Food Safety

Dossier – Legionella, the first waterborne pathogen

June 12, 2025

110

Legionella pneumophila, the most significant environmental waterborne pathogen within the European Union, continues to present substantial public health challenges due to severe respiratory infections and associated morbidity and mortality. This review systematically examines the escalating relevance of Legionella as a food safety concern, particularly within food processing environments. It critically analyzes the implementation dynamics of the revised EU Drinking Water Directive (DWD) 2020/2184 and its practical implications, a topic of direct interest to water supply entities and food control authorities.

The DWD’s mandate for comprehensive risk assessment and monitoring programs across drinking water distribution systems, extending beyond conventionally recognized high-risk premises, is discussed. This analysis underscores the strategic alignment of Hazard Analysis and Critical Control Points (HACCP) principles with water safety management, and explores alternative control strategies such as Copper-Silver Ionization (CSI), presenting a robust framework for controlling Legionella proliferation in food industry settings relevant to R&D departments.

The review also evaluates recent advancements in molecular detection methods, including real-time PCR and immunomagnetic separation, as rapid and sensitive alternatives to traditional culture techniques, offering enhanced tools for surveillance and outbreak response pertinent to national public health officials. Finally, it addresses persistent implementation challenges, particularly concerning the harmonization of testing methodologies and the establishment of appropriate action levels across EU Member States, providing insights valuable for top officials within Ministries of Health.

Table of Contents

Introduction

Legionnaire’s disease, a severe form of pneumonia caused predominantly by Legionella pneumophila serogroup 1, continues to pose significant challenges to public health throughout Europe (Yao et al., 2024). This bacterium in fact:

is ubiquitous in freshwater environments, its natural habitat, which comprise spring waters (including thermal springs), rivers, lakes and the muddy sediments associated with them;
is capable of colonising artificial water systems through water infrastructure, such as distribution networks, reservoirs and pipework;
proliferates in the presence of favourable conditions which, in addition to temperature, include the presence of biofilm, water stagnation, deposits and sediments, as well as specific chemical characteristics of the water;
creates particular risks in food processing facilities, where complex water distribution networks, temperature fluctuations and aerosol generation provide ideal conditions for bacterial amplification (Walker & McDermott, 2021);
must therefore be addressed through the adoption of preventive strategies based on monitoring of the physico-chemical parameters of water throughout its entire course of abstraction and extraction, treatment, storage and distribution.

The European Centre for Disease Prevention and Control (ECDC) has identified Legionella as having the highest health burden among all waterborne pathogens in the EU, necessitating comprehensive regulatory approaches to risk management (van den Berg et al., 2023).

Legionella spp. and food safety

The presence of Legionella spp. in water systems represents a risk that is as serious as it is peculiar from the perspective of food safety, in consideration of the following:

contagion does not occur through the ingestion of water contaminated by legionella, as legionellosis is transmitted by inhalation through exposure to contaminated aerosols;
water is therefore a vector for the pathogen, and is simultaneously classified as ‘food‘ – according to regulation (EC) 178/02, known as General Food Law, article 2.1 – after the point of compliance defined by directive (EU) 2020/2184;
the risk of Legionella contamination in water systems intended for the supply of drinking and process water in agri-food supply chains therefore requires the adoption of adequate prevention, monitoring and control measures, according to the principles of Hazard Analysis and Critical Control Points (HACCP), in order to protect public health and ensure compliance with food safety requirements.

The significance of Legionella therefore extends beyond the direct risks of contamination, as:

food processing environments use extensive water systems for production, cleaning and cooling processes, creating multiple opportunities for biofilm formation and bacterial proliferation (Steinert et al., 2002);
recent outbreaks linked to wastewater treatment plants at food industries have in turn highlighted the potential for transmission via aerosols over considerable distances, affecting both workers and surrounding communities (van den Berg et al., 2023).

The revised EU Drinking Water Directive: a paradigm shift

The revised Drinking Water Directive (EU) 2020/2184, which Member States were required to transpose into national legislation by 12 January 2023, marks the most significant update to EU water quality standards in more than 20 years.

A fundamental innovation of the revised DWD is the mandatory implementation of risk-based approaches to water safety, encompassing three key elements:

first, a risk assessment of the catchment area for abstraction points identifies potential contamination sources;
second, water suppliers must conduct risk assessments and implement risk management strategies throughout the supply chain, from abstraction through treatment and distribution;
third, and most significantly for Legionella control, the directive requires risk assessment of domestic distribution systems, with particular focus on priority premises including hospitals, healthcare facilities, and other buildings housing vulnerable populations (Directive EU 2020/2184).

The directive specifically addresses Legionella through several mechanisms, granting Member States:

the responsibility to ensure that risk assessments of domestic distribution systems include monitoring of parameters listed in Part D of Annex I, which explicitly mentions Legionella (Article 10);
the flexibility to focus monitoring efforts either on Legionella pneumophila or on all Legionella species, in recognition of the ongoing scientific debate regarding the most effective monitoring strategy.

France and Germany‘s successful focus on L. pneumophila testing has demonstrated reduced outbreak frequencies compared to broader screening approaches (Tata et al., 2023).

Legionella in food processing environments: unique challenges

Food processing facilities present distinctive challenges for Legionella control due to their complex water usage patterns and environmental conditions:

these facilities typically maintain extensive hot and cold water systems, cooling towers, hot water cylinders and specialised equipment requiring water for operation;
the integration of HACCP principles with water safety management has emerged as a critical strategy for controlling Legionella risks in these environments (Awuchi, 2023).

Multiple water sources within food production settings can harbour Legionella, including process water systems, equipment cooling circuits, and employee welfare facilities. The bacterium’s ability to survive in biofilms at temperatures between 20-45°C, combined with the presence of nutrients and protective microbial communities, creates persistent colonisation risks (Martin et al., 2019). Recent studies have demonstrated that dishwashing units with shower-type sprayers in mass catering establishments represent previously underrecognised sources of Legionella exposure, generating significant aerosols during operation (Ay, 2024);

The implementation of water management programmes based on HACCP methodology requires systematic hazard analysis at each stage of water use within food facilities. Critical control points typically include temperature maintenance in hot water systems (>60°C at heat exchangers, >50°C at outlets), adequate biocide residuals in cooling systems, and regular cleaning and flushing protocols for infrequently used outlets (as water stagnation could occur. ASHRAE, 2018). The effectiveness of these measures depends on comprehensive validation through environmental monitoring and continuous verification procedures.

Detection methods: evolution and current standards

The landscape of Legionella detection has evolved significantly, driven by the need for rapid, accurate methods to support risk assessment and outbreak investigation. Traditional culture methods using buffered charcoal yeast extract (BCYE) agar remain the gold standard for regulatory compliance, providing quantitative results and isolates for epidemiological typing. However, the 7-10 day incubation period and inability to detect viable but non-culturable (VBNC) cells represent significant limitations (Párraga-Niño et al., 2024).

Molecular methods, particularly quantitative PCR (qPCR), offer results within 24 hours and demonstrate superior sensitivity for detecting both culturable and VBNC Legionella. Recent validation studies have shown qPCR performance parameters exceeding 94% for sensitivity, specificity, and accuracy, though standardisation of genome unit correlations with colony-forming units remains challenging (Donohue et al., 2023). The integration of viability dyes such as propidium monoazide (PMA) with qPCR protocols enables differentiation of viable from non-viable cells, addressing concerns about false-positive results from dead cellular material.

Emerging technologies include immunomagnetic separation assays providing results within 24 hours, lateral flow devices for field screening, and most probable number (MPN) liquid culture methods offering improved recovery of stressed organisms. The Legiolert system, based on MPN methodology, has gained acceptance for its simplicity and ability to provide confirmed L. pneumophila results within seven days, with studies demonstrating equivalent or superior performance compared to traditional culture (Walker & McDermott, 2021).

Fourier-transform infrared spectroscopy (FT-IR) coupled with machine learning algorithms represents a cutting-edge approach for rapid Legionella identification and serotyping. Multi-centre validation studies have demonstrated the ability to discriminate between L. pneumophila serogroup 1, serogroups 2-15, and non-pneumophila species with high accuracy, providing valuable tools for outbreak investigation and source attribution (Tata et al., 2023).

Implementation challenges and solutions

The transposition of the revised DWD into national legislation has revealed significant implementation challenges across EU Member States. Primary concerns include the establishment of appropriate action levels for Legionella detection, harmonisation of testing methodologies, and resource allocation for expanded monitoring programmes. The directive’s flexibility in allowing member states to determine specific monitoring approaches has resulted in varied interpretations and potential inconsistencies in public health protection.

Laboratory capacity represents a critical constraint, particularly in regions with limited access to accredited facilities capable of performing Legionella analysis. The requirement for ISO 11731 compliance for culture methods necessitates significant investment in equipment, training, and quality assurance systems. Development of regional reference laboratories and inter-laboratory proficiency schemes has emerged as a strategy to ensure analytical reliability while building local capacity.

The integration of building water systems risk assessment with existing food safety management systems requires coordinated approaches between environmental health authorities and food safety inspectorates. Successful implementation depends on clear guidance documents, training programmes for risk assessors, and establishment of multidisciplinary teams capable of addressing both microbiological and engineering aspects of water system management.

Water quality parameters and Legionella control

Recent research has elucidated critical water quality factors influencing Legionella proliferation and detection in distribution systems. Total organic carbon (TOC) concentrations positively correlate with L. pneumophila detection frequency, providing nutrients supporting biofilm development and protecting bacteria from disinfectant action (Donohue et al., 2023). Heterotrophic plate counts serve as indicators of general microbial water quality and biofilm presence, with elevated counts associated with increased Legionella risk.

Disinfectant type and concentration significantly impact Legionella control efficacy. While free chlorine demonstrates superior planktonic inactivation rates, monochloramine shows enhanced penetration of biofilms and longer-lasting residual effects in distribution systems. The effectiveness varies with pipe materials, with copper surfaces providing additional antimicrobial effects compared to polyvinyl chloride (Buse et al., 2019). Understanding these interactions enables optimisation of disinfection strategies tailored to specific system characteristics.

Temperature management remains the primary control measure, with the directive emphasising maintenance of cold water below 20°C and hot water above 50°C throughout distribution systems. However, energy conservation efforts and concerns about scalding risks have led to exploration of alternative control strategies, including copper-silver ionisation, chlorine dioxide application, and point-of-use filtration for high-risk locations.

Copper-silver ionisation: an alternative control strategy

Copper-silver ionisation (CSI) has emerged as a promising alternative or complementary approach for Legionella control, particularly in complex building water systems where temperature maintenance proves challenging. This technology, which releases positively charged copper and silver ions into water systems, has demonstrated efficacy against both planktonic and biofilm-associated Legionella for over three decades (LeChevallier, 2023). The antimicrobial mechanism involves electrostatic bonding between ions and negatively charged bacterial cell walls, resulting in protein denaturation and disruption of cellular permeability. Importantly, copper and silver ions demonstrate long-term efficacy in penetrating and disrupting established biofilms, typically requiring 30-45 days for complete penetration (Shih & Lin, 2010).

A systematic review of CSI effectiveness found that this technology successfully reduced Legionella colonisation when copper concentrations were maintained between 0.2-0.8 mg/L and silver concentrations between 0.01-0.08 mg/L, levels well below regulatory limits (Cachafeiro et al., 2007). The technology has proven particularly effective in alkaline water conditions and systems with complex distribution networks where thermal disinfection is impractical. However, successful implementation requires careful attention to water chemistry parameters, including pH, alkalinity, organic carbon content, and the presence of competing ions that may reduce efficacy (LeChevallier, 2023).

Long-term studies have demonstrated the sustained effectiveness of CSI systems when properly maintained, with some facilities achieving consistent Legionella control for over five years. The technology offers several advantages over traditional thermal methods, including continuous disinfection, reduced energy consumption, and compatibility with lower hot water temperatures. However, challenges include the need for regular monitoring of ion concentrations, potential development of bacterial resistance with prolonged use at suboptimal concentrations, and concerns about regulatory compliance in certain jurisdictions. The European Union has imposed restrictions on copper use as a biocide, though these relate to registration requirements rather than safety concerns, highlighting the importance of understanding local regulatory frameworks when implementing CSI systems.

Future directions and research needs

The implementation of the revised DWD has highlighted several areas requiring further research and development. Standardisation of rapid detection methods remains a priority, with need for validated protocols establishing equivalence between molecular and culture-based results. Development of quantitative microbial risk assessment (QMRA) models specific to Legionella in food processing environments would enable evidence-based establishment of control limits and action levels.

Climate change impacts on Legionella ecology and transmission require investigation, particularly regarding increased ambient temperatures, extreme weather events affecting water systems, and changing patterns of water usage. The emergence of non-pneumophila species as causative agents of legionellosis necessitates evaluation of current monitoring strategies focused predominantly on L. pneumophila.

Artificial intelligence and machine learning applications show promise for predictive modelling of Legionella risk based on water quality parameters, system characteristics, and environmental conditions. Integration of real-time monitoring technologies with automated control systems could enable proactive intervention before hazardous conditions develop.

Conclusions

The revised EU Drinking Water Directive represents a watershed moment in the regulation of Legionella risk, establishing comprehensive frameworks for risk-based management of water systems across all sectors, including food production. The directive’s emphasis on preventive approaches aligns with established food safety principles, enabling integration of water safety and HACCP systems. However, successful implementation requires continued investment in laboratory infrastructure, development of harmonised methodologies, and cultivation of multidisciplinary expertise spanning microbiology, engineering, and public health.

The burden of Legionnaires’ disease in Europe demands sustained commitment to evidence-based control strategies. As climate change and demographic shifts create conditions favouring Legionella proliferation, the importance of robust regulatory frameworks and effective implementation becomes ever more critical. The food industry, with its complex water systems and potential for widespread exposure, must remain at the forefront of efforts to prevent environmental waterborne disease transmission.

Dario Dongo e Ylenia Desiree Patti Giammello

References

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