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

The primary objective was to compare microbiological results of the University of Minnesota Tri-plate and the 3M Petrifilm Staph Express (STX) Count Plate to standard culture techniques for identification of clinical mastitis caused by Staphylococcus aureus. The secondary objective was to evaluate the Tri-plate's ability to differentiate Streptococcus spp. from other gram-positive organisms. The tests were evaluated using clinically positive mastitic milk samples (n = 282) to determine their ability to diagnose the pathogens of interest. A Tri-plate was classified positive for Staph. aureus when at least 1 colony exhibiting beta-hemolysis was present on the Factor media portion of the plate. When the plate was used in this manner and read by a trained laboratory technician, the sensitivity of the Tri- plate was 97.9% and the specificity was 81.8%. When the Tri- plate was evaluated by the laboratory technician for its ability to diagnose Streptococcus spp., both sensitivity and specificity of the test were very good (92.6 and 89.5%, respectively). Using the Petrifilm, samples were classified as positive for Staph. aureus if any red-violet colonies were present on the Petrifilm after an initial 24-h incubation. When used in this manner, the Petrifilm had a sensitivity of 97.4% and a specificity of 76.1%. Further evaluation of the Petrifilm was done using the STX disk, which was used to confirm the presence of Staph. aureus. When using the presence of 1 pink colony on the disk, the sensitivity of the Petrifilm was 92.1% and the specificity was 93.1%. Both the Triplate and the 3M STX Petrifilm successfully diagnosed Staph. aureus in clinical milk samples when used in a laboratory setting and the Tri-plate successfully differentiated Streptococcus spp. from other gram-positive organisms.