2/17/2026: 

Bovine milk quality is important for both animal and human health. Since many years, Royal GD offers bulk tank milk (BTM) udder health monitoring programs, consisting of six or 10 BTM samples per year. It is an accessible monitoring tool for udder health and for environmental and milking hygiene. Bacteria being monitored include coliforms, - with specific attention for Klebsiella spp. -, Staphylococcus aureus, streptococci and streptococci-like bacteria, - with specific attention for Streptococcus uberis, Streptococcus dysgalactiae, and Streptococcus agalactiae.

The latter, S. agalactiae, is a long-known udder pathogen in dairy cattle, which was considered for a long time to be strictly cow-bound and highly contagious with serious consequences with respect to subclinical and clinical mastitis, but with good managerial tools to be combatted. Over the years, the prevalence of S. agalactiae decreased in many regions and was at some point even considered eradicated. In recent years, however, an increase in prevalence of S. agalactiae was described in several countries with new insights in the epidemiology of the pathogen leading to renewed attention for the pathogen.

In that same period Dutch dairy farmers reported that results of the BTM program showed alternating positive and negative culture results for S. agalactiae, referred to by them as ‘flashing light’ results. Alternating positive and negative culture results for S. agalactiae-positive herds may be due to the difficulty of reading agar plates with high numbers of other streptococci(-like) colonies. Intrigued by this phenomenon we studied consecutive BTM samples from the same herds and evaluated the added value of a commercially available chromogenic medium selective for S. agalactiae. During the 10 sampling rounds in the one-year study period from August 2020 until July 2021, BTM samples collected from 2,395 herds, varying from 1,443 to 2,180 herds per sampling round, were examined for S. agalactiae using both modified Edwards medium and the chromogenic BrillianceTM GBS Agar.

The study showed that the sensitivity of S. agalactiae culture from BTM samples using BrillianceTM GBS Agar is higher than using modified Edwards medium. Additionally, the use of BrillianceTM GBS Agar makes the work of technicians easier and quicker. We found that for optimal sensitivity of S. agalactiae and of other streptococci(-like) bacteria, the combination of the two agars is needed, as it is implemented in our laboratory.

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Authors: Annet Heuvelink, Alessandro Bellato, Manon Holstege and Theo Lam.