Antibiotics and genetic variation in bacteria

Objectives• Antibiotics • Antibiotics don’t work against viruses. • Antibiotics may be used to treat bacterial disease. One way, in which antibiotics function is by preventing the formation of bacterial cell walls, resulting in osmotic lysis.

• Genetic variation in bacteria• DNA is the genetic material in bacteria as well as in most other organisms.• Mutations are changes in DNA and result in different characteristics.• Mutations in bacteria may result in resistance to antibiotics.• Resistance to antibiotics may be passed to subsequent generations by vertical gene transmission.

• Resistance may also be passed from one species to another when DNA is transferred during conjugation. This is horizontal gene transmission. 

• Antibiotic resistance in terms of the difficulty of treating tuberculosis and MRSA.

Natural selection

• Survival of the fittest• Individuals vary• Individuals who are best suited to the

environment survive to reproduce and pass on their genes.

Causes of variation in bacteria

• Mutation

• conjugation

mutation

• A change to the DNA of a gene

• Can change the amino acid sequence

• This could give an altered protein

Types of mutation

• DNA base sequence can be changed by:• Original sequence: CAT CAT CAT CAT CAT CAT CAT CAT• • A deletion: CAT CTA CTA CTA CTA CTA CTA CTA• • An addition: CAT CAT TCA TCA TCA TCA TCA

TCA• • A substitution CAT CAT GAT CAT CAT CAT CAT CAT

Animation Mcgraw Hill -conjugation

Vertical and horizontal gene transmission

Antibiotics

• chemicals that act against bacteria.• Often they stop or slow the bacterial cells from reproducing by binary fission

• Antibiotics may be used to treat bacterial disease. 

• Example-penicillin

How do antibiotics work?

• prevent important cross linkages between peptides in bacterial cell walls from forming causing them to be weak.

• In water the lower water potential inside the bacterium causes inward movement of water molecules by osmosis.

• the bacterium will swell and burst (Lysis will occur) destroying the cell.

Antibiotic resistance• A chance mutation occurred • Bacterium able to grow in the presence of penicillin. • It had become resistant to it. • the bacterium is clearly at an advantage with this gene. • This means that the resistant bacterium will survive and pass on its

resistance gene from one generation to the next-vertical gene transmission.

• The resistant form would now be selected for in the presence of penicillin so the frequency of the gene will increase in the population.

• As the gene for resistance to penicillin is found on a plasmid it could also be passed from cell to cell by conjugation-horizontal gene transmission.

Antibiotic halos around antibiotic discs. Only bacteria resistant to the antibiotic can grow up to disc.

Treating infections like TB

• The susceptable bacteria are killed first• The remaining bacteria are likely to be the ones

with most resistance to the antibiotic. • The resistant strains survive and are more likely

to be spread to other people. • The selection pressure promotes the resistant

strains. • These strains can pass the resistant genes to

other species during conjugation.

Treating TB

• Take antibiotics for 6-9 months• Patients with TB are given a cocktail of several

different antibiotics to ensure that at least one is effective.

• Patients may have to report to a clinic to be observed taking their medication.

MRSA and antibiotics

• Most people carry Staphylococcus on skin/throats

• Staphylococcus infections can be treated with antibiotics.

• MRSA occurs when the strain of Staphylococcus is resistant to one or more antibiotics.

• MRSA infections are a problem in hospitals

Issues

• Antibiotics are no longer prescribed for slight infections

• Antibiotics should not be used to treat viral infections

• Patients should take the whole course of antibiotics.

• Antibiotics are used by farmers to reduce disease and hence improve productivity