Why selecting the right type of agar media is critical

There are several types of agar media used in a microbiology laboratory for culturing molds. These media may differ in their water activity, pH, nutrient content or composition. Molds differ in their growth requirements. Therefore, no single medium is suitable for each and every mold out there.

How would one select the media to use then?

It is easy to select the media to use if one is looking for a specific type of mold. However, in most mold investigation projects, one is interested in knowing the the kinds of viable molds present in the air and their concentrations. The problem of using a single type of media is that some molds may not grow well (or may not grow at all) in the selected media. Hence, although such molds may be the dominant contaminants in the air, they may end up being missed or underestimated. The solution, therefore, is to use more than one type of media or select one that is known to support a wide range of environmental molds. A good example is Malt Extract Agar (MEA). The problem with this media is that it also supports the growth of bacteria. If the environment sampled is contaminated with bacteria, the bacteria grow faster than molds and interfere with mold growth. This problem can be overcome by incorporating a suitable antibiotic or other suitable compounds (e.g., Rose Bengal) into MEA to suppress bacterial growth. Rose Bengal not only suppresses the growth of bacteria but also restricts the spread of fast growing molds thus making it easy for colony counting.

What about culturing of bulk samples?

The same applies to culturing of bulk samples such as pieces of building material or dust. Direct culturing of such material in a single type of media could give erroneous results. If a single media is to be used to culture these types of samples, it is recommended that a lab performs a direct microscopic examination of the samples before culturing. Direct microscopy allows identification of the dominant contaminant (at least to genus level) regardless of whether the mold is dead or cannot grow on media used.

Demonstrating the effect of media on mold growth

To demonstrate how results from a single media can be misleading, examine the 4 petridishes. Two bulk samples were cultured onto 2 different media (DG18 and MEA) after serial dilution. Sample 1 was cultured in petridishes marked “A”. Direct micrsocopic examination of sample 1, indicated it had Stachybotrys as the dominant mold and some slight growth of Penicillium. After incubation, Stachybotrys did not show up at all in DG18 but both Stachybotrys (cream colonies with dark centres) and Penicillium (blue colonies) appeared on MEA. The second sample had Stachybotrys only. After plating onto DG18 and MEA and incubation (see petridishes marked "B"), Stachybotrys appeared on MEA and but not on DG18. These observations clearly indicate how wrong conclusions can be made if the right type of media is not used either in air sampling or culturing of bulk samples.

References

Microorganisms in home and indoor work environments: diversity, health impacts, investigation and control. Flanning Brian, Samson, Robert A., and Miller, David J (Ed.), Taylor and Francis, 2001.

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