Flashcards in Antibiotics and Infectious Disease Definitions (Chapter 10) Deck (41)
What is an antibiotic?
A drug that kills or stops the growth of bacteria, without harming the cells of the infected organism
How do antibiotics work?
They interfere with some aspect of growth or metabolism of the target bacterium e.g. DNA/protein/bacterial cell wall synthesis and enzyme actions
Describe the structure of bacterial cell walls
- They are made of peptidoglycans (long molecules containing peptides and sugars), held together by cross-links that form between them
What happens when a newly formed bacterial cell is growing?
1) it secretes enzymes called autolysins, which make little holes in its cell wall
2) these holes allow the wall to stretch so that the new peptidoglycan chains can link together
How does penicillin work?
1) Penicillin prevents the peptidoglycan chains from linking up, but the autolysins keep making new holes
2) ∴ the cell wall becomes progressively weaker
3) as bacteria live in watery environments and take up water by osmosis, water enters the cells
4) when the cell walls are weakened they cannot withstand the pressure potential exerted on them by the cell contents ∴ the cells burst and die
What does the way penicillin work mean for its use?
- It is only effective against bacteria while they are growing
- It does not affect human cells as our cells don't have walls
- It does not affect eukaryotic cells as they have different proteins from bacteria
Why do Penicillin and other antibiotics not affect viruses?
1) they do not have cell walls
2) they are not living or growing
3) they are inside host cells
4) antibiotics do not act on the protein coat
5) when viruses replicate, they use the host cell's mechanisms for transcription and translation - antibiotics do not bind to the proteins that host cells use in these processes
Why does penicillin have no effect on M.TB?
1) The thick cell wall of this bacterium is not very permeable
2) The bacterium has a gene that codes for an enzyme that catalyses the breakdown of penicillin
How can a bacterium prevent being affected by an antibiotic?
1) proteins in the membranes can inactivate antibiotics
2) some bacterial membranes have proteins that pump out antibiotics if they enter cytoplasm
3) sometimes, the antibiotic simply cannot bind to the intended site of action
What does it mean if a bacterium is sensitive?
That it is susceptible to that antibiotic
What antibiotic resistance mechanisms do soil bacteria have?
They have acquired a gene that codes for an enzyme called beta-lactamase which breaks down a structure in penicillin
What exists to control viral infections?
What is an infectious disease?
Diseases caused by pathogens (can be transmitted from infected humans/animals to uninfected humans)
What are non-infectious diseases?
Diseases not caused by pathogens
What are some examples of non-infectious diseases?
1) long-term degenerative diseases e.g lung cancer
2) inherited or genetic diseases e.g. sickle cell anaemia
3) deficiency diseases caused by malnutrition
4) mental diseases
What is a disease?
An illness or disorder of the body or mind that leads to poor health - each disease is associated with a set of signs and symptoms
Why can some infectious diseases only spread by direct contact?
Because the pathogen cannot survive outside the human body
In what mediums can certain pathogens survive in?
Water, human food, faeces or animals
What are carriers?
- People without symptoms, who may spread a pathogen even though they do not have the disease themselves
- They can be difficult to trace as a source of infection
What happens when someone takes Penicillin to treat a bacterial infection?
1) bacteria that are susceptible to Penicillin will die
2) in most cases, if the dose is followed correctly, this will be the entire population of bacteria
When does antibiotic resistance arise?
When an existing gene within the bacterial genome changes spontaneously to give rise to a nucleotide sequence that codes for a slightly different protein that is not affected by or protects from the antibiotic (i.e. when there is a mutation in the bacterial genome)
How can bacteria become resistant to antibiotics?
1) if the dose of Penicillin is not followed (e.g. bc people stop taking it when they feel better as the symptoms disappear) then some susceptible bacteria survive
2) if any mutations occur these might confer resistance
3) these bacteria have a selective advantage and so will survive and reproduce while the others are killed
4) the next time there is an infection with this strain of bacteria, penicillin might not be effective
What does a mutant gene have an immediate effect on any bacterium possessing it?
Bc bacteria only have one copy of each gene, since they only have a single loop of double-stranded DNA
Describe vertical transmission of antibiotic resistance
1) bacteria reproduce asexually by binary fission (DNA is replicated and each daughter cell receives one copy)
2) this happens rapidly in ideal conditions
3) ∴ a large population of penicillin-resistant bacteria would result
Describe horizontal transmission of antibiotic resistance
1) if genes for resistance occur on plasmids or part of the DNA from the bacterial chromosome, these can be transferred between bacteria
2) this happens during conjugation, when a tube forms between 2 bacteria to allow the movement of DNA
3) during conjugation, plasmids are transferred from the donor bacterium to the recipient bacterium
What is the consequence of horizontal transmission?
- It is possible for antibiotic resistance to arise in one species and be passed onto another
- Antibiotic resistance may first appear in non-pathogenic bacteria, but then be passed to a pathogenic species
What happens the more we misuse antibiotics?
The greater the selection pressure we exert on bacteria to evolve resistance to them
Describe multiple resistance of bacteria
1) if bacteria have plasmids carrying resistance genes for several different antibiotics, they have multiple resistance
2) problem e.g. with MRSA which caused dangerous infections after surgery
How can antibiotics be carefully chosen?
Testing antibiotics against the strain of bacterium isolated from people ensures that the most effective antibiotic can be used in treatment