MadSci Network: Microbiology

Re: How can I test a sample of air for bacterial contamination?

Date: Thu Oct 26 02:01:54 2000
Posted By: Mike Conrad, Post-doc/Fellow, Microbiology, UNC
Area of science: Microbiology
ID: 971926093.Mi

Question: How can I test a sample of air for bacterial contamination? Can I use a Petri dish for this purpose? How would I go about doing it?

Yes you can use a petri dish to get a quick and simple indication of the microorganisms in the air. In fact, Louis Pasteur did something like this in 1859 when he was trying to disprove the theory of "spontaneous generation". He found that broth left open outside became populated with microorganisms faster then when it was left inside.

What you suggest, with a petri dish, has been used in a science project by a student at Bridgeforth Middle School. You can see how she did it in her lab report at

She left petri dishes open for 6 hours in 1) in a hospital ER and waiting room 2) in an airplane and 3) in a classroom. She also had control plates which were kept covered. Then the experimental plates were covered and allowed to grow for a week in a warm room. Bacteria colonies are visible after two days and after a week molds and fungi are visible. Then she identified the microorganisms that grew. Here's what she found:

  1. Nothing grew on the plates left in the hospital. You might expect microbes to be found in the air in a hospital. But hospitals try very hard to design ventilation systems which minimize microbial circulation, and this hospital's efforts were successful.
  2. Three different kinds of colonies grew in the plates left in the airplane. There has been concern that the highly recirculated air in an airplane could carry microbes. Here, microbes were found.
  3. The largest number of microorganisms grew in the plates left open in the classroom. With all those kids running around, lots of microorganisms could have been brought in and stirred up.
You could repeat this with more plates, different locations, and outdoors, etc. You ought to try this since it's so simple. But tests with only a petri dish just give give a rough indication of the airborne load of micro-organisms. Quantitation of how many organisms are in a certain volume of air can not be done this way. Quantitative experiments are more difficult but they give more useful results.

Getting quantitative measurements involves sucking a known volume of air, say a cubic meter, through an ultra-filter. Ultra filters are filters with a pore size of 0.45 or 0.22 microns, for example. These filters are small enough to catch bacteria. Then the filter is taken and placed, face up, on a petri dish. After growth, each bacteria will give rise to a colony. The colonies can be counted and you can calculate the number of bacteria per volume of air.

If you want to do this yourself, you can find ultra filters in most microbiology or tissue culture laboratories. A good university microbiology lab should have ultra-filters and they aren't very expensive, they ought to give you a few. 0.45 microns is small enough for the average bacteria while 0.22 is more complete. Controlling the amount of air pulled through the filter is more complicated, but with some ingenuity, you should be able to do it.

Good luck, Mike Conrad.

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