How do bacteria acquire resistance to antibiotics?

Spread of mutations via vertical and horizontal transmission

Once a bacterium has a mutation that provides resistance to an antibiotic, it can spread through replication or through direct transmission to other bacterial cells.

Bacteria have DNA, just as in eukaryotes (animals, plants, algae), although the bacterial chromosome is circular rather than linear. Bacteria also have smaller circular sections of DNA not attached to the main chromosome; this is known as a plasmid. Some bacterial species have many of these. Mutation can occur on either chromosome or plasmid.

In vertical gene transmission, the mutation is spread with the whole bacterial genome (meaning all genes in the bacteria – plasmids and chromosome) via replication or binary fission.

1. Many bacteria have mutations in their genome

2. If a mutation provides resistance to a prevalent antibiotic, bacteria containing that mutation are more likely to survive and replicate

3. In the next generation, there will be many more bacteria containing that mutation.

In horizontal gene transmission, the mutation is spread via replication and transmission of the plasmids between bacterial cells. Two cells join together and transfer the plasmid containing the mutation in a process known as conjugation.

Examples of antbiotic resistance in bacteria:


Very ancient human infectious disease that can infect many parts of the body, causing particularly dangerous illness if it infects the lungs.

Individuals can be infected for many years before they have any symptoms, and many more may never have symptoms at all.

Tuberculosis is treated with a course of four different antibiotics over 6 months.

MDR (multi drug resistant) and XDR (extensively drug resistant) tuberculosis is present globally.


MRSA stands for methicillin resistant Staphylococcus aureus.

S. aureus is an opportunistic pathogen – it can cause infections in many parts of the body but is most commonly found in wounds.

S. aureus can acquire resistance to most antibiotics very quickly, and some worry that types of S. aureus could become ‘super bugs’ that are resistant to lots of different antibiotics.

MRSA is a big problem in hospitals, where it can spread very quickly (often through contaminated water – it grows well in warm water pipes).

In both cases, using cocktails of drugs (giving patients many different antibiotics in one go) is the best way to treat resistant bacteria.

It is unlikely that a bacteria will be resistant to all the drugs given

While treating the patient, this also reduces the chances of a drug-resistant strain spreading to other individuals.

However, people may need to take these drugs for a long time, and they can cause unpleasant side effects.

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