cocci

Cocci are spherical or nearly spherical bacteria that represent a morphological category rather than a single taxonomic group. They can occur as single cells or in characteristic arrangements depending on how they divide and adhere: in pairs as diplococci, in chains as streptococci, in irregular clusters as staphylococci, or in tetrads and other groupings.

In terms of staining, cocci are commonly classified as Gram-positive or Gram-negative. Gram-positive cocci have a thick peptidoglycan cell wall and include the genera Staphylococcus, Streptococcus, and Enterococcus. Gram-negative cocci include Neisseria and Moraxella, among others. Many cocci are harmless residents of the human microbiota, while others are notable pathogens. Examples of medically important cocci include Staphylococcus aureus (skin and invasive infections), Streptococcus pyogenes (pharyngitis and invasive disease), Streptococcus pneumoniae (pneumonia and meningitis), Neisseria meningitidis (meningitis) and Neisseria gonorrhoeae (gonorrhea). The balance between commensal and pathogenic cocci depends on host factors and microbial virulence traits.

Diagnosis and identification combine microscopy with culture and biochemical tests. Laboratory methods typically use Gram staining, assessment of colony morphology and hemolysis on blood agar, catalase testing (Staphylococci are usually catalase positive), and additional tests or molecular methods (such as MALDI-TOF) for species-level identification. Antibiotic susceptibility testing guides treatment, reflecting varying resistance patterns among cocci species.