The quest for the perfect test: Phenotypic versus genotypic identification of coagulase-negative staphylococci associated with bovine mastitis

Coagulase-negative staphylococci (CNS) are a frequent cause of bovine intramammary infection and the objective of this paper is to discuss the utility of phenotypic identification systems used for species identification of CNS relative to use of genotypic identification. Mastitis control programs have been developed for major mastitis pathogens but few are specifically targeted for control of CNS. Few documented differences in treatment outcomes of mastitis caused by different species of CNS have been published, and at least one study has reported no differences in bacterial cures of mastitis caused by CNS based on genotypic identification. A number of commercial identification kits for species identification of CNS have been evaluated by mastitis researchers. Most phenotypic systems are considered to accurately identify >80% of staphylococci but have not been designed to detect all taxa that have been associated with bovine mastitis. Typical results were observed in the evaluation of agreement between 2 systems used for identification of staphylococci (n =54) isolated from cases of mastitis. Satisfactory agreement (Kappa>0.87) was achieved for API Staph but low agreement at the species level was seen for the BBL Crystal Gram-Positive system (Kappa=0.25). Results of this small study are typical of similar studies and confirm that differences occur among phenotypic identification systems. In spite of the limited precision of some phenotypic identification systems, their consistent use with an adequate number of isolates in the diagnostic algorithm is probably sufficient for most mastitis control programs, which are currently not based on species level identification. However, genotypic identification will be useful for advancing knowledge of the role of CNS in bovine mastitis.