These are substances that can destroy/inhibit growth of microorganisms, mostly produced by bacteria.
They work by stopping bacteria making cell walls - they inhibit synthesis and assembly of peptide cross-linkages so walls are weakened and cannot withstand pressure that occurs when water tries to move in by osmosis. The walls are inelastic so resist expansion. Without this they are unable to stop water moving in so osmotic lysis occurs and kills bacterium. It's only effective if bacteria are growing. Resistance:
- Due to chance mutation in bacteria.
- Occurs with penicillin - mutation resulted in certain bacteria being able to make new protein, an enzyme that broke down AB before it could work (penicillinase).
- Presence of AB does not lead to mutation.
- Genefor penicillinase then passed down by vertical gene transmission. Frequency of alleles increases so resistant form becomes dominant.
- Alleles carried on plasmids which transfer by conjugation which leads to some bacteria accumulating DNA with resistance.
- To identify resistant form place in petri dish with different antibiotics, as bacteria grow they form a cloudy covering.
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