Teaching Under the Microscope

by Christine Dunnington Fenton M.Sc.

Member of the New Zealand Microbiological Society

The most favoured practical lesson for Microbiology must be environmental sampling. You know the one, open agar plates on the windowsill, swabs taken from all sorts of places then smeared on a plate. Incubate, and then look at all the pretty colonies. Yes, it allows students to 'see' the unseen - all the microbe life from supposedly clean surfaces and the fuzzy bits that we breathe in. But is it good Science and more importantly, is it safe? Can concentrating unknown microorganisms and exposing the plates for inspection by inexperienced and curious students be wise?

I personally have known a fourth form boy ask which colonies should he lick so he can get a day off school! A horrifying thought. The normal environmental sample could concentrate pathogens and other nuisance organisms in enormously high numbers. For instance Serratia marcescens has known carcinogenic properties and has red to pink colonies - very attractive.

Mucor, Penicillium and Aspergillus moulds are fascinating and they grow very easily on bread but can affect people with asthma and allergies. Any of which could be present on an environmental sample.

"The Guidance Manual for NZ Schools" (Safety and Biotechnology Education, Ministry of Education), recommends that you NEVER work with unknown living materials.

Rather than doing the typical environmental sample you purchase known microorganisms to work with. Bakers' yeast (Saccharomyces cerevisiae ) and yoghurt bacteria from commercial home yoghurt packs can be used safely. Another alternative is to use laminated colour photographs of the cultures grown on petri dishes or a web camera with the display piped through the computer onto a monitor in the classroom.

However, I feel like the traditional environmental sample can still be done safely by following these precautions:

NEVER sample from body fluids. This includes swabs taken from the mouth, nose or sneezing and coughing directly onto plates.

NEVER sample from sinks, toilets, rubbish bins and taps.

ALWAYS use sterile swabs to take samples with. Clean or new does NOT mean sterile.

Once you have exposed your petri dish plates then seal them with "parafilm" or sellotape or even glad wrap so that the lid cannot be removed.

ALWAYS incubate at room temperature. Be aware that this is not a "safe" temperature - some pathogens can grow at temperatures below 37o C - they just reproduce at a slower rate.

ALWAYS adequately destroy plates after examination. Do not just thrown in the bin! (yes, this happens). Either pressure cook the plates at 15 lbs pressure for 15 minutes then discard or bag the plates up and microwave until the plastic melts, then discard. Alternatively you can incinerate or soak in 10% bleach (NOTE: household bleach is only 3% sodium hypo-chlorite) for 3 days. Anything else is unacceptable and unwise.

Presenting Microbiology to the students in an interesting way to spark the interest of budding young Geneticists and Microbiologists is of course very important. But for many the lab practicals done at High School will be the only time they are exposed to life under the microscope so let's keep it as safe as possible.