Cambridge International A Level Biology Revision Guide

Chapter 10: Infectious diseases
Vertical transmission
resistant
parent cell
Vertical transmission
Horizontal transmission Horizontal transmission
resistant resistant
parent cell
non-resistantresistant
cell cell
non-resistant
cell
bacterial
chromosome
and plasmid
replicate
bacterial
chromosome
and plasmid
replicate
cell division
conjugating bacteria
cell division
conjugating bacteria
Daughter cells each receive Daughter a copy cells A each single receive DNA strand a copyof the A plasmid single DNA is transferred. strand of the plasmid is transferred.
of the plasmid and are resistant. of the plasmid Each and bacterium are resistant. then synthesises Each bacterium complementary then synthesises a complementary
strand. Both cells are now resistant. strand. Both cells are now resistant.
Figure 10.15 Vertical and horizontal transmission of resistance in bacteria.
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The more we misuse antibiotics, the greater the
selection pressure we exert on bacteria to evolve resistance
to them. Antibiotic-resistant strains of bacteria are
continually appearing (Figure 10.16). Antibiotic-resistant
infections increase the risk of death, and are often
associated with long stays in hospital, and sometimes
serious complications.
There is a constant race to find new antibiotics as
resistant strains keep arising.
Where there is widespread use of antibiotics, such
as in hospitals or on farms, resistance quickly spreads
among different species of bacteria. Resistance may first
appear in a non-pathogenic bacterium, but then be passed
to a pathogenic species. Bacteria living where there is
widespread use of antibiotics may have plasmids carrying
resistance genes for several different antibiotics, giving
multiple resistance. This presents major problems for
doctors. For example, methicillin-resistant Staphylococcus
aureus (MRSA) has become a problem in hospitals
around the world and in prisons in the USA. It is now also
infecting people in the general population. MRSA caused
Figure 10.16 The grey areas on the agar jelly in this Petri dish
are colonies of the bacterium Escherichia coli. The white discs
are pieces of card impregnated with different antibiotics.
Where there are clear areas around the discs the antibiotic has
prevented the bacteria from growing. However, you can see
that this strain of E. coli is resistant to the antibiotics on the
discs at the bottom left and has been able to grow right up to
the discs.