Legionella occurs naturally in freshwater environments but can also colonize engineered water systems. Building plumbing, cooling towers, decorative fountains, and hot tubs can support bacterial growth under suitable environmental conditions. Risk increases when system conditions favor persistence and amplification, particularly where sediment, biofilm, water stagnation, and inadequate control measures are present.

People can develop Legionnaires’ disease after inhaling aerosols containing the bacteria. Exposure may occur around showers, cooling towers, spas, and other devices that generate water droplets. Infection risk depends on several factors, including exposure conditions and individual susceptibility.

For microbiology laboratories, the principal challenge is to recover and identify Legionella from complex environmental samples while accounting for competing microorganisms and limitations associated with each analytical method. For facility managers and quality assurance teams, the objective is to use reliable results to understand contamination patterns, assess control measures, and guide appropriate corrective action.

In the United States, environmental Legionella testing should follow a validated procedure appropriate to the intended application, alongside applicable public health directions and jurisdictional requirements. CDC environmental sampling and laboratory procedures are particularly relevant during investigations of Legionnaires’ disease. The international equivalent for culture-based enumeration is ISO 11731, published by the International Organization for Standardization.

ANSI/ASHRAE Standard 188, issued by ASHRAE, provides a framework for managing legionellosis risk associated with building water systems. Its applicability depends on the jurisdiction, adopted codes, and other requirements relevant to the facility. OSHA workplace safety requirements may also apply to employee exposure risks. These frameworks serve different purposes and should not be treated as interchangeable laboratory test methods.

The appropriate approach depends on whether testing is being conducted for routine water management, suspected exposure, outbreak investigation, or verification following remediation.

Environmental sampling should be designed to identify potential sources of exposure, determine the distribution of Legionella within the water system, and evaluate whether remediation has been effective. The CDC recommends developing the sampling plan using environmental assessment findings, previous results, building characteristics, epidemiological information, and the water system's configuration.

Potential sampling locations include:

• Building water entry points.
• Water heaters and storage tanks.
• Representative faucets and showers.
• Hot-water return lines.
• Cooling towers and associated water-treatment equipment.
• Hot tubs, decorative fountains, and locations with visible biofilm.

For cooling towers, spas, and decorative fountains, biofilm swabs from relevant surfaces, including waterlines and jets, can complement bulk-water samples. The sampling plan should represent individual systems separately when a facility contains multiple water systems or devices.

The required sample volume depends on the water source, analytical method, and testing objective. CDC guidance recommends collecting approximately one liter of water, when feasible, during environmental investigations, while smaller volumes may be sufficient for certain routine testing applications. For microbiological analysis, Legionella present in water samples may be concentrated by membrane filtration, while other samples may be diluted or directly plated, depending on their characteristics and the selected testing method. To minimize interference from non-target microorganisms and improve the recovery of Legionella, portions of the samples may undergo heat treatment, acid treatment, or a combination of both, as specified in the analytical procedure. Therefore, sample volume, preparation, and pretreatment methods should be determined according to the selected testing procedure rather than applying a uniform approach to all samples.

Samples should be collected, transported, and processed according to the laboratory's validated procedure. Collection conditions, sample identity, transport information, and any deviations should be documented to maintain traceability.

Culture-based methods and nucleic acid amplification tests are commonly used for environmental Legionella testing. They provide different types of information and should be selected according to the investigation objective.

Culture-based testing:

Culture-based testing involves processing the sample, inoculating suitable media, incubating under the conditions specified by the validated method, and examining presumptive colonies for confirmation. It can demonstrate viable, culturable organisms and provide isolates for further identification and characterization. Selective culture media help recover Legionella while limiting interference from competing microorganisms. Media selection and sample pretreatment should reflect the source water and the laboratory's validated procedure.

Molecular testing:

PCR-based methods detect specific genetic targets and can support screening or prioritization of environmental samples during investigations. However, detection of DNA does not necessarily demonstrate that organisms are viable. Results expressed in genomic units are not directly interchangeable with culture results expressed as colony-forming units.

Table 1: Comparison of Culture-Based and PCR-Based Testing Methods for Legionella Detection

The CDC emphasizes that method selection should account for sensitivity, specificity, laboratory capability, sample handling, and the intended use of the result.

Table 2: Recommended Legionella Testing Methods Based on Testing Objectives

Culture and PCR can serve complementary purposes. The source material identifies PCR as an option for screening and prioritizing samples for culture where appropriate. The current CDC guidance should be verified before making this recommendation definitive.

Step 1: Sample collection:

Identify sampling locations according to the investigation objectives and potential sources of exposure. Collect water or biofilm samples using applicable CDC guidance or a validated sampling procedure. Record the sampling location, sample type, collection conditions, and relevant observations.

Step 2: Sample transport and preparation:

Transport samples according to the laboratory's validated procedure. Depending on the selected method, sample preparation may involve concentration, pretreatment, or other processing steps to improve recovery and manage interfering microorganisms.

Step 3: Laboratory analysis:

For culture-based testing, inoculate appropriate media, such as buffered charcoal yeast extract (BCYE) agar and suitable selective formulations. Alternatively, use a validated PCR assay when appropriate for the investigation.

Step 4: Organism identification and confirmation:

Evaluate presumptive colonies using validated identification and confirmation procedures. For PCR-based testing, interpret results according to the assay's target specificity, performance characteristics, and intended use.

Step 5: Quality control and reporting:

Confirm that required quality-control checks meet acceptance criteria and document any deviations. Reports should identify the method used, the findings, and relevant limitations. Interpret the results in the context of the sampling plan and the facility's water management program.

Sample volume and collection techniques should follow the established sampling plan and laboratory testing method. Due to the varying characteristics and complexity of different sample matrices, laboratories should select the most appropriate sampling and analytical procedures for each sample type. For environmental investigations, CDC guidance recommends collecting approximately 1 L of water when feasible, although smaller volumes may be sufficient for certain routine testing applications.

Use suitable sterile containers and documented collection procedures. Collect surface swabs using a technique appropriate for the target surface and sample type.

For each sample, record the location, collection date, sample type, collection conditions, and relevant water-system observations. Follow the laboratory's validated instructions for transport, temperature management, preservation where applicable, and delivery time.

Sampling should be performed by trained personnel. During a suspected outbreak or facility-associated investigation, coordinate the sampling plan with the responsible public health authority and facility team.

Accurate documentation helps laboratories interpret findings and supports decisions about corrective action.

Testing frequency should be based on the facility's water management program, environmental risk assessment, system performance, and applicable public health or regulatory requirements. Routine monitoring and outbreak-related sampling serve different purposes and should not be assigned the same schedule by default.

Routine testing can establish baseline performance, identify potential growth patterns, and help validate whether water management controls are working. Additional testing may be appropriate after remediation, when results indicate inadequate control, or when an investigation identifies a potential exposure source.

The CDC's routine-testing guidance recommends considering the facility population, previous disease associations, consistency of control performance, and the need for additional information about the water management program. It also advises coordinating testing associated with public health investigations with the relevant authority.

Facilities should document the rationale for their testing schedule and revise it when risk assessments, system conditions, results, or applicable requirements change. A positive result should not be interpreted in isolation or used as the sole measure of health risk.

Effective control requires a documented water management program that identifies conditions supporting bacterial growth and establishes appropriate preventive and corrective measures. The CDC identifies water management as the primary strategy for controlling Legionella growth and spread in building water systems.

Important control activities include:

• Monitoring system conditions, including temperature and disinfectant residuals where relevant.
• Managing sediment, biofilm, corrosion, and scale.
• Reducing water stagnation and addressing poorly circulating sections of the system.
• Maintaining cooling towers, plumbing, storage tanks, and aerosol-generating devices.
• Implementing appropriate cleaning, disinfection, and water-treatment procedures.
• Documenting maintenance, monitoring, deviations, and corrective actions.

If results indicate inadequate control, the response should begin with a review of sampling and laboratory procedures, system operation, treatment performance, and implementation of the existing water management program. Corrective treatment may be needed, but the underlying conditions that allowed growth must also be addressed to reduce the likelihood of recolonization.

Following remediation, follow-up testing should use representative sampling locations and the procedure specified in the corrective-action plan. The CDC routine-testing toolkit advises waiting at least [VERIFY: 48 hours] after the system returns to normal operating conditions before collecting follow-up samples under its described approach. This timing should not be treated as a universal requirement for every system or public health investigation.

A laboratory result describes the sample analyzed and must be interpreted alongside the sampling plan, water system conditions, and method limitations. A single result cannot independently establish the health risk associated with an entire building or predict whether disease will occur.

Interpretation should consider:

• The measured concentration and reporting units.
• The location and type of sample.
• The species or serogroup identified, where available.
• Previous results and changes in system performance.
• Relevant operational parameters and treatment records.
• Whether the result indicates that established control measures may not be working.

Where disease is suspected or an outbreak is being investigated, environmental findings should be considered with epidemiological evidence and clinical laboratory findings. Comparison of environmental and clinical isolates can help investigators evaluate whether a water system is associated with cases

Reliable Legionella management depends on representative environmental sampling, appropriate culture or molecular methods, careful interpretation of results, and effective water management. CDC guidance provides a practical foundation for US environmental investigations, while the applicable jurisdictional requirements and validated laboratory procedures determine how testing and corrective actions should be implemented. BCYE agar, GVPC selective agar, and BCYE agar without cysteine are relevant culture media for the workflows described; exact HiMedia catalogue numbers require confirmation before publication.