While air sampling is the most popular method for assaying indoor environmental quality, it is not infallible in determining the source of microbial contamination. Moreover, it is not possible to collect all microbes using a single sampling method as the techniques used to collect, culture, and analyze samples vary greatly. Microbes may be culturable, nonculturable, or nonviable. Fungal and bacterial fragments can be allergenic. Different agars will support the growth of different fungal species, depending on the agar formulation and moisture availability. Incubation time and temperature also favor selective organisms. Ultimately, the investigator is faced with a plethora of questions that can be addressed in an open dialogue with an experienced analytical laboratory.

What is the ideal sampling matrix for my microbial investigation? Once the investigator has identified the nature of the complaints and developed a hypothesis of the causal factors, sampling protocols need to be developed. Below is a list of common sample matrices and what analytical methods can be performed on each.

• Air Monitoring Cassettes – There are numerous designs on the market, but they all supply non-viable, fungal spore data. Other particulates that can be identified from these cassettes include skin cells, pollen, hyphal fragments, and insect parts. Bacterial cells are too small to be routinely quantified on this matrix. Also, there are new viable cassettes on the market, but the current consensus is that only a portion of the fungal spores on the cassette can be cultured and should be used for qualitative, not quantitative analyses.

• Impaction Plates – Air monitoring with agar plates has a tremendous amount of variability. There are literally thousands of different agar plates on the market that can, for example, sample a broad range of fungi (MEA w/chloramphenicol) to a very specific Cryptococcal yeast infection (Birdseed Agar) to bacteria that can cause staph infections (Mannitol Salts Agar). Also, variable incubation times and temperatures of these plates can further select for, or exclude, an individual organism.

• Dust – Dust samples can be manipulated by the lab to generate a wide variety of data. Dust can be homogenized and cultured on a wide variety of media for bacterial or fungal viable results, analyzed under the microscope for the presence of large biological contaminants (fungal spores, hyphal fragments, etc.), or assayed for allergens.

• Bulk - Bulk samples (drywall, wallpaper, etc.) do not need to be removed from site for nonviable microscopic examinations but should be sent to the lab for viable bacterial and fungal analyses, or mycotoxin assays. Tape or swab samples of the affected material are more economical matrices for nonviable microscopic analyses.

• Tape – Tape lifts are simple but effective matrices to sample contaminated bulk objects but can only be used for non-viable, fungal analyses.

• Swabs – Swab samples are versatile matrices to sample contaminated bulk objects. Swabs can be used for fungal microscopic analyses or viable bacterial and fungal assays. Additionally, swabs are the only matrix appropriate for the 3M Environmental Petrifilm plates. Petrifilm is a rapid, diagnostic system for the identification of E. coli /coliforms, lactic acid bacteria, Enterobacteriaceae, Staphylococcus aureus, and Listeria sp.

• Contact Plates - RODAC (Replicate Organism Detection and Counting) contact plates are convex agar plates that are “rolled” over a surface to recover viable bacteria or fungi from environmental samples. Contact plates are very sensitive and are typically used to quantify sanitized surfaces or qualify contaminated surfaces.

• Water – Water analyses are typically performed for drinking water, recreational waters (pools, hot tubs, lakes) and storm waters where fecal bacteria are the target analytes. Legionella from air handling and water cooler systems are also targeted. Also, water samples have very specific holding times, shipping protocols, and sampling techniques that vary with the desired analyte(s).

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