Conventional Identification of Streptococcus uberis Isolated from Bovine Mastitis in Argentinean Dairy Herds

Abstract

The objective of this study was to evaluate a conventional scheme for identifying Streptococcus uberis strains isolated from bovine mastitis. Seventy-five gram-positive, catalase-negative cocci were collected from cows with mastitis from 19 dairy herds located in the east-central region of Argentina. Five American Type Culture Collection strains and bovine isolates were identified by the API 20 Strep system and by restriction fragment length polymorphism analysis of 16S rDNA. A conventional scheme based on 11 biochemical tests was selected for identification of Strep. uberis strains: the Christie–Atkins–Munch-Petersen reaction; hydrolysis of Arg, esculin, and sodium hippurate; growth in inulin, mannitol, raffinose, salicin, and sorbitol; and growth at 45°C and in 6.5% NaCl. Reference strains and 25 bovine isolates were classified accurately to the species level by the conventional scheme in a blind assay. Each reference strain and each bovine isolate were identified as belonging to the same species following these 3 methods. The remaining 50 isolates identified as Strep. uberis by the API 20 Strep system and 16S rDNA RFLP were assayed by the conventional scheme. This scheme correctly identified 47 (94%) of 50 isolates as Strep. uberis by comparing their biochemical profile with that of the reference strain. Three (6%) of the 50 isolates were classified as Strep. uberis by the API 20 Strep system and by 16S rDNA RFLP and were identified as Enterococcus faecalis by the conventional scheme. Thirty percent of the Strep. uberis strains showed biochemical profiles identical to the Strep. uberis American Type Culture Collection 27958 strain. Seventy percent of the Strep. uberis strains demonstrated variability compared with the reference strain, resulting in 19 different biochemical profiles. The conventional scheme proposed in this study resulted in a relatively low number of misidentifications and could biochemically identify not only typical, but also atypical Strep. uberis strains. This conventional scheme can be considered an adequate method for identifying Strep. uberis strains isolated from bovine mastitis because of its affordable cost in developing countries, and it may contribute to determining the frequency of isolation of Strep. uberis strains in Argentinean dairy herds.