ICMSF Methods Studies. XV. Comparison of Four Media and Methods for Enumerating Staphylococcus aureus in Powdered...

Area of Science:

• Food Microbiology
• Microbial Enumeration Techniques
• Dairy Science

Background:

• Accurate enumeration of Staphylococcus aureus (S. aureus) is crucial for food safety, particularly in dried milk products.
• Evaluating diverse microbiological media and methods is essential for reliable S. aureus detection.

Purpose of the Study:

• To assess the efficacy of four different media for enumerating S. aureus in dried milk.
• To compare methods involving inoculation into liquid milk followed by drying versus direct inoculation into dried milk powder.
• To evaluate the impact of storage duration on S. aureus viability and detection.

Main Methods:

• Seven S. aureus strains were inoculated into milk (then dried) or directly into dried milk powder.
• Enumeration was performed using direct plating, enrichment, and most probable number (MPN) methods across 14 laboratories.
• Baird-Parker medium, Hauschild pork plasma fibrinogen medium, and Giolitti and Cantoni's broth (with and without Tween 80) were evaluated.

Main Results:

• Direct plating on agar in 14-cm petri dishes showed comparable utility to enrichment methods.
• Baird-Parker medium, Hauschild medium, and MPN using Giolitti and Cantoni's broth with Tween 80 demonstrated equal sensitivity for S. aureus in dried milk powder.
• S. aureus survived longer when inoculated into milk that was subsequently dried compared to direct inoculation into dried milk powder.

Conclusions:

• Several methods, including direct plating and specific selective media, are effective for S. aureus enumeration in dried milk.
• Hauschild medium may reduce the need for confirmatory tests, though variability was observed.
• The method of sample preparation (milk drying vs. direct inoculation) significantly impacts S. aureus survival and detection.