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Laboratory evaluation of 3M Petrifilms and University of Minnesota Bi-plates as potential on-farm tests for clinical mastitis

J. L. McCarron^*,1, G. P. Keefe^*,, S. L. B. McKenna^*, I. R. Dohoo^*, and D. E. Poole^*,

^* Department of Health Management,
Maritime Quality Milk, and
Centre for Veterinary Epidemiological Research, University of Prince Edward Island, Charlottetown, PEI, C1A 4P3 Canada

¹ Corresponding author: jmccarron{at}upei.ca

The objective was to determine test characteristics and compare 2 potential on-farm culture systems for clinical mastitis, the Minnesota Easy Culture System II Bi-plate and Petrifilm. The tests were evaluated using clinically positive mastitic milk samples (n = 282) to determine their ability to differentiate appropriate treatment groups; all cases that had gram-positive growth were considered treatment candidates (n = 161), whereas cases that grew gram-negative organisms only or yielded no bacterial growth were classified as no treatment (n = 121). For Petrifilm, both undiluted and 1:10 diluted milk samples were used. To create treatment categories, 2 types of Petrifilms were used, Aerobic Count (AC) and Coliform Count (CC). Both Bi-plates and Petrifilms were read after 24 h of incubation. Analysis was conducted at various colony count thresholds for the Petrifilm test system. The combination of Petrifilms that had the highest sensitivity classified a case as gram-negative if there were 20 colonies present on the CC. If there were <20 colonies present on the CC and >5 colonies present on the AC, a case would be classified as gram-positive. The Bi-plate had a sensitivity of 97.9% and a specificity of 68.6%. The Petrifilm test system had a sensitivity of 93.8% and a specificity of 70.1%. There was no significant difference in the sensitivities between the tests. All Bi-plates and Petrifilms were read by a laboratory technician and a group of masked readers with limited microbiology training. Kappa values for the masked readers were 0.75 for Bi-plates and 0.84 and 0.86 for AC and CC Petrifilms, respectively. The Bi-plate and Petrifilm were able to successfully categorize clinical cases of mastitis into 2 treatments based on their ability to detect the presence of a gram-positive organism. Neither method had the ability to determine if a sample was contaminated. The results of this study indicate that both tests were able to appropriately categorize cases, which could potentially result in a reduction in the quantity of antibiotics used to treat clinical cases of mastitis.

Key Words: mastitis • culture • Bi-plate • Petrifilm

This article has been cited by other articles:

				J. L. McCarron, G. P. Keefe, S. L. McKenna, I. R. Dohoo, and D. E. Poole Evaluation of the University of Minnesota Tri-plate and 3M Petrifilm for the isolation of Staphylococcus aureus and Streptococcus species from clinically mastitic milk samples J Dairy Sci, October 1, 2009; 92(10): 5326 - 5333. [Abstract] [Full Text] [PDF]