How to identify science fair bacteria in agar nutrient gel

April 2, 2009 8:08 AM

What kinds of bacteria make red and yellow spots in agar nutrient gel?

So my Kindergartener recently did a science fair project where he made up some agar gel and then tested cleaning his hands with various things to see which worked best. All manner of things grew in there, and I'd like to know what exactly.

Some bits grew kinda black and moldy, other bits white and spidery, and then there were some very vivid small red and yellow dots.

Any idea which kind of bacteria produce these different growths?

posted by zeoslap to Science & Nature (12 answers total)

Best answer: Nutrient agar supports growth of a very, very broad range of both bacteria and fungi. Without pictures, it'll be very difficult to tell. A description of white and spidery might mean a fungus of some variety, or it might suggest a Proteus species... I couldn't say. Even with pictures, it might be possible to narrow it down a bit, but probably not entirely. When students learn to identify microbial species, it often involves starting the cultures on something like nutrient agar, and then moving to more and more specific nutrient sources until where the microbe does/doesn't live tells you what it is.

Err... Why yes, I am a microbiologist.
posted by amelioration at 8:22 AM on April 2, 2009

Response by poster: I'll post a pic when I get home, but the ones I was really curious about were about 1-2mm across after sitting around for about 5 days and some were bright red and others yellow. These guys were the hardiest (the sole two spots in the sample where we used Purell)
posted by zeoslap at 8:46 AM on April 2, 2009

in my completely inexpert opinion, few of those sound like bacteria. Mainly fungi. the white spidery stuff I would guess is aspergillus, your common bread moldy thing. The red and yellow spots i would probably guess could be a candida or some other colony yeastie. the black mold is not descriptive enough for me to even wildly speculate. In conclusion, don't eat it.
posted by Cold Lurkey at 8:47 AM on April 2, 2009

Response by poster: But it said it was nutritious on the box! So what would it look like if it were bacteria as opposed to fungus?
posted by zeoslap at 8:55 AM on April 2, 2009

Best answer: I know it's tempting to eat, but you must maintain your anti-microbial resistance (ah ah ah). Again this is inexpert opinion but bacteria tend to make rather low lying slimy looking colonies that don't spread much, while yeast colonies have a raised puffy appearance, sometimes they send out little tentacles either on top of or through the agar. Also, as these are from post purell treatment, I'd be more inclined to guess fungus. You could try subcloning these little guys on more diagnostic media such as amelioration suggest, but that is probably outside the purview of this little household experiment.
posted by Cold Lurkey at 9:06 AM on April 2, 2009

Best answer: Serratia marcescens is a red bacterium that will grow on starchy substrates like bread or agar. It's a common contaminant of petri plates in the laboratory and while it's normally not a problem for most people, it is occasionally pathogenic in some people. (Don't panic about your kid's experiment, just don't eat the agar!)

Fun historical theory: S. marcescens colonies growing on eucharist wafers, looking like spots of blood, may have been the cause of the "miracle of Bolsena" in 1263.
posted by Quietgal at 9:07 AM on April 2, 2009 [2 favorites]

If your yellow colonies were a pale creamy color, really more of a beigy-white, and not a bright vivid yellow, I'd put some money on them being Staphylococcus epidermis. It's a very common, generally non-pathogenic bug.
posted by amelioration at 9:41 AM on April 2, 2009

Best answer: Rhodotorula is a common red yeast that might be your red colonies.
posted by pombe at 9:54 AM on April 2, 2009

Best answer: If your yellow colonies were a pale creamy color, really more of a beigy-white, and not a bright vivid yellow, I'd put some money on them being Staphylococcus epidermis.

Yellow colonies could also be Staphylococcus aureus, which was named for its color. Although usually thought of as inhabiting the nose and other moist areas of the body, it can also be on the hands, as this article shows. The colonies are red in the picture because of the type of agar they were using but would normally be yellow or white.

Fun historical theory: S. marcescens colonies growing on eucharist wafers, looking like spots of blood, may have been the cause of the "miracle of Bolsena" in 1263.

Wouldn't Proteus mirabilis be more appropriate?
posted by TedW at 10:25 AM on April 2, 2009

Response by poster: Yeah could well be Staphylococcus aureus because one of the tests involved swabbing his hands after he rubbed his snotty nose :)

Thanks for all the replies, really interesting, thanks :)
posted by zeoslap at 12:23 PM on April 2, 2009

Best answer: You need this book, or one like it. I found a similar book at my public library but, being rather easily grossed out, had no desire to go on a microbial "safari" (I already have to throw away moldy kitchen leftovers).
posted by bad grammar at 6:27 PM on April 2, 2009

Response by poster: Thanks for the tip bg, my local library has it :)
posted by zeoslap at 9:40 AM on April 3, 2009