Testing the Effectiveness of Certain Foods on the Growth of Lactobacillus

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ibbioq
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Testing the Effectiveness of Certain Foods on the Growth of Lactobacillus

Post by ibbioq »

Hi! I'm working on my IB Biology Extended Essay, but I'm running into some problems with the setup of my experiment. My research question for the science project I'm working on is looking at the effectiveness of some common prebiotic foods (asparagus, bananas, flax seeds, etc.) in promoting the growth of the probiotic bacteria Lactobacillus acidophilus. I'm planning on using a probiotic supplement of Lactobacillus and dissolving it in a tomato juice/yeast extract/milk broth medium (with each tube containing a different one of the foods), leaving the tube in a warm/dark place for around 24 hours, and then swabbing the culture on an MRS agar petri dish. The different foods I'm testing are bananas, asparagus, and flax seeds because they are all known to contain Lactobacillus as well as promote the growth of it. I've swabbed my Petri dishes and let them sit in a warm oven for hours now, but I'm not seeing any growth. Did I do something wrong? Is there a better way to be testing this?

Thank you in advance, and I appreciate any help I can get!
koneill18
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Re: Testing the Effectiveness of Certain Foods on the Growth of Lactobacillus

Post by koneill18 »

Hello!

This sounds like an interesting project idea! Have you seen any growth on your agar plates since making this post? Bacteria usually take at least a full 24 hours to grow on a plate, so don't be discouraged if you don't see any growth after only a few hours. Do you have your oven set to 37 degrees celsius to simulate body temperature? If the bacteria still isn't growing, you might need to check the pH of your growth media to make sure it's ideal for Lactobacillus growth.

If you didn't do this already, I would also recommend pulverizing the foods before adding them to the tubes with Lactobacillus. When you eat prebiotic foods, they've already been chewed up and partially digested by the time they reach the Lactobacillus in the gut. Mashing up the food before you add it to the tube with the Lactobacillus would simulate this process.

Also, do you have access to a spectrophotometer? Spectrophotometers measure the turbidity of liquid bacterial cultures, which allows you to quantify bacterial growth. That could be an alternative to using agar plates.

I hope this helps! Don't hesitate to ask any more questions that you have!
brandimiller610
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Re: Testing the Effectiveness of Certain Foods on the Growth of Lactobacillus

Post by brandimiller610 »

Hi ibbioq,

I hope you are having a great day! I would like to add a couple of notes to the excellent answer given by koneill18.

First, Lactobacillus take at least 24-48 hours to grow pure colonies, depending on the strain. So, if you are not seeing growth after 24 hours, don't be discouraged; it is possible that your plates just need more incubation!

Second, make sure that your tomato juice/yeast extract/milk broth medium that you are suspending your bacteria in has a pH within the optimal growth conditions for L. acidophilus (between ~4.5 and 6.5, depending on the strain). For optimal growth, I suggest cultivating your bacteria in MRS media supplemented with your prebiotics (one in each tube, as you described). I also think homogenizing (i.e. blending/mashing) your prebiotics is a good idea for optimizing the growth.

You might also consider extending the exposure time of the Lactobacillus to the prebiotics (i.e. >24 hrs). As koneill18 mentioned, it might be good to use liquid cultures and a spectrophotometer to indirectly measure the bacterial growth (using optical density [OD600]). That way, you can determine if the bacteria are growing more in comparison to the controls. However, since the OD does not account for live versus dead cells, you should still plate the bacteria and count the colonies in addition to measuring OD.

Hope this helps! Best of luck on your project! Let us know if you have any more questions/concerns.

-Brandi
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