By Bill Dembowski  (Elton, Pennsylvania, USA)



I have a liking for small projects. Nothing too large, complicated, or life altering; just something structured enough to make me feel as though I have put my arms around something. This Autumn and Winter I got my arms around some airborne fungi.

I took my samples using 90mm Petri plates containing potato dextrose agar, the traditional medium for growing mold. The plates were exposed outdoors for 10 minutes each, one every five days, for a total of 24 samples. I kept the plates covered while traveling to and from the exposure site (my backyard) and they were allowed to “incubate” at room temperature under the watchful eye of my cautiously understanding wife.

Plates exposed only a week or two apart can be striking different, as seen below. Plate Numbers indicate the date of exposure:


[FIGURE  1] Plate 2004-1205


[FIGURE  2] Plate 2004-1220


[FIGURE  3] Plate 2004-1225


On the other hand, some plates exposed nearly a month apart were almost identical in appearance:

[FIGURE  4] Plates 2004-1210  &  2005-0104


Although the naked eye can easily detect differences in the various growth centers, positive identification requires microscopic examination. Like most microscopic subjects, identification can be difficult to say the least. One of the major keys to the identification process is getting a good look at the subject. That can prove to be more difficult than it sounds. The trick is to transfer a sample from the Petri dish to the microscope slide without damaging the structure of the mold. It is the spores and their arrangement on the hyphae (the plant-like stalks of the mold) that set most of the various fungi apart. The sample below was transferred by simply uprooting part of the colony with fine-point forceps and making a water preparation. The jumble of hyphae and spores may be interesting to view, but contributes little to their identification.


[FIGURE  5] Unidentified mold hyphae and spores


Here is a reasonably intact example of the familiar Penicillium mold. Its colonies are distinctly green (See Figure 2) and are among the most commonly encountered. In this case I used a scalpel to transfer the sample and part of the substrate material onto a well slide with no fluid added. A word of caution: When handling molds, it is essential to keep an alcohol lamp handy to sterilize your instruments before each use. Cross contamination is impossible to avoid without it.


[FIGURE  6] Fragment of Penicillium mold


Another method of transferring mold from a plate to the slide is by using a piece of transparent tape; the clear type, not the milky type. You simply press a piece of tape to the mold colony and then affix the tape, sticky side up, to a slide. Some recommend adding a drop of water, others not. In either case, I have not found this method to be entirely satisfactory. Spores adhere to the tape quite nicely and, as such, make the tape an excellent collection device for such purposes. But, once again, I find that the integrity of the overall structure is usually lost. Perhaps the reader will have better luck.

There are, however, a few exceptions to the “don’t damage the structure” rule. Some molds have very distinctive spore cases which can themselves provide identification. One intriguing example is Alternaria which was always a pleasure to find.


[FIGURE  7] Mold of genus Alternaria


I called a halt to the project after 24 sample collections, the last 4 of which were negative; probably due to cold temperatures and significant snow cover (February in Western Pennsylvania). I found the project so interesting that I will probably do some indoor sampling as my next mini-project (after all, I have 6 unused plates) to see how the results compare.

NOTE: All photomicrographs were made with a Leica CME, 40x objective, and eyepiece mounted Moticam 1000 camera.

Those who are interested in studying molds will find (as always) that the Internet is a treasure trove of information. Here is one site that I found particularly helpful in identification:


The author, Bill Dembowski, welcomes comments and suggestions.  


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