Differential development of Legionella sub-populations during short- and long-term starvation

Legionella pneumophila response to shifts in biofilm structure mediated by hydrodynamics

Preventing legionellosis in water systems demands effective hydrodynamic management and biofilm mitigation. This study investigates the complex relationship between hydrodynamics (80 RPM and stagnation), biofilm mesoscale structure and Legionella pneumophila colonization, by addressing three key questions: (1) How do low flow vs stagnation conditions affect biofilm response to L. pneumophila colonization?, (2) How do biofilm structural variations mediate L. pneumophila migration across the biofilm?, and (3) Can specific hydrodynamic conditions trigger L. pneumophila entrance in a viable but nonculturable (VBNC) state? It was found that Pseudomonas fluorescens biofilms exhibit different responses to L. pneumophila based on the prevailing hydrodynamic conditions. While biofilm thickness and porosity decreased under shear (80 RPM), thickness tends to significantly increase when pre-established 80 RPM-grown biofilms are set to stagnation upon L. pneumophila spiking. Imposing stagnation after the spiking also seemed to accelerate Legionella migration towards the bottom of the biofilm. Water structures in the biofilm seem to be key to Legionella migration across the biofilm. Finally, shear conditions favoured the transition of L. pneumophila to VBNC states (∼94 %), despite the high viable cell counts (∼8 log10 CFU/cm2) found throughout the experiments. This research highlights the increased risk posed by biofilms and stagnation, emphasizing the importance of understanding the mechanisms that govern Legionella behaviour in diverse biofilm environments. These insights are crucial for developing more effective monitoring and prevention strategies in water systems.

Occurrence, predictive models and potential health risk assessment of viable but non-culturable (VBNC) pathogens in drinking water

Viable but non-culturable (VBNC) pathogens are prevalent in drinking water systems and can resuscitate under favorable conditions, thereby posing significant public health risks. This study investigated the occurrence of VBNC Escherichia coli and Pseudomonas aeruginosa in source water, tap water, and potable water in eastern China, using propidium monoazide-quantitative PCR and culture-based methods. Multiple linear regression (MLR) and artificial neural network (ANN) models were developed based on conventional water quality indicators to predict VBNC pathogen concentrations. The results indicated that drinking water treatment plants effectively reduced VBNC pathogens by 1-3 log units, however, concentrations ranging from 100 to 102 CFU/100 mL remained in tap and potable water, with detection rates between 83.33% and 100%. Furthermore, potable water contained a higher concentration of VBNC pathogens than tap water, suggesting a potential risk of microbial leakage from water dispensers. The constructed ANN models outperformed than MLR models, with R values greater than 0.8, indicating a strong correlation between measured values and model predictions for VBNC pathogens. ANN models also demonstrated superior accuracy than MLR models in predicting VBNC pathogens across different type of drinking water, achieving accuracies of 88.89% for Escherichia coli and 77.78% for Pseudomonas aeruginosa. The QMRA revealed that annual infection risks and disease burdens from VBNC pathogens in potable water were greater than those in tap water, with both exceeding acceptable safety thresholds. This study emphasizes that the risks posed by VBNC pathogens deserve attention and model predictions provide critical evidence for health risk identification.

Occurrence of Uncultured Legionella spp. in Treated Wastewater Effluent and Its Impact on Human Health (SCA.Re.S Project)

Wastewater treatment plants (WWTPs) provide optimal conditions for the environmental spread of Legionella. As part of the Evaluation of Sanitary Risk Related to the Discharge of Wastewater to the Ground (SCA.Re.S) project, this study was conducted to evaluate the presence of Legionella in WWTP effluent and in groundwater samples collected from two wells located downstream from the plant. The samples were analyzed to determine the concentrations of Legionella spp using the standard culture-based method and molecular techniques, followed by genomic sequencing analysis. Legionella was detected only with the molecular methods (except in one sample of effluent positive for L. pneumophila serogroup 6), which showed viable Legionella pneumophila and L. non-pneumophila through the use of free DNA removal solution in both the effluent and groundwater, with concentrations that progressively decreased downstream from the plant. Viable L. pneumophila appeared to be slightly more concentrated in warm months. However, no significant differences (p ≥ 0.05) in concentrations between cold and warm months were observed. A genotypic analysis characterized the species present in the samples and found that uncultured Legionella spp, as yet undefined, constituted the prevalent species in all the samples (range 77.15-83.17%). WWTPs play an important role in the hygienic and sanitary quality of groundwater for different uses. The application of Legionella control systems during the purification of effluents is warranted to prevent possible outbreaks of legionellosis.

Legionella colonization and 3D spatial location within a Pseudomonas biofilm

Biofilms are known to be critical for Legionella settlement in engineered water systems and are often associated with Legionnaire's Disease events. One of the key features of biofilms is their heterogeneous three-dimensional structure which supports the establishment of microbial interactions and confers protection to microorganisms. This work addresses the impact of Legionella pneumophila colonization of a Pseudomonas fluorescens biofilm, as information about the interactions between Legionella and biofilm structures is scarce. It combines a set of meso- and microscale biofilm analyses (Optical Coherence Tomography, Episcopic Differential Interference Contrast coupled with Epifluorescence Microscopy and Confocal Laser Scanning Microscopy) with PNA-FISH labelled L. pneumophila to tackle the following questions: (a) does the biofilm structure change upon L. pneumophila biofilm colonization?; (b) what happens to L. pneumophila within the biofilm over time and (c) where is L. pneumophila preferentially located within the biofilm? Results showed that P. fluorescens structure did not significantly change upon L. pneumophila colonization, indicating the competitive advantage of the first colonizer. Imaging of PNA-labelled L. pneumophila showed that compared to standard culture recovery it colonized to a greater extent the 3-day-old P. fluorescens biofilms, presumably entering in VBNC state by the end of the experiment. L. pneumophila was mostly located in the bottom regions of the biofilm, which is consistent with the physiological requirements of both bacteria and confers enhanced Legionella protection against external aggressions. The present study provides an expedited methodological approach to address specific systematic laboratory studies concerning the interactions between L. pneumophila and biofilm structure that can provide, in the future, insights for public health Legionella management of water systems.

Antimicrobial effects of automobile screenwashes against Legionella pneumophila

AIMS

Legionella pneumophila (Lp), a human pathogen, has been detected in windscreen wiper fluid reservoirs (WWFRs) where commercial screenwashes (CSWs) are commonly added. Limited information is available on CSWs against planktonic Lp; however, responses of sessile Lp and planktonic Lp pre-acclimated in nutrient-limited water to CSWs remain unknown. This study thus investigates the antibacterial effects of CSWs on sessile and starved planktonic Lp, in comparison with unstarved Lp.

METHODS AND RESULTS

Lp biofilms were produced on glass and WWFR materials of high-density polyethylene (HDPE) and polypropylene (PP). Planktonic Lp with and without acclimation in tap water were prepared. Log reductions in cell counts averaged 0.4-5.0 for ten brands of CSWs against sessile Lp and 1.0-3.9 and 0.9-4.9, respectively, against starved and unstarved planktonic Lp for five CSWs. Both biofilm formation and acclimation in tap water enhanced Lp resistance to CSWs. Significantly different log-reduction values among CSW brands were observed for sessile Lp on HDPE and planktonic Lp regardless of acclimation (p<0.05).

CONCLUSIONS

Biofilm formation, starvation acclimation, and CSW brand are crucial factors influencing Lp response to CSWs.

SIGNIFICANCE AND IMPACT OF STUDY

This study advances the knowledge of Lp reaction in anthropogenic water systems with CSWs.

Risk Exposure to Legionella pneumophila during Showering: The Difference between a Classical and a Water Saving Shower System

The increase in legionellosis incidence in the general population in recent years calls for a better characterization of the sources of infection, such as showering. Water-efficient shower systems that use water-atomizing technology have been shown to emit slightly more inhalable particles in the range of bacterial sizes than the traditional systems; however, the actual rate of bacterial emission remains poorly documented. The aim of this study was to assess the aerosolisation rate of the opportunistic water pathogen Legionella pneumophila during showering with one shower system representative of each technology. To achieve this objective, we performed controlled experiments inside a glove box and determined the emitted dose and viability of airborne Legionella. The bioaerosols were sampled with a Coriolis^® Delta air sampler and the total number of viable (cultivable and noncultivable) Legionella was determined by flow cytometry and culture. We found that the rate of viable and cultivable Legionella aerosolized from the water jet was similar between the two showerheads: the viable fraction represents 0.02% of the overall bacteria present in water, while the cultivable fraction corresponds to only 0.0005%. The two showerhead models emitted a similar ratio of airborne Legionella viable and cultivable per volume of water used. Therefore, the risk of exposure to Legionella is not expected to increase significantly with the new generation of water-efficient showerheads.

Variable Legionella Response to Building Occupancy Patterns and Precautionary Flushing

When stay-at-home orders were issued to slow the spread of COVID-19, building occupancy (and water demand) was drastically decreased in many buildings. There was concern that widespread low water demand may cause unprecedented Legionella occurrence and Legionnaires’ disease incidence. In lieu of evidenced-based guidance, many people flushed their water systems as a preventative measure, using highly variable practices. Here, we present field-scale research from a building before, during, and after periods of low occupancy, and controlled stagnation experiments. We document no change, a > 4-log increase, and a > 1.5-log decrease of L. pneumophila during 3- to 7-week periods of low water demand. L. pneumophila increased by > 1-log after precautionary flushing prior to reoccupancy, which was repeated in controlled boiler flushing experiments. These results demonstrate that the impact of low water demand (colloquially called stagnation) is not as straight forward as is generally assumed, and that some flushing practices have potential unintended consequences. In particular, stagnation must be considered in context with other Legionella growth factors like temperature and flow profiles. Boiler flushing practices that dramatically increase the flow rate and rapidly deplete boiler temperature may mobilize Legionella present in biofilms and sediment.

A Novel Flow Cytometric Approach for the Quantification and Quality Control of Chlamydia trachomatis Preparations

Chlamydia trachomatis is an obligate intracellular pathogenic bacterium with a biphasic developmental cycle manifesting two distinct morphological forms: infectious elementary bodies (EBs) and replicative intracellular reticulate bodies (RBs). Current standard protocols for quantification of the isolates assess infectious particles by titering inclusion-forming units, using permissive cell lines, and analyzing via immunofluorescence. Enumeration of total particle counts is achieved by counting labeled EBs/RBs using a fluorescence microscope. Both methods are time-consuming with a high risk of observer bias. For a better assessment of C. trachomatis preparations, we developed a simple and time-saving flow cytometry-based workflow for quantifying small particles, such as EBs with a size of 300 nm. This included optimization of gain and threshold settings with the addition of a neutral density filter for small-particle discrimination. The nucleic acid dye SYBR® Green I (SGI) was used together with propidium iodide and 5(6)-carboxyfluorescein diacetate to enumerate and discriminate between live and dead bacteria. We found no significant differences between the direct particle count of SGI-stained C. trachomatis preparations measured by microscopy or flow cytometry (p > 0.05). Furthermore, we completed our results by introducing a cell culture-independent viability assay. Our measurements showed very good reproducibility and comparability to the existing state-of-the-art methods, indicating that the evaluation of C. trachomatis preparations by flow cytometry is a fast and reliable method. Thus, our method facilitates an improved assessment of the quality of C. trachomatis preparations for downstream applications.

Population analysis of Legionella pneumophila reveals a basis for resistance to complement-mediated killing

Legionella pneumophila is the most common cause of the severe respiratory infection known as Legionnaires' disease. However, the microorganism is typically a symbiont of free-living amoeba, and our understanding of the bacterial factors that determine human pathogenicity is limited. Here we carried out a population genomic study of 902 L. pneumophila isolates from human clinical and environmental samples to examine their genetic diversity, global distribution and the basis for human pathogenicity. We find that the capacity for human disease is representative of the breadth of species diversity although some clones are more commonly associated with clinical infections. We identified a single gene (lag-1) to be most strongly associated with clinical isolates. lag-1, which encodes an O-acetyltransferase for lipopolysaccharide modification, has been distributed horizontally across all major phylogenetic clades of L. pneumophila by frequent recent recombination events. The gene confers resistance to complement-mediated killing in human serum by inhibiting deposition of classical pathway molecules on the bacterial surface. Furthermore, acquisition of lag-1 inhibits complement-dependent phagocytosis by human neutrophils, and promoted survival in a mouse model of pulmonary legionellosis. Thus, our results reveal L. pneumophila genetic traits linked to disease and provide a molecular basis for resistance to complement-mediated killing.

Legionella and Biofilms-Integrated Surveillance to Bridge Science and Real-Field Demands

Legionella is responsible for the life-threatening pneumonia commonly known as Legionnaires’ disease or legionellosis. Legionellosis is known to be preventable if proper measures are put into practice. Despite the efforts to improve preventive approaches, Legionella control remains one of the most challenging issues in the water treatment industry. Legionellosis incidence is on the rise and is expected to keep increasing as global challenges become a reality. This puts great emphasis on prevention, which must be grounded in strengthened Legionella management practices. Herein, an overview of field-based studies (the system as a test rig) is provided to unravel the common roots of research and the main contributions to Legionella’s understanding. The perpetuation of a water-focused monitoring approach and the importance of protozoa and biofilms will then be discussed as bottom-line questions for reliable Legionella real-field surveillance. Finally, an integrated monitoring model is proposed to study and control Legionella in water systems by combining discrete and continuous information about water and biofilm. Although the successful implementation of such a model requires a broader discussion across the scientific community and practitioners, this might be a starting point to build more consistent Legionella management strategies that can effectively mitigate legionellosis risks by reinforcing a pro-active Legionella prevention philosophy.

Detection and distribution of vbnc/viable pathogenic bacteria in full-scale drinking water treatment plants

Viable but non-culturable (VBNC) bacteria have attracted widespread attention since they are inherently undetected by traditional culture-dependent methods. Importantly, VBNC bacteria could resuscitate under favorable conditions leading to significant public health concerns. Although the total number of viable bacteria has been theorized to be far greater than those that can be cultured, there have been no reports quantifying VBNC pathogenic bacteria in full-scale drinking water treatment plants (DWTPs). In this work, we used both culture-dependent and quantitative PCR combination with propidium monoazide (PMA) dye approaches to characterize cellular viability. Further, we established a method to quantify viable pathogens by relating specific gene copies to viable cell numbers. Ratios of culturable bacteria to viable 16S rRNA gene copies in water and biological activated carbon (BAC) biofilms were 0-4.75% and 0.04-56.24%, respectively. The VBNC E. coli, E. faecalis, P. aeruginosa, Salmonella sp., and Shigella sp. were detected at levels of 0-10³ cells/100 mL in source water, 0-10² cells/100 mL in chlorinated water, and 0-10³ cells/g in BAC biofilms. In addition, differences between the total and viable community structures after ozonation and chlorination were investigated. The relative abundance of opportunistic pathogens such as Mycobacterium, Sphingomonas, etc. increased in final water, likely due to their chlorine resistance. In summary, we detected significant quantities of viable/VBNC opportunistic pathogens in full-scale DWTPs, confirming that traditional, culture-dependent methods are inadequate for detecting VBNC bacteria. These findings suggest a need to develop and implement rapid, accurate methods for the detection of VBNC pathogenic bacteria in DWTPs to ensure the safety of drinking water.

Viability and infectivity of viable but nonculturable Legionella pneumophila strains induced at high temperatures

Thermal disinfection is commonly used to prevent the proliferation of culturable Legionella in engineered water systems (EWS). In response to such stress, culturable Legionella populations can switch into a viable but nonculturable (VBNC) state. The importance of such VBNC Legionella cells is currently hotly debated. Here, we investigated the stress response patterns and transitions of the bacteria to the VBNC state at 55 °C, 60 °C and 70 °C on two L. pneumophila strains for >80 days using a combination of cell-based viability indicators. Complete loss of culturability at 55 °C, 60 °C and 70 °C occurred after 3-8 h, 60 min and <2 min, respectively. In contrast, L. pneumophila strains required 9 days at 55 °C, 8 h at 60 °C and 20 min at 70 °C to achieve a 2 log reduction in cells with intact membranes and high esterase activity; a 4 log reduction was achieved only after 150, 8-15 and 1-4 days, respectively. In parallel, the presence of diagnostic outer-membrane epitopes (OMEs) and changes in the infectivity patterns of the two strains towards amoebae and THP-1 cells were assessed. OMEs were more persistent than viability indicators, showing their potential as targets for VBNC Legionella detection. L. pneumophila strains infected amoebae and THP-1 cells for at least 85 days at 55 °C and 60 °C and for up to 8 days at 70 °C. However, they did so with reduced efficiency, requiring prolonged co-incubation times with the hosts and higher Legionella cell numbers in comparison to culturable cells. Consequently, infection of amoebae by thermally induced VBNC L. pneumophila with lowered virulence can be expected in EWS. Although the gold standard method cannot detect VBNC Legionella, it provides important information about the most virulent bacterial subpopulations. Our results indicate that a prolonged thermal regime ≥60 °C at the central parts of warm water systems is not only effective against culturable L. pneumophila but in the long run even against VBNC cells.

Floc-size effects of the pathogenic bacteria in a membrane bioreactor plant

The size nature of sludge flocs could affect the occurrence and distribution of bacterial pathogens in wastewater treatment plants (WWTPs). In this study, the floc-size dependence of bacterial pathogens in the activated sludge of a WWTP was investigated using high-throughput metagenomic sequencing approaches. The results showed that a total of 423 pathogenic species belonging to 123 genera were identified in the three size-fractionated flocs. Also, we found that all the pathogens on the WHO's global priority pathogens list were detected in the size-fractionated flocs, with relative abundance of 0.4%, 0.3% and 0.3% for large-size, medium-size and small-size flocs, respectively, indicating the severe human and environmental health risks of activated sludge. Importantly, our results revealed that the pathogenic species showed a clear floc-size dependent distribution manner, leading to significant differences (P < 0.05) of pathogenic communities among the size-fractionated flocs. Additionally, by partitioning pathogens based on the occurrence and significant difference in abundances, we suggested the following distribution features: 1) large flocs-associated pathogens, such as Borrelia recurrentis, Actinobacillus ureae and Campylobacter gracilis; 2) medium flocs-associated pathogens, such as Mycobacterium szulgai and Ureaplasma urealyticum; and 3) small flocs-associated pathogens, such as Rickettsia akari, Staphylococcus anginosus and Helicobacter cinaedi. Overall, this study provides a comprehensive understanding of pathogens in activated sludge, which is expected to aid in assessment and management of pathogen risks.

Persistent presence of outer membrane epitopes during short- and long-term starvation of five Legionella pneumophila strains

BACKGROUND

Legionella pneumophila, the causative agent of Legionnaire's disease, may enter a viable but non-culturable (VBNC) state triggered by environmental stress conditions. Specific outer-membrane epitopes of L. pneumophila are used in many diagnostic applications and some of them are linked to important virulence-related factors or endotoxins. However, it is not clear how the presence and status of these epitopes are influenced by environmental stress conditions. In this study, changes of outer membrane epitopes for monoclonal antibodies (mAb) from the Dresden panel and the major outer membrane protein MOMP were analysed for five L. pneumophila strains during short- and long-term starvation in ultrapure water.

RESULTS

With ELISA and single cell immuno-fluorescence analysis, we could show that for most of the investigated mAb-strain combinations the total number of mAb-stained Legionella cells stayed constant for up to 400 days. Especially the epitopes of mAb 3/1, 8/5, 26/1 and 20/1, which are specific for L. pneumophila serogroup 1 subtypes, and the mAb 9/1, specific for serogroup 6, showed long-term persistence. For most mAb- stained cells, a high percentage of viable cells was observed at least until 118 days of starvation. At the same time, we observed a reduction of the fluorescence intensity of the stained cells during starvation indicating a loss of epitopes from the cell surface. However, most of the epitopes, including the virulence-associated mAb 3/1 epitope were still present with high fluorescence intensity after 400 days of starvation in up to 50% of the starved L. pneumophila population.

CONCLUSIONS

The results demonstrate the continuous presence of outer membrane epitopes of L. pneumophila during short-term and long-term starvation. Thus, culture-independent mAb-based diagnostic and detection tools, such as immuno-magnetic separation and microarray techniques are applicable for both L. pneumophila in the culturable and the VBNC state even after long-term starvation but nevertheless require careful testing before application. However, the mere presence of those epitopes is not necessarily an indication of viability or infectivity.