16. Genetic Variation in Bacteria

ADAPTION - organisms adjust to the changing environment they live in.

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  • It increases the long term reproductive success of a species by helping its members survive long enough to breed.
• Bacteria's adaption to a changing environment is seen as their ability to develop resistance to antibiotics.
• Diversity in organisms arises due to changes in its DNA, which occurs in 2 ways:
  • MUTATION - changing the quantity/structure of DNA of an organism.
  • SEXUAL REPRODUCTION - recombining the existing DNA of 2 individuals.

Mutations - changes in DNA that results in different characteristics, they arise in different ways. E.g.

• Any differences in the base sequence of a DNA molecule, may result in a different sequence of amino acids being coded for - leads to a different polypeptide and protein.
• If this protein is an enzyme, its likely to disrupt the metabolic pathway leading to the production od other substances, including proteins.
• Proteins are responsible for the characteristics of an organism.

Conjugation occurs when one bacterial cell transfers DNA to another bacterial cell.

1. One cell produces a thin projection that meets another cell - forms a thin conjugation tube between the 2 cells.

2. The donor cell replicates its PLASMID (small circular pieces of DNA).

3. The circular DNA is broken to make it linear, before it passes along the tube into the recipient cell.

4. Contact between the cells is brief - only a portion of the donor's DNA is transferred.

5. The recipient cell acquires new characteristics from the donor cell.

HORIZONTAL GENE TRANSMISSON - DNA in the form of genes, can be passed from one species to another.

VERTICAL GENE TRANSMISSION - genes are passed down from one generation to another of the same species.

How do they work?

• They prevent bacteria making CELL WALLS.
• Water constantly enters a cell by osmosis, the cell wall prevents OSMOTIC LYSIS (cell bursting).
• The cell wall is made of a tough material that isnt easily stretched, as water enters the cell, the contents expand and push against the cell wall.
• The cell wall resists expansion (its inelastic), so it stops the further entry of water and prevents osmotic lysis.
• They inhibit the synthesis and assembly of important peptide cross linkages in bacterial cell walls - weakens the walls, making them unable to withstand pressure.
• They are only effective when the bacteria are growing as they inhibit the formation of cell walls.

• The presence of antibiotics doesn't cause the bacteria to mutate - they occur randomly and are rare.
  • As theres a large number of bacteria, the number of mutations is LARGE.
• The gene for antibiotic resistance is passed on from one generation to the next by vertical gene transmisson.
• The allele for carrying antibiotic resistance is carried in the PLASMID which can be transferred cell to cell by conjugation.
  • Can lead to certain bacteria accumulating DNA that gives them resistance to a range of antibiotics - 'super bugs'.
• New mutations arise randomly all the time.
• The more we use antibiotics, the greater chance mutant bacteria will gain advantage over the normal variety.
• The longer we continue using an antibiotic, the greater the chance the mutant will out compete and replace the normal variety.

• The antibiotic treatment for TB is 6-9 months, they initially destroy the least resistant strains of the bacteria.
• After a few months, the patient feels better as most of the bacteria have been killed so the temptation is to stop taking the antibiotics.
  • This is the worst thing to do as the few bacteria that remain are the MOST resistant, they then can multiply and spread.
    • Leads to the development of strains of the bacteria that don't respond to the antibiotic, they can then interchange genes for resistance with other strains by conjugation.
    • Multiple antibiotic resistant strains of TB have been developed.
• To overcome this, 3 or 4 antibiotics are used to ensure at least one will be effective.

STAPHYLOCCOUS - if is becomes a health risk, it can be cleared up with a treatment of antibiotics.

• MRSA is the name given to any strain of this bacteria that is resistant to one or mroe antibiotics.
• Its especially prevalent in hospitals as:
  • People in hospitals are older, sicker and weaker - more vunerable to infection.
  • Many people live close together and are examined by doctors who have just touched other patients - perfect for the transmisson of diseases.
  • Most antibiotics are used in hospitals so any mutant strains have an advantage over non-mutant strains.