Tuesday, May 26, 2009

Growing Bacteria Cultures

The ingredients and the bacterial smear of the window.

We tried a couple of formulas for homemade growth medium in which we could grow bacteria cultures. In real labs, a vegetable product called agar is used to make a gel. The agar is melted and poured into Petri dishes, and then chilled until it solidifies again. To get bacteria samples, a sterile cotton swab is rubbed across a surface. Then the dish is streaked by rubbing the swab in a zigzag pattern across the agar. It is set aside in a warm place and allowed to grow for several days.

The first formula was from The Science of Life by Frank G. Bottone, Jr. It used combined flavored Jello and SlimFast diet drink. Instead of Petri dishes, we used small plastic cups (the kind they put ketchup in at fast food places) covered with clear plastic wrap held on with a rubber band. For the sterile swabs, we took Q-Tips and dipped them in a cup of boiling water. Sadly, this formula grew mold but no bacteria.


Stirring the gelatin.

We had better luck with the second formula. It came from a website called Science in the Real World and was designed by biologist Teresa Thiel of the University of Missouri. It used unflavored gelatin, beef boullion cubes and sugar. We used bacteria from yogurt, a toilet, the stem of our hydroponic tomato plant, the inside of one of our mouths, the inside of our refrigerator, the inside of our fish tank, a window, the inside of one of our navels, some dirt from our back yard, the kitchen counter, a sock, and one of our fingers.

The naval smear and the finger smear at 7 days.


The dirt smear after 2 days and after 7 days. The fuzz on the right is mold.

The plant smear and window smear (see top) at 7 days.

At the end of the week, we're going to dispose of the smears and bleach out the box so that any possible escaped bacteria won't start infecting our house.

6 comments:

Tess said...

Hi Kathy, Great blog! I really enjoyed your post about growing bacteria. I tried several agar home recipes and never found one that would grow anything besides mold. I finally broke down and bought some from here: http://www.stevespanglerscience.com/product/1551, It was worth the five bucks for me not to have disappointed children again. This time it worked! We got tons of colonies and we took photos every day to compare them. The kids loved this experiment. They swabbed everything in the house and found the most bacteria on the soap dispenser!

Kathy Ceceri said...

Thanks, Tess! I have a link in the Microbiology sidebar for Home Science Tools' agar and dishes. Their agar comes as a gel, which you just warm to melt, as opposed to Steve Spanglers' powder. Both look to cost about the same. If you've got a link to your bacteria photos I'd love to see them!

BTW, here's a GeekDad post I wrote this week about Steve Spangler, who just set the world record for largest physics lesson: http://www.wired.com/geekdad/2009/05/worlds-largest-physics-lesson/

Box Turtle maniac said...

why do the Q-tips have to be sterile? you are trying to grow bacterium not clean a wound!

Kathy Ceceri said...

I guess to see whether you get different microorganisms from different sources. If the Qtips are contaminated, you are also growing whatever they already had on them.

Anonymous said...

Hey Kathy, I was wondering how you discarded the agar? Did you disinfect the agar and then discard it? And how?

Anonymous said...

ewwww much gross ahah i need to grow bacteria for school