Characterisation of staphylococci associated with clinical and subclinical bovine mastitis

Abstract

Attempts were made to identify 900 species of staphylococci or micrococci recovered from samples of bovine milk examined for mastitis pathogens. The presence and identity of haemolysins was recorded together with results of disc diffusion antibiotic sensitivity tests. The occurrence of clinical mastitis was also noted and somatic cell counts (SCC) were performed on milk samples which were normal in appearance.

Eight hundred and thirty-one coagulase positive staphylococci were obtained, of which 810 were S. aureus and 21 were S. intermedius. Of 65 coagulase negative staphylococci the species of 19 could not be determined by the identification systems used. The remainder were identified as S. hyicus sub sp. hyicus (1), S. hyicus sub sp. chromogenes (19), S. haemolyticus (17), S. hominis (3), S. epider-midis (4), S. capitis (1) and either S. hominis or S. warneri (1). Four other isolates could not clearly be assigned to the genus Staphylococcus or Micrococcus and were designated “irregular strains”. No micrococci were identified.

The presence of alpha (α), beta (β), or delta (δ) haemolysins occurring singly or in various combinations was identified in 98.3% of coagulase positive staphylococci and in 60% of coagulase negative staphylococci. Epsilon (ε) haemolysin was detected in 47.6% of the coagulase negative staphylococci and in 9.5% of S. intermedius.

All staphylococci were sensitive to tetracycline (30 μg), novobiocin (1.6 μg), nafcillin (30 μg), methicillin (10 μg) and cephalothin (30 μg) and variable numbers of each species were sensitive to penicillin (2 iu) and streptomycin (10 μg). One non-identified species of coagulase negative staphylococcus was sensitive to erythromycin (0.4 μg), the remaining staphylococci were resistant. Each of the four “irregular strains” was sensitive to erythromycin and novobiocin.

Clinical mastitis was associated with 30.6% of coagulase positive staphylococci, 15.3% of coagulase negative staphylococci, and two of the four “irregular strains” (50%). Subclinical mastitis as determined by SCC of 500 × 10³ or greater was associated with 92.7% of coagulase positive and 37.5% of coagulase negative staphylococci.