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Adaptation and selection
Antibiotics are chemicals that weaken bacterial cell walls. They bring
about osmotic lysis. Bacterial cells are no longer restrained by a strong
cell wall, they swell & burst. Viruses have different covering and are
therefore unaffected by antibiotics.
Mutations are changes in DNA that result in different characteristics.
Mutations arise in many ways, including errors when the DNA is
replicated. Changes to base sequences changes the sequence of
amino acids being coded for, this produces new proteins/ or no protein
Random mutations cause some bacterial cells to make new proteins
which could be an enzyme. The enzyme could be penicillinase.
Mutations are rare & random. Most mutations are harmful causing the
bacteria to die.
Occasionally mutations are advantageous, e.g. producing the enzyme
penicillinase in the presence of penicillin is an advantage. Penicillin kills
normal types of bacteria & therefore only the mutants will survive. These
mutations are passed on vertically from one generation to another as
the bacteria reproduces asexually by mitosis. This is called vertical
gene transmission. Resistant forms of a bacteria survive, so the
frequency of an advantageous allele (brought about by mutation) in
Some bacteria can conjugate by forming a conjugations tube. The
plasmid of the donor cell starts to replicate. The replicated plasmid
moves through the conjugation tube as a linear strand. This linear strand
forms a circle (new plasmid). This is called horizontal gene
transmission. Advantageous mutations of plasmids are passed on
within a generation horizontally. Allele frequency is also increased by
conjugation. Some bacteria can accumulate several genes in this way
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These are called super bugs
an example is MRSA.
Aiding resistance :
There are 2 main events that give rise to this:
1. Overuse...as more mutations and conjugations can occur.
2. Incompletion of a course of antibiotics... the more resistant
bacteria remain and increase their population size.
Methicillin Resistant Staphylococcus Aureus (MRSA)
Staphylococcus is a common and generally harmless throat bacterium
but can become a health risk.…read more