A problem encountered with this method is that phenotypic properties can often vary according to culture conditions Serology Most bacterial surface molecules can act as antigens ie induce the production antibodies. If the molecules between strains are similar the antibodies will cross react. The streptococci can be classified on the basis of their acid-extracted polysaccharide antigens and this can be used to define species. The chemical composition of various bacterial components is often used in taxonomy. Especially used for the Corynebacteria Lipid composition - eg the archaeobacteria group have ether linked lipids whereas other eubacteria have acyl linked lipids.
Mycolic acids are useful for mycobacteria.
Phospholipids and glycolipids also used. Not always reliable as unrelated organisms can give very similar patterns and conversely related organisms can give different patterns Fermentation product profiles - gas chromatograph analysis of fatty acids produced as end products of protein or CHO metabolism. Useful for Bacteroides , Porphyromonas and Clostridium Cytochrome composition - presence and structure of different types cytochromes. The sequence of related proteins can then be compared and the degree of similarity calculated. Nucleic acids The composition of DNA is useful in taxonomy.