Gram stain and shape
Gram-positive cocci
Spherical bacteria that hold the crystal violet and read purple: the staphylococci, streptococci, and enterococci.
How this group is recognized
Gram-positive cocci are the classic first branch of bacterial identification. After the stain itself, the next clue is arrangement: staphylococci divide in multiple planes and pile into clusters, while streptococci and enterococci divide in one plane and line up in pairs or chains.
The catalase test then splits the group. Catalase-positive clusters point to the staphylococci, where coagulase separates Staphylococcus aureus from the coagulase-negative species. Catalase-negative chains point to the streptococci, where hemolysis pattern and specific tests carry the workup from there.
PetriKey lists 11 gram-positive cocci. Shape is read from each organism's classification; organisms whose card does not state a shape stay on the main Gram-positive page.
All 11
Organisms and their lab clues
Enterococcus faecalis/faecium Bile-esculin and VRE vocabulary
Catalase negative. Bile-esculin vocabulary.
Staphylococcus aureus Gram-positive cocci in clusters
Catalase positive. Coagulase positive.
Staphylococcus epidermidis Coagulase-negative skin flora clue
Catalase positive. Coagulase negative.
Staphylococcus saprophyticus Coagulase-negative, novobiocin-resistant clue
Catalase positive. Coagulase negative.
Streptococcus agalactiae Group B beta-hemolytic streptococcus
Catalase negative. Beta-hemolysis.
Streptococcus anginosus group Abscess-forming viridans (milleri) streptococci
Variable hemolysis on blood agar. Small (<0.5 mm) colonies vocabulary.
Streptococcus gallolyticus (bovis) Group D strep linked to colon cancer
Bile-esculin positive. Group D antigen.
Streptococcus mutans Dental caries viridans streptococcus
Alpha-hemolytic (green) vocabulary. Optochin-resistant (contrast with pneumococcus).
Streptococcus pneumoniae Lancet diplococci and capsule clue
Optochin sensitivity is commonly taught. Bile solubility is a classic lab distinction.
Streptococcus pyogenes Group A beta-hemolytic streptococcus
Catalase negative. Beta-hemolysis.
Viridans streptococci Alpha-hemolytic optochin-resistant oral strep
Alpha-hemolytic (green) colonies. Optochin-resistant (pneumococcus is sensitive).