A comparison of methods used in species identification of coagulase-negative staphylococci isolated from the milk of sheep.

Thirty reference strains of staphylococci, 76 strains isolated from confirmed cases of subclinical mastitis and 10 strains from the teat tip skin and the mouth of sucking lambs were assigned to species using four biochemical methods. These were the commercially available micromethods API Staph and Staph-Zym; the specialized laboratory method Rosco Set and Reactions in Standard Laboratory Culture Media (SLCM). The Rosco Set assigned species to all the strains, the SLCM to 96.5%, the Staph-zym to 86.1% and the API Staph to 81.4%. API Staph and the SLCM favoured the 'aureus' group with the 'epidermidis' second, while the reverse was true for the other two methods. All the identification methods assigned the same species to only 29% of the tested isolates, causing considerable variation in the number of species identified within each group. This variation indicates that reported species prevalence from cases of ovine mastitis should be viewed with caution.