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Objectives The genus Streptococcus contains species of important zoonotic pathogens such as those that cause bovine mastitis. Unfortunately, many Streptococcus species have developed antibiotic resistance. Phage lysins are considered promising alternatives to antibiotics because it is difficult for bacteria to develop lysin resistance. However, there remains a lack of phage lysin resources for the treatment of streptococci-induced mastitis. Methods We identified the prophage lysin Lys0859 from the genome of the Streptococcus suis SS0859 strain. Lys0859 was subsequently characterized to determine its host range, MIC, bactericidal activity in milk, and ability to clear biofilms in vitro. Finally, to determine the effects of Lys0859 on the treatment of both bovine mastitis and S. suis infection in vivo , we established models of Streptococcus agalactiae ATCC 13813-induced mastitis and S. suis serotype 2 SC19 systemic infection. Results Our results demonstrate that Lys0859 possesses broad-spectrum lytic activity against Streptococcus and Staphylococcus species isolated from animals with bovine mastitis and 15 serotypes of S. suis isolated from swine. Intramammary and intramuscular injection of Lys0859 reduced the number of bacteria in mammary tissue by 3.75 and 1.45 logs compared with the PBS group, respectively. Furthermore, 100 μg/mouse of Lys0859 administered intraperitoneally at 1 h post-infection protected 83.3% (5/6) of mice from a lethal dose of S. suis infection. Conclusions Overall, our results enhance the understanding and development of new strategies to combat both streptococci-induced mastitis and S. suis infection. [ABSTRACT FROM AUTHOR]