Classification of bacteria
The purpose of classification of microorganisms is to define the pathogenic potential. For example, a Staphylococcus aureus isolated from blood is more likely to be acting as a pathogen than Staphylococcus epidermidis from the same site. Some bacteria have the capacity to spread widely in the community and cause serious disease, for example Corynebacterium diphtheriae and Vibrio cholerae. Bacteria are identified using a series of physical immunological or molecular characteristics.
• Gram reaction: Gram-positive and Gram-negative bacteria respond to different antibiotics. Other bacteria (e.g. mycobac- teria) may require special staining techniques.
• Cell shape (cocci, bacilli or spirals).
• Endospore: presence, shape and position in the bacterial cell
(terminal, subterminal or central).
• Atmospheric preference: aerobic organisms require oxygen; anaerobic ones require an atmosphere with very little or no oxygen. Organisms that grow in either atmosphere are known as facultative anaerobes. Microaerophiles prefer a reduced oxygen tension; capnophiles prefer increased carbon dioxide.
• Fastidiousness: requirement for special media or intracellular growth.
• Key enzymes: for example, lack of lactose fermentation helps identify salmonellae, urease helps identify Helicobacter.
• Serological reactions: interaction of antibodies with sur- face structures (e.g. subtypes of salmonellae, Haemophilus, meningococcus and many others).
• DNA sequences: 16S ribosomal DNA sequences are now a key element in classification.
The classification systems used are very effective, but it is important to remember that these are generalizations and that there can be considerable variation in clinical behaviour of dif- ferent strains of bacteria within a species as well as similarities across species. For example, some strains of E. coli may cause similar diseases to Shigella sonnei, and toxin-producing C. diph- theriae causes different disease from non-toxin producers.