated alongside the TEMPO STA and the direct plate count method.

Results: Food analysis and the data demonstrated that the PTSBS pre-enrichment media for MPN prediction of S. aureus was more selective and less cumbersome to use and provided more accurate result interpretation than the GC pre-enrichment broth, with 80% proportion of overall agreement. Results for the TEMPO STA proved comparable to the reference method for S. aureus, with 90% proportion of overall agreement.

Significance: The study indicated the PTSBS performed better, overall, when evaluating accuracy and ease of use once compared to the GC broth for MPN prediction of S. aureus. Furthermore, the TEMPO STA yielded results in 24 hours that were comparable to the current four day enumeration method.