Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
406
As bacteria have only a single loop of DNA, they have
only one copy of each gene, so the mutant allele will have
an immediate effect on the phenotype of any bacterium
possessing it. These individuals have a tremendous
selective advantage. The bacteria without this allele
will be killed, while those bacteria with resistance to
penicillin can survive and reproduce. Bacteria reproduce
very rapidly in ideal conditions, and even if there was
initially only one resistant bacterium, it might produce ten
thousand million descendants within 24 hours. A large
population of a penicillin-resistant strain of Staphylococcus
would result.
Such antibiotic-resistant strains of bacteria are
continually appearing (Figure 17.12). By using antibiotics,
we change the environmental factors which exert selection
pressures on bacteria. A constant race is on to find new
antibiotics against new resistant strains of bacteria.
Alleles for antibiotic resistance often occur on plasmids
(Figure 1.30, page 21). Plasmids are quite frequently
transferred from one bacterium to another, even between
different species. Thus it is even possible for resistance to
a particular antibiotic to arise in one species of bacterium,
and be passed on to another. The more we use antibiotics,
the greater the selection pressure we exert on bacteria to
evolve resistance to them.
QUESTION
17.4 Suggest how each of the following might decrease the
chances of an antibiotic-resistant strain of bacteria
developing:
a limiting the use of antibiotics to cases where there
is a real need
b regularly changing the type of antibiotic that is
prescribed for a particular disease
c using two or more antibiotics together to treat a
bacterial infection.
Industrial melanism
One well-documented case of the way in which changing
environmental factors may produce changes in allele
frequencies is that of the peppered moth, Biston betularia
(Figure 17.13), in the UK and Ireland. This is a nightflying
moth which spends the day resting underneath
the branches of trees. It relies on camouflage to protect
it from insect-eating birds that hunt by sight. Until 1849,
all specimens of this moth in collections had pale wings
with dark markings, giving a speckled appearance. In
1849, however, a black (melanic) individual was caught
near Manchester (Figure 17.14). During the rest of the 19th
century, the numbers of black Biston betularia increased
dramatically in some areas, whereas in other parts of the
country the speckled form remained the more common.
Figure 17.12 The red gel in each of these Petri dishes has been
inoculated with bacteria. The small light blue circles are discs
impregnated with antibiotics. Bacteria that are resistant to an
antibiotic are able to grow right up to the disc containing it.
See also Figure 10.16, page 215.
Figure 17.13 Dark form of peppered moth on dark and pale
tree bark.