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Background  Coagulase negative  Staphylococcus  (CNS) species are currently the most prevalent intra-mammary pathogens causing subclinical mastitis and occasional clinical mastitis or persistent infection in lactating dairy cattle. More than 10 CNS species have been identified, but they are generally managed as one group on most dairies in the United States. However, improved management decisions and treatment outcomes may be achieved with better understanding of the prevalent species, pathogenicity and strain diversity within and across dairies.    Methodology  A total of 604 CNS isolates were cultured from milk samples collected during a dry-cow treatment clinical trial conducted on 6 dairy herds in 4 states in the US. All the study cows were randomized to receive 1 of the 3 different intra-mammary antimicrobial infusions (Quatermaster, Spectramast DC or ToMorrow Dry Cow) at dry-off. Milk samples were collected at dry-off, calving (0–6 days in milk, DIM), post-calving (7–13 DIM) and at mastitis events within the first 100 DIM. The CNS isolates were identified to species level by partial sequencing of the  rpoβ  gene, and genetic relatedness within species was investigated by phylogenetic analysis of the pulse-field gel electrophoresis profiles of the isolates.    Results  The major CNS species identified were  S. chromogenes  (48.3%),  S. haemolyticus  (17.9%),  S. simulans  and  S. epidermidis  (each at 6.5%). Other CNS species identified at lower frequencies included  S. hominis ,  S. auricularis ,  S. sciuri ,  S. spp  KS-SP,  S. capitis ,  S. cohnii ,  S. warneri ,  S. pasteuri, S. xylosus, S. hyicus, S. equorum, S. microti, S. rostri, S. gallinarum, S. saprophyticus  and  S. succinus . Phylogenetic analyses of the major species types demonstrated an association between genetic relatedness and epidemiological distributions of  S. chromogenes, S. simulans, S. haemolyticus  and  S. auricularis.  Additionally, identical strains of  S. chromogenes  and  S. simulans  were isolated from the same udder quarter of several cows at consecutive sample stages. The rest of the minor species had no deducible genetic-epidemiological link.    Discussion  The observed association between genetic and epidemiological distributions indicated animal-adapted nature of four CNS species, suggesting possible host-adapted and environmental transmission of these species. Multi-stage isolation of the same udder quarter strain was evidence for chronic intra-mammary infection.    Conclusion  The different CNS species and strains circulating on US dairy herds were genetically diverse. Four species identified were likely udder-adapted pathogens, 2 of which caused persistent infection. Our findings are important in guiding the design of effective mastitis control strategies.

Molecular epidemiology of coagulase-negativeStaphylococcusspecies isolated at different lactation stages from dairy cattle in the United States