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Evaluation of Petrifilm for the isolation of Staphylococcus aureus from milk samples
B O Silva1, D Z Caraviello, A C Rodrigues
1Department of Dairy Science, University of Wisconsin, Madison 53706, USA.
Abstract:
Four experiments were conducted to evaluate the Petrifilm Staph Express Count plate (3M, Minneapolis, MN) for diagnosis of mastitis caused by Staphylococcus aureus. The objective of experiment 1 was to determine the sensitivity of Petrifilm compared with results of standard and augmented microbiological techniques, and the objective of experiment 2 was to compare microbiological results of composite and quarter milk samples processed using Petrifilm. Experiment 3 was conducted to determine the specificity of the Petrifilm method based on different interpretation parameters, and the objective of experiment 4 was to determine the repeatability of reading Petrifilm Staph Express plates. Results of standard microbiological techniques used for experiments 1 and 2 were compared with results of samples preprocessed using centrifugation or preincubation. The prevalence of recovery of Staph. aureus from milk samples processed using Petrifilm was significantly greater than the prevalence of milk samples processed using standard microbiological techniques. The sensitivity of isolation of Staph. aureus was 65.6, 75.0, 84.4, and 87.5% for standard, centrifugation, incubation, and Petrifilm methods, respectively. The occurrence of a distinct pink zone surrounding a colony was highly specific for Staph. aureus, and the specificity was 98.5 and 96.0% for experiments 3 and 4, respectively. The use of a weak pink zone to diagnose Staph. aureus resulted in a high rate of false-positive results. The interpretation of results of Petrifilm Staph Express was associated with the person that read the plates. Results from all 4 experiments indicate potential for the Petrifilm products as a diagnostic tool in some herd situations when Staph. aureus is the pathogen of interest. Results also indicate the need for standardization of interpretive criteria for personnel working with the products.
Insights
The Petrifilm Staph Express Count plate shows potential for diagnosing Staphylococcus aureus mastitis, offering higher sensitivity than standard methods. However, clear interpretation guidelines are needed for accurate results.
Area of Science:
- Veterinary Microbiology
- Dairy Science
- Food Safety
Background:
- Mastitis, a significant concern in dairy herds, is often caused by Staphylococcus aureus.
- Accurate and rapid diagnosis of Staphylococcus aureus is crucial for effective mastitis management.
- Traditional microbiological techniques can be time-consuming and may lack optimal sensitivity.
Purpose of the Study:
- To evaluate the diagnostic performance of the Petrifilm Staph Express Count plate for Staphylococcus aureus.
- To compare the sensitivity and specificity of the Petrifilm method against standard microbiological techniques.
- To assess the impact of different milk sample types and interpretation parameters on diagnostic accuracy.
Main Methods:
- Four experiments were conducted involving Petrifilm Staph Express Count plates and standard microbiological methods.
- Milk samples were analyzed using composite and quarter samples, with and without pre-processing (centrifugation, pre-incubation).
- Sensitivity, specificity, and repeatability of the Petrifilm method were determined, with attention to colony appearance and reader interpretation.
Main Results:
- The Petrifilm method demonstrated significantly greater recovery of Staphylococcus aureus compared to standard techniques.
- Sensitivity for Staphylococcus aureus isolation was highest with the Petrifilm method (87.5%), followed by incubation, centrifugation, and standard methods.
- A distinct pink zone was highly specific (98.5%), while weak pink zones led to false positives; interpretation varied between readers.
Conclusions:
- The Petrifilm Staph Express Count plate shows promise as a diagnostic tool for Staphylococcus aureus in certain herd settings.
- The Petrifilm method offers improved sensitivity for Staphylococcus aureus detection in milk samples.
- Standardization of interpretive criteria is essential for consistent and reliable results across different personnel.
