Mold Cultures

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Dec. '02

Overview

The use of growth media (i.e. culture plates, strips, etc.) during fungal investigations has wide-spread use in the indoor air quality industry, however, it is envirochex's position that relying solely on this method of testing often creates misleading information.  

It is important to appreciate what can be obtained from such test methods as well as what variables are involved.

What is a Culture?
In simple terms, an organism is cultured, or 'grown' under conditions which are typically controlled.

Microbiologists utilize a growth medium (generally an agar) which supplies nutrients and moisture for the organism.  Numerous types of media are available and some may include added compounds to enhance growth or suppress competing organisms.

In most cases the growth media is placed in a petri-dish to contain and protect the specimen.  Special media-coated strips inside of tubes are also commercially available.

Once transferred to the media, the organism is allowed to grow and then observed both macroscopically and microscopically.  Since the microbiologist can see what the organism looks like undisturbed, a more accurate identification is possible. 

In some instances, the quantity of colonies formed on the culture plate relative to the size of the field specimen are reported.  In these cases, microbiologists report 'Colony Forming Units' per unit of measure of the field specimen (i.e. cubic meters of air, square centimeters, grams, etc.)  Quantified results are therefore listed as CFU/m³; CFU/cm²; CFU/g, etc.

Macro View - Molds in Petri Dish

Microscopic View

Agar (ahg-ar)
A solid culture medium that provides nutrients and moisture for microbial growth.  Many different recipes are used which include a seaweed derived powder to solidify the mixture.

Two common fungal agars include Malt Extract Agar (MEA) and Potato Dextrose Agar (PDA).  

Benefits of Culture Plate Analysis
From the standpoint of general testing, the primary benefit of using culture plates is to allow fungal identification to the species level by the microbiologist.  

In other words, if it is advantageous to determine which species of Aspergillus is present, it is typically necessary to culture the organism for a more detailed analysis by the microbiologist.

Since the microbiologist knows he is working with the genus Aspergillus, selection of preferred media, incubation conditions, growth times, etc. are more easily controlled resulting in more accurate results.  

Drawbacks of Culture Plate Testing
Using culture media adds numerous variables to the testing methodologies which can dramatically increase the occurrence of false negative and/or false positive results.  

Since most experienced investigators try to minimize variables, relying solely on culture testing becomes a questionable method for general fungal assessments. 

In addition, some organisms grow slowly (days or weeks); therefore, fast turnaround times are not available.  

Typical Variables of Culturing
The following variables highlight some of the issues encountered when growth of the organisms on culture plates under laboratory conditions are compared to conditions of the environment from which the specimen was obtained:

• Contamination - Contamination of the media during field use can result in a false positive and generally requires additional specimens (control blanks) to be submitted to the lab.

• Spore Viability - The viability of mold spores is dependent on many factors; especially when considering field environmental conditions.  Since non-viable spores will not grow on the media, the results are subject to significant false negatives (remember, non-viable spores still retain their adverse health properties and may outnumber the viable types on many projects).

• Media Compatibility - Fungi grow best on media that provides optimum nutrition which can vary by species (some species grow well on one type of media, but poorly on others).  In addition, the media's water activity and pH also impact growth.  During preliminary assessments, multiple genera are usually present and it is often important to identify all of them.  Since there is no 'perfect' media for all types of mold, multiple types of media are typically required. 

• Competition - Competition exists at the microorganism level and some fungi may over-grow or inhibit competing species.  In addition, many fungal organisms are susceptible to infestation by mites which can completely strip a colony from the culture plate.  

• Temperature - Temperature impacts a number of species of fungi and temperature conditions in the field environment are seldom static.  Since laboratories typically incubate culture specimens at a specific temperature, the resulting growth may not accurately reflect field conditions.  

• Light - Many species of fungi grow well in the dark, others tolerate daylight, and some sporulate best under cycles of light including near ultraviolet.  The light/dark conditions used by the laboratory can have an impact on field-comparative growth.

• Time -  Some types of fungi grow faster than others; therefore, the duration of culture must be sufficient to identify all types present.

• Handling - Most laboratories will emphasize the importance of temperature control and expedited shipping when dealing with specimens on culture media to avoid exposing the specimen to unknown conditions that may dramatically influence growth results.

False Results

False Negative: Improper interpretation of the absence of something in a specimen.  

Let's say the microbiologist only reports Aspergillus and Cladosporium colonies on a petri dish.  

We cannot say there is no Stachybotrys present since Stachybotrys spores may have been there, but no colony appeared because of non-viable spores, improper media compatibility, or other variables which can create a false negative result.  

False Positive:  Improper interpretation of the presence of something in a specimen.  

Let's say the microbiologist reports Cladosporium colonies on a petri dish taken from an indoor air test.  

We cannot say there is an indoor source colony of Cladosporium since the spores may have come from an outdoor source organism and created a false positive result for the indoor sample.

Summary
Culture-based testing can provide specific information concerning fungal organisms, however, it's use in obtaining the best overall view of conditions is severely limited due to the introduction of numerous variables leading to false negative and false positive results.

Client's should question any investigator who relies solely on results from growth media during an initial site assessment without a complete understanding of the investigator's objectives.  Note that there are instances during initial site surveys when benefits are gained from recovery of both culture and non-culture specimens at the same time.

Remember, it's the information you desire that dictates what type of test should be conducted and all test methods are subject to limitations.

Focal Points represent editorial comments, opinions, and select positions of envirochex.  Information presented is not intended to serve as expert advice; be critical of any specific company's claims, statements, or positions; or be technically comprehensive on any issue.