Which Antibiotic is Most Effective

Student

Practical 6.3

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Which antibiotic is most effective?

Purpose

Safety

• Toin oinves vestiga tigateth tetheef eeffect fectofd ofdiff ifferen erent t antibioticsonbacteria.

Wear eye protection. The microorganisms are a potential biological hazard. Use aseptic techniques when transferring the bacteria to the Petri dishes. Clean the bench with antibacterial disinfectant. Do NOT open the Petri dishes once they have been incubated.

Introduction When a bacterial inection is diagnosed it is useul to be able to tell to which antibiotics it is most susceptible. In some cases this inormation is known, but in other cases tests need to be carried out to fnd out which antibiotic will be most eective. In this activity you will be testing the eectiveness o several types o antibiotics on bacteria. The standard method o doing this is to put discs o chromatography blotting paper soaked in the various antibiotics onto an agar plate that has been inoculated with the bacteria. Alternatively a Mast ring (a r ing o paper with several ‘arms’, each treated with a dierent antibiotic) can be used.

Procedure Youwillneed: • Agar Agarplate platesee seededw dedwitha ithakno knownb wnbacte acterium rium

•

• Bu Buns nsen enb bur urne nerr

• Au Auto tocl clav aved edf for orce ceps ps

• Benc Benchspr hsprayo ayofdis fdisinf infectan ectant, t,1% 1%Vi Virkon rkonor or

• Mast Mastrin ringora gorantib ntibioti iotic-imp c-impregn regnated atedpape paper r

equivalent

1

Marker pen

discs

• So Soap apo or rha hand ndw was ash h

• Adh dhes esiv ive eta tape pe

•

• Ey Eye epr prot otec ecti tion on

Pap apeer rttowels

Wash your hands with the soap or handwash. Spray the working area thoroughly with the disinectant spray. Leave or at least 10 minutes, then wipe with a paper towel.

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Work very close to a lit Bunsen burner. Prepare an agar plate seeded with bacteria. This may have already been done or you. I not, ollow the instructions in the section ‘Pouring agar plates’ in Practical 4.3 Edexcel AS Biology. Label the Petri dish on the base at the edge with your name, the date and the type o bacterium it is inoculated with.

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Flame the orceps and then use them to pick up an antibiotic disc or Mast r ing. Raise the lid o the Petri dish and place the Mast ring frmly in the centre o the agar; i individual discs are used they will need to be spaced evenly around the dish.

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Tape the dish securely with two pieces o adhesive tape (but do not seal it completely), then keep it upside down at room temperature or 48 hours. Edexcel practical materials created by Salters-Nufeld Advanced Biology, ©University o York Science Education Group.

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2 of 2

Student

m ost effective? Practical 6.3 (cont.) Which antibiotic is most

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Wash your hands with soap or handwash and clean the bench again using the Virkon spray.

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Ater incubation, look careully at the plate but do not open it . Where bacteria have grown the plate will look opaque, but where the antibiotics have inhibited growth, clear zones called inhibition zones will be seen. Measure the diameter o the inhibition zones in millimetres and use this inormation to decide which antibiotic is most eective at inhibiting the growth o the bacterium.

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Collect data rom other members o the class who used the other bacterial cultures.

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Write a brie report o the results, comparing the dierent antibiotics antibiotics and the eects on the dierent bacterial cultures.

Questions 1

Are the inhibition zones circular? I not, what is a sensible measuring strategy?

2

What actors determine the diameter o the inhibition zones?

3

I class data are shared: a what is the overall spread o the data  b do all individual results show the same trends – i not, why not, and how could this

variability be represented on your graphs? 4

I you were working in a hospital laboratory, and and you had just carried out this test on bacteria isolated rom sick patients, would you always choose the antibiotic that gave the biggest inhibition zone? Are there any other actors you would need to consider?

Edexcel practical materials created by Salters-Nufeld Advanced Biology, ©University o York Science Education Group.

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