<b>Identification of </b><b>the genetic determinants</b><b> for the </b><b>g</b><b>rowth of </b><b>Streptococcus dysgalactiae</b><b> </b><b>in milk</b>

Streptococcus dysgalactiae is a pathogen that causes mastitis and milk contamination in dairy cows, resulting in significant economic losses in dairy production worldwide. This study aimed to identify the genetic determinants<i> </i>of <i>S.</i> <i>dysgalactiae</i> 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. Although the inactivation of dppA did not affect growth in milk, it significantly impaired its tolerance of lactoferrin, an antimicrobial protein in cow colostrum milk, and its ability to adhere to mammary epithelial cells. Moreover, another lipoprotein gene, mtsA, which is involved in metal transport, was identified in S. dysgalactiae. Unlike dppA, the deletion of mtsA not only reduced lactoferrin tolerance and cell adhesion but also resulted in growth inhibition in milk. Further studies revealed that this effect was caused by a disturbed iron utilization. This study identified the critical factors and mechanisms that enable <i>S. dysgalactiae</i> to grow in milk and cause infections, which might lead to novel strategies to mitigate risks to the dairy industry.