Aspergillus is one the most common types of moulds in indoor environment. Some members of Aspergillus group are recognized health hazards and are of great concern if they appear in built environment. The most important species as concerns health are Aspergillus clavatus, Aspergillus fumigatus, Aspergillus niger and Aspergillus versicolor.
Aspergillus clavatus is often associated with allergic diseases in workers of malt-houses. Aspergillus fumigatus is the most important and well known potential pathogen for humans with weak immunity. It is of great concern in hospital environments. Spores of Aspergillus fumigatus are allergenic and have the ability to bind on lung epithelium causing complications in the health status of asthmatic individuals.
Due to their small size (2-3.5 microns), about 70% of Aspergillus fumigatus spores are able to penetrate into the trachea and primary bronchi. Aspergillus niger and Aspergillus fumigatus are often associated with two respiratory disease conditions: allergic and invasive aspergilloses. Aspergillus versicolor is one of the most common Aspergillus species found in damp indoor environments and is a major producer of one of the liver damaging and carcinogenic mycotoxin.
Sampling For Aspergillus Species
It is rare for an indoor air quality investigation to target for Aspergillus species only. However, situations may arise when sampling is specifically targeted to Aspergillus species or a single species such as Aspergillus fumigatus.
Although sampling for non-viable analysis can be used to sample for members of the genus Aspergillus, it is limited in that species of Aspergillus, Penicillium and even some other unrelated genera are difficult to differentiate based on spore characteristics only. Therefore, sampling for viable analysis or for both viable and non-viable analysis is recommended.
Combining sampling for viable and non-viable analysis has one major advantage. Non-viable sampling could give a better estimation of the total spore concentration for Aspergillus/Penicillium and the results could be obtained within the same day of sampling. Sampling for viable analysis on the other hand could give an estimate of the viable portion of the Aspergillus/Penicillium groups detected with the non-viable sampling.
It is however, important to note that results for non-viable and viable samples sometimes show no correlation. This is understandable because apart from the differences in sampling devices, there are other factors that could influence the results obtained with either viable or non-viable samplers.
For example, spores reported as Aspergillus/Penicillium species may belong to species totally unrelated to either Aspergillus or Penicillium species. In this article we will discuss sampling airborne Aspergillus species for viable (culture) analysis.
The important points to consider when sampling air for members of the genus Aspergillus are:
- Efficiency of the air sampler in collecting small size spores.
- Selection of sampling media.
- Selection of sampling time.
- Selection of sampling location.
- Sampling limitations.
Efficiency of the Air sampler
The size of spores of Aspergillus species ranges from 2 to 10 micrometres (microns). A sampling pump that is most efficient at this size range should be selected. Most volumetric samplers are less efficient for spore sizes less than 5 microns.
Selection of Sampling Media
Aspergillus species can grow over a wide range of substrates. However, with the exception of a few species, they generally grow well on low water activity substrates (that is they are xerophilic). One of the media recommended for xerophilic moulds is Dichloran 18% Glycerol Agar. This is a general-purpose low water activity medium that selectively support the growth of xerophilic moulds or moulds that prefer dry environments. A second advantage of DG18 is that it restricts the colony diameter thus preventing colonies from overgrowing each other and hence easy to enumerate. DG18 also inhibits bacterial growth ensuring that bacteria do not interfere with mould growth.
Selection of Sampling Time
Selection of sampling time is based on the environment being sampled (i.e. whether heavily contaminated or not) and the type of sampler (most samplers have manufacturers recommended sampling time). If the sampling time selected is too long in a heavily contaminated environment then the colonies may be too many to be accurately counted.
Sometimes it is hard to know how contaminated the environment being sampled is. In this case, initial non-viable air samples may be taken and analysed immediately to give an idea of the level of contamination.
Selection of Sampling Location
The sampling location and height from where the samples are taken can influence the results obtained. The location should generally correspond broadly with the breathing zone or about a height of 1.2 meters. If one is interested in detecting the source of Aspergillus growth,samples should be taken near the suspected sources of contamination. Multiple samples are recommended since they would highlight variation of airborne mould concentrations with time and also location.
Air sampling on growth media has one major limitation. Only those mould propagules (spores and hyphal fragments) that can grow on media are detected. Although this may not apply to species of Aspergillus, it is estimated that only 10-15% of airborne mould propagules may be viable.
To see guidelines for interpreting non-viable air sample results click Guidelines for Interpreting Non-viable Air Samples.
To see guidelines for interpreting viable air sample results, click Guidelines for Interpreting Viable Air Samples.
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