Science Buddies: "Ask an Expert"

hi im doing the science fair project: "how do preservatives affect bacterial growth" and i used sodium benzoate as my preservative and e coli as my bacteria. i filled each dish with a certain concentration of the sodium (like one plate had 5% of soidum benzoate in the whole mixture). also, i made my agar out of unflavored Knox gelatin... i made the plates 4 days ago and i just put the e coli in the plates yesterday. its around 55-60 degrees F where im storing the plates right now, and i suspect that is why my bacteria is not growing, however, its been more than 24 hours since i put bacteria on the plates and i still have no growth... so i put all my plates under a lamp about 6 hours ago and i just checked my plates again and they all melted!!! what do i do now? my science fair project is due in two weeks! but my results and conclusion portion of my project is due next week! can i refrigerate the plates again to solidify the agar?

(i have done prior research before starting the experiments and on several websites and science journals, the addition of sodium benzoate inhibits the growth of bacteria (gram negative bacteria to be exact, which is also the type of bacteria that the strain of e coli im using is), so would it be correct for me look for these results if my experiment does end up working out? please please please help...im really stressing about this..aaahh...)

THANK YOU FOR ALL THE HELP AND INPUT FROM ANYONE! THANK YOU!

Hi,

Sorry to hear about the melt-down! You probably needed to make the gelatin more concentrated.

So, you have plates with and without sodium benzoate, right? The preservative will kill the E coli or at least stop them from growing. But, if the gelatin melted then the E coli got mixed in with it and will be a bit difficult to see.

Bacteria can also grow in liquid culture, so you should still be able to see a difference in the cloudiness of the gelatin. The benzoate gelatin should look clearer than the one without the preservative.

E coli likes to grow at 98.6F, human body temperature, so you really have to keep the plates warmer than 60F or they will take a LONG time to grow. Don't put the plates in the refrigerator. Put them under the lamp and check the temperature with a thermometer so you don't heat the plants too much. Cover them with aluminum foil so they are dark.

If you still have your original E. coli culture left, why don't you make some more plates using a HIGHER concentration of gelatin with or without sodium benzoate. If you streak some more E coli on these plates and keep them at a temperature of around 90F, then you will have results by next week. They grow pretty fast when they are warm enough.

One way or another, you should get the results you need in time. Take pictures of the plates before, during and at the end of the experiment to show the growth with and without benzoate.

Good luck!

Sybee