Polymerase chain reaction-based DNA fingerprinting of Enterococcus faecium isolated from intramammary infections of dairy cows

Abstract Twenty of 71 cows in a dairy herd that calved between 1 November 1990 and 31 January 1991 had clinical or subclinical mastitis associated with Enterococcus faecium during early lactation. Forty-two E. faecium isolates were cultured from samples of mammary secretions collected from these 20 cows during this period. In the following calving season between 1 November 1991 and 31 January 1992, five isolates of E. faecium were identified from mammary secretion of four of 85 cows. All four cows had E. faecium intramammary infections within 14 days after calving. Isolates were subtyped using a polymerase chain reaction (PCR)-based DNA fingerprinting method. In addition, plasmid analysis was performed. PCR-based DNA fingerprinting differentiated the 47 isolates into four distinct subtypes. Five of six clinical mastitis isolates of E. faecium belonged to Subtype 2, and E. faecium Subtype 1 was the only subtype that was isolated from both calving seasons. PCR-based DNA fingerprinting was useful in tracing an infection of the same subtype in a cow over a period of time. New intramammary infections by different subtypes were identified also. Two plasmids of 4.8- and 5.1-kb were observed in seven strains identified as Subtype 3 by PCR-based DNA fingerprinting. Results of this study suggest that E. faecium can be an important cause of mastitis during early lactation. PCR-based DNA fingerprinting was useful for subtyping E. faecium isolates. Use of this technique could aid in understanding the epidemiology of bovine mastitis and facilitate development of more effective methods to control intramammary infections in dairy cows.