It is difficult to classify all bacteria into clear-cut categories.  Various classification methods are used, depending in part, on the purpose.


  1. Nomenclature
  2. Bacteria are given 2 names (usually Latin) denoting their genus and species
  3. Ex.  Escherichia coli
  4. The name is italicized or underlined because it’s Latin.


  1. The first name refers to the genus (plural:  genera) and is capitalized,



  1. The last name refers to the species (plural:  species) and is not




  1. Phenotypic Classification—usually presumptive
  2. Macroscopic morphology
  3. Colonial appearance



  1. Microscopic appearance
  2. Gram stain:  Gram-positive vs. Gram-negative, Gram-variable


  1. Influences choice of antibiotic—in general, Gram-positive bacteria are

more susceptible to penicillin G than Gram-negative bacteria.  Why?