Beta-hemolytic Streptococcus are spherical bacteria that produce hemolysins capable of completely lysing (bursting open) red blood cells. When grown on sheep blood agar, colonies of beta-hemolytic Streptococcus are encircled by visible areas of clearing where beta hemolysis has occurred.

Beta-hemolytic streptococci may be pathogens or non-pathogens. Group A Streptococcus (Streptococcus pyogenes) causes "strep throat". Strep throat may lead to delayed sequelae such as rheumatic fever, glomerulonephritis, and/or scarlet fever.

The bacitracin susceptibility test is used to distinguish Group A streptococci, which cause ninety percent of acute streptococcal infections in humans, from other streptococci. When grown on blood agar, Group A streptococci are sensitive to (killed by) the antibiotic bacitracin . A sterile disk impregnated with bacitracin is placed on the first sector of an isolation plate before incubation. A zone of inhibition (area with no growth) will be seen around the disk after incubation if the organism is a Group A beta-hemolytic Streptoccus. Other beta-hemolytic streptococci are resistant to (not killed by) bacitracin. Their colonies will thus grow right up to the disk of bacitracin.

Blood agar plate Group A Streptococcus and Group C Streptococcus with bacitracin disks