Gram-positive cocci are often isolated from clinical samples. It is important to be able to distinguish these organisms from one another because while some are normal flora, others can be pathogenic.

Gram-positive bacteria have a thick peptidoglygan wall and appear purple when stained; Gram-negative bacteria turn pink. Staph aureus is immediately recognizable when viewed under a microscope ...

"Normal flora" include Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and anaerobes such as Bacteroides species, anaerobic gram-positive cocci, and Fusobacterium species.