A field study evaluation of Petrifilm™ plates as a 24-h rapid diagnostic test for clinical mastitis on a dairy farm

Clinical mastitis is one of the most common and expensive diseases of dairy cattle. To make an informed treatment decision, it is important to know the causative pathogen. However, no detection of bacterial growth can be made in approximately 30% of all clinical cases of mastitis. Before selecting the treatment regimen, it is important to know whether the mastitis-causing pathogen (MCP) is Gram-positive or Gram-negative. The aim of this field study was to investigate whether using two 3M Petrifilm™ products on-farm (which conveys a higher degree of sample freshness but also bears a higher risk for contamination than working in a lab) as 24-h rapid diagnostic of clinical mastitis achieved results that were comparable to the conventional microbiological diagnostic method. AerobicCount (AC)-Petrifilm™ and ColiformCount (CC)-Petrifilm™ were used to identify the total bacterial counts and Gram-negative bacteria in samples from clinical mastitis cases, respectively. Missing growth on both plates was classified as no bacterial detection. Growth only on the AC-Petrifilm™ was assessed as Gram-positive, and growth on both Petrifilm™ plates was assessed as Gram-negative bacterial growth. Additionally, milk samples were analysed by conventional microbiological diagnostic method on aesculin blood agar as a reference method. Overall, 616 samples from clinical mastitis cases were analysed. Using the reference method, Gram-positive and Gram-negative bacteria, mixed bacterial growth, contaminated samples and yeast were determined in 32.6%, 20.0%, 2.5%, 14.1% and 1.1% of the samples, respectively. In 29.7% of the samples, microbiological growth could not be identified. Using the Petrifilm™ concept, bacterial growth was detected in 59% of the culture-negative samples. The sensitivity of the Petrifilm™ for Gram-positive and Gram-negative MCP was 85.2% and 89.9%, respectively. The specificity was 75.4% for Gram-positive and 88.4% for Gram-negative MCP. For the culture-negative samples, sensitivity was 41.0% and specificity was 91.0%. The results indicate that the Petrifilm™ concept is suitable for therapeutic decision-making at the farm level or in veterinary practice. As this concept does not allow any statement about the genus or species of microorganisms, relevant MCP should be assessed periodically at the herd level with conventional microbiological diagnostics.