Science Buddies: "Ask an Expert"

my project is due after christmas break so i really don't have time to spare- i have all my materials and have made my agar plates. I only have enough supplies to do 5 plates with thyme, 5 with basil, 5 with cloves and 5 control. I have sterile syringes to use

also- should i do a simple dilution or a serial dilution? And would i dilute it with moremilk?

Hi,

You are very well prepared to complete your project on time, but you are right; you don’t have time to spare.

You should do a serial dilution 1:10 and 1:100 dilution, and maybe a 1:1000 dilution. You can do the dilutions in sterile water. If you set up a pilot experiment tomorrow with just a positive control, you can get experience in setting up the experiment and verify that you are working in the right dilution range. You will have results by Tuesday and you can set up your definitive experiment on Wednesday. That will give you a week to finish your board.

While you are waiting for your results, you can write the sections for your board and plan the layout.

https://www.sciencebuddies.org/science- ... oard.shtml


Donna Hardy

My project was having some difficulty and not proceeding as hoped, and I was afraid all the bacteria on my slant would die. For this reason, I streaked a clean agar plate with a lot of it. If I am lucky and it is not dead and grows, will I be ablee to pick the colonies off the plate and use them in my experiement as planned originally?

Hi,

Yes, it is always better to use young, actively growing bacteria for an experiment like yours. Hopefully you will have some colonies to select from on Tuesday or Wednesday. Fortunately, Bacillus brevis forms spores when the culture gets old, so I'm sure you will have viable colonies. But you will get faster results starting with young vegetative cells rather than dormant spores. . Transferring the culture to a plate will revive the culture and if you transfer a small bit of colony from the new plate to the milk, I'm sure you will get good results.


Donna Hardy

Hi!
So I finished my experiment and am working on graphs. I did, however, have some difficulty counting. I probably should have diluted the acteria more ( I streaked the plates with the 1:100) because counting was very difficult. On some plates there were tons and tons of tiny colonies, while one have very few HUGE colonies. Also, In the first zigzag, the colonies often merged together, making counting difficult and potentially inaccurate. I held each dish under a bright lamp and used a pin to count, but I am afraid my data was affected. Should I be worried about it, or should I include it in my conclusion that I should have diluted? Also, can the number of colonies go down? mine seem to have, and I don't know if that is normal.
Thanks again!

Hi,

It’s not possible to accurately count more plates with more than 300 colonies; these plates are described as “TNTC” or “too numerous to count,” and all you can do is estimate the number of bacteria as greater than 300 x the dilution factor. This would definitely a topic for discussion in your conclusion section. If something does not work out right, you should always think of what you would do differently next time, and making additional dilutions would be the obviously solution. The number of colonies cannot decrease with incubation time, but if the colonies grow together, you won’t be able to tell how many you had at the beginning.

I hope this helps.

Donna Hardy

So, should I mark the colony count as going down on my table and graph and put it in my graph description that the plates above 300 are TNTC so are approximate and that their numbers appeared to go down because they grew together? This is the only reasonable approach I can think of.

Hi,

I think this would be a reasonable approach. If you had a TNTC plate and the dilution was 1:100, then you could report results as >300,000. If all of your results are >300,000, then you could also make a qualitative description of the results. What were your results? Can you post the data?

Donna Hardy