Introduction There are 6 classic Gram-positive groups of bacteria that cause disease in humans 2 are cocci one of these is streptococcus the other is staphylococcus Streptococci vs staphylococcus on Gram stain, strep looks like a "strip" of bacteria, while staph looks like a cluster strep is catalase negative, staph is catalase positive Lancefield antigens strep are characterized by the antigenic properties of the C carbohydrate in the cell wall so-called Lancefield antigens are given letter names (A, B, C, D, etc. through S) S. pneumoniae and viridans group streptococci do not have Lancefield antigens Significant human streptococci pathogens S. pyogenes (group A) S. agalacticae (group B) S. bovis (group D) enterococci (group D) S. pneumoniae viridans group streptococci