Name Reason Alternative Microscopic Approach

(Ex. M. tuberculosis)

Too much lipid in cell wall so dye cannot penetrate Acid-fast stain
Treponema pallidum Too thin to see Dark-field microscopy or fluorescent antibody
Mycoplasma pneumoniae No cell wall; very small None
Legionella pneumoniae Poor uptake of red counterstain Prolong time of counterstain
Chlamydiae Intracellular; very small Look for inclusion bodies in cytoplasm, but insensitive
Rickettsiae Intracellular; very small Giemsa or other tissue stains


B. Cell shape:  cocci (round), bacilli (rods), spiral-shaped (spirochetes), pleomorphic

C. Cell arrangement:  clusters, chains, pairs

  1. Determined by orientation and degree of attachment of the cells during

cell division.

D. Biotyping

1. Based on the presence or absence of specific biochemical markers, like:

a) ability to ferment specific sugars.

b) production of specific end products.

c) relationship to oxygen

E. Serotyping

  1. Based on detection of unique antigens using specific antibodies.
  2. Advantages
  •  Fast
  • Can be used for organisms that can’t be identified by biotyping.
  • Can be used for organisms difficult or impossible to grow.

F. Antibiograms

  1. Tests for patterns of susceptibility to different antibiotics.

G. Phage typing

  1. Tests for patterns of susceptibility to infection by bacteriophages.

Analytic Classification

    1. Analysis of specific cell wall or whole cell constituents; labor intensive; not used typically in clinical labs.


Genotypic Classification

The development of molecular techniques based on analysis of nucleic acids is changing the way bacterial classification and identification are done.  The advantages of these techniques include their specificity and the ability to detect organisms that cannot be cultured.  These can also be considered as disadvantages.  Techniques include:

  1. guanine + cytosine content.
  2. DNA hybridization.
  3. nucleic acid sequence analysis. Ex. Amplification of ribosomal (16S) or other unique sequences by PCR.

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