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What is Biofilm? The slime that haunts some mechanical systems.

It’s NOT a documentary about Abe Lincoln…

A biofilm is any group of microorganisms in which cells stick to each other and can adhere to a surface. These cells are frequently embedded within a self-produced matrix. Biofilm extracellular polymeric substance, which is also referred to as a slime, is a conglomeration composed of extracellular DNA, proteins, and polysaccharides. Biofilms may form on non-living surfaces (not just living) and can be prevalent in industrial and hospital environments as well as in nature.

Formation of a biofilm begins when free-floating microorganisms attach to a surface.  If these first colonists are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion structures such as pili.  

Biofilm

Some species are not able to attach to a surface on their own but are instead able to anchor themselves to the matrix or directly to earlier colonists.  Once colonized, the biofilm grows through a combination of cell division and recruitment.

When a biofilm is established, it may only change in shape and size.  During this final phase, dispersion enables biofilms to spread and colonize new surfaces. Dispersed cells are found to be highly virulent against macrophages.  (A macrophage is a type of white blood cell that cleans the body of unwanted microscopic particles, such as bacteria and dead cells.)  They can be irritants or the source of infections, some even deadly (like Legionella pneumophila, the organism responsible for Legionnaire’s disease).

Biofilms are usually found on solid surfaces submerged in or exposed to an aqueous solution, although they can form as floating mats on liquid surfaces and also on the surface of leaves, particularly in high humidity climates. Given sufficient resources for growth, a biofilm will quickly grow to be visible to the naked eye. Biofilms can contain many different types of microorganisms, e.g. bacteria, archaea, protozoa, fungi and algae; each group performs specialized metabolic functions.

Bacteria living in a biofilm usually have significantly different properties from free-floating bacteria of the same species, as the dense and protected environment of the film allows them to cooperate and interact in various ways. This can mean increased resistance to detergents and antibiotics, as the dense extracellular matrix and the outer layer of cells protect the interior of the community.

Biofilms can grow in many moist and warm environments and on virtually every non-shedding wet surface. Biofilms in cooling or heating water systems have been known to reduce heat transfer. The buildup of biofilms can affect the heat flow across a surface and increase surface corrosion.

Within an HVAC system, the most likely locations for biofilms to develop include the air conditioning coils and the condensate pans.  While some EPA registered antimicrobials can be applied in these locations, established biofilms can disperse infectious airborne particles throughout the HVAC system, and they are more difficult to remove.  It’s important to attack them early, while detergents are still effective, and before more corrosive cleaning agents are required that may harm the aluminum fins of the coils.  While some contractors will recommend the installation of UVC lights to destroy these microorganisms, this method can be costly and not necessarily effective in all environments, as it’s reactive rather than proactive.  Others may go so far as applying a sanitizer, but the EPA has no registered antimicrobial products for application to ductwork, and they allow them only on specific locations like the coils and pans themselves. The key is in preventative maintenance through regular inspections and thorough source removal with full HVAC system cleaning, as recommended by the National Air Duct Cleaners Association, NADCA.

 

Sources:

The Microbiology of HVAC Biofilm, by Robert Scheir Indoor Environment Connections, Vol 12, Issue 8, June 2011.

Biofilm:  Wikipedia.com.

Wisconsin Protocol for Cleaning Air Conditioners & Heat Pumps & Legionella Bacteria, InspectAPedia.com.

Using Chemical Products in HVAC Systems:  NADCA Provides Guidance, NADCA Position Paper, NADCA.com.

EPA Pesticide Registration Manual:  Chapter 4- Additional Considerations for Antimicrobial Products:  http://www.epa.gov/pesticides/bluebook/chapter4.html

 

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