4.3.14 Evolution of Resistance

Antibiotics

• When humans experience a pathogenic bacterial infection they are often prescribed drugs known as antibiotics by a healthcare professional
• Antibiotics are chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue
  • Antibiotics are derived from naturally occurring substances that are harmful to prokaryotic cells (structurally or physiologically) but usually do not affect eukaryotic cells
  • The aim of antibiotic use is to aid the body’s immune system in fighting a bacterial infection

• Penicillin is a well-known example; it was the first antibiotic to be discovered in 1928 by Sir Alexander Fleming

Drug resistance in microorganisms

• Within a bacterial population, there is variation caused by mutations (as occurs in populations of all species)
• A chance mutation might cause some bacteria to become resistant to an antibiotic (eg. penicillin)
• When the population is treated with this antibiotic, the resistant bacteria do not die
  • For example, a mutation may change an existing gene within the bacterial genome, causing it to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by the antibiotic being used

• This means the resistant bacteria can continue to reproduce with less competition from the non-resistant bacteria, which are now dead
• Therefore the genes for antibiotic resistance are passed on with a much greater frequency to the next generation
  • As bacteria only have one copy of each gene, a mutant gene will have an immediate effect on any bacterium possessing it

• Over time, the whole population of bacteria becomes antibiotic-resistant because the antibiotic-resistant bacteria are best suited to their environment
• This is an example of evolution by natural selection
• Some pathogenic bacteria have become resistant to penicillin as they have acquired genes that code for the production of the enzyme β-lactamase (also known as penicillinase), which breaks down penicillin

Bacteria evolve rapidly as they reproduce quickly and acquire random mutations – some of which confer resistance

How bacteria inherit antibiotic resistance

• There are two ways in which a bacterium inherits resistance to an antibiotic:
  • Vertical transmission
  • Horizontal transmission

• In vertical transmission:
  • Bacteria reproduce asexually by binary fission (the DNA of the bacterial chromosome is replicated and the bacterial cell divides in two, with each daughter cell receiving a copy of the chromosome)
  • Bacteria reproduce like this very rapidly (on average, every 20 minutes)
  • If one bacterium contains a mutant gene that gives it antibiotic resistance, all of its descendants (millions of which can be produced in a matter of hours) will also have the antibiotic resistance
  • This form of transmission enables antibiotic resistance to spread within a bacterial population

• In horizontal transmission:
  • Plasmids (the small rings of DNA present in bacterial cells) often contain antibiotic-resistant genes
  • These plasmids are frequently transferred between bacteria (even from one species to another)
  • This occurs during conjugation (when a thin tube forms between two bacteria to allow the exchange of DNA) – DNA from the bacterial chromosome can also be transferred in this way
  • In this way, a bacterium containing a mutant gene that gives it antibiotic resistance could pass this gene on to other bacteria (even those from a different species). This is how ‘superbugs’ with multiple resistance have developed (e.g. methicillin-resistant Staphylococcus aureus – MRSA)
  • This form of transmission enables antibiotic resistance to spread within or between bacterial populations

Antibiotic resistance in bacteria can spread by vertical or horizontal transmission

• Antibiotic resistance in bacteria is an example of natural selection that humans have helped to develop. This is due to the overuse of antibiotics in situations where they were not really necessary or the incorrect use of antibiotics, for example:
  • For treatment of non-serious infections
  • Routine treatment of animals in agriculture
  • Failure to finish the prescribed course of antibiotics

• A variety of steps can be taken to reduce cases of antibiotic resistance, including:
  • Only prescribing antibiotics when absolutely necessary
  • Ensure patients complete courses of antibiotics
  • Rotate which antibiotics are used so that one type is not continuously used in the treatment of a specific disease
  • Hold back some antibiotics from being used at all so they are available as a 'last resort'
  • More investment in research into new antibiotics

Pesticide resistance in insects

• Pesticides are chemicals that kill pests of any kind, including insect pests, pathogenic organisms or weeds
• There are various types of pesticides, including:
  • Insecticides (kill insect pests)
  • Herbicides (kill plant pests)
  • Fungicides (kill fungal pests)
  • Molluscicides (kill slug and snail pests)
  • Rodenticides (kill rodent pests)

• A major global use of pesticides is in the control of insect pests that consume or otherwise damage human food crops (e.g. Colorado beetles that eat potato crops) or insects that are vectors of disease (e.g. Anopheles mosquitos that transmit malaria)
• In a similar way to antibiotic resistance in bacteria, insecticides that are sprayed on crops act as selective agents
  • A selective agent is any environmental factor that influences the survival of a particular species and so drives natural selection in that species
  • For example, any insect that has a mutation making them resistant to the insecticide will survive and reproduce, passing on the resistant gene