6.8 Antibiotics

Syllabus

• _{Distinguish between bacteriostatic and bactericidal}

antibiotics.

• _{Practical: Describe how to investigate the effect of}

different antibiotics on bacteria

• _{Describe how an understanding of the contributory}

• _{Penicillin, the first antibiotic, was discovered in 1928}

when Alexander Fleming noticed that some bacteria he had left in a petri dish had been killed by a mould.

Making Pencillin

• _{Unfortunately, Fleming could not extract the}

penicillium “juice” that killed the bacteria, so he could not prove that it would actually kill bacteria and make people better

• _{10 years later two other scientists, Chain and Florey,}

How Antibiotics Work

• _{The antibiotics are used to control bacterial}

infections

• _{Bacteriostatic prevent bacteria from reproducing.}

This is because they prevent cell division.

• _{Bactericidal antibiotics kill bacteria. The bacterial}

Evolution of Antibiotic Resistant

Bacteria

• _{When you are given an antibiotic almost all the}

bacteria are killed. However, the ones that are left are likely to be resistant to the antibiotic. The

antibiotic acts as the selection pressure.

• _{These bacteria then survive and pass on their}

antibiotic resistance alleles to the next generation – a process known as natural selection.

• _{These antibiotic resistant bacteria may be passed on}

How to prevent bacteria developing

antibiotic resistance

• _{Only use antibiotics when necessary – not as a}

preventative medicine. This means the antibiotic is no longer a selection pressure; this prevents the evolution of antibiotic resistant bacteria.

• _{Always finish the course, so all the bacteria are killed.}

MRSA - A Bacteria that is resistant to

many antibiotics

• _{MRSA - Methicillin Resistant Staphylococcus aureus,}

is an example of a species of bacteria that has

developed resistance to almost all different types of antibiotics. MRSA now causes a thousand deaths

every year.

• _{Hospital use many types of antibiotics to treat}

How to prevent further spread of

MRSA

• _{Strict hygiene practices employed by hospitals e.g. isolation of} suspected MRSA cases.

• _{Medical staff must wash their hands between patients,} particularly when dealing with open wounds.

• _{Hospital staff must wear suitable clothing (e.g. no ties;} disposable aprons and gloves)

• _{Use of antiseptics to clean medical equipment. Alcohol rubs} can be used to clean hands.

• _{The controlled use of antibiotics – only if necessary}

Investigating the effect of different

antibiotics on bacterial growth

• _{Aseptic technique is used. All apparatus is sterilized}

at high temperature. A wire loop is used to spread a bacterial culture over the agar in the agar plate.

• _{Antibiotic discs are placed on the agar using}

tweezers

• _{The plates are then incubated for 24 – 36 hours} • _{At 25 – 30 °C}

•

_{Independent variable:}

•

_{Dependent variable:}

•

_{Control variables (increasing validity)}

•

_{Precision – a measure of the detail in the}

Aseptic Technique

• _{Petri dish and other vessels must be open for the}

minimum amount of time possible.

• _{On opening a test tube or bottle, the neck must be}

immediately warmed by flaming with the vessel held as near to horizontal as possible and so that any

movement of air is outwards from the vessel.

• _{All items which come into contact with microorganisms}

•

_{When using the Petri dish, limit exposure of}

the sterile inner surfaces to contamination

from the air.

•

_{Independent variable:}

•

_{Dependent variable}

•

_{Control variables:}

• _{Independent variable: Type of antibiotic} • _{Dependent variable: Zone of inhibition}

• _{Control variables: concentration of antibiotic; disc}

size; concentration and species of bacteria; temperature

• _{Reliability increased by repeates}

• _{Validity: Having good aseptic technique to prevent}

plate contamination; even distribution of discs; age of antibiotic – if it is out of date likely to be less

Discussing results

1. No clear zone “zone of inhibition”; bacteria is

resistant is resistant to the antibiotic

2. Zone of inhibition is where the antibiotic kills

bacteria

3. Compare clear zone: the size of zone of

Gram positive bacteria are more

susceptible to antibiotics

• _{Bacteria are classified by gram staining. This is a}

process that can be used to determine resistance to antibiotics. It involves staining a group of bacteria with four different liquids. Crystal violet, iodine and safranin. Then it goes through an alcohol wash.

• _{The bacteria that retain the purple stain from the}

crystal violet are gram-positive, and those that take on the pink stain from the safranin are

•

_{Gram positive bacteria have a thick layer of}

peptidoglycan

, which absorbs the gram stain.

Gram-negative bacteria have a thick phospholipid

bilayer and thin layer of peptidoglycan

on the

outside, which is selectively permeable - not

everything can pass through it including gram

stain.

•

_{The gram-positive bacteria}

_{(since things can pass}

through it easily) are much more susceptible to