The genetic material in bacteria, although not carried in a nucleus, is still DNA. As in other organisms this DNA is comprised of genes which carry codes to make proteins and these proteins define the characteristics of the bacteria.

A mutation in DNA changes the base sequencing. Many mutations can be deadly. However, some can change a protein and thereby a characteristic in the organism. If the characteristic is a useful one, for example resistance to antibiotics in a bacterium, then this organism will survive and spread the mutation on to further generations.

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Vertical gene transmission

Vertical gene transmission is the process by which genes are passed on through reproduction. A bacterium reproduces asexually which means that all the daughter cells it produces are genetically identical to the parent. Therefore, a bacterium carrying an antibiotic resistant gene will pass this on to its offspring. Such a gene could be located in either the bacterial chromosome or in the small rings of DNA called plasmids. Either way, both are passed onto the daughter cell.

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Horizontal gene transmission

Antibiotic resistant genes can also be passed on horizontally. This is where two bacteria join together, a process known as conjugation. A copy of the plasmid only is passed from one bacterium to the other through a pilus. This form of gene transmission is not limited to members of the same species: plasmids can be passed on to completely different species too.