Our April session had two activities using agar; they will be blogged separately. This one is about cultivating micro-organisms from our environment.
We swabbed surfaces in the our homes, and tried to cultivate microorganisms from the swabs.
Angie made up nutrient agar using chicken stock as a food source. The recipe used was:
- 2g agar powder (standard food-grade agar)
- 2tsp sugar
- 100ml chicken stock made up to half usual concentration. Don't use stock with herbs - you don't want bits in it!
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| Nutrient agar sterilised in the pressure cooker. |
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| Very tasty - if you're a microbe! |
This was sterilised it in the pressure cooker for 15 minutes, and was poured into petri dishes when it had cooled to 40C - 45C. If it was poured while too hot, too much condensation developed in the petri dish later. Lids were put on the agar plates immediately, to protect them from contamination by airborne particles.
Another batch was made using beef broth (Bovril), and each petri dish was labelled either BB (Beef broth) or CB (Chicken broth).
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| Taking swabs from the compost caddy |
To take samples, we slightly moistened a clean cotton bud from a freshly-opened pack, and rubbed it on the surface to be tested. We lifted the lid of the petri dish and wiped the cotton bud across the agar surface in a zig-zag motion. Lids were put back on immediately, and taped in 2 or 3 places around the edge. Each petri dish was labelled on the back with the date and surface tested.
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| Innoculating the agar plate with swab |
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| Labelling petri dishes after innoculation |
Everyone took some agar plates home in a ziplock bag with some clean cotton buds, so they could swab some surfaces in their own homes.
Disposal
We practised safe disposal technique. Once taped shut, the petri dishes would not be opened again. They were to be kept in the open air, at temperatures below 25C, to reduce the risk of encouraging harmful organisms to grow. After 3-5 days, people were asked to take some photos and send them to Angie. After that, the agar plates would be put in ziplock bags and immersed in bleach for a couple of hours at least, then put in the dustbin.
Results
One petri dish was kept as a control - that stayed beautifully clear, with only a tiny white spot appearing after 5 days. One left open to the air for 4 hours developed all sorts of fungal and bacterial growths. Swabs taken grew all sorts of impressive-looking nasties - interestingly, while scrapings from under 5 year-old Tommy's fingernails leaped into an early lead alongside swabs from the phone keypad, they were overtaken by swabs from the toilet flush handle.
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| The control stayed clear for several days, which was good evidence that the pressure cooker sterilisation and general handling had been effective in minimising contamination. |
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| First set of swabs taken from Angie's house before the session. |
Moulds started to out-compete bacterial colonies after a few days, and we saw that some colonies of microbes got an early start and seemed to thrive, but then were overtaken and apparently killed by mould. You can see some more photos of these plates below.
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| Phone keypad |
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| Swabs from two different children's hands! |
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| Swab from stair banister. |
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| Stair banister swab (being bleached prior to disposal) |
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| Leo's cat's paw - back view |
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| Leo's cat's paw - top view |
Leo's cat kindly donated a paw swab, above, and below are swabs Leo took from Angie's door, and his own hand:
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