Polymerase Chain Reaction (PCR) and Culture Testing
By Dr. Michael Berg, EMLab P&K Senior Molecular Biologist
Today, home inspectors and other mold investigators have a variety of choices and tools available for the detection and quantification of mold in indoor environments. The visual inspection remains a key component of the evaluation and many professionals include indoor and outdoor air sampling (typically non-viable spore trap analysis) and testing in their assessment to confirm or revise their initial findings. Some testing laboratories, such as EMLab P&K, offer additional tools and analyses to help the investigator with the mold assessment.
In this article, we want to focus on some other tools for the detection and quantification of mold. In particular, culture of viable spores and the Polymerase Chain Reaction (PCR). The use of culture media to grow bacteria or molds from air samples is well established and has been standardized (e.g. Andersen sampler, BioCassette™ etc.). The main advantage of culture analysis compared to a visual inspection or spore trap analysis is that frequently the molds can be better characterized. Experienced mycologists are able to identify most molds to the species level based on growth characteristics. The main drawback is the time it takes to culture the fungi and limitation of the sample size that can be examined. We should also remember that only viable spores grow in culture, and some fungi require special media to grow or may not grow well in culture. This could mean there may be other mold spores in the air that just do not grow on an agar plate. Cultures can also be useful for surface samples. While a tape lift is easy and quicker than culturing, it frequently does not allow the more detailed characterization possible for many mold species through culture.
DNA-based technology such as the polymerase chain reaction (PCR) provides another valuable tool for the detection and quantification of molds. There are advantages and disadvantages that mold investigators need to consider. Most or all commercial laboratories use the mold-specific quantitative PCR technology developed by Steve Vesper and his group at the Environmental Protection Agency (EPA) in Cincinnati. In contrast to visual inspection, microscopy and culture, PCR can only detect those molds that the assay is designed to detect (usually one species or a group of closely related species). One advantage of the methodology is that air samples can not be overloaded by longer sampling times or large numbers of spores. This makes PCR a good tool for the detection of a specific organism of concern when longer sampling times are applied. For example, it would be sensible to use PCR for the detection of nosocomial organisms (opportunistic pathogens) such as Aspergillus species in operating rooms. Another advantage is the accurate species identification by PCR especially when speed is important. The environmental relative moldiness index (ERMI) is a specific application based on the use of mold-specific quantitative PCR. The theory behind the ERMI is that the "moldiness" of a home can be determined from the composition and concentration of mold spores in house dust. A number of studies from the Cincinnati group (EPA) using the ERMI have been published thus far.
Choosing the most appropriate method for the right application is not always easy. As a laboratory that offers the complete spectrum of test methods including spore trap, culture and PCR, EMLab P&K does not promote one method over the other. We do hope that we can help mold investigators to choose the most appropriate method for a specific application by highlighting specifics of each test procedure and pointing out advantages and disadvantages.
This article was originally published on September 2009.