Legionella: A Key Organism in Water Management

1. Introduction to the Special Issue

Legionella is a critical organism in the fields of water management, environmental microbiology, and public health. Since the Legionella pneumophila outbreak in Philadelphia during the late summer of 1976, significant global progress has been achieved in these fields [1,2]. These advances include improved detection methods, innovative disinfection technologies, enhanced diagnostics and therapies for Legionnaires’ disease, updated guidelines, new recommendations, and the implementation of official policies at both national and international levels.

Water contamination and disease epidemiology are closely linked to urbanization processes and water management practices, both locally and globally. Nowadays, different engineered water environments have attracted the attention of public health researchers and operators, including cooling towers and even swimming pools [3,4]. This Special Issue brings together contributions from multiple disciplines, highlighting developments in water management and public health tools aimed at reducing exposure risks and preventing disease. It also introduces new perspectives on microbial biology, methodologies, epidemiological research, and regulatory frameworks.

Human health is intrinsically connected to water quality—from drinking water in buildings to wastewater management—on both local and planetary scales. Understanding the diversity of Legionella species, their characteristics, and their roles in environmental niches and disease remains a central topic.

The role of Legionella in biofilms and ecological niches provides a foundation for risk assessment across different infection routes and for evaluating clinical impacts within an epidemiological context. Addressing Legionella-related challenges requires an integrated approach rooted in microbiology, while also encompassing chemistry, physics, engineering, and seemingly unrelated fields such as economics, sociology, communication, public health, environmental sciences, management, policy, narrative medicine, and the humanities.

Collaboration among researchers with diverse expertise fosters dialogue and advances knowledge in this multidisciplinary domain, firmly grounded in the One Health perspective.

2. Overview of the Articles Published in This Special Issue

According to the study by Papadaki et al. (Contribution 1), 44.59% of the 1081 water samples collected and analyzed over a six-year period tested positive for Legionella. Overall, 16.46% of the total samples and 14.6% of the hotel samples (analogous to buildings considered in the Italian mathematical model) exceeded the Greek and EU threshold action limits.

This exceedance rate is lower than the 30–60% of buildings assumed in the mathematical model to be contaminated above 1000 cfu/L. However, it is important to consider that the sampled facilities were selected based on their inclusion in routine inspections, reports of a Travel Associated LD (in the case of hotels), or targeted sampling. As a result, these buildings may have had more robust water management practices and regular monitoring in place, potentially contributing to better control of water quality.

The finding that 97% of all Legionella-positive samples in this study were the highly pathogenic L. pneumophila serogroups 1–15 underscores the critical importance of active water management and routine monitoring for L. pneumophila.

The sensitivity analyses published alongside the Italian mathematical model demonstrate that even under the water conditions observed in the Greek study—where 14.6% of hotels exceeded the Legionella action limit and 93% of the positive samples were L. pneumophila serogroups 1–15—targeting L. pneumophila specifically (rather than Legionella spp. broadly) still delivers comparable health outcomes at lower economic and total costs. When both assumptions are applied simultaneously in the model, the cost per case avoided with an L. pneumophila-focused monitoring approach is 7% lower, with virtually the same number of LD cases prevented.

The consistent detection of Legionella via PCR in 100% of the effluent samples designated for agriculture re-use (Macrì et al., Contribution 2), is an important call to action for considering alternative methods for assessing viable, culturable Legionella prior to the release of reclaimed water. This is essential not only for protecting public health but also for building public trust in this critical and emerging water source.

It is worth noting that the well-founded critiques by the authors and previous researchers regarding the limitations of culture methods in this matrix, as a result of high background interference, are based solely on the ISO 11731 spread-plate culture method. Preliminary findings from wastewater trials in the Netherlands used a modified protocol of the liquid culture method for detecting L. pneumophila; these offer significant promise as methods that reducs background interference and reliably quantify culturable L. pneumophila in this matrix, and should continue to be investigated [5].

The physiological differences between Legionella spp. and L. pneumophila—which must be considered to inform Legionella management and control strategies—are demonstrated by the differing percentages of detectable bacteria following each treatment step in this study. Notably, 100% of the Legionella spp. PCR samples remained positive throughout all stages of the treatment process. In contrast, L. pneumophila detections declined significantly, with only 17% of samples testing positive after the final treatment stage, down from 100% at the start.

Vincenti et al. (Contribution 3) demonstrated that buildings equipped with hot water recirculation systems and storage tanks are particularly susceptible to colonization by Legionella spp. To mitigate this risk, it is essential to maintain appropriate water temperatures throughout the system.

New technologies, such as the hot water circuit described by Vincenti et al., serve as an important reminder that effective Legionella control is achievable in all types of buildings—not just hospitals. Proactive water management is not only significantly safer but also far more cost-effective than reactive remediation carried out after a case of Legionnaires’ disease has already occurred.

Romano Spica et al. (Contribution 4) developed a mathematical simulation model to evaluate the effectiveness and cost-efficiency of Legionella monitoring strategies in Italian non-hospital buildings. Using epidemiological, environmental, and economic data, the model compares targeting Legionella pneumophila versus all Legionella species. Results show that focusing on L. pneumophila provides comparable health protection while optimizing total costs, even under extreme assumptions. The tool can support policymakers in selecting surveillance strategies and can be adapted to other countries and different epidemiological scenarios

LeChevallier et al. (Contribution 5) highlight that routine monitoring programs can benefit from focusing analyses on the microorganisms that pose the greatest public health risk, whereas outbreak investigations and research contexts may require broader detection methods capable of identifying a wider range of microbial species. This review supports prioritizing Legionella pneumophila as the primary target for surveillance in drinking water distribution systems. Drawing on data from 930 U.S. outbreaks (1973–2023), the authors conclude that monitoring additional Legionella species provides only a minimal incremental value for public health protection. Since L. pneumophila is the predominant cause of disease and is associated with simpler analytical and remediation procedures, focusing monitoring efforts on this species can increase efficiency by reducing unnecessary costs, particularly in non-healthcare settings.

3. Conclusions

This Special Issue offers a diverse set of high-value contributions that expand current knowledge and open new perspectives on the management of Legionella, its microbial characteristics, and associated health risks. The overall approach is grounded in a public health framework, emphasizing the need for appropriate and effective interventions that also ensure a reasonable and well-balanced cost–benefit ratio. Readers will find insights that support the design of future research as well as the development of suitable preventive strategies.