Role of biofilms in the survival of Legionella pneumophila in a model potable-water system

Legionella in Hot Water Heat Pump (HWHP) Systems

It is anticipated that by 2028 there will be a significant increase in the use of HWHP systems in Great Britain (GB). Such systems are considered a better, energy-efficient alternative to fossil fuel-based burners and furnaces, as they use electricity. There are concerns that these systems are susceptible to microbial contamination because they hold water at lower temperatures. In particular, the concern is regarding Legionella contamination, as it can potentially cause disease in the general public and those who are maintaining and replacing these systems. Therefore, this review was focused on understanding the potential risk posed by their increased use and maintenance requirements. This review was approached systematically but was not a full systematic review. There were 61 papers that were considered potentially relevant to the research questions. Of these, 40 papers were considered relevant to the topic of Legionella in HWHP and underwent full article assessment and data extraction. The remaining papers were considered useful for background information. The scope of this review established that Legionella are a known risk in hot water systems that can be carried over to HWHP systems, yet there is minimal evidence to suggest that the current control measures are being appropriately applied to reduce the risk of exposure. When considering countrywide legislation and guidance, it appears that the risk is considered lower in single- or multi-family homes that do not require a centralised system. This review included the assessment of information regarding the safety of working with HWHP systems with regards to maintenance and replacement. The authors found a lack of information regarding these safety concerns. This review is among the first to systematically evaluate the risks of Legionella contamination in HWHP systems.

A trickling biofilm chamber to investigate the survival of Legionella pneumophila in evaporative cooling systems

As biofilms are a crucial factor in the proliferation of Legionella pneumophila in evaporative cooling systems (ECS), the impact of biocides on L. pneumophila containing biofilms in ECS needs to be evaluated by cultivation-independent methods. Therefore, a trickling biofilm chamber that simulates the spraying of process water was developed. Through this setup, the cultivation of Legionella-containing biofilms was possible. To demonstrate a potential application of the biofilm chamber, experiments using oxidizing and non-oxidizing biocides were conducted. Differences in cell survival were observed, alongside variations in the efficacy of culture and flow cytometry as analytical methodologies for assessing both intact and total cell populations. These findings also highlight the benefits of flow cytometry as a culture-independent analytical approach. This proof-of-principle study illustrates the need of the biofilm chamber for conducting experiments related to biofilm growth and biocide impact.

Effects of magnetically treated water on the survival of bacteria in biofilms

The goal of this study was to evaluate if a magnetic water treatment device could be used to mitigate biofilms in water systems. Magnetic treatment was applied to water upstream of a modified Robbins device in which Pseudomonas fluorescence biofilms were formed. Duration of magnetic treatment, system flow rate, and field strength were varied to assess the impacts on the biofilm. A control system was concurrently established in which no magnetic treatment was applied. After treatment, the number of viable cells in the biofilm was reduced by up to 2.46 log10 CFU cm-2 depending on the operational conditions. Increased cell stress, and ultimately death, was observed during treatment as indicated by an elevated AMPi stress index. These results indicate that magnetic water treatment may be an effective technology to decrease the extent of biofilms in water systems and a reduced need for chemical treatment. A mechanism is proposed in which metabolic processes are hindered due to the magnetic field effects on ions in the water. However, a mechanistic investigation remains outside the scope of this study. Future studies should aim to characterize both the impacts of treatment on the matrix and cellular processes to determine a mechanism for the observed effects.

Legionella pneumophila, a Rosetta stone to understanding bacterial pathogenesis

Legionella pneumophila is an environmentally acquired pathogen that causes respiratory disease in humans. While the discovery of L. pneumophila is relatively recent compared to other bacterial pathogens, over the past 50 years, L. pneumophila has emerged as a powerhouse for studying host-pathogen interactions. In its natural habitat of fresh water, L. pneumophila interacts with a diverse array of protozoan hosts and readily evolve to expand their host range. This has led to the accumulation of the most extensive arsenal of secreted virulence factors described for a bacterial pathogen and their ability to infect humans. Within amoebae and human alveolar macrophages, the bacteria replicate within specialized membrane-bound compartments, establishing L. pneumophila as a model for studying intracellular vacuolar pathogens. In contrast, the virulence factors required for intracellular replication are specifically tailored to individual host cells types, allowing the pathogen to adapt to variation between disparate niches. The broad host range of this pathogen, combined with the extensive diversity and genome plasticity across the Legionella genus, has thus established this bacterium as an archetype to interrogate pathogen evolution, functional genomics, and ecology. In this review, we highlight the features of Legionella that establish them as a versatile model organism, new paradigms in bacteriology and bacterial pathogenesis resulting from the study of Legionella, as well as current and future questions that will undoubtedly expand our understanding of the complex and intricate biology of the microbial world.

Dynamics of drinking water biofilm formation associated with Legionella spp. colonization

Understanding how Legionella spp. proliferate in multispecies biofilms is essential to develop strategies to control their presence in building plumbing. Here, we analyzed biofilm formation and Legionella spp. colonization on new plumbing material during 8 weeks. Biofilm formation was characterized by an initial increase in intact cell concentrations up to 9.5 × 105 cells/cm2, followed by a steady decrease. We identified Comamonas, Caulobacter, Schlegella, Blastomonas and Methyloversatilis as pioneer genera in the biofilm formation process. Importantly, L. pneumophila was the dominant Legionella spp. and rapidly colonized the biofilms, with culturable cell concentrations peaking at 3.1 × 104 MPN/cm2 after 4 weeks already. Moreover, several Legionella species co-occurred and had distinct dynamics of biofilm colonization. Vermamoeba vermiformis (V. vermiformis) was the dominant protist identified with 18S rRNA gene amplicon sequencing. Together our results highlight that biofilm formation upon introduction of new building plumbing material is a dynamic process where pathogenic Legionella species can be part of the earliest colonizers.

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.

Ecology of Legionella pneumophila biofilms: The link between transcriptional activity and the biphasic cycle

There has been considerable discussion regarding the environmental life cycle of Legionella pneumophila and its virulence potential in natural and man-made water systems. On the other hand, the bacterium's morphogenetic mechanisms within host cells (amoeba and macrophages) have been well documented and are linked to its ability to transition from a non-virulent, replicative state to an infectious, transmissive state. Although the morphogenetic mechanisms associated with the formation and detachment of the L. pneumophila biofilm have also been described, the capacity of the bacteria to multiply extracellularly is not generally accepted. However, several studies have shown genetic pathways within the biofilm that resemble intracellular mechanisms. Understanding the functionality of L. pneumophila cells within a biofilm is fundamental for assessing the ecology and evaluating how the biofilm architecture influences L. pneumophila survival and persistence in water systems. This manuscript provides an overview of the biphasic cycle of L. pneumophila and its implications in associated intracellular mechanisms in amoeba. It also examines the molecular pathways and gene regulation involved in L. pneumophila biofilm formation and dissemination. A holistic analysis of the transcriptional activities in L. pneumophila biofilms is provided, combining the information of intracellular mechanisms in a comprehensive outline. Furthermore, this review discusses the techniques that can be used to study the morphogenetic states of the bacteria within biofilms, at the single cell and population levels.

Legionella maioricensis sp. nov., a new species isolated from the hot water distribution systems of a hospital and a shopping center during routine sampling

Two Legionella-like strains isolated from hot water distribution systems in 2012 have been characterized phenotypically, biochemically and genomically in terms of DNA relatedness. Both strains, HCPI-6^T and EUR-108, exhibited biochemical phenotypic profiles typical of Legionella species. Cells were Gram-negative motile rods which grew on BCYEα agar but not on blood agar and displayed phenotypic characteristics typical of the family Legionellaceae, including a requirement for l-cysteine and testing catalase positive. Both strains were negative for oxidase, urease, nitrate reduction and hippurate negative, and non-fermentative. The major ubiquinone was Q12 (59.4 % HCPI-6^T) and the dominant fatty acids were C_16 : 1 ω7c (28.4 % HCPI-6^T, ≈16 % EUR-108), C_16 : 0 iso (≈22.5 % and ≈13 %) and C_15 : 0 anteiso (19.5 % and ≈23.5 %, respectively). The percent G+C content of genomic DNA was determined to be 39.3 mol %. The 16S rRNA gene, mip sequence and comparative genome sequence-based analyses (average nucleotide identity, ANI; digital DNA-DNA hybridization, dDDH; and phylogenomic treeing) demonstrated that the strains represent a new species of the genus Legionella. The analysis based on the 16S rRNA gene sequences showed that the sequence similarities for both strains ranged from 98.8-90.1 % to other members of the genus. The core genome-based phylogenomic tree (protein-concatemer tree based on concatenation of 418 proteins present in single copy) revealed that these two strains clearly form a separate cluster within the genus Legionella. ANI and dDDH values confirmed the distinctiveness of the strains. Based on the genomic, genotypic and phenotypic findings from a polyphasic study, the isolates are considered to represent a single novel species, for which the name Legionella maioricensis sp. nov. is proposed. The type strain is HCPI-6^T (=CCUG 75071^T=CECT 30569^T).

Proteome Exploration of Legionella pneumophila To Identify Novel Therapeutics: a Hierarchical Subtractive Genomics and Reverse Vaccinology Approach

Legionella pneumophila is the causative agent of a severe type of pneumonia (lung infection) called Legionnaires' disease. It is emerging as an antibiotic-resistant strain day by day. Hence, identifying novel drug targets and vaccine candidates is essential to fight against this pathogen. Here, attempts were taken through a subtractive genomics approach on the complete proteome of L. pneumophila to address the challenges of multidrug resistance. A total of 2,930 proteins from L. pneumophila proteome were investigated through diverse subtractive proteomics approaches, e.g., identification of human nonhomologous and pathogen-specific essential proteins, druggability and "anti-target" analysis, subcellular localization prediction, human microbiome nonhomology screening, and protein-protein interaction studies to find out effective drug and vaccine targets. Only three fulfilled these criteria and were proposed as novel drug targets against L. pneumophila. Furthermore, outer membrane protein TolB was identified as a potential vaccine target with a better antigenicity score. Antigenicity and transmembrane topology screening, allergenicity and toxicity assessment, population coverage analysis, and a molecular docking approach were adopted to generate the most potent epitopes. The final vaccine was constructed by the combination of highly immunogenic epitopes, along with suitable adjuvant and linkers. The designed vaccine construct showed higher binding interaction with different major histocompatibility complex (MHC) molecules and human immune TLR-2 receptors with minimum deformability at the molecular level. The present study aids the development of novel therapeutics and vaccine candidates for efficient treatment and prevention of L. pneumophila infections. However, further wet-lab-based phenotypic and genomic investigations and in vivo trials are highly recommended to validate our prediction experimentally. IMPORTANCE Legionella pneumophila is a human pathogen distributed worldwide, causing Legionnaires' disease (LD), a severe form of pneumonia and respiratory tract infection. L. pneumophila is emerging as an antibiotic-resistant strain, and controlling LD is now difficult. Hence, developing novel drugs and vaccines against L. pneumophila is a major research priority. Here, the complete proteome of L. pneumophila was considered for subtractive genomics approaches to address the challenge of antimicrobial resistance. Our subtractive proteomics approach identified three potential drug targets that are promising for future application. Furthermore, a possible vaccine candidate, "outer membrane protein TolB," was proposed using reverse vaccinology analysis. The constructed vaccine candidate showed higher binding interaction with MHC molecules and human immune TLR-2 receptors at the molecular level. Overall, the present study aids in developing novel therapeutics and vaccine candidates for efficient treatment of the infections caused by L. pneumophila.

Influence of Metal Concentration and Plumbing Materials on Legionella Contamination

Legionella colonization of water supply pipes is a significant public health problem. The objective of this work was to evaluate Legionella colonization in hotel hot water systems and to investigate the relationship between metal concentrations, piping materials (galvanized iron pipes and plastic pipes), and Legionella proliferation. Concentrations of calcium and magnesium ions and the presence of Legionella pneumophila were determined in a total of 108 water samples from the hot water systems of four hotels in Split-Dalmatia County over a 12-month period, and additional data on piping materials were collected. L. pneumophila was isolated in 23.1% of all samples-in 28.8% (15/52) of water samples from galvanized iron pipes and in 17.8% (10/56) of samples from plastic pipes. L. pneumophila serogroups 2-14 were isolated from all samples. This study found higher prevalence of L. pneumophila at higher concentrations of Ca and Mg ions (except for Mg and plastic pipes). The metal parts of the water supply may be important factors in Legionella contamination due to the possibility of lime scale or roughness of the pipes. Higher Ca and Mg ion concentrations increased the risk of Legionella colonization.

Migration of Acanthamoeba through Legionella biofilms is regulated by the bacterial Lqs-LvbR network, effector proteins and the flagellum

The environmental bacterium Legionella pneumophila causes the pneumonia Legionnaires' disease. The opportunistic pathogen forms biofilms and employs the Icm/Dot type IV secretion system (T4SS) to replicate in amoebae and macrophages. A regulatory network comprising the Legionella quorum sensing (Lqs) system and the transcription factor LvbR controls bacterial motility, virulence and biofilm architecture. Here we show by comparative proteomics that in biofilms formed by the L. pneumophila ΔlqsR or ΔlvbR regulatory mutants the abundance of proteins encoded by a genomic ‘fitness island’, metabolic enzymes, effector proteins and flagellar components (e.g. FlaA) varies. ∆lqsR or ∆flaA mutants form ‘patchy’ biofilms like the parental strain JR32, while ∆lvbR forms a ‘mat‐like’ biofilm. Acanthamoeba castellanii amoebae migrated more slowly through biofilms of L. pneumophila lacking lqsR, lvbR, flaA, a functional Icm/Dot T4SS (∆icmT), or secreted effector proteins. Clusters of bacteria decorated amoebae in JR32, ∆lvbR or ∆icmT biofilms but not in ∆lqsR or ∆flaA biofilms. The amoeba‐adherent bacteria induced promoters implicated in motility (P_flaA) or virulence (P_sidC, P_ralF). Taken together, the Lqs‐LvbR network (quorum sensing), FlaA (motility) and the Icm/Dot T4SS (virulence) regulate migration of A. castellanii through L. pneumophila biofilms, and – apart from the T4SS – govern bacterial cluster formation on the amoebae.

The Impact of Pipe Material on the Diversity of Microbial Communities in Drinking Water Distribution Systems

As many cities around the world face the prospect of replacing aging drinking water distribution systems (DWDS), water utilities must make careful decisions on new pipe material (e.g., cement-lined or PVC) for these systems. These decisions are informed by cost, physical integrity, and impact on microbiological and physicochemical water quality. Indeed, pipe material can impact the development of biofilm in DWDS that can harbor pathogens and impact drinking water quality. Annular reactors (ARs) with cast iron and cement coupons fed with chloraminated water from a municipal DWDS were used to investigate the impact of pipe material on biofilm development and composition over 16 months. The ARs were plumbed as closely as possible to the water main in the basement of an academic building to simulate distribution system conditions. Biofilm communities on coupons were characterized using 16S rRNA sequencing. In the cast iron reactors, β-proteobacteria, Actinobacteria, and α-proteobacteria were similarly relatively abundant (24.1, 22.5, and 22.4%, respectively) while in the cement reactors, α-proteobacteria and Actinobacteria were more relatively abundant (36.3 and 35.2%, respectively) compared to β-proteobacteria (12.8%). Mean alpha diversity (estimated with Shannon H and Faith's Phylogenetic Difference indices) was greater in cast iron reactors (Shannon: 5.00 ± 0.41; Faith's PD: 15.40 ± 2.88) than in cement reactors (Shannon: 4.16 ± 0.78; Faith's PD: 13.00 ± 2.01). PCoA of Bray-Curtis dissimilarities indicated that communities in cast iron ARs, cement ARs, bulk distribution system water, and distribution system pipe biofilm were distinct. The mean relative abundance of Mycobacterium spp. was greater in the cement reactors (34.8 ± 18.6%) than in the cast iron reactors (21.7 ± 11.9%). In contrast, the mean relative abundance of Legionella spp. trended higher in biofilm from cast iron reactors (0.5 ± 0.7%) than biofilm in cement reactors (0.01 ± 0.01%). These results suggest that pipe material is associated with differences in the diversity, bacterial composition, and opportunistic pathogen prevalence in biofilm of DWDS.

Human- and infrastructure-associated bacteria in greywater

Greywater, the wastewater from sinks, showers and laundry, is an understudied environment for bacterial communities. Most greywater studies focus on quantifying pathogens, often via proxies used in other wastewater, like faecal indicator bacteria; there is a need to identify more greywater-appropriate surrogates, like Staphylococcus sp. Sequencing-based studies have revealed distinct communities in different types of greywater as well as in different parts of greywater infrastructure, including biofilms on pipes, holding tanks and filtration systems. The use of metagenomic sequencing provides high resolution on both the taxa and genes present, which may be of interest in cases like identifying pathogens and surrogates relevant to different matrices, monitoring antibiotic resistance genes and understanding metabolic processes occurring in the system. Here, we review what is known about bacterial communities in different types of greywater and its infrastructure. We suggest that wider adoption of environmental sequencing in greywater research is important because it can describe the entire bacterial community along with its metabolic capabilities, including pathways for removal of nutrients and organic materials. We briefly describe a metagenomic dataset comparing different types of greywater samples in a college dormitory building to highlight the type of questions these methods can address. Metagenomic sequencing can help further the understanding of greywater treatment for reuse because it allows for identification of new pathogens or genes of concern.

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.

Efficient cooling tower operation at alkaline pH for the control of Legionella pneumophila and other pathogenic genera

Efficient control of pathogenic bacteria, specifically Legionella pneumophila, is one of the main concerns when operating industrial cooling towers. Common practices to limit proliferation involves use of disinfectants, leading to formation of disinfection by-product and increase in water corrosiveness. A disinfectant-free Legionella control method would make the industry more environmentally friendly. A pilot-scale cooling tower (1 m³/h) operated with demineralized water was used to investigate the potential of high-pH conditioning as a disinfectant-free alternative for control of L. pneumophila and other pathogens. One control experiment was performed under standard full-scale operation involving sodium hypochlorite dosage. Thereafter 3 alkaline pHs of the cooling water were tested: 9.0, 9.4 and 9.6. The tests lasted between 25 and 35 days. The cooling water from the basins were analysed for total cell count by flow cytometry, L. pneumophila concentration by plate count and occasional qPCR analyses targeting the mip-gene, bacterial and eukaryotic community analyses with 16S and 18S rRNA gene amplicon sequencing, relative abundance of eukaryotic to prokaryotic DNA by qPCR of the 16S and 18S rRNA gene. The L. pneumophila analyses showed considerable growth at pH 9.0 and pH 9.4 but was maintained below detection limit (< 100 CFU/L) at pH 9.6 without disinfection. Interestingly, the results correlated with the overall abundance of protozoa in the water samples but not directly with the relative abundance of specific reported protozoan hosts of Legionella. The pathogenicity based on 16S rRNA gene amplicon sequencing of the cooling water DNA decreased with increasing pH with a strong decline between pH 9.0 and pH 9.4, from 7.1% to 1.6% of relative abundance of pathogenic genera respectively. A strong shift in microbiome was observed between each tested pH and reproducibility of the experiment at pH 9.6 was confirmed with a duplicate test lasting 80 days. High-pH conditioning ≥ 9.6 is therefore considered as an efficient disinfectant-free cooling tower operation for control of pathogenicity, including L. pneumophila.

Are pathogenic Legionella non-pneumophila a common bacteria in Water Distribution Networks?

The present study analyzes at the national level, the presence of circulating Legionella in the artificial aquatic systems of different facilities of all of them state-owned centers throughout Spain for 12 months. 1754 water samples from various state-owned centers were collected from January to December 2014. Samples were collected from the cooling towers and evaporative condensers (CTC), and water distribution networks such as domestic hot water (DHW), cold water for human consumption (CW), sprinkler irrigation systems (SIS), fire sprinkler systems (FSS), and water from decorative fountains (DF). All these facilities are considered, according to current regulations, as potential amplifying systems for bacteria and possible sources of infection by the generation of droplets and aerosols. The isolation and counting of Legionella in water samples was carried out using microbiological culture following the international normative UNE-EN-ISO 11,731:2007 (ISO 11,731:1998) and UNE-EN ISO 8199:2008 (ISO 8199:2005).The quantification of Legionella colonization, the annual distribution, and the geographical distribution of the Legionella isolates recovered in the water were analyzed. Besides, molecular techniques were used for the characterization of the Legionella non-pneumophila isolates. Legionella was recovered from 15.79% of the analyzed water samples. High colonization was more frequently detected in water samples from CTC, DHW, CW, and DF. Regarding the geographic distribution, positive samples of Legionella were obtained in 14 of the 18 Spanish locations analyzed. Legionella non-pneumophila was the most prevalent and was isolated from water samples from 13 different geographical locations (72%). Legionella anisa and Legionella jordanis were the most frequently non-pneumophila species isolated. Legionella donaldsonii was isolated for the first time in the water distribution networks in Spain. Legionella pneumophila sg 2-14 was detected in 13 locations and Legionella pneumophila sg 1 in 11 locations. Therefore, our study concludes that the presence of Legionella pneumophila and Legionella non-pneumophila species in these systems can be a potential threat to public health and should be examined thoroughly with complementary techniques, such as molecular techniques as a screen for routine diagnosis.

Causes, Factors, and Control Measures of Opportunistic Premise Plumbing Pathogens—A Critical Review

This review critically analyses the chemical and physical parameters that influence the occurrence of opportunistic pathogens in the drinking water distribution system, specifically in premise plumbing. A comprehensive literature review reveals significant impacts of water age, disinfectant residual (type and concentration), temperature, pH, and pipe materials. Evidence suggests that there is substantial interplay between these parameters; however, the dynamics of such relationships is yet to be elucidated. There is a correlation between premise plumbing system characteristics, including those featuring water and energy conservation measures, and increased water quality issues and public health concerns. Other interconnected issues exacerbated by high water age, such as disinfectant decay and reduced corrosion control efficiency, deserve closer attention. Some common features and trends in the occurrence of opportunistic pathogens have been identified through a thorough analysis of the available literature. It is proposed that the efforts to reduce or eliminate their incidence might best focus on these common features.

Pathogenicity and Virulence of Legionella: Intracellular replication and host response

Bacteria of the genus Legionella are natural pathogens of amoebae that can cause a severe pneumonia in humans called Legionnaires’ Disease. Human disease results from inhalation of Legionella-contaminated aerosols and subsequent bacterial replication within alveolar macrophages. Legionella pathogenicity in humans has resulted from extensive co-evolution with diverse genera of amoebae. To replicate intracellularly, Legionella generates a replication-permissive compartment called the Legionella-containing vacuole (LCV) through the concerted action of hundreds of Dot/Icm-translocated effector proteins. In this review, we present a collective overview of Legionella pathogenicity including infection mechanisms, secretion systems, and translocated effector function. We also discuss innate and adaptive immune responses to L. pneumophila, the implications of Legionella genome diversity and future avenues for the field.

Differential Bacterial Predation by Free-Living Amoebae May Result in Blooms of Legionella in Drinking Water Systems

Intracellular growth of pathogenic Legionella in free-living amoebae (FLA) results in the critical concentrations that are problematic in engineered water systems (EWS). However, being amoeba-resistant bacteria (ARB), how Legionella spp. becomes internalized within FLA is still poorly understood. Using fluorescent microscopy, we investigated in real-time the preferential feeding behavior of three water-related FLA species, Willaertia magna, Acanthamoeba polyphaga, and Vermamoeba vermiformis regarding Legionella pneumophila and two Escherichia coli strains. Although all the studied FLA species supported intracellular growth of L. pneumophila, they avoided this bacterium to a certain degree in the presence of E. coli and mostly fed on it when the preferred bacterial food-sources were limited. Moreover, once L. pneumophila were intracellular, it inhibited digestion of co-occurring E. coli within the same trophozoites. Altogether, based on FLA–bacteria interactions and the shifts in microbial population dynamics, we propose that FLA’s feeding preference leads to an initial growth of FLA and depletion of prey bacteria, thus increases the relative abundance of Legionella and creates a “forced-feeding” condition facilitating the internalization of Legionella into FLA to initiate the cycles of intracellular multiplication. These findings imply that monitoring of FLA levels in EWS could be useful in predicting possible imminent high occurrence of Legionella.

Unravelling the importance of the eukaryotic and bacterial communities and their relationship with Legionella spp. ecology in cooling towers: a complex network

BACKGROUND

Cooling towers are a major source of large community-associated outbreaks of Legionnaires' disease, a severe pneumonia. This disease is contracted when inhaling aerosols that are contaminated with bacteria from the genus Legionella, most importantly Legionella pneumophila. How cooling towers support the growth of this bacterium is still not well understood. As Legionella species are intracellular parasites of protozoa, it is assumed that protozoan community in cooling towers play an important role in Legionella ecology and outbreaks. However, the exact mechanism of how the eukaryotic community contributes to Legionella ecology is still unclear. Therefore, we used 18S rRNA gene amplicon sequencing to characterize the eukaryotic communities of 18 different cooling towers. The data from the eukaryotic community was then analysed with the bacterial community of the same towers in order to understand how each community could affect Legionella spp. ecology in cooling towers.

RESULTS

We identified several microbial groups in the cooling tower ecosystem associated with Legionella spp. that suggest the presence of a microbial loop in these systems. Dissolved organic carbon was shown to be a major factor in shaping the eukaryotic community and may be an important factor for Legionella ecology. Network analysis, based on co-occurrence, revealed that Legionella was correlated with a number of different organisms. Out of these, the bacterial genus Brevundimonas and the ciliate class Oligohymenophorea were shown, through in vitro experiments, to stimulate the growth of L. pneumophila through direct and indirect mechanisms.

CONCLUSION

Our results suggest that Legionella ecology depends on the host community, including ciliates and on several groups of organisms that contribute to its survival and growth in the cooling tower ecosystem. These findings further support the idea that some cooling tower microbiomes may promote the survival and growth of Legionella better than others. Video Abstract.

Characterization of Legionella pneumophila Populations by Multilocus Variable Number of Tandem Repeats (MLVA) Genotyping from Drinking Water and Biofilm in Hospitals from Different Regions of the West Bank

The West Bank can be considered a high-risk area for Legionnaires' disease (LD) due to its hot climate, intermittent water supply and roof storage of drinking water. Legionella, mostly L. pneumophila, are responsible for LD, a severe, community-acquired and nosocomial pneumonia. To date, no extensive assessment of Legionella spp and L. pneumophila using cultivation in combination with molecular approaches in the West Bank has been published. Two years of environmental surveillance of Legionella in water and biofilms in the drinking water distribution systems (DWDS) of eight hospitals was carried out; 180 L. pneumophila strains were isolated, mostly from biofilms in DWDS. Most of the isolates were identified as serogroup (Sg) 1 (60%) and 6 (30%), while a minor fraction comprised Sg 8 and 10. Multilocus Variable number of tandem repeats Analysis using 13 loci (MLVA-8(12)) was applied as a high-resolution genotyping method and compared to the standard Sequence Based Typing (SBT). The isolates were genotyped in 27 MLVA-8(12) genotypes (Gt), comprising four MLVA clonal complexes (VACC 1; 2; 5; 11). The major fraction of isolates constituted Sequence Type (ST)1 and ST461. Most of the MLVA-genotypes were highly diverse and often unique. The MLVA-genotype composition showed substantial regional variability. In general, the applied MLVA-method made it possible to reproducibly genotype the isolates, and was consistent with SBT but showed a higher resolution. The advantage of the higher resolution was most evident for the subdivision of the large strain sets of ST1 and ST461; these STs were shown to be highly pneumonia-relevant in a former study. This shows that the resolution by MLVA is advantageous for back-tracking risk sites and for the avoidance of outbreaks of L. pneumophila. Overall, our results provide important insights into the detailed population structure of L. pneumophila, allowing for better risk assessment for DWDS.

Isolation and identification of Legionella spp. from hot spring water in Algeria by culture and molecular methods

AIMS

Due to infectious risk associated with the presence of Legionella in warm water, we determined the prevalence of living Legionella spp. in hot spring water in Algeria.

METHODS AND RESULTS

Detection of Legionella by culture was done by using two methods, direct culture on agar plates and co-culture with amoeba. Fifty samples were taken from different hot springs in northern Algeria, including swimming pools, showers and thermal sources. Legionella pneumophila serotypes were predominant, accounting for 60% of positive samples. Direct method allowed the isolation of 13 L. pneumophila only of 50 samples (26%), whereas co-culture using a panel of three free living amoeba allowed the isolation of 119 Legionella species from the same samples (80%) CONCLUSIONS: Amoeba co-culture allowed the isolation of several Legionella sp., while direct culture allowed the isolation of L. pneumophila only. Remarkably, Legionella longbeachae, usually isolated from soil and compost, was isolated for the first time in thermal water in three samples using Vermamoeba vermiformis co-culture.

SIGNIFICANCE AND IMPACT OF THE STUDY

The presence of Legionella in the water of hot springs in Algeria, which are mainly frequented by individuals at risk of Legionellosis, requires urgent control measures.

Legionella pneumophila Attachment to Biofilms of an Acidovorax Isolate from a Drinking Water-Consortium Requires the Lcl-Adhesin Protein

Human infection by Legionella pneumophila (Lpn) only occurs via contaminated water from man-made sources, and eradication of these bacteria from man-made water systems is complicated by biofilm colonization. Using a continuously fed biofilm reactor model, we grew a biofilm consortium from potable water that was able to prolong recovery of Lpn CFU from biofilms. This effect was recreated using a subset of those species in a simplified consortium composed of eight bacterial isolates from the first biofilm reactor. In the reactor with the eight-species consortium, Lpn biofilm CFU was relatively stable over a 12-day trial. An isolate of Acidovorax from the consortium was, as a single species biofilm, able to promote Lpn surface attachment. Other isolates from the Pelomonas genus grew as equally robust biofilms alone, but did not promote surface attachment of Lpn. This attachment was disrupted by cationic polysaccharides and loss of the Lpn Lcl collagen-like adhesin protein. This work demonstrates that, while Lpn was fairly incompetent at attachment to surfaces to form a biofilm alone, pre-existing biofilms allowed attachment of Lpn as secondary colonizers. In addition, we demonstrate that initial attachment of Lpn to Acidovorax biofilms is likely via the Lcl-adhesin protein.

Legionellosis and Recent Advances in Technologies for Legionella Control in Premise Plumbing Systems: A Review

This review discusses Legionella, among the most prolific and publicly well-known waterborne pathogens, and advances in potential treatment technologies. The number of cases associated with Legionella continues to rise, as does its public awareness. Currently, cases associated with premise plumbing account for the largest number of legionellosis cases in the United States. So, while it is important to understand Legionella as such, it is also important to investigate how to treat drinking water in premise plumbing for Legionella and other waterborne pathogens. While there are currently several methods recognized as potential means of inactivating waterborne pathogens, several shortcomings continue to plague its implementation. These methods are generally of two types. Firstly, there are chemical treatments such as chlorine, chlorine dioxide, monochloramine, ozone, and copper-silver ionization. Secondly, there are physical treatments such as thermal inactivation and media filtration. Their shortcomings range from being labor-intensive and costly to having negative health effects if not properly operated. Recently developed technologies including ultraviolet (UV) irradiation using light emitting diodes (LEDs) and innovative carbon nanotube (CNT) filters can better control waterborne pathogens by allowing for the simultaneous use of different treatment measures in plumbing systems.

Effect of temperature on opportunistic pathogen gene markers and microbial communities in long-term stored roof-harvested rainwater

Roof-harvested rainwater (RHRW) has received increasing attention in recent years as an alternative water source for domestic use, yet its biological stability during storage is not fully understood. This study investigated the effects of temperature (4 °C, 20 °C and 30 °C) on the microbiological characteristics of RHRW over a storage period of 60 days by targeting different microbial groups including total bacteria and fecal indictor Escherichia coli, bacterial opportunistic pathogen genera and species (Legionella spp, Legionella pneumophila, Mycobacterium spp, Mycobacterium avium, Pseudomonas aeruginosa), and two amoebas (Acanthamoeba and Vermamoeba vermiformis). The rainwater chemistry demonstrated no obvious change during storage. The highest biomass was observed in RHRW stored at 30 °C, as measured by heterotrophic bacterial counts, adenosine triphosphate, and 16S rRNA gene numbers. Gene markers of E. coli, Legionella spp., P. aeruginosa, and V. vermiformis were detected in fresh RHRW and can persist during RHRW storage; whereas P. aeruginosa was the only species demonstrated significant regrowth at higher storage temperatures (P < 0.05). Acanthamoeba spp. was only detected in RHRW after 50 days of storage at three investigated temperatures, highlighting increased health risks in long-term stored RHRW. Bacterial community compositions were significantly different in RHRW stored at different temperatures, with increased variations among triplicate storage bottles noted at higher temperatures along with storage time. The results provide insights into RHRW storage practices in terms of mitigating microbial contamination risks.

Presence of Legionella spp. in cooling towers: the role of microbial diversity, Pseudomonas, and continuous chlorine application

Legionnaires' disease (LD) is a severe pneumonia caused by several species of the genus Legionella, most frequently by Legionella pneumophila. Cooling towers are the most common source for large community-associated outbreaks. Colonization, survival, and proliferation of L. pneumophila in cooling towers are necessary for outbreaks to occur. These steps are affected by the chemical and physical parameters of the cooling tower environment. We hypothesize that the bacterial community residing in the cooling tower could also affect the presence of L. pneumophila. A 16S rRNA gene targeted amplicon sequencing approach was used to study the bacterial community of cooling towers and its relationship with the Legionella spp. and L. pneumophila communities. The results indicated that the water source shaped the bacterial community of cooling towers. Several taxa were enriched and positively correlated with Legionella spp. and L. pneumophila. In contrast, Pseudomonas showed a strong negative correlation with Legionella spp. and several other genera. Most importantly, continuous chlorine application reduced microbial diversity and promoted the presence of Pseudomonas creating a non-permissive environment for Legionella spp. This suggests that disinfection strategies as well as the resident microbial population influences the ability of Legionella spp. to colonize cooling towers.

Influence of surface copper content on Stenotrophomonas maltophilia biofilm control using chlorine and mechanical stress

This work aimed to evaluate the action of materials with different copper content (0, 57, 96 and 100%) on biofilm formation and control by chlorination and mechanical stress. Stenotrophomonas maltophilia isolated from drinking water was used as a model microorganism and biofilms were developed in a rotating cylinder reactor using realism-based shear stress conditions. Biofilms were characterized phenotypically and exposed to three control strategies: 10 mg l^-1 of free chlorine for 10 min, an increased shear stress (a fluid velocity of 1.5 m s^-1 for 30s), and a combination of both treatments. These shock treatments were not effective in biofilm control. The benefits from the use of copper surfaces was found essentially in reducing the numbers of non-damaged cells. Copper materials demonstrated better performance in biofilm prevention than chlorine. In general, copper alloys may have a positive public health impact by reducing the number of non-damaged cells in the water delivered after chlorine exposure.

Wide-scale study of 206 buildings in the Netherlands from 2011 to 2015 to determine the effect of drinking water management plans on the presence of Legionella spp

Previous analysis of the Dutch National Legionella Outbreak Detection Program 2002-2012 has shown that buildings required to maintain a Legionella control plan for their drinking water installation are more likely to test positive for Legionella spp. Than buildings without such a plan (38% versus 22% of samples). To clarify this discrepancy, we analysed the results of mandatory water sample testing conducted as part of risk assessments in 206 buildings in the Netherlands from 2011 to 2015. Of the 6171 samples analysed, 16.2% exceeded the Dutch drinking water standard for Legionella spp. of 100 CFU/litre. In buildings with ≤50 tap points, the average percentage of samples containing ≥100 CFU/litre was 28.2%, and from buildings with >50 tap points, it was 12.2%. Analysis of serial samples (taken every 6 months) from each building showed that 33.2% of all buildings tested positive for at least one sample every 6 months. The overall increase was 4.4% per year. Analysis of Legionella subgroups showed that while the majority of positive samples contained L. non-pneumophila (96.9%), some samples did contain L. pneumophila serogroup 1 (1.0%) and serogroups 2-14 (2.1%). Our data suggest that the Dutch mandatory risk assessment and drinking water management plan is not sufficiently effective in preventing the proliferation of Legionella spp. and may even contribute to proliferation. This analysis should now be expanded to include other areas of the Netherlands in order to understand the geographical differences that we observed in our results, and why smaller buildings appear to be more likely to test positive for Legionella spp.

Stenotrophomonas maltophilia Encodes a VirB/VirD4 Type IV Secretion System That Modulates Apoptosis in Human Cells and Promotes Competition against Heterologous Bacteria, Including Pseudomonas aeruginosa

Stenotrophomonas maltophilia is an emerging opportunistic and nosocomial pathogen. S. maltophilia is also a risk factor for lung exacerbations in cystic fibrosis patients. S. maltophilia attaches to various mammalian cells, and we recently documented that the bacterium encodes a type II secretion system which triggers detachment-induced apoptosis in lung epithelial cells. We have now confirmed that S. maltophilia also encodes a type IVA secretion system (VirB/VirD4 [VirB/D4] T4SS) that is highly conserved among S. maltophilia strains and, looking beyond the Stenotrophomonas genus, is most similar to the T4SS of Xanthomonas To define the role(s) of this T4SS, we constructed a mutant of strain K279a that is devoid of secretion activity due to loss of the VirB10 component. The mutant induced a higher level of apoptosis upon infection of human lung epithelial cells, indicating that a T4SS effector(s) has antiapoptotic activity. However, when we infected human macrophages, the mutant triggered a lower level of apoptosis, implying that the T4SS also elaborates a proapoptotic factor(s). Moreover, when we cocultured K279a with strains of Pseudomonas aeruginosa, the T4SS promoted the growth of S. maltophilia and reduced the numbers of heterologous bacteria, signaling that another effector(s) has antibacterial activity. In all cases, the effect of the T4SS required S. maltophilia contact with its target. Thus, S. maltophilia VirB/D4 T4SS appears to secrete multiple effectors capable of modulating death pathways. That a T4SS can have anti- and prokilling effects on different targets, including both human and bacterial cells, has, to our knowledge, not been seen before.

Legionella pneumophila as a Health Hazard to Miners: A Pilot Study of Water Quality and QMRA

Legionella pneumophila (L. pneumophila), the causative agent of legionellosis, is an aquatic bacterium that grows in warm water. Humans are only presented with a health risk when aerosolized water containing L. pneumophila is inhaled. In mining operations, aerosolized water is used as dust control and as part of the drilling operations, a currently ignored exposure route. This study characterized L. pneumophila concentrations in the mine's non-potable water and the relationship between L. pneumophila and chlorine concentrations. These concentrations informed a quantitative microbial risk assessment (QMRA) model to estimate the infection risk to miners exposed to aerosolized water containing L. pneumophila. Fourteen water samples were collected from seven locations at a mine and analyzed for temperature, pH, chlorine, and L. pneumophila serogroup. Most samples (93%) tested positive for L. pneumophila cells. The faucet from the sprinkler system on the adit level (entrance to the underground mine levels) showed the highest concentration of L. pneumophila (8.35 × 104 MPN/L). Disability adjusted life years (DALYs) were estimated in the QMRA model and showed that the risk for all miners was significantly lower (p < 0.0001) with the ventilation system on than when the system was off. Our study showed that the use of a ventilation system at the adit level of the mine reduced the risk of infection with aerosolized L. pneumophila.

Attenuated Legionella pneumophila Survives for a Long Period in an Environmental Water Site

Legionella pneumophila is known as a human pathogen and is ubiquitous in natural and artificial aquatic environments. Many studies have revealed the virulence traits of L. pneumophila using clinical strains and a number of studies for characterizing environmental strains are also reported. However, the association between the virulence and survivability in the environment is unclear. In the present study, L. pneumophila was isolated from environmental water sites (Ashiyu foot spa, water fountain, and public bath), and the serogroups of isolated strains were determined by serological tests. Isolated strains were found to belong to serogroups SG1, SG2, SG3, SG4, SG5, SG8, SG9, and SG13. Untypeable strains were also obtained. Isolated strains were used for intracellular growth assay in a human monocytic cell line, THP-1. Among these strains, only an untypeable strain, named AY3, failed to replicate in THP-1. In addition, AY3 was maintained for a long period in an environmental water site, Ashiyu foot spa 2. Further, we compared the characteristics of several strains isolated from Ashiyu foot spa 2 and a clinical strain, Togus-1. AY3 failed to replicate in THP-1 cells but replicated in an amoeba model, Dictyostelium discoideum. Compared with Togus-1, the culturable cell number of environmental strains under stress conditions was higher. Moreover, biofilm formation was assessed, and AY3 showed the same degree of biofilm formation as Togus-1. Biofilm formation, replication in amoebae, and resistance against stress factors would explain the predominance of AY3 at one environmental site. Although the mechanism underlying the difference in the ability of AY3 to replicate in THP-1 cells or amoebae is still unclear, AY3 may abandon the ability to replicate in THP-1 cells to survive in one environment for a long period. Understanding the mechanisms of L. pneumophila in replication within different hosts should help in the control of Legionnaires' disease, but further study is necessary.

Migration of Acanthamoeba castellanii Through Legionella Biofilms

The amoeba-resistant bacterium Legionella pneumophila infects humans through aerosols and thereby can cause a life-threatening pneumonia termed Legionnaires' disease. In the environment L. pneumophila forms and colonizes biofilms, which usually comprise complex multispecies communities. In these biofilms L. pneumophila persists and replicates intracellularly in protozoa, such as the amoeba Acanthamoeba castellanii. The interactions between sessile L. pneumophila in biofilms and their natural protozoan hosts are not understood on a molecular level. Here, we describe a method to visualize by confocal microscopy the formation and architecture of mono-species L. pneumophila biofilms. Furthermore, we describe and quantify the migration or "grazing" of A. castellanii in the biofilm. This allows investigating on a molecular and cellular level L. pneumophila biofilm formation and Legionella-amoeba interactions within biofilms.

Long-term persistence of infectious Legionella with free-living amoebae in drinking water biofilms

Prolific growth of pathogenic Legionella pneumophila within engineered water systems and premise plumbing, and human exposure to aerosols containing this bacterium results in the leading health burden of any water-related pathogen in developed regions. Ecologically, free-living amoebae (FLA) are an important group of the microbial community that influence biofilm bacterial diversity in the piped-water environment. Using fluorescent microscopy, we studied in-situ the colonization of L. pneumophila in the presence of two water-related FLA species, Willaertia magna and Acanthamoeba polyphaga in drinking water biofilms. During water flow as well as after periods of long-stagnation, the attachment and colonization of L. pneumophila to predeveloped water-biofilm was limited. Furthermore, W. magna and A. polyphaga showed no immediate interactions with L. pneumophila when introduced to the same natural biofilm environment. A. polyphaga encysted within 5-7 d after introduction to the tap-water biofilms and mostly persisted in cysts till the end of the study period (850 d). W. magna trophozoites, however, exhibited a time delay in feeding on Legionella and were observed with internalized L. pneumophila cells after 3 weeks from their introduction to the end of the study period and supported putative (yet limited) intracellular growth. The culturable L.pneumophila in the bulk water was reduced by 2-log over 2 years at room temperature but increased (without a change in mip gene copies by qPCR) when the temperature was elevated to 40 °C within the same closed-loop tap-water system without the addition of nutrients or fresh water. The overall results suggest that L. pneumophila maintains an ecological balance with FLA within the biofilm environment, and higher temperature improve the viability of L. pneumophila cells, and intracellular growth of Legionella is possibly cell-concentration dependent. Observing the preferential feeding behavior, we hypothesize that an initial increase of FLA numbers through feeding on a range of other available bacteria could lead to an enrichment of L. pneumophila, and later force predation of Legionella by the amoeba trophozoites results in rapid intracellular replication, leading to problematic concentration of L. pneumophila in water. In order to find sustainable control options for legionellae and various other saprozoic, amoeba-resisting bacterial pathogens, this work emphasizes the need for better understanding of the FLA feeding behavior and the range of ecological interactions impacting microbial population dynamics within engineered water systems.

Vermamoeba vermiformis as etiological agent of a painful ulcer close to the eye

In the present article, we report on the identification of Vermamoeba (Hartmannella) vermiformis as the etiological agent of a tissue infection close to the eye of a female patient. Laboratory examination revealed no involvement of any pathogenic bacteria or fungi in the tissue infection. V. vermiformis was identified by cultivation and morphology of trophozoites and cysts as well as phylogenetic analysis of nuclear 18S rDNA. The lesion improved in the course of 4 weeks by application of zinc paste.

Environmental pharmacology: source, impact and solution

Environmental pharmacology is the knowledge, study and the methods implemented for amalgamating the presence of pharmaceutical products and their metabolites in the environment. Pharmaceutical and house care products and their metabolites gain access to the environment through various means and affect the flora and fauna and modulate the ecosystem. The effect on wildlife, biofilms and human are being studied to gain knowledge of sources and causations. Potential risks of development of acute and chronic toxicity, carcinogenicity, interference with hormone and immune systems and drug resistance are of major concern. They may alter the genome and can affect future generations leaving them vulnerable to disease. There are regulations in good manufacturing practices and disposal which take into account the environmental risks but the knowledge for stakeholders and their implementation is very restricted. Ecopharmacology and ecopharmacovigilance are propagators of green healthcare. A strategy towards human health risk assessment and ecotoxicological hazard evaluation must be developed and risk minimization measures to be sought for and applied.

Environmental surveillance of Legionella spp. colonization in the water system of a large academic hospital: Analysis of the four-year results on the effectiveness of the chlorine dioxide disinfection method

The prevention of Legionella colonization of water systems is one of the goals of hospital management. Among chemical disinfection methods, chlorine dioxide (ClO₂) has been largely used to control Legionella spp. in water systems. We performed a retrospective study to analyse data deriving from the surveillance plan of the water system in a large academic hospital in Rome (Italy) during the period August 2011 and August 2018. We collected the data deriving from the routine water samples used to monitor Legionella spp. colonization. Data from the water samples collected from 163 selected sampling points (hot water tanks, the return loop and distal outlets) was analysed using a life table analysis in order to investigate the duration of the effectiveness of the ClO₂ method in eradicating Legionella spp. The colonization of the water sample by Legionella spp. was considered as the outcome. Our results show that in 81,59% of the sampling points Legionella spp. were never detected at four years of follow up. Chemical and physical characteristics of the water were also compared between the samples which were positive for Legionella spp. and those which were not. No association was found between these factors. The knowledge of the duration over time of the effectiveness of the ClO₂ disinfection method could support decision-making processes in the framework of Risk Management activities in hospitals. Future studies could also be conducted in hospitals to compare the long-term cost-effectiveness of different Legionella spp. colonization prevention methods.

Vermamoeba vermiformis: a Free-Living Amoeba of Interest

Free-living amoebae are protists that are widely distributed in the environment including water, soil, and air. Although the amoebae of the genus Acanthamoeba are still the most studied, other species, such as Vermamoeba vermiformis (formerly Hartmannella vermiformis), are the subject of increased interest. Found in natural or man-made aquatic environments, V. vermiformis can support the multiplication of other microorganisms and is able to harbor and potentially protect pathogenic bacteria or viruses. This feature is to be noted because of the presence of this thermotolerant amoeba in hospital water networks. As a consequence, this protist could be implicated in health concerns and be indirectly responsible for healthcare-related infections. This review highlights, among others, the consequences of V. vermiformis relationships with other microorganisms and shows that this free-living amoeba species is therefore of interest for public health.

Microbial Ecology and Water Chemistry Impact Regrowth of Opportunistic Pathogens in Full-Scale Reclaimed Water Distribution Systems

Need for global water security has spurred growing interest in wastewater reuse to offset demand for municipal water. While reclaimed (i.e., nonpotable) microbial water quality regulations target fecal indicator bacteria, opportunistic pathogens (OPs), which are subject to regrowth in distribution systems and spread via aerosol inhalation and other noningestion routes, may be more relevant. This study compares the occurrences of five OP gene markers ( Acanthamoeba spp., Legionella spp., Mycobacterium spp., Naegleria fowleri, Pseudomonas aeruginosa) in reclaimed versus potable water distribution systems and characterizes factors potentially contributing to their regrowth. Samples were collected over four sampling events at the point of compliance for water exiting treatment plants and at five points of use at four U.S. utilities bearing both reclaimed and potable water distribution systems. Reclaimed water systems harbored unique water chemistry (e.g., elevated nutrients), microbial community composition, and OP occurrence patterns compared to potable systems examined here and reported in the literature. Legionella spp. genes, Mycobacterium spp. genes, and total bacteria, represented by 16S rRNA genes, were more abundant in reclaimed than potable water distribution system samples ( p ≤ 0.0001). This work suggests that further consideration should be given to managing reclaimed water distribution systems with respect to nonpotable exposures to OPs.

Insights into the long-term persistence of Legionella in facilities from whole-genome sequencing

We investigated the value of whole-genome sequencing (WGS) and single nucleotide polymorphism (SNP) analyses in determining the relationships among and evolutionary rates of Legionella species with long-term persistence in three healthcare facilities. We examined retrospective clinical and environmental isolates of Legionella micdadei and Legionella pneumophila serogroup 1 isolates with identical PFGE DNA fingerprints sampled over the course of up to 18 years. WGS analyses demonstrated that heterogeneous populations of Legionella were present within each facility despite displaying the same PFGE profiles. Additionally, clustering of some clinical isolates with those from a separate but related institution exposed a source of infection not previously detected, underscoring the importance of considering phylogenetic relationships when assessing epidemiological links. The data supported an average substitution rate of 0.80 SNPs per genome per year for L. micdadei but a reliable estimate for L. pneumophila serogroup 1 could not be obtained due to complicating factors such as non-chronological links among isolates and inadequate sampling depths. While the substitution rate for L. micdadei is consistent with previous estimates for L. pneumophila, the lack of a temporal signal in our sequence data for L. pneuomphila serogroup 1 isolates suggests either insufficient change to provide an estimate or variable evolutionary rates, which could reflect the presence of both actively dividing and viable but non-culturable Legionella spp. in the built environment. This study highlights the increased discriminatory power of WGS SNP analysis as compared to PFGE, emphasizes the need for extended sampling, and provides insight into the evolution of Legionella from longitudinal investigations.

Fouling Development in A/O-MBR under Low Organic Loading Condition and Identification of Key Bacteria for Biofilm Formations

Membrane fouling in membrane bioreactors (MBR) remains a major issue and knowledge of microbes associated with biofilm formation might facilitate the control of this phenomenon, Thus, an anoxic/oxic membrane bioreactor (A/O-MBR) was operated under an extremely low organic loading rate (0.002 kg-COD·m⁻³·day⁻¹) to induce membrane fouling and the major biofilm-forming bacteria were identified. After operation under extremely low organic loading condition, the reactor showed accumulation of total nitrogen and phosphorus along with biofilm development on the membrane surface. Thus, membrane fouling induced by microbial cell lysis was considered to have occurred. Although no major changes were observed in the microbial community structure of the activated sludge in the MBR before and after membrane fouling, uncultured bacteria were specifically increased in the biofilm. Therefore, bacteria belonging to candidate phyla including TM6, OD1 and Gammaproteobacteria could be important biofilm-forming bacteria.

Legionella spp. Risk Assessment in Recreational and Garden Areas of Hotels

Several Travel-associated Legionnaires’ disease (TALD) cases occur annually in Europe. Except from the most obvious sites (cooling towers and hot water systems), infections can also be associated with recreational, water feature, and garden areas of hotels. This argument is of great interest to better comprehend the colonization and to calculate the risk to human health of these sites. From July 2000–November 2017, the public health authorities of the Island of Crete (Greece) inspected 119 hotels associated with TALD, as reported through the European Legionnaires’ Disease Surveillance Network. Five hundred and eighteen samples were collected from decorative fountain ponds, showers near pools and spas, swimming pools, spa pools, garden sprinklers, drip irrigation systems (reclaimed water) and soil. Of those, 67 (12.93%), originating from 43 (35.83%) hotels, tested positive for Legionella (Legionella pneumophila serogroups 1, 2, 3, 6, 7, 8, 13, 14, 15 and non-pneumophila species (L. anisa, L. erythra, L. taurinensis, L. birminghamensis, L. rubrilucens). A Relative Risk (R.R.) > 1 (p < 0.0001) was calculated for chlorine concentrations of less than 0.2 mg/L (R.R.: 54.78), star classification (<4) (R.R.: 4.75) and absence of Water Safety Plan implementation (R.R.: 3.96). High risk (≥10⁴ CFU/L) was estimated for pool showers (16.42%), garden sprinklers (7.46%) and pool water (5.97%).

Factors Mediating Environmental Biofilm Formation by Legionella pneumophila

Legionella pneumophila (L. pneumophila) is an opportunistic waterborne pathogen and the causative agent for Legionnaires' disease, which is transmitted to humans via inhalation of contaminated water droplets. The bacterium is able to colonize a variety of man-made water systems such as cooling towers, spas, and dental lines and is widely distributed in multiple niches, including several species of protozoa In addition to survival in planktonic phase, L. pneumophila is able to survive and persist within multi-species biofilms that cover surfaces within water systems. Biofilm formation by L. pneumophila is advantageous for the pathogen as it leads to persistence, spread, resistance to treatments and an increase in virulence of this bacterium. Furthermore, Legionellosis outbreaks have been associated with the presence of L. pneumophila in biofilms, even after the extensive chemical and physical treatments. In the microbial consortium-containing L. pneumophila among other organisms, several factors either positively or negatively regulate the presence and persistence of L. pneumophila in this bacterial community. Biofilm-forming L. pneumophila is of a major importance to public health and have impact on the medical and industrial sectors. Indeed, prevention and removal protocols of L. pneumophila as well as diagnosis and hospitalization of patients infected with this bacteria cost governments billions of dollars. Therefore, understanding the biological and environmental factors that contribute to persistence and physiological adaptation in biofilms can be detrimental to eradicate and prevent the transmission of L. pneumophila. In this review, we focus on various factors that contribute to persistence of L. pneumophila within the biofilm consortium, the advantages that the bacteria gain from surviving in biofilms, genes and gene regulation during biofilm formation and finally challenges related to biofilm resistance to biocides and anti-Legionella treatments.

Effect of temperature and colonization of Legionella pneumophila and Vermamoeba vermiformis on bacterial community composition of copper drinking water biofilms

It is unclear how the water-based pathogen, Legionella pneumophila (Lp), and associated free-living amoeba (FLA) hosts change or are changed by the microbial composition of drinking water (DW) biofilm communities. Thus, this study characterized the bacterial community structure over a 7-month period within mature (> 600-day-old) copper DW biofilms in reactors simulating premise plumbing and assessed the impact of temperature and introduction of Lp and its FLA host, Vermamoeba vermiformis (Vv), co-cultures (LpVv). Sequence and quantitative PCR (qPCR) analyses indicated a correlation between LpVv introduction and increases in Legionella spp. levels at room temperature (RT), while at 37°C, Lp became the dominant Legionella spp. qPCR analysis suggested Vv presence may not be directly associated with Lp biofilm growth at RT and 37°C, but may contribute to or be associated with non-Lp legionellae persistence at RT. Two-way PERMANOVA and PCoA revealed that temperature was a major driver of microbiome diversity. Biofilm community composition also changed over the seven-month period and could be associated with significant shifts in dissolved oxygen, alkalinity and various metals in the influent DW. Hence, temperature, biofilm age, DW quality and transient intrusions/amplification of pathogens and FLA hosts may significantly impact biofilm microbiomes and modulate pathogen levels over extended periods.

A human time dose response model for Q fever

The causative agent of Q fever, Coxiella burnetii, has the potential to be developed for use in biological warfare and it is classified as a bioterrorism threat agent by the Centers for Disease Control and Prevention (CDC) and as a category B select agent by the National Institute of Allergy and Infectious Diseases (NIAID). In this paper we focus on the in-host properties that arise when an individual inhales a dose of C. burnetii and establish a human time-dose response model. We also propagate uncertainty throughout the model allowing us to robustly estimate key properties including the infectious dose and incubation period. Using human study data conducted in the 1950's we conclude that the dose required for a 50% probability of infection is about 15 organisms, and that one inhaled organism of C. burnetti can cause infection in 5% of the exposed population. In addition, we derive a low dose incubation period of 17.6 days and an extracellular doubling time of half a day. In conclusion this paper provides a framework for detailing the parameters and approaches that would be required for risk assessments associated with exposures to C. burnetii that might cause human infection.

Bacterial adhesion capacity on food service contact surfaces

The aim of this study was to analyse the adhesion of E. coli, P. aeruginosa and S. aureus on food contact materials, such as polyethylene terephthalate, silicone, aluminium, Teflon and glass. Surface roughness, streaming potential and contact angle were measured. Bacterial properties by contact angle and specific charge density were characterised. The bacterial adhesion analysis using staining method and scanning electron microscopy showed the lowest adhesion on smooth aluminium and hydrophobic Teflon for most of the bacteria. However, our study indicates that hydrophobic bacteria with high specific charge density attach to those surfaces more intensively. In food services, safety could be increased by selecting material with low adhesion to prevent cross contamination.

Biofilm Composition and Threshold Concentration for Growth of Legionella pneumophila on Surfaces Exposed to Flowing Warm Tap Water without Disinfectant

Legionella pneumophila in potable water installations poses a potential health risk, but quantitative information about its replication in biofilms in relation to water quality is scarce. Therefore, biofilm formation on the surfaces of glass and chlorinated polyvinyl chloride (CPVC) in contact with tap water at 34 to 39°C was investigated under controlled hydraulic conditions in a model system inoculated with biofilm-grown L. pneumophila The biofilm on glass (average steady-state concentration, 23 ± 9 pg ATP cm-2) exposed to treated aerobic groundwater (0.3 mg C liter-1; 1 μg assimilable organic carbon [AOC] liter-1) did not support growth of the organism, which also disappeared from the biofilm on CPVC (49 ± 9 pg ATP cm-2) after initial growth. L. pneumophila attained a level of 4.3 log CFU cm-2 in the biofilms on glass (1,055 ± 225 pg ATP cm-2) and CPVC (2,755 ± 460 pg ATP cm-2) exposed to treated anaerobic groundwater (7.9 mg C liter-1; 10 μg AOC liter-1). An elevated biofilm concentration and growth of L. pneumophila were also observed with tap water from the laboratory. The Betaproteobacteria Piscinibacter and Methyloversatilis and amoeba-resisting Alphaproteobacteria predominated in the clones and isolates retrieved from the biofilms. In the biofilms, the Legionella colony count correlated significantly with the total cell count (TCC), heterotrophic plate count, ATP concentration, and presence of Vermamoeba vermiformis This amoeba was rarely detected at biofilm concentrations of <100 pg ATP cm-2 A threshold concentration of approximately 50 pg ATP cm-2 (TCC = 1 × 106 to 2 × 106 cells cm-2) was derived for growth of L. pneumophila in biofilms.IMPORTANCELegionella pneumophila is the etiologic agent in more than 10,000 cases of Legionnaires' disease that are reported annually worldwide and in most of the drinking water-associated disease outbreaks reported in the United States. The organism proliferates in biofilms on surfaces exposed to warm water in engineered freshwater installations. An investigation with a test system supplied with different types of warm drinking water without disinfectant under controlled hydraulic conditions showed that treated aerobic groundwater (0.3 mg liter-1 of organic carbon) induced a low biofilm concentration that supported no or very limited growth of L. pneumophila Elevated biofilm concentrations and L. pneumophila colony counts were observed on surfaces exposed to two types of extensively treated groundwater, containing 1.8 and 7.9 mg C liter-1 and complying with the microbial water quality criteria during distribution. Control measures in warm tap water installations are therefore essential for preventing growth of L. pneumophila.

Prevalence of Legionella species isolated from shower water in public bath facilities in Toyama Prefecture, Japan

AIMS

We investigated the prevalence of Legionella spp. isolated from shower water in public bath facilities in Toyama Prefecture, Japan. In addition, we analyzed the genetic diversity among Legionella pneumophila isolates from shower water as well as the genetic relationship between isolates from shower water and from stock strains previously analyzed from sputum specimens.

METHODS

The isolates were characterized using serogrouping, 16S rRNA gene sequencing, and sequence-based typing.

RESULTS

Legionella spp. were isolated from 31/91 (34.1%) samples derived from 17/37 (45.9%) bath facilities. Isolates from shower water and bath water in each public bath facility were serologically or genetically different, indicating that we need to isolate several L. pneumophila colonies from both bath and shower water to identify public bath facilities as sources of legionellosis. The 61 L. pneumophila isolates from shower water were classified into 39 sequence types (STs) (index of discrimination = 0.974), including 19 new STs. Among the 39 STs, 12 STs match clinical isolates in the European Working Group for Legionella Infections database. Notably, ST505 L. pneumophila SG 1, a strain frequently isolated from patients with legionellosis and from bath water in this area, was isolated from shower water.

CONCLUSIONS

Pathogenic L. pneumophila strains including ST505 strain were widely distributed in shower water in public bath facilities, with genetic diversity showing several different origins. This study highlights the need to isolate several L. pneumophila colonies from both bath water and shower water to identify public bath facilities as infection sources in legionellosis cases.

Multiplication of Legionella pneumophila Sequence Types 1, 47, and 62 in Buffered Yeast Extract Broth and Biofilms Exposed to Flowing Tap Water at Temperatures of 38°C to 42°C

Legionella pneumophila proliferates in freshwater environments at temperatures ranging from 25 to 45°C. To investigate the preference of different sequence types (ST) for a specific temperature range, growth of L. pneumophila serogroup 1 (SG1) ST1 (environmental strains), ST47, and ST62 (disease-associated strains) was measured in buffered yeast extract broth (BYEB) and biofilms grown on plasticized polyvinyl chloride in flowing heated drinking water originating from a groundwater supply. The optimum growth temperatures in BYEB were approximately 37°C (ST1), 39°C (ST47), and 41°C (ST62), with maximum growth temperatures of 42°C (ST1) and 43°C (ST47 and ST62). In the biofilm at 38°C, the ST47 and ST62 strains multiplied equally well compared to growth of the environmental ST1 strain and an indigenous L. pneumophila non-SG1 strain, all attaining a concentration of approximately 10⁷ CFU/cm^-2 Raising the temperature to 41°C did not impact these levels within 4 weeks, but the colony counts of all strains tested declined (at a specific decline rate of 0.14 to 0.41 day^-1) when the temperature was raised to 42°C. At this temperature, the concentration of Vermamoeba vermiformis in the biofilm, determined with quantitative PCR (qPCR), was about 2 log units lower than the concentration at 38°C. In columns operated at a constant temperature, ranging from 38 to 41°C, none of the tested strains multiplied in the biofilm at 41°C, in which also V. vermiformis was not detected. These observations suggest that strains of ST47 and ST62 did not multiply in the biofilm at a temperature of ≥41°C because of the absence of a thermotolerant host.

IMPORTANCE

Growth of Legionella pneumophila in tap water installations is a serious public health concern. The organism includes more than 2,100 varieties (sequence types). More than 50% of the reported cases of Legionnaires' disease are caused by a few sequence types which are very rarely detected in the environment. Strains of selected virulent sequence types proliferated in biofilms on surfaces exposed to warm (38°C) tap water to the same level as environmental varieties and multiplied well as pure culture in a nutrient-rich medium at temperatures of 42 and 43°C. However, these organisms did not grow in the biofilms at temperatures of ≥41°C. Typical host amoebae also did not multiply at these temperatures. Apparently, proliferation of thermotolerant host amoebae is needed to enable multiplication of the virulent L. pneumophila strains in the environment at elevated temperatures. The detection of these amoebae in water installations therefore is a scientific challenge with practical implications.