Which organisms are Gram-positive cocci?

Gram-Positive Cocci: Organism Classification

Gram-positive cocci are bacteria that retain crystal violet stain and appear purple under microscopy, organized into several clinically important genera based on their arrangement patterns and biochemical characteristics.

Major Clinically Relevant Organisms

Cocci in Clusters

• Staphylococcus aureus (including MSSA and MRSA)
• Coagulase-negative Staphylococci (CoNS) - including various species that are part of normal skin flora but can cause opportunistic infections 1, 2

Cocci in Chains

• Streptococcus species:
  • Streptococcus pneumoniae (pneumococcus)
  • Streptococcus pyogenes (Group A Strep)
  • Streptococcus agalactiae (Group B Strep)
  • Other species including S. morbillorum, S. salivarius, S. sanguis, and S. milleri group 3, 2

Cocci in Pairs or Tetrads

• Enterococcus species:
  • Enterococcus faecalis (most common)
  • Enterococcus faecium
  • Enterococcus avium
  • Enterococcus durans
  • Enterococcus casseliflavus 3, 4

Anaerobic Gram-Positive Cocci (GPAC)

These organisms account for 25-30% of all anaerobic bacteria isolated from clinical specimens and are increasingly recognized as important pathogens 5:

• Peptostreptococcus anaerobius
• Finegoldia magna - frequently isolated in pure culture from serious infections 6, 5
• Parvimonas micros (formerly Peptostreptococcus micros) - strongly proteolytic, important in periodontitis and deep organ abscesses 6, 5
• Anaerococcus species:
  • A. hydrogenalis
  • A. lactolyticus
  • A. octavius
  • A. prevotii
  • A. tetradius
  • A. vaginalis 7, 5
• Peptoniphilus species:
  • Pn. asaccharolyticus
  • Pn. harei
  • Pn. indolicus
  • Pn. ivorii
  • Pn. lacrimalis 7, 5

Less Common Organisms

• Aerococcus viridans 3
• Gemella haemolysans 3

Clinical Context

Morphological arrangement on Gram stain provides immediate diagnostic clues: Gram-positive cocci in clusters strongly suggest Staphylococcus species (with 98% specificity for S. aureus when present) 1, while chains suggest Streptococcus species, and pairs suggest Enterococcus or Streptococcus pneumoniae.

The anaerobic gram-positive cocci are part of normal mucocutaneous flora but cause deep organ abscesses, obstetric/gynecological sepsis, and intraoral infections when they become pathogenic 6, 5. Their clinical significance has historically been underestimated because they grow slowly and are often isolated alongside other pathogens in polymicrobial infections 5.

Important caveat: Multi-drug resistance is common, particularly among staphylococcal species, with penicillin resistance rates exceeding 90% in many S. aureus and CoNS isolates 2. Susceptibility testing should guide definitive therapy rather than relying solely on organism identification.