Gram stain and shape
Gram-negative cocci
A short, high-yield list: the Neisseria diplococci and Moraxella catarrhalis, all reading pink in pairs.
How this group is recognized
Gram-negative cocci form one of the smallest branches in bacterial identification, which is exactly why exams like it. The classic members are diplococci: two cells paired with flattened adjacent sides, often described as coffee-bean shaped in lab teaching.
All three organisms here are oxidase positive, so the discriminating work falls to sugar utilization patterns and the clinical setting each organism is associated with in coursework.
PetriKey lists 3 gram-negative cocci. Shape is read from each organism's classification; organisms whose card does not state a shape stay on the main Gram-negative page.
All 3
Organisms and their lab clues
Moraxella catarrhalis Oxidase-positive Gram-negative diplococcus
Oxidase positive. Diplococcus morphology.
Neisseria gonorrhoeae Oxidase-positive Gram-negative diplococcus
Oxidase positive. Diplococci morphology.
Neisseria meningitidis Gram-negative diplococci
Oxidase positive. Chocolate agar vocabulary.