Science Buddies: "Ask an Expert"

Dear Life Technologies Scientist,'
Our project is about simply Bacteria and testing solutions that are effective at eliminating bacteria. We made our own Petri dishes with sealed/sterilized plastic containers and made a nutrient agar from this website: http://www.ehow.com/how_6317533_make-nu ... -home.html
We have placed the raw ground beef bacteria onto the Sterilized by boiling water Petri dish slides with a sterilized q-tip and have left them for sealed under a heated lamp for 24hours. Do you know how much longer will it take for colonies to be visible on a Petri dish which had water applied/how many colonies might be visible if the heated lamp temp is around 80 degrees fahrenheit?
Additionally, we did a quick experimental run prior to this one and did the same experiment except we used baby wipes to sterilize the Petri dishes and did not add nutrients into our agar. We observed that no bacteria have grown for 3days... Do baby wipes inhibit bacteria growth along with no sugar/amino acid sources in our agar?
Finally, we have questions about the solutions we used and how they eliminate bacteria. We know that meat bacteria along with many others are vulnerable to changes in temperature (High), but how does 91percent alcohol affect bacterial cells in terms of their functions? How does hand sanitizer (gel form) work in order to kill bacteria? Can distilled water or pH 0 and 14 solutions kill meat bacteria?
Wait one more question about bacteria! Do you know what pH, temp, and salinity levels that typical meat bacteria (e.coli) can withstand without the cell dying and typically what part of the bacteria is affected the most from changes in pH, Temp, and salinity levels.
-Thank You Life Technologies Scientist for your time

Hi Skyhawk,

Those are some great questions - it sounds like you have a great project planned.

Do you know how much longer will it take for colonies to be visible on a Petri dish which had water applied/how many colonies might be visible if the heated lamp temp is around 80 degrees fahrenheit? I would think 24-48 hours would be plenty of time.

Do baby wipes inhibit bacteria growth along with no sugar/amino acid sources in our agar? Correct - with no/little nutrition and wiping with baby wipes, I would expect the result you got = no bacterial growth. Whether or not the wipes are sterilizing completely... depends on the wipe and what solution it is soaked in. Many of these have a good amount of alcohol in them, with a detergent, so they sterilize pretty well.

Finally, we have questions about the solutions we used and how they eliminate bacteria. We know that meat bacteria along with many others are vulnerable to changes in temperature (High), but how does 91percent alcohol affect bacterial cells in terms of their functions? The high alcohol denatures (unfolds) proteins that make up the cells, most notably those in the cell membrane - causing the cells to open up, then die.

How does hand sanitizer (gel form) work in order to kill bacteria? Usually these have a high alcohol %.

Can distilled water or pH 0 and 14 solutions kill meat bacteria? The extreme pH can kill bacteria, yes. Distilled water alone, most likely. Can you do a google search to find this answer for sure? It has to do with the osmolality of the water - it will make the cells burst, I think. They prefer a salt solution.

Wait one more question about bacteria! Do you know what pH, temp, and salinity levels that typical meat bacteria (e.coli) can withstand without the cell dying and typically what part of the bacteria is affected the most from changes in pH, Temp, and salinity levels.
I bet you can find the first half of this answer on google too. The second part - it's the cell membrane that is taking the most damage. Then it can't hold the cell together.

Let me know if you need more help with that, ok?

Tonya

Dear Life Technologies Scientist,

We used plastic sealed containers with the homemade agar inside and we have left them out under a heated lamp for 48hours. So far we do not see any bacteria on the surface. Does having a plastic container affect growth or does bacteria from meat from the store not contain much bacteria?

-Thank You

Dear Life Technologies Scientist,

If we did the experiment again and used the same procedure except in a glass sealed container, will this be able to grow bacteria in around 24hours?

-Thank You

Plastic or glass should be ok. I have used both. And yes, meat from the store (raw) has bacteria. How did you prepare the meat bacteria? Did you drop meat right into the agar?

Maybe we need a positive control that your nutrient mix is good. Maybe you can try a separate container and try to grow bacteria from other sources - like swabbing something that looks dirty, or using dirt from outside, etc.

Dear Life technologies scientist,

we prepared the meat by letting it sit out at room temperature for 8 hours and then used a sterile q-tip to wipe solutions on the cutting board with bacteria on it which were separated into 6 groups with 6 diffferent solutions applied.. and we just got some bacteria colonies in our plastic Petri dishes so its good we got bacteria colonies at last
We are now in the stage of analyzing our data and found that Surgical soap, mouth wash, 91percent alcohol, and hand sanitizer caused no colonies to grow.
We googled around a bit and found that bacteria solutions can be Bacteriostatic or Bactericides-which was preventing growth for Bacteriostatic and killing with Bactericides. We need help on determining if Surgical soap, typical mouth wash, 91percent alcohol, or isopropanol hand sanitizer are either Bacteriostatic or Bactericides-or maybe both?

Thank You so much for your help!
-Skyhawk

YAY!! It's growing! That's great news.

T find out the answer - you could look around on google and see if you find it anywhere, OR you could devise a test. Can you figure out the test?

HINT - If the product you use is bacteriostatic - the bacteria can't grow but they don't die. So if you then put the bacteria into good growth medium, without the inhibitor, they maybe should grow again? Whereas if you had killed them, you aren't brining themback to life, right?