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• The human immune system
• Infection of the body by bacteria
• The inflammatory response
• Immune evasion by bacteria
• Kochs Postulates
• Bacteria
• Normal flora of the human body
• How are bacteria identified?
• Mycobacteria
• Immune reactions to mycobacteria
• Treatment mycobacteria with antibiotics
• Pathology of mycobacterial diseases

Bacteria

What are bacteria?

Bacteria are unicellular organisms, i.e. organisms that consist of one single cell. They come in many shapes and sizes. Common shapes are rod-shaped (bacillus), sphere-shaped (coccus) and helix-shaped (spirilla). These shapes are caused by the growth of the cell wall of the bacterium. The cell wall usually acts to protect the bacterium against invasion, by other organisms or by chemicals. However, sometimes, in particular environments, bacteria can exist without cell walls.

How do bacteria multiply?

Bacteria multiply in a straightforward manner (apologies to molecular biologists for the simplification). Each single celled bacterium grows until there is enough material to form two separate bacteria. The one parent bacterium then splits into two progeny bacteria. This process is known as binary fission. The time that it takes one bacterium to accumulate enough material to split is known as the generation length. This generation length varies greatly between different genus and species of bacteria, from as short as twenty minutes for E. coli to as long as twenty four hours for Mycobacterium tuberculosis. The population growth curve for bacteria is an exponential curve. With each generation, the number of bacteria doubles. In ideal circumstances (i.e. a ready supply of nutrients and a benign environment), a single E. coli bacterium can grow to become over one million bacteria in as little as three and a half hours. For one Mycobacterium tuberculosis bacterium to generate the same number of bacteria, again under ideal circumstances, may take as long as twenty days.

In practice, however, this is not always the case, since the circumstances in which bacteria grow are not always ideal. Nutrients may not be plentiful, the temperature may be too warm or too cold, the acidity of the environment may be too high or too low, there may be chemicals present which inhibit the growth of bacteria or there may be organisms present that consume and destroy bacteria.

How do bacteria evolve?

Every bacterium has a set of genes that completely describe the bacterium, and which dictate the physical, chemical and biological characteristics of the bacterium. These genes are made of the chemicals DNA and RNA. This set of genes is known as the genotype of the bacterium. Usually, when a parent bacterium splits into two bacteria, the two progeny bacteria are genetically identical, i.e. they have the same genotype.

However, this is not always the case. There are several situations in which the genes(genotype) of a bacterium can change.

• Mutation. Mutation happens when there is a genetic "error" in the copying of the genes from parent to progeny bacterium. This results in a progeny bacterium which has a different genotype to that of its parent. Mutation rates vary between different genus and species of bacteria. Statistically, random mutations may occur as often as one in every million (* 1,000,000) multiplications, or as seldom as one in every billion (* 1,000,000,000) multiplications. However, since most bacterial populations in the human body number in the millions (* 1,000,000) or billions (* 1,000,000,000), it is statistically probable that there will be many mutations with each new generation.
• Transduction. Bacteria, like humans, can be attacked by viruses. These bacterial viruses are known as bacteriophages. These bacteriophages invade bacteria, and can change their DNA. They may also carry DNA from one bacterium to another. These actions alter the genotype of the bacterium. This process is known as Transduction.
• Conjugation. Sometimes bacteria may join together and exchange DNA. This changes the genotype of the bacteria. This process is known as Conjugation.

Why are the above important? Because they allow the bacteria to adapt to their environment. Changes in the genotype may allow the bacteria to obtain nutrition from sources they were unable to feed from before, they may allow the bacteria to survive in a more hostile environment, and they may allow the bacteria to avoid the action of destructive chemicals (e.g. antibiotics) or allow them to produce chemicals that protect them from attack by organisms that are capable of destroying them.