A number of methods can be used to test air for mold or other microbial contamination. One of the oldest methods of testing air for microbial contamination is the settle plates method. Though the method is semi-quantitative, it is still considered a useful method. In industries such as food, pharmaceutical and cosmetics the method is used to assess the likely number of microorganisms depositing onto the product or surface in a given time. The method involves opening and exposing petri dishes containing agar medium suitable for growth of microorganisms of interest. If one is interested in testing for mold, agar plates containing malt extract agar (MEA) supplemented with some antibiotics to suppress bacterial growth would be used. The agar plates are left open at table-top level at selected points in the room for half-hour to 4 hours. This allows mold spores and fragments to settle onto agar media by gravity. Mold test kits (involving growth media) are settle plates.
Settle Plates Results
The number of microorganisms deposited onto the agar surface of the plate over the period of exposure is determined by incubation of the agar plates at 25ºC for 5- 7 days and counting colonies that develop. The results can be expressed as number of colony forming units (CFUs) per unit time. The counted colonies can then be further characterised to genera or species. Higher numbers of CFUs and/or presence of potential pathogenic or toxigenic molds such Aspergillus fumigatus and Stachybotyrs chartarum are indicators of a problem.
Disadvantages of Settle Plates
Settle plate method is an extremely useful method for assessing air contamination by microorganisms. It is easy to conduct and very cost effective. However, only viable microorganisms would be detected by this method and hence it may give a false impression that the air is “clean” if most of the airborne microorgainisms are dead. False negatives may also be obtained from buildings with:
- very restricted mold growths.
- very still air in undisturbed rooms.
- species of poorly culturable molds (e.g., Stachybotrys chartarum).
- molds consisting of species with poor airborne dissemination (e.g., Aureobasidium on windowsills, Cladosporium on painted cold air vents, Fusarium and many other wet-spored fungi).