Bacteria are not easily classified according to their
forms, and only recently has enough been learned about
their biochemical and metabolic characteristics to develop
a satisfactory overall classification comparable to
that used for other organisms. Early systems for classifying
bacteria relied on differential stains such as the
Gram stain. Key bacterial characteristics used in classifying
bacteria were:
1. Photosynthetic or nonphotosynthetic
2. Motile or nonmotile
3. Unicellular or multicellular
4. Formation of spores or dividing by transverse
binary fission
With the development of genetic and molecular approaches,
bacterial classifications can at last reflect true
evolutionary relatedness. Molecular approaches include:
(1) the analysis of the amino acid sequences of key proteins;
(2) the analysis of nucleic acid base sequences by
establishing the percent of guanine (G) and cytosine (C);
(3) nucleic acid hybridization, which is essentially the
mixing of single-stranded DNA from two species and
determining the amount of base-pairing (closely related
species will have more bases pairing); and (4) nucleic
acid sequencing especially looking at ribosomal RNA.
Lynn Margulis and Karlene Schwartz proposed a useful
classification system that divides bacteria into 16 phyla,
according to their most significant features. Table 34.1
outlines some of the major features of the phyla we
describe.