The objective of trapping and enumeration of air-borne fungal spores in indoor environments may be 1) to determine the load of allergenic spores and their composition, 2) to determine whether there were hidden sources of fungal amplification, 3) to assess the effectiveness of remediation. Regardless of the objective, the methods of trapping and enumeration are the same.
Fungal spores trapping
Generally fungal spores are trapped by impacting air onto some inert media or some suitable growth agar media. Enumeration of fungal spores trapped on inert media is performed by direct microscopic examination without culturing. This is referred to as total fungal spore count or non-viable analyses. When air is impacted on growth agar media any fungal spores or hyphal fragments that can germinate on that media develops into colonies (often referred to as colony forming units) that are then enumerated and identified if necessary.
Fungal Spores Trapping for Total fungal Spore Counts (Non-viable).
The common air samplers for total fungal spores counts are slit and circular type of samplers. Slit samplers include Air-O-Cell, BioCell, VersaTrap, Allergenco, BioSis, and Burkard. Examples of circular samplers include Cyclex, Cyclex-d and Micro-5. These samplers (cassettes) are attached to a pump that can draw air at the rate recommended by the manufacturer of the cassette.
Fungal spores Enumeration Using Air-O-Cell Cassettes
The Air-O-Cell cassettes are opened in the laboratory by cutting around the sealing band. The glass cover slip (containing the sample trace) is removed and slowly placed at an angle with the media collection side facing either upwards or downwards. If the glass cover slip is to be placed with the media facing down, one to two (1-2) drops of lacto phenol cotton blue is placed in the center of the microscope slide before placing the glass cover slip. The edges of the cover glass can then be secured with a drop of nail polish. If the cover glass is to be placed with the media collection side facing up, it can first be secured with nail polish and then 1-2 drops of lactophenol cotton blue is placed in the middle of the sample. A second clean cover glass is then placed to cover the sample collection media and spread out the stain. Care should be taken not to trap air bubbles as these could interfere with the analyses.
The sample deposition trace is identified at lower magnification (10X or 20X) objective and spore counting is performed at a minimum magnification of X400. There has been a lot of debate as to what magnification should be used during identification and enumeration. Some analysts have recommended use of 100X oil immersion. However, it’s important to note that use of 100X objective with oil can be very messy and in most cases may not add quality to the results. In our opinion, the most important considerations when it comes to spore enumeration and identification are a good quality microscope and a well trained analyst.
In the past laboratories were using a variety of methods to analyze spore traps. This made comparison of results (and even terms used in the reports) difficult. The ASTM International has now released a new standard “Standard Test Method for Categorization and Quantification of Airborne Fungal Structures in an Inertial Impaction Sample by Optical Microscopy”. While this standard may not resolve all limitations of spore traps it will ensure that labs that use the standard analyze samples in a similar manner. The standard, however, may not improve the precision or accuracy of results, since, as it’s stated in the standard itself “…the detector in this method is the analyst, and therefore results are subjective, depending on the experience, training, qualification, and mental and optical fatigue of the analyst”.
Enumeration and Identification of Fungal Colony Forming Units.
Enumeration of colonies is performed under a stereo microscope or a suitable colony counter. Counting of colony forming units (CFU) is straight forward and easier than enumeration of total fungal spore counts. However, accuracy may be affected by:
- Colony density: If there are too many colonies, they tend to merge and this makes counting very difficult and inaccurate.
- Overgrowth: Some fungi grow faster than others thus masking the slow growing fungi
Counting is easier in some media such as dichloran 18% glycerol (DG18) agar which restricts the growth of colonies and significantly reduces the number of merging colonies. One drawback in using DG18 is that it can completely inhibit the growth of some fungi especially the hydrophilic species such as Stachybotrys and Chaetomium.
Malt extract agar (MEA) is commonly used in indoor air quality surveys. MEA is a nutrient rich medium and has high water activity, which favors hydrophilic and fast growing species which may mask, inhibit or entirely suppress the growth of other fungi.
Simultaneous use of MEA and DG18 is recommended since MEA supports the growth of a wide range of hydrophilic and mesophilic fungi while DG18 supports the growth of xerophilic and moderately xerophilic fungi. Therefore, use of MEA or DG18 alone may significantly reduce the number of species recovered.
For air samples collected using Andersen and other similar samplers, the colony count is adjusted (positive hole correction) before calculating the number of CFU per cubic meter of air. This adjustment is however not performed for air samples collected using the Reuter Centrifugal Sampler (RCS).