Classification of Bacteria

  • Morphology
    • Cocci
      • Spherical shape
    • Rod/bacilli
      • Elongated with straight sides
    • Coccobacilli
      • Mixed shape
  • Gram Stain
    • Positive
      • Thick wall of peptidoglycan (polymer of sugars + amino acids) + phosopholipid bilayer forming the plasma membrane
      • Stain purple
    • Negative
      • Outer membrane (lipopolysaccharide & protein) + thin peptidoglycan + plasma membrane
      • Stain pink
    • Variable (Coccobacilli)
      • Acinobacter
  Cocci Rods Coccobacilli
Gram positive Staphylococcus








Gram negative Neisseria


E coli, Proteus, Klebsiella

Serratia, Enterobacter, Shigella, Salmonella, Yersinia, Pseudomonas, Stenotrophomonas, Bacteroides






  • Growth requirements
    • Aerobic
    • Anaerobic
      • only grow and reproduce in the absence of oxygen, usually found in GI tract include oral cavity
        • Clostridium, Bacteroides, Peptostreptococcus, Actinomyces
  • Biochemical Reactions
    • Coagulase Test
      • Enzome that converts fibrinogen to fibrin
      • Used to differentiate between Staphylococcus species
      Coagulase Positive Coagulase Negative
      Staph aureous

      Some Staph intermedius

      Most other species i.e. S. epidermidis, S. capitis

      These are generally not pathological


      Hemolysis Test

      • Streptococcal species further classified based on their ability to hemolyze RBCs
      • Can be tested using known Strep bacterial on blood agar
      Alpha haemolytic Beta haemolytic Gamma haemolytic
      Partial hemolysis – green tinge around the colonies

      -S. pneumonia

      -Viridans Strep

      Complete hemolysis – area of clearing around colonies

      -S pyogenes

      -S agalactiae

      No hemolysis

      -S bovis



      Lancefield Test

      • Used to distinguish Streptococcus/Enterococcus according to specific carbohydrates present on the bacterial wall
        • Group A – S pyogenes [Beta haemolytic]
        • Group B – S agalactiae [Beta haemolytic]
        • Group D – Enterococcus, S bovis [Gamma haemolytic]
        • No Lancefield Antigens – S pneumonia, Viridans strep – [Alpha haemolytic]


      Lactose Fermentation

      • Ability to ferment lactose is a major differentiating characteristic of gram negative rods
      • Usually tested by culturing unknown bacteria on MacConkey agar
      • Lactose Fermenters/Positives turn red with a red halo
        • E coli
        • Klebsiella
        • Enterobacter
      • Lactose non-fermenters/Negative appear translucent
        • Almost all other gram negative rods
      • Lactose slow fermenters
        • Turn red bit without a halo
          • Citrobacter
          • Serratia
  • Serotype i.e. Streptococcus – see above
    • Group A
    • Group B
    • Group C
  • Atypical Bacteria
    • Mycoplasma – lack cell wall, highly pleomorphic
    • Chlamydia – obligate intracellular parasites, lack peptidoglycan in cell wall, multiphased life-cycle
    • Rickettsia – require arthropod vectors, obligate intracellular parasites
    • Legionella – doesn’t grow on traditional media
  • Antibiotic Resistance Pattern
    • MSSA – Methicillin-sensitive Staph aureus
    • MRSA – Methicillin- resistant Staph aureus
    • VRE – Vancomycin-resistent enterococcus
    • ESBL – Extended-spectrum beta-lactamase producing member of the enterobacteriaceae family
  • rRNA Sequence Analysis

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