The genetic determinants involved in metal utilization play a critical role in the growth of Streptococcus dysgalactiae subsp. dysgalactiae in milk

Streptococcus dysgalactiae subsp. dysgalactiae is a pathogen that causes mastitis in dairy cows, resulting in significant economic losses in dairy production worldwide. This study aimed to identify the genetic determinants of S. dysgalactiae that are required for its growth in milk and adhesion to mammary epithelial cells. Transcriptome analysis revealed 936 genes that were significantly differentially expressed in milk compared with optimal medium, among which the gene with the greatest upregulation was dppA, which putatively encodes a lipoprotein tethered to the membrane with a lipid anchor. The dppA gene deletion significantly impaired the tolerance of lactoferrin, an antimicrobial protein in cow milk, and the ability to adhere to mammary epithelial cells, but did not affect growth in milk. Moreover, another lipoprotein gene, mtsA, which is involved in metal transport, was identified in S. dysgalactiae. Unlike dppA, the deletion of mtsA not only impacts lactoferrin tolerance and cell adhesion but also results in growth inhibition in milk. Further studies suggested that this effect was caused by a disturbed iron utilization. This study identified critical genetic determinants required for growth in milk and cell adhesion, which might provide novel insights into the mechanisms of S. dysgalactiae pathogenesis.