Understanding the Differences Between Two Legionella Bacteria Testing Methods per CDC

Part 2 of a 3-Part Series of interpreting the CDC’s latest Legionella bacteria sampling guidance 

By Hilary Nardone, Environmental Group Training Manager
Certified ASSE 12080 Legionella Water Safety and Management Specialist 

Image via CDC

In addition to recommending routine environmental Legionella sampling, the CDC’s “Toolkit for Controlling Legionella in Common Sources of Exposure” (Legionella Control Toolkit) discusses multiple Legionella bacteria testing methods and the differences between them.  Traditional culture and polymerase chain reaction (PCR) are the most common types of testing methods, yet they provide insight into your water systems in distinct ways.  Below are the major differences between the two method types to help you in determining which testing methods is the best choice for your facility:

Traditional Culture (spread plate)  Polymerase Chain Reaction (PCR) 
Detects viable bacteria  May not differentiate between live and dead bacteria 
Detects all Legionella species  Detects Legionella-specific DNA or RNA 
Results typically reported in colony forming units (CFU) per volume with limit of detection  Results typically reported in genomic units (GU) which is not directly equivalent to CFU 
Results typically reported in 7-14 days  Results typically reported in 2 – 48 hours 
Is subject to skill, experience and procedural rigor of the laboratory  Is useful for negative screening 
May be preferred for evaluating growth trends  May be preferred for evaluating whether remediation was successful 
Yields isolate for additional characterization   

While both testing methods are recognized for their ability to identify Legionella bacteria, the traditional culture method is considered the “gold standard” in the industry.  In fact, the CDC’s Environmental Legionella Isolation Techniques Evaluation (ELITE) program issues certifications to labs that have “demonstrated the ability to isolate Legionella from simulated environmental samples by culture.”[1]  Labs who are ELITE-certified have proven credibility when analyzing Legionella samples during a case or outbreak investigation.

In recent years, various new alternative methods have been developed to perform Legionella bacteria analysis with different characteristics and turnaround times. However, per the CDC, these methods should be validated against a standard method by a third party.

Barclay Water Management, Inc. is an industry leader in Legionella bacteria risk minimization by offering sampling and testing programs to detect Legionella bacteria in potable and non-potable water systems.  We can help determine the appropriate number and location of samples to ensure our customer’s unique needs are met.  We then provide each customer with results along with result interpretations and relevant recommendations.  Barclay’s third-party laboratory is accredited by the CDC ELITE Legionella Program and the New York Environmental Laboratory Approval Program.

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