The Added Value of Brilliance GBS Agar for Screening Bulk Tank Milk for Streptococcus agalactiae

This study aimed to evaluate the added value of chromogenic Brilliance^TM GBS Agar to the generally used modified Edwards medium for the isolation of S. agalactiae from bovine bulk tank milk (BTM) samples in programs to monitor udder health and hygiene in dairy farms. During 10 BTM sampling rounds in a one-year study period, BTM samples were examined for the presence of S. agalactiae using both, modified Edwards medium and Brilliance^TM GBS Agar. The prevalence of S. agalactiae per sampling round based on modified Edwards medium alone varied from 0.7% to 2.1%, and based on the combination of modified Edwards medium and Brilliance^TM GBS Agar from 2.3% to 2.8% of samples. Of 1,352 herds with culture results available for all 10 sampling rounds, 59 (4.4%) tested positive for S. agalactiae at least once; 79.7% of these 59 herds were identified on modified Edwards medium and 93.2% on Brilliance^TM GBS Agar. Using both agars, the percentage of herds with at least one negative culture result between positive results decreased from 68.1% to 53.2% of positive herds, thus leading to a more consistent herd-level S. agalactiae diagnosis. However, for 10.6% of the herds, an irregular S. agalactiae culture result was introduced by using both agars. The use of Brilliance^TM GBS Agar makes the work of technicians easier and quicker in comparison with modified Edwards medium. For optimal sensitivity of S. agalactiae and of other streptococci(-like) bacteria, the combination of the two agars is needed.