Antimicrobial effect investigation - Help please, any bio teacher ?

Here's a picture of the mark scheme :

What type of Bacteria should I use ? And for Mp12, what should I write ?

If you're going to use a screenshot, then at least make sure it's legible! How in earth can we help you if we can't even read the document you've uploaded?

I honestly dont know how to make it clearer, in the gallery its very clear. Is this better ? https://imgur.com/a/rWVGbFC
Here's a link to the document https://qualifications.pearson.com/content/dam/pdf/International%20Advanced%20Level/Biology/2013/Exam%20materials/WBI06_01_msc_20140306.pdf
Q3c

Yes, the imgur link is legible.

For bacterial choice, you need to write about what qualities you're investigating and how the selection of bacteria needs to meet that. For instance, it's obvious you wouldn't want to use a pathogenic bacteria from a safety point of view. But that leaves a lot more. What are you intending to do with the bacteria? Is it going to be in a particular environment, such as high salt, high/low temperature?

Until you can answer these questions you can't make a meaningful selection of bacteria and be able to justify that choice.

The only types of bacteria I know of are harmful ( or pathogenic ) and non pathogenic. Even having answered these questions I wouldn't be able to choose a particular type of bacteria. There's nothing on this anywhere in the specification or the book, my teacher didn't tell me about it either. All I would've written was that the same volume and type of bacteria should be used in all the petri dishes and that an air gap should be left to prevent the growth of anaerobic harmful bacteria. Would that be enough ?

If you were asking me these questions then
The investigation is studying the antimicrobial properties of secretions from 2 frogs of different species. The bacteria would be put in a petri dish that contains agar and it would be mixed with the agar then filter paper disks which were soaked in the secretions would be placed on top and the petri dishes would be covered and incubated at 25c for 24 hrs.

OK - so there are some clues there, in red. What steps are you taking to avoid growing pathogenic bacteria? What other steps could you take as well?

When covering the petri dish with adhesive tape, I'd leave an air gap to prevent the growth of anaerobic harmful bacteria. And when incubating the dishes I'd make sure to incubate them at a temperature less than 37c to prevent the growth of pathogenic bacteria.
Is that it ?

Yes, that's it - I was particularly pointing to the incubation temperature of 25. You'd also avoid any sort of selective medium like blood/chocolate agar for the same reason.

Q6 is the selection of bacteria. For your information, you'd usually select a non-pathogenic strain of E.Coli. For Q12, you need to say how you're going to set up the experiment to grow your bacteria. Will you have a lawn, streak a plate, make a broth, etc etc?

You are great Thank you but one more thing please, just to make sure I get this mark, is that ( Mp12 ) covered by saying that I'll transfer a measured volume of the bacteria culture to a sterile petri dish containing the agar and that I'll mix them together and then Ill place the soaked disks on top?

You also need time and temperature of inoculation, plus mentioning aseptic technique and why that is important is also required.

Hello! I actually had a quick question on this investigation please.
Why do we flip the petri dish before incubation ?
And there was a question on variables to be controlled and how they can be controlled and one of the variables was O2 concentration, another was Antibiotic concentration .
How do I control these when I have different antibiotics ?

You inoculate them upside down so that any condensation which forms on the lid doesn't drip onto your plate and interfere with microbial growth. Also, if the lid was contaminated, the condensation would similarly be contaminated, and this would ruin your plate. You also always label the dish, not the lid. Lids can inadvertently get swapped between plates, so labelling the actual dish prevents this.

Variables - have you got any ideas about this (I'll help you if not, but I want to see if you can help yourself first )

Got it, thank you!

For the variables, we were allowed to choose whichever and write how we'd control them so I wrote temperature and control it by a thermostatically controlled incubator.
I also know that you can control the type of bacteria by taking it from the same strain (?). The problem is that the mark scheme does not say how it just says -appropriate method.

Other variables I said are
pH of agar using a buffer

For O2 concentration first I thought we can use an oxygen probe to measure the conc in the growing medium but tbh that is a memorized statement as I do not even know what that looks like. Also I realized that since they will be exposed to air there will also be O2 in the room so I thought put all the petri dishes in the same room .

For the concentration I wanna say we can titrate the antibiotics to find the concentration and then change them by dilution so they can all be the same but that sounds like a lot of work so I don't know

This is all very good For the oxygen concentration, the oxygen in the room isn't relevant because they'll be inoculating in sealed petri dishes. So how would you ensure all your dishes had the same 'amount ' of oxygen in them in this circumstance. Think about amount of substance present in a gas and PV=NRT...

Do you have a copy of the question paper I can look at. I've tried to find it on Edexcel's website but it wasn't easy. I just want to check something before giving you a complete answer. I've never taught the international qualification.

I'm sorry but I'm not familiar with the formula
and ofc, here's a link to the QP : https://qualifications.pearson.com/content/dam/pdf/International%20Advanced%20Level/Biology/2013/Exam%20materials/WBI06_01_que_20150513.pdf

And the marking scheme: https://qualifications.pearson.com/content/dam/pdf/International%20Advanced%20Level/Biology/2013/Exam%20materials/WBI06_01_msc_20150812.pdf

Thank you However, the question paper you've just sent me doesn't have the frog question on it. Are you sure you've sent the right one. I have found on edexcel the frog question now. Please confirm. Meanwhile:

OK - PV=NRT is just a fancy way of relating the amount of substance in a gas to the pressure, temperature, volume and a constant. Why it's relevant to you is that if you keep the pressure and temperature constant, the volume of the gas directly affects the amount of substance in it (and with that, oxygen concentration). Thus, the way to control the initial oxygen level in the petri dish is to pour exactly the same amount of agar, leaving the same 'air space' in each. However, and this is the important bit, there are much better variables to talk about than oxygen concentration. Can you think of any?