Bacterial Identification & Classification

Bacterial Identification Methods

Bergey's Manual

Physiological/Biochemical Characteristics


Analysis of the enzymes & other biological properties


Examples of tests:


  • Fermentation of sugats
  • Capacity to digest/metabolize complex polymers
  • Production of gas
  • Presence of enzymes
  • Sensitivity to antimicrobic drugs

Chemical Analysis


Analysis of the types of specific structural substances contained in the bacteria. Such as:


  • Peptides in the Cell Wall
  • Lipids in the Membranes

Macroscopic Morphology


Analysis of the appearance of colonies, including:


  • Texture
  • Size
  • Shape
  • Pigment
  • Speed & Patterns of Growth (In broth & gelatin media)

Serological Analysis


Analysis of the reaction of the immune system to the antigens present on the bacterial surface


Response facilitates identification of bacteria in specimens & cultures

Microscopic Morphology


Analysis of:

  • Combinations of cell shape & size
  • Gram-Stain reaction
  • Acid-fast reaction
  • Special structures (Ex. Endospores, granules & capsules)

Genetic & Molecular Analysis

Firmicutes Gram-Positive thick-skinned cells

Tenericutes


-Class 1. Mollicutes: Bacteria that lack a cell wall (Mycoplasmas)

Gracilicutes Gram-Negative thin-skinned cells

Mendosicutes


-Class 1. Archaebacteria: Bacteria w/ atypical compounds in the cell wall & membranes

Class 2. Anoxyphotobacteria Gram-Negative Photosynthetic bacteria that do not produce oxygen (Purple & Green Bacteria)

Class 3. Oxyphotobacteria Gram-Negative Photosynthetic bacteria that evolve oxygen (Cyanobacteria)

Class 1. Scotobacteria Gram-Negative Non-photosynthetic bacteria

Class 1. Firmibacteria Gram-Positive Rods or Cocci

Class 2. Thallobacteria Gram-Positive Branching cells (Actinomycetes)

G + C Base Composition Analysis of the overall percentage of guanine & cytosine


Indicator of relatedness since the pair does not change rapidly


Significant differences = Genetic distinction


Ex. Escherichia (48-52% G+C) and Pseudonomas (58-70% G+C) are not closely related

DNA Analysis w/ Genetic Probes


Analysis of the exact order & arrangement of the DNA code


Hybridization allows identification of bacterial species by analyzing segments of its DNA


Probes = Small DNA/RNA fragments complementary to specific DNA squences


Test Sequence


1) Unknown test DNA are extracted & bound to special blotter paper


2) Probes are added to the blotter paper


3) After several probes are added, they are observed for visible signs of fixing


4) Binding of probes to several areas of test DNA = close correspondence

Nucleic Acid Sequencing & rRNA Analysis


Comparison in the sequence of nitrogen bases in ribosomal RNA (rRNA)


Ribosomes have the same function in all cells & are more or less stable


Any major differences in the sequence/signature of the rRNA likely indicates some distance in ancestry


Effective for differentiating general group differences & identifying differences between species