Penicillin works by interfering with the synthesis of bacterial cell walls. It inhibits enzymes involved in creating cross-links between polymers that make up the cell wall. This disrupts the cell wall structure and kills bacteria. Penicillin is ineffective against viruses, as they do not have cells or metabolism. While initially effective, overuse of penicillin has led to the rise of resistant bacterial strains through natural selection, as bacteria carrying resistance genes are able to survive and pass on those genes.

1. Penicillin as an antibiotic
• In general, antibiotics bring about their
effects in a number of ways:
– Inhibiting protein synthesis
– Interfering with the synthesis of bacterial cell
walls
– Interfering with the functioning of the cell
membrane
– Inhibiting enzyme activity – will disrupt
metabolism
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2. The mode of action of penicillin
• Broad spectrum bacteria
• Interferes with synthesis of cell wall – inhibits enzymes
involved in the synthesis of cross-links between
peptidoglycan polymers in bacterial cell walls
• Strains of Mycobacterium tuberculosis are resistant to
penicillin because it possesses enzymes that break down
penicilllin (penicillinase)
• Antibiotics are ineffective against viruses:
– Viruses do not have any form of cell structure or
metabolism
– Replicates only within living host cells
– Eukaryotic mechanisms which are not affected by
antibiotics
– Absence of cell wall means penicillin has no effect on
viruses
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4. The causes and effects of antibiotic
resistance
• Natural selection can change the frequency of alleles in
the population so that most of the bacteria in the
population are resistant, by the following steps:
– Within natural populations of bacteria, some individuals have
alleles of genes which give resistance to a particular antibiotic
– Bacteria cause an infection, leading to treatment of the infected
person with antibiotic
– The antibiotic will kill susceptible bacteria, but resistant bacteria
will survive
– Only the resistant bacteria will reproduce – increase in the
frequency of the bacteria that are resistant to that particular
antibiotic
– Increase in allele frequency for the allele of the gene that gives
resistance in the population of bacteria
– People infected in the future are infected by bacteria more likely
to carry the alleles for resistance
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5. • Alleles of genes that cause resistance arise
fro the first time by mutation
• Such genes located on plasmid – can
rapidly spread from one bacterial species
to another since plasmids are naturally
exchanged between species
• Bacteria can acquire resistance to a
number of antibiotics because plasmids
may contain a number of different
antibiotic resistance genes
• Such populations of bacteria may cause
infections that are difficult to treat
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