Interpretation of gram stains

Gram positive

Aerobic

In clusters

  • Coagulase (+): Staphylococcus
    aureus
  • Coagulase (-): Staphylococcus
    lugdunensis and other coagulase-
    negative staphylococci

In pairs/chains

  • Optochin sensitive: Streptococcus pneumoniae
  • Alpha-hemolytic: Viridans group Streptococcus, Enterococcus
  • Beta-hemolytic:
    • Group A Strep (Streptococcus
      pyogenes)
    • Group B Strep (Streptococcus
      agalactiae)
    • Group C, D, G Strep
  • Anaerobic: Peptostreptococcus spp. and many others

Gram negative cocci

Aerobic

  • Diplococcus: Neisseria meningitidis, N. gonorrhoeae, Moraxella catarrhalis
  • Cocco-bacillus: Haemophilus influenzae, Acinetobacter

Gram-positive rods

Aerobic

  • Large: Bacillus spp
  • Cocco-bacillus: Listeria
    monocytogenes, Lactobacillus spp
  • Small, pleomorphic: Corynebacterium
    spp
  • Branching filaments: Nocardia spp,
    Streptomyces spp

Anaerobic

  • Large: Clostridium spp
    Small, pleomorphic: P. acnes, Actinomyces spp

Gram-negative rods

Aerobic

Lactose fermenting (Lactose positive):

  • Enterobacter spp, Escherichia coli, Klebsiella spp
  • Citrobacter spp, Serratia spp
    Non lactose-fermenting (Lactose negative):
  • Oxidase (-): Acinetobacter spp,
    Burkholderia spp, E. coli, Proteus spp, Salmonella spp, Shigella spp, Serratia spp*, Stenotrophomonas maltophilia
  • Oxidase (+): P. aeruginosa, Aeromonas spp.

Anaerobic

Bacteroides spp, Fusobacterium spp, Prevotella spp.

Interpretations of Key Phrases

  • “Gram positive cocci in clusters” may suggest Staphyloccocus species.
  • “Gram positive cocci in pairs and chains” may suggest Streptococcus species or
    Enterococcus species.
  • “Gram negative coccobacilli” may suggest Haemophilus species.
  • “Lactose-positive gram negative rods” may suggest Enterobacteriaceae, such as E. coli,
    Klebsiella, or Enterobacter spp.
  • “Lactose-negative gram negative rods” may suggest Pseudomonas.
  • “Branching Gram positive rods, modified acid fast stain positive” may suggest Nocardia
    or Streptomyces species.
  • “Acid fast bacilli” may suggest Mycobacterium species.
  • “Yeast” suggests Candida spp. “Germ-tube negative yeast” suggests non-albicans
    Candida yeast. “Germ-tube positive yeast” is identified as Candida albicans or Candida
    dubliniensis.
  • “Round Yeast” suggests Cryptococcus spp
  • “Fungal elements or hyphal elements” suggest filamentous fungi (moulds).