AQA AS Biology Unit 2 - Antibiotic Resistance

Revision cards covering how antibiotics work, antibiotic resistance and natural selection, horizontal and vertical gene transmission, TB and MRSA.

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—Antibiotics are chemicals that either kill or inhibit the growth of bacteria.  Different types of antibiotics kill or inhibit the growth of bacteria in different ways. Some prevent growing bacterial cells from forming the bacterial cell wall, which usually gives the cell structure and support. This can lead to osmotic lysis:

1.The antibiotics inhibit enzymes that are needed to make the chemical bonds in the cell wall.

2.This prevents the cell from growing properly and weakens the cell wall.

3.Water moves into the cell by osmosis.

4.The weakened cell wall can’t withstand the increase in pressure and bursts (lyses).

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The genetic material in bacteria is the same as in most other organisms. Mutations are changes in the base sequence of an organism’s DNA.  —

If a mutation occurs in the DNA of a gene it could change the protein and cause a different characteristic.  —

Some mutations in bacterial DNA mean that the bacteria are not affected by a particular antibiotic any more – they’ve developed antibiotic resistance.

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Vertical Gene Transmission

—Vertical gene transmission is where genes are passed on during reproduction.

1.Bacteria reproduce asexually, so each daughter cell is an exact copy of the parent.

2.This means that each daughter cell has an exact copy of the parent cell’s genes, including any that give it antibiotic resistance.

3.Genes for antibiotic resistance can be found in the bacterial chromosome or in plasmids.

4.The chromosome and any plasmids are passed on to the daughter cells during reproduction.

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Horizontal Gene Transmission

 1.Two bacteria join together in a process called conjugation and a copy of a plasmid containing the antibiotic resistance allele is passed from one cell to the other via the conjugation tube or pilus.

2.Plasmids can be passed on to a member of the same species or a totally different species.

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Natural Selection in Bacteria

1.Some individuals in a population have alleles that give them resistance to an antibiotic.

2.The population is exposed to that antibiotic, killing bacteria without the antibiotic resistance allele.

3.The resistant bacteria survive and reproduce without competition, passing on the allele that gives antibiotic resistance to their offspring.

4.After some time most organisms in the population will carry the antibiotic resistance allele.

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—Methicillin-resistant Staphylococcus aureus (MRSA) is a strain of the Staphylococcus aureus bacterium that has evolved to be resistant to a number of commonly used antibiotics, including methicillin.

—Staphylococcus aureus causes a range of illnesses from minor skin infections to life threatening diseases such as meningitis and septicaemia. —

Some strains are resistant to nearly all the antibiotics that are available. —It can take a long time for clinicians to determine which antibiotics, if any, will kill the strain each individual is infected with. During this time the patient may become very ill and even die. —Drug companies are trying to develop alternative ways of treating MRSA to combat the emergence of resistance.

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—TB is a lung disease caused by bacteria. —TB was once a major killer in the UK, but the number of people dying from TB decreased with the development of specific antibiotics that killed the bacterium. Also the number of people catching TB dropped due to the vaccine and herd immunity. —

Recently, some populations of TB have evolved resistance to the most effective antibiotics. Natural selection  has lead to populations that are resistant to a range of different antibiotics – so the strains are multidrug – resistant. —TB treatment now involves taking a combination of different antibiotics for about 6 months to combat this.

—TB is becoming harder to treat as multidrug-resistant strains are evolving quicker than drug companies can develop new antibiotics.

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reli helpful! thnks 

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