** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** MASTITIS IS ONE OF THE MOST IMPORTANT DISEASES OF DAIRY CATTLE AND, IN THE PAST FEW DECADES, NON-AUREUS STAPHYLOCOCCI (NAS) HAVE BECOME THE MOST COMMON BACTERIA ISOLATED FROM SUBCLINICAL MASTITIS CASES IN DAIRY COWS AROUND THE WORLD. IN GENERAL, HEIFERS HAVE BEEN IDENTIFIED AS MORE PRONE TO NAS INTRAMAMMARY INFECTIONS (IMIS) COMPARED TO OLDER COWS. AMONG MOST STUDIES, STAPHYLOCOCCUS CHROMOGENES IS THE MOST PREVALENT NAS SPECIES, REPRESENTING AS MUCH AS 50% OF ALL NAS ISOLATES IN SOME NORTH AMERICAN STUDIES. HOWEVER, THE OVERALL IMPACT OF S. CHROMOGENES IMIS ON DAIRY INCOME AND MILK QUALITY REMAINS TO BE DETERMINED. WE HYPOTHESIZE THAT STRAIN SPECIFIC DIFFERENCES AMONG S. CHROMOGENES ISOLATES ARE ASSOCIATED WITH UDDER HEALTH OUTCOMES. THEREFORE, THE OVERALL GOAL OF THIS WORK IS TO INCREASE THE UNDERSTANDING OF STRAIN SPECIFIC DIFFERENCES AMONG S. CHROMOGENES ISOLATES AND DETERMINING IF THESE DIFFERENCES ARE CONSISTENTLY ASSOCIATED WITH SPECIFIC UDDER HEALTH OUTCOMES. SPECIFIC OBJECTIVES OF THIS PROJECT INCLUDE TO FOLLOWING: 1) DETERMINE PREDICTABLE MOLECULAR PATTERNS OF S. CHROMOGENS ISOLATES; 2) DETERMINE WHEN S. CHROMOGENES MAMMARY INFECTIONSOCCUR IN THE HEIFER TO BETTER FOCUS IMPLEMENTATION OF PREVENTIVE MANAGEMENT STRATEGIES; AND 3) DETERMINE IF PHENOTYPIC BEHAVIORS OF S. CHROMOGENES INTRAMAMMARY INFECTIONS ARE REPRODUCIBLE USING A HEIFER CHALLENGE MODELS. WE EXPECT TO FIND GENOMIC LEVEL VARIABILITY THAT EXPLAINS PHENOTPYIC VARIABILITY IN S. CHROMOGENES ISOLATES AND WE EXPECT OUR CHALLENGE STUDY TO SHOW THESE STRAINS HAVE REPRODUCIBLE BEHAVIOR IN THE CONTEXT OF UDDER HEALTH OUCTOMES.THE PROPOSED RESEARCH WILL PROVIDE NEW DATA ESSENTIAL TO UNDERSTANDING THE ASSOCIATIONS BETWEEN S. CHROMOGENES IMIS OR SKIN COLONIZATION ON UDDER HEALTH, AND FURTHER EXAMINE WHETHER SOME STRAINS ARE BENEFICIAL TO MAMMARY HEALTH.