Student_118
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Hi so for my Biology project I'm doing the effect of antibiotics on bacteria. I am supposed to use serial dilutions but am I suppose to do serial dilutions for the bacteria or the antibiotics? I'm really confused... Also what is the purpose of serial dilutions?
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alldaydreaming
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Hi there,

Do you have a method sheet? And which exam board are you doing?
I'm a Y13 student doing AQA Biology and we did a practical involving the effects of antibiotics on bacteria and measured inhibition zones etc. If that sounds familiar?
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usycool1
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(Original post by Student_118)
Hi so for my Biology project I'm doing the effect of antibiotics on bacteria. I am supposed to use serial dilutions but am I suppose to do serial dilutions for the bacteria or the antibiotics? I'm really confused... Also what is the purpose of serial dilutions?
I'd presume it'd be for the antibiotics, to see how the concentration of the antibiotic affects bacterial growth.
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Reality Check
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(Original post by Student_118)
Hi so for my Biology project I'm doing the effect of antibiotics on bacteria. I am supposed to use serial dilutions but am I suppose to do serial dilutions for the bacteria or the antibiotics? I'm really confused... Also what is the purpose of serial dilutions?
A serial dilution allows you to massively dilute something (in powers of 10) accurately. You usually take an 1mL 'aliquot' (=portion) of the thing you want to dilute and add that to 999mL of dilutant to make a 10-3 dilution. You can see that if you repeat this a couple of times, you're going to end up with an incredibly dilute sample.

It's often used where you're going to be manually counting colonies of bacteria - if you used the sample 'neat' it would be impossible to manually count them as there'd be millions of them. By diluting to 10-9 or 10-12, you can reach a dilution where you can accurately count a hundred or so and then by working backwards, calculate the number of bacteria in the original sample per unit volume.

Does this make sense to you?
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Student_118
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(Original post by alldaydreaming)
Hi there,

Do you have a method sheet? And which exam board are you doing?
I'm a Y13 student doing AQA Biology and we did a practical involving the effects of antibiotics on bacteria and measured inhibition zones etc. If that sounds familiar?
Yes that sounds sort of familiar but I'm growing my bacteria in liquid culture and measuring inhibition using a colorimeter, is that correct?

Do you use serial dilutions in your method at all?
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Student_118
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(Original post by Reality Check)
A serial dilution allows you to massively dilute something (in powers of 10) accurately. You usually take an 1mL 'aliquot' (=portion) of the thing you want to dilute and add that to 999mL of dilutant to make a 10-3 dilution. You can see that if you repeat this a couple of times, you're going to end up with an incredibly dilute sample.

It's often used where you're going to be manually counting colonies of bacteria - if you used the sample 'neat' it would be impossible to manually count them as there'd be millions of them. By diluting to 10-9 or 10-12, you can reach a dilution where you can accurately count a hundred or so and then by working backwards, calculate the number of bacteria in the original sample per unit volume.

Does this make sense to you?
I'm still confused because I'm using liquid culture to grow bacteria and adding the antibiotics then measuring inhibition by using a colorimeter so I don't think I count them? My teacher knows my method but said I have to do serial dilutions and I don't know why or what she means or whether it's on the bacteria or the antibiotics?
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Davwardo
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(Original post by Student_118)
I'm still confused because I'm using liquid culture to grow bacteria and adding the antibiotics then measuring inhibition by using a colorimeter so I don't think I count them? My teacher knows my method but said I have to do serial dilutions and I don't know why or what she means or whether it's on the bacteria or the antibiotics?
Apologies if I am no help - I'm a CCEA student and we dont do projects so I may be of no use.
You mention you will be using a colourimeter, to prevent you counting them. You are "doing the effects of antibiotics on bacteria", I interpret this to mean "You are going to use a series of dilutions of the antibiotic and use a constant volume of bacteria to see what effects the antibiotics have on the bacteria." By this I would assume you culture bacteria in a liquid culture then sample them and putting them into different concentrations of antibiotic (with one sample in no antibiotics to serve as a control), and using the colourimetry method to see relatively the effects when compared to the control (the one sample not put into antibiotics)
By a reasonable hypothesis I would suggest that the amount of bacteria in a sample will decrease as the concentration of antibiotics increases.

I think thats what you're trying to get at.
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alldaydreaming
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(Original post by Student_118)
Yes that sounds sort of familiar but I'm growing my bacteria in liquid culture and measuring inhibition using a colorimeter, is that correct?

Do you use serial dilutions in your method at all?
We didn't use serial dilutions in the method last year. We basically had a paper disc with different antibiotics on and put it in the petri dish with the bacteria in. But when we did our inhibition zones, we measured the diameter of the clear zone and and calculated it's area using the πr^2 formula for area of a circle.

If you get to decide on your own method, I'd use the method I've (briefly) described above - it's a hell of a lot easier and it's the method recommended by AQA.

But if you were to use a colorimeter, it would probably be to check the turbidity (cloudiness) of the culture once you've added the antibiotic? This is because, the higher the concentration of bacteria the cloudier the solution would be. Just to clarify, would you be adding antibiotic directly to your bacteria? And when measuring inhibition, I'm guessing you're just looking for a reduction in turbidity for the most effective antibiotics?
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Student_118
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(Original post by alldaydreaming)
We didn't use serial dilutions in the method last year. We basically had a paper disc with different antibiotics on and put it in the petri dish with the bacteria in. But when we did our inhibition zones, we measured the diameter of the clear zone and and calculated it's area using the πr^2 formula for area of a circle.

If you get to decide on your own method, I'd use the method I've (briefly) described above - it's a hell of a lot easier and it's the method recommended by AQA.

But if you were to use a colorimeter, it would probably be to check the turbidity (cloudiness) of the culture once you've added the antibiotic? This is because, the higher the concentration of bacteria the cloudier the solution would be. Just to clarify, would you be adding antibiotic directly to your bacteria? And when measuring inhibition, I'm guessing you're just looking for a reduction in turbidity for the most effective antibiotics?
My teacher suggested using liquid culture and it's too late to turn back sadly. My brother used your method for his project so it's a little bit different and he has no idea how to help me because he didn't have to use serial dilutions either
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Student_118
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(Original post by Davwardo)
Apologies if I am no help - I'm a CCEA student and we dont do projects so I may be of no use.
You mention you will be using a colourimeter, to prevent you counting them. You are "doing the effects of antibiotics on bacteria", I interpret this to mean "You are going to use a series of dilutions of the antibiotic and use a constant volume of bacteria to see what effects the antibiotics have on the bacteria." By this I would assume you culture bacteria in a liquid culture then sample them and putting them into different concentrations of antibiotic (with one sample in no antibiotics to serve as a control), and using the colourimetry method to see relatively the effects when compared to the control (the one sample not put into antibiotics)
By a reasonable hypothesis I would suggest that the amount of bacteria in a sample will decrease as the concentration of antibiotics increases.

I think thats what you're trying to get at.
Thank you so much! Makes way more sense now!
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