Microbe handprint of child shows what grew

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Credit: Tasha Sturm

Microbiologist Tasha Sturm took a handprint from her eight-year-old son on a Tryptic Soy Agar (TSA) plate after he had been playing outside and let the culture grow for two days and this was the result – a colourful collection of bacteria, yeasts and fungi.

Some, like the organism in the close-up below, were unknown (from the upper left of the plate above) – although Sturm said she could not be sure it came from her son’s hand or was a contaminant.

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Credit: Tasha Sturm

Sturm believes the culture below is of Bacillus sp. She writes:

The big blob in the lower right is the image “unknown from handprint #2” and is most likely a Bacillus sp which is commonly found in dirt…..saw a fair # of what looks like Bacillus. The other image is either a yeast or Bacillus as well. We do this at the beginning of the semester and this is pretty common. The white being Staph and the colored colonies probably yeast. The colored colonies tend to show up after the plates are refrigerated or left at room temp for a few days.

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Credit: Tasha Sturm

She also explains how she created the culture.

Tryptic Soy Agar (TSA) can be purchased through a number of companies (Fisher Scientific, Hardy Diagnostics, Neogen). Most sell it in the powdered form, add water, autoclave, cool to about 55 degrees then pour into the plate, cover with the lid then let solidify. Once the plates are cool then place the hand on the plate making sure to gently pressing the fingers/palm to make contact with the agar. Cover the plate with the lid and place in a 37 degree C incubator for 24-48 hrs……incubate agar side up. This will grow the normal flora on the hand like Staph., Micrococcus, etc. Take the plate out and let it incubate/set out with the lid on at room temp (22 degrees C) for several days (3+ days). Normal flora will continue to grow (slowly) and yeast/fungi will start to grow….usually colored colonies (red/pink/yellow). It will also help bacteria like Serratia turn red. Once grown the plate should be treated as a Biohaz and disposed of properly. The plate should not be opened if mold/fungi is present without proper respiratory protection.

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