Validation of the diagnostic accuracy of the 3M™ Petrifilm™ coliform and aerobic count plates to measure colostrum bacterial contamination on Scottish dairy farms

A calf should receive 10–12 % of its bodyweight in high quality colostrum ( >50 g/L immunoglobulin) shortly after birth in order to confer passive immunity, with calves ideally receiving their first feed of colostrum in the first 1–2 hours of life (Godden et al., 2019). It is recommended (not validated) that total counts for bacteria and coliforms in colostrum should not exceed 100,000 colony forming units per mL (CFU/mL) and 10,000 CFU/mL, respectively. This study was conducted between March and September 2023. Eighty-five colostrum samples from 5 commerical dairy farms in Dumfries and Galloway in Scotland were purposively selected. Samples were collected from various stages during the colostrum harvest, storage and feeding process to provide multiple samples with variable (low, medium and high) bacterial contamination. The objective of this study was to validate the diagnostic accuracy of the 3 M Petrifilm™ Coliform Count Plate (CCP) and Aerobic Count Plate (ACP) for colostrum bacteriology. The Petrifilm™ Aerobic Count Plates were compared to 5 % sheep blood agar (SBA) (total bacteria counts, TBC), and the Petrifilm™ Coliform Count Plates were compared to MacConkey plates (MAC) (total coliform counts, TCC) and test sensitivity and specificity were calculated.

Colostrum bacterial contamination was highly variable between farms (TBC median and interquartile range= 50,000CFU/mL and 546000CFU/mL; TCC median and interquartile range=100,750CFU/mL and 188,500CFU/mL). Overall correlation between the Petrifilms™ and reference plate measures was moderate (Spearman rho=0.38–0.47). The statistical accuracy of the Petrifilm™ systems and the test sensitivity were improved by lowering the bacterial cutpoint from 100,000CFU/mL to 91,000 CFU/mL for the aerobic counts plates and from 10,000CFU/mL to 1500 CFU/mL for the coliform count plates. The Petrifilm™ system could be used as a monitoring tool to provide quick and statistically accurate results for TBC and TCC but should not be seen as a comprehensive solution for poor colostrum hygiene.