Chapter 10.2 Flashcards
Antibiotics
are drugs that kill or stop the growth of bacteria (prokaryotes) but do not harm the cells of the infected organism
-Some antibiotics are derived from living organisms, whilst other are made synthetically
Antibiotics work by interfering with the growth or metabolism of the target bacterium. Antibiotics target a variety of processes including:
- synthesis of bacterial cell walls
- activity of proteins in bacterial cell surface membranes
- bacterial enzyme action
- bacterial DNA synthesis
- bacterial protein synthesis
How penicillin affects bacteria
- Bacterial cell walls are composed of peptidoglycans (long molecules of peptides and sugars)
- These peptidoglycan molecules are held together by cross-links that form between them
- When a new bacterial cell is growing, it secretes enzymes known as autolysins that create small holes in the bacterial cell wall
- These holes allow the bacterial cell wall to stretch, with new peptidoglycan molecules then joining up via the cross-links described above
- Penicillin stops these cross-links forming by inhibiting the enzymes that catalyse their formation
- However, the autolysins keep creating holes in the bacterial cell wall, making the walls weaker and weaker
- As bacteria live in watery environments and take up water by osmosis, their weakened cell walls eventually burst as they can no longer withstand the pressure exerted on them from within the cell
penicillin is only effective against
bacteria that are still growing (autolysins no longer create holes and no more cross-links between peptidoglycan molecules are formed once the growth of a bacterium is complete, as the bacterial cell wall no longer needs to expand)
Penicillin is not effective against all bacteria (eg. tuberculosis) because the bacteria may have:
- Thick cell walls which reduce permeability
- Enzymes which breakdown penicillin
How penicillin works
- Penicillin inhibits the strengthening of the cell wall (by stopping the formation f cross-links between peptidoglycan molecules)
- The cell wall bursts, because it can no longer withstand the pressure inside the cell
Penicillin (and other antibiotics) do not affect
viruses as they do not have cells (or cell walls) and therefore cannot be targeted in any of the ways that an antibiotic targets a bacterial cell
-When a virus replicates, it uses the host cell’s mechanisms for transcription and translation, so not even these processes can be targeted as antibiotics do not bind to the proteins that host cells use in these processes
Resistance to Antibiotics: An Outline
- 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
- 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 Steps:
- A population of bacteria in the gut, some have antibiotic resistance
- When exposed to an antibiotic, Bacteria causing illness, as well as healthy gut bacteria, are killed
- When reduced competition dor nutrients, Antibiotic-resistant Bacteria Multiply, forming a larger population that is difficult to control
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 method of spreading antibiotic resistance within a bacterial population is known as vertical transmission
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 method of spreading antibiotic resistance within or between bacterial populations is known as horizontal transmission
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 to animals in agriculture
- Failure to finish the prescribed course of antibiotics
Commonly prescribed antibiotics are becoming less effective for many reasons, the main being:
- Overuse of antibiotics and antibiotics being prescribed when not necessary
- Large scale use of antibiotics in farming to prevent disease when livestock are kept in close quarters, even when animals are not sick
- Patients failing to complete the full course of antibiotics prescribed by doctors
- These factors have led to a reduction in the effectiveness of antibiotics, and an increase in the incidence of antibiotic resistance
- These bacteria are commonly known as superbugs
