Notes

Staphylococcus Aureus Confirmation Count Plate
Staphylococcus aureus Confirmation Count Plate is a sample-ready selective culture system for the isolation and enumeration of bacteria. It contains a modified Baird-Parker medium that is selective and differential for Staphylococcus spp. and a Staph Express disk that contains deoxyribonuclease and a dye, which react to produce a pink zone around colonies of Staph. aureus. It is marketed for use in mastitis identification and monitoring of herds to prevent spread of disease in the milking parlor (Mason et al., 1999).

A number of supplementary tests are available for the detection of Staph. aureus in milk samples, such as mannitol and coagulase reactions, and hemolytic patterns. These tests are intended to help identify the bacteria to aid in a correct diagnosis of mastitis, and to differentiate bacterial isolates from other species using their morphology and hemolytic characteristics. Other auxiliary methods such as indole, esculin, motility, and triple sugar iron slants can be used to identify Staphylococcus and Streptococcus.


The characterization of mastitis pathogens is essential in the control of mastitis, and rapid and accurate methods for their detection are required. Many of the specialized, augmented cultures required for this purpose are time-consuming and require a high level of skill to perform. These methods are also labor-intensive and expensive, and may limit their use in the field due to limited availability of trained personnel and laboratory facilities.

In the field, a simple and inexpensive alternative to augmented methods is desired. The Petrifilm Staph Express Count plate (3M, Minneapolis, MN) is a sample-ready chromogenic, differential, Staphylococcus-specific medium. It contains a modified Baird-Parker agar that is selective and differential for Staphylococcus, and a Staph Express disk that has a deoxyribonuclease which binds to the chromogenic dye, producing a pink zone surrounding Staph. aureus colonies. This is a very sensitive method for the detection of Staph. aureus, and results are reported in 24 to 26 h as opposed to 48 h for standard culture methods.

Experiments were performed to determine the sensitivity, specificity, and predictive value of the Petrifilm Staph Express Count plates. Milk samples from herds with clinical mastitis were collected and inoculated on the Petrifilm Staph Express agar, and on the Baird-Parker agar with and without the Staph Express disk. Milk Test kits of both the agar plates and the agar with the Staph Express disk were compared to standard and augmented methods for mastitis identification, using composite and quarter milk samples.

The sensitivity of the Petrifilm Staph Express plate was determined by comparing the proportion of Staph. aureus isolated from each sample by the 3 methods. The specificity of the Petrifilm Staph Express was evaluated by determining the percentage of bacteriologically negative samples that contained Staph. aureus, and by examining the reproducibility of the method between several individuals who read the plates. The predictive value of the Petrifilm Staph Expres was evaluated by comparing the predicted probability that a positive test is Staph. aureus with the proportion of Staph. aureus that had a pink zone in the agar with and without the Staph Expres disk.



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