A Survey of Coliforms and Staphylococcus aureus in Cheese Using Impedimetric and Plate Count Methods ¹

Area of Science:

• Food Microbiology
• Dairy Science
• Analytical Chemistry

Background:

• Accurate microbial enumeration is crucial for food safety and quality control in the dairy industry.
• Traditional plate count methods can be time-consuming, necessitating faster and reliable alternatives.
• Impedance microbiology offers a rapid method for detecting microbial contamination in food matrices.

Purpose of the Study:

• To evaluate the efficacy of impedance microbiology (Bactometer) against traditional plate counts for coliforms and Staphylococcus aureus in cheese.
• To determine the correlation coefficients between the two methods across various cheese types and contamination levels.

Main Methods:

• Analysis of 256 cheese samples for coliforms using violet red bile agar (plate count) and Bactometer Coliform Medium (impedance).
• Analysis of 200 cheese samples for Staphylococcus aureus using Baird-Parker medium (plate count) and Bactometer S.aureus Medium (impedance).
• Inoculation studies with Escherichia coli and staphylococci in cheese samples to validate method correlations (R values).

Main Results:

• High correlation coefficients (R=-.91 for coliforms, R=0.98 for S. aureus) were observed between plate and impedance methods.
• Coliform counts ranged from 10^2 to 10^7 colony forming units/gram (CFU/g) in 54% of samples, with highest levels in cream and fresh cheeses.
• Only 2% of samples exceeded 10^3 CFU/g for Staphylococcus aureus; isolated strains were coagulase-positive.

Conclusions:

• The impedance method (Bactometer) demonstrates a strong correlation with traditional plate counts for enumerating coliforms and Staphylococcus aureus in cheese.
• Impedance microbiology is a viable and efficient alternative for rapid microbial assessment in cheese products.
• The findings support the adoption of impedance methods for routine quality control in the dairy industry.