Streptococcus agalactiae

sector Strep (streptococci), small bright green - blue colonies, marked with the number 10.

Streptococcus agalaciae belongs to the so-called infectious causative agents of bovine mastitis, is well adapted to the mammary gland and has an affinity for milk. It does not reproduce in the external environment, but is able to survive for several weeks.


The source of infection is the infected mammary gland of dairy cows, other animals, humans, or the infected environment. A constant source of infection in the herd are weakly infected dairy cows, which secrete bacteria irregularly and may not be detected by microbiological test. In practise it means, that any dairy cow coming from a farm where S. agalactiae occurs is considered to be suspected of being infected.


The clinical signs of mastitis caused by S. agalactiae are diverse and latent, subclinical forms often developing acute catarrhal mastitis after delivery.


On the PM test, Streptococcus agalactiae grows in small colonies of blue-green colour, within 24 hours of applying the milk to petri dish. Then it is possible to check it in our cooperating laboratory (MALDI TOF about 1-2 hours).  It is important to test the antibiotics sensitivity within the next day. From the point of view of good practice in antibiotic therapy with regard to the development of resistance, we choose narrow-spectrum antibiotics, such as benzylpenicillin, according to the results of the antibiogram.