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How To Collect Mold And Bacteria Samples From Air

Collecting mold and bacteria samples from air is very easy. The first thing to decide is the kind of results you want and for what purpose. This will help you to decide on the sampling equipment and the media to use. For example, if sampling for a specific bacterium or fungus, you would want to use a sampling agar media that is suitable for the growth of the target organism. The efficiency of the air sampling pumps for the collection of the target organism has also to be considered.

Sampling equipment can be expensive. However, for a company that does not collect air samples every other week, renting the equipment is a better option. Sampling media can be obtained directly from the manufacturers or their resale agents. For companies collecting only a few samples in a month, sampling media can be obtained from a good mold testing laboratory.

Colony Forming Units (CFU)

Colony Forming Units - CFUWhat are colony forming units? Colony forming units, usually abbreviated as CFU, refer to individual colonies of bacteria, yeast or mold. A colony of bacteria or yeast refers to a mass of individual cells of same organism, growing together. For moulds, a colony is a group of hyphae (filaments) of the same mould growing together. Colony forming units are used as a measure of the number of microorganisms present in or on surface of a sample. Colony forming units may be reported as CFU per unit weight, CFU per unit area, or CFU per unit volume depending on the type of sample tested. To determine the number of colony forming units, a sample is prepared and spread or poured uniformly on a surface of an agar plate and then incubated at some suitable temperature for a number of days. The colonies that form are counted. CFU is not a measure for individual cells or spores as a colony may be formed from a single or a mass of cells or spores.

Moulds commonly found in carpet and mattress dust

A number of moulds are frequently found in carpet and mattress dust. Eurotium repens is the most frequently detected mould in mattress dust. Others include Aureobasidium pullulans, Alternaria alternata, Penicillium chrysogenum, Aspergillus penicilloides and Aspergillus restrictus.

More than 100 species of moulds have been recorded from carpet dust. As with mattress dust, the most frequently isolated mould in carpet dust is Eurotium repens. The others are Penicillium chrysogenum, Alternaria alternata, Aureobasidium pullulans and Phoma herbarum.

Concentrations of these moulds in carpet and mattress dust can be as high as 70 million colony forming units per gram of dust. Such high concentrations of moulds are likely to cause respiratory allergy or irritating symptoms. Therefore, it is import to regularly HEPA vacuum the carpets, mattresses and upholstered furniture to reduce the dust and spore concentration. If people are suffering from reoccurring respiratory allergy or irritating symptoms in a building where there is no visible mould, it is suggested that dust be tested for the types and concentrations of mould present.

How Fast Does Mould Grow On Building Materials?

Under experimental conditions moulds are found to cover artificially inoculated building materials in 4-10 days. For example, fast growing strains of Stachybotrys chartarum produce visible mould growth on new water-damaged gypsum boards in 5 days. Slow growing strains of Stachybotrys take 14 days to completely cover the test materials. These observations suggest that visible mould growth could appear within 1-3 weeks after water damage. Under natural conditions mould growth is influenced by a number of factors.

Key factors that determine how fast mould grows

  1. Type of mould: All moulds don’t grow at the same rate. Some moulds grow faster than others. For example, under the same conditions of growth, Ulocladium would grow faster than Stachybotrys.
  2. Temperature: Moulds grow faster at or closer to their optimal temperature for growth. For most indoor moulds the optimal temperature is around 25 °C. However, mould can still grow at temperatures as low as -7 °C. Some species of Cladosporium and Penicillium are capable of growing on wood at -5 °C although spore germination requires at least 0 °C. At low temperatures mould growth is very slow.
  3. Water activity: Growth rate of mould increases with increasing water activity. Most indoor moulds have their optimal water activity at 0.96-0.98. Lowering the water activity of the material lowers the growth rate significantly. However, a few species such as some species of Aspergillus, Penicillium, Eurotium and Wallemia are able to grow at lower water activities. Germination of spores requires slightly higher water activity than the minimum required for growth.

    4. White wood-rotting fungus

  4. Age of the spores: Older spores require longer time to germinate than relatively younger spores.
  5. The spore load: If a building is full of settled viable spores, mould would cover a wider area in a shorter time, than, if there were a few settled spores.
  6. Composition of the building material: Most building materials derived from plants are highly susceptible to mould attack. For instance, wallpaper allow fast growth for cellulolytic moulds such as Chaetomium and Stachybotrys.
    • pH of the material: pH of the material strongly influences the rate of mould growth, sporulation and metabolite production. Some moulds such as Aspergillus fumigatus grow best in the pH range from 4-7 and less well at acidic and basic extremes.
    • Nutrient availability in the material: Growth could be unrestricted if all nutrients are in excess but restricted when not all nutrients are in excess.
    • Presence of antimicrobial compounds in building materials: Some building materials contain antimicrobial compounds. These compounds slow down mould growth.

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  7. Competition with other micro-organisms: In a moisture damaged environment, there is a great diversity of microorganism. These microorganisms (including bacteria) compete for nutrients and also produce by-products that may inhibit the growth of other microorganisms. Fast growing moulds tend to over-grow slow growers.

    8. Picture of black mold in basement

Conclusion

Since there are many factors that influence mould growth in buildings, it is hard to say when mould growth started. However, if we know where and when the moisture problem started, we could speculate how long the mould growth has been there.

References

  1. Nielsen, K. F. (2002). Mould growth on building materials. Secondary metabolites, mycotoxins, and biomarkers. Ph.D. Thesis. BioCentrum-DTU, Technical University of Denmark.
  2. Nielsen, K. F., Holm, G., Uttrup, L. P & Nielsen, P. A. (2004). Mould growth on building materials under low water activities. Influence of humidity and temperature on fungal growth and secondary metabolism. International Biodeterioration & Biodegredation, 54(4) 325-336

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For more information on mould growth, please visit http://www.moldbacteria.com/ or call 905-290-101.

Mould Exposure

The best strategy to determine if building occupants are exposed to hazardous mould is to take air samples. It is important to note that even in rooms with visible mould growth air sampling may give very low spore counts. Two methods are widely used in sampling air for mould.

  1. Impacting air on some growth media. This method is used when one is interested in determining the concentration of viable mould spores/fragments in the air.
  2. Impacting air on some inert sticky surface. In this case the mould spores and other particulate are directly counted under a microscope regardless of whether the spores are viable or not.

Both methods have limitations. Therefore, whether to use the first, second or both methods depends on the type of data required, which in turn depends on the objective of the investigation.

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