Encapsulation of streptococci isolated from bovine milk

Capsule expression was assessed by India ink staining in 95 streptococcal and enterococcal isolates representing 8 species. Organisms were evaluated after primary culture on blood agar and after storage in 10% skim milk at -80 degrees C. Seventeen of 95 primary culture samples were positive for an unstained halo (capsule) by India ink. Only 3 isolates were positive for capsule after being maintained for an extended period of time. To confirm results of India ink tests, an unencapsulated and encapsulated strain of each species was observed by electron microscopy. Cells of encapsulated strains were surrounded by thread-like fibers indicative of capsule, whereas cells of unencapsulated strains had a smooth surface. The importance of capsule in the pathogenesis of streptococcal mastitis is unknown. Studies on capsule expression by streptococci may facilitate investigations on the pathogenesis of streptococcal mastitis.