Occurrence and Control of Legionella in Recycled Water Systems

A synergistic anti-bacterial and anti-adhesion activity of tea tree (Melaleuca alternifolia) and lemon eucalyptus tree (Eucalyptus citriodora Hook) essential oils on Legionella pneumophila

Legionella pneumophila is a Gram-negative bacterial pathogen that colonizes natural and artificial water systems and has the ability to form a biofilm. The biofilm protects L. pneumophila from various environmental factors and makes it more resistant to chlorine-based disinfectants. This study investigated the anti-bacterial properties of tea tree (Melaleuca alternifolia (Maiden and Betche) Cheel) oil and lemon eucalyptus tree (Eucalyptus citriodora Hook) essential oils (EOs) and their synergistic, additive inhibitory and anti-adhesive effects against L. pneumophila biofilm formation on polystyrene. The minimum effective concentration (MEC) for tea tree is 12.8 mg ml-1 and for lemon eucalyptus tree EO 6.4 mg ml-1. In the checkerboard assay, different combinations of these two EO show synergistic and additive anti-microbial activity. The minimum anti-adhesive concentration (MAC) for tea tree is 12.8 mg ml-1 and for lemon eucalyptus tree EO 6.4 mg ml-1. A combination of 3.2 mg ml-1 tea tree EO and 0.8 mg ml-1 lemon eucalyptus tree EO showed the strongest anti-adhesive effect against L. pneumophila on polystyrene. The tested oils and their combination showed intriguing potential to inhibit L. pneumophila biofilm formation.

Numerical simulation for spatial distribution of water aerosol produced from nozzle spray and health risk related to Legionella pneumophila in spray scenarios

Water spray facilities are widely used in public places for sprinkling or beautifying the environment. However, the potential health risk induced by water aerosols increasingly calls for attention. In this study, the spatial distribution of water aerosols was investigated through the molecular sieve adsorption method, and predicted by discrete phase model (DPM). On this basis, the health risk regarding Legionella pneumophila for specific spray scenarios was evaluated by quantitative microbial risk assessment (QMRA). The results showed that the original droplet size can be described by the Rosin_Rommaler distribution (R²>0.99). The spatial distribution of water aerosols produced from a nozzle spray can be well predicted by the DPM. The concentration of water aerosols showed a sharp decline within 5 m from the nozzle and was not significantly different within 5 m (p>0.05) as for various spray scenarios. However, the difference was significant outside 5 m (p<0.05). Furthermore, a safe contact distance of exceeding 8 m is proposed in spray scenarios considering the risk threshold of 0.0001. Sensitivity analysis demonstrated the concentration of Legionella pneumophila in water aerosols as the critical factor affecting the health risk.

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.

Bioaerosol in a typical municipal wastewater treatment plant: concentration, size distribution, and health risk assessment

An investigation on bioaerosol in a wastewater treatment plant (WWTP) located in Xi'an, China, was conducted to understand the characteristics of bioaerosol released from wastewater and sludge treatment facilities because the bioaerosols may pose a threat to human health. Using the Andersen impactor sampler collection and colony-counting method, bioaerosol concentrations and size distributions were detected. The risk quotient method was used to evaluate the health risks associated with inhalation of bioaerosol for WWTP staff, based on the average daily dose rates of exposure. The health risk in relation to Legionella pneumophila was quantitatively calculated using quantitative microbial risk assessment (QMRA), based on the assumption of the percentage. The maximum concentration of airborne bacteria (3,767 ± 280 colony forming units (CFU)/m³) and fungi (8,775 ± 406 CFU/m³) occurred from the aerated grit chamber and sludge thickening house, respectively, which all exceeded 500 CFU/m³ as the acceptable guideline proposed by the American Conference of Governmental Industrial Hygienists. The particle size of airborne bacteria was mainly distributed in the first three stages (>3.3 µm), while that of airborne fungi was from the second to the fourth stage (2.1-7.0 µm). The hazard index exposure to bioaerosol for adult males and females by inhalation were higher than 1. The proportion of L. pneumophila should be strictly controlled below 10^-8, based on the QMRA approach.

Bacterial viability and diversity in a landscape lake replenished with reclaimed water: a case study in Xi'an, China

To understand the characteristics of bacterial viability and diversity in landscape waters replenished with reclaimed water, the typical landscape lake using reclaimed water was investigated in this study. Samples were collected from a reclaimed water inlet (P1), a reclaimed water distribution outlet (P2), and a landscape lake replenished by reclaimed water (P3). By means of measuring adenosine triphosphate (ATP), flow cytometry (FCM), and 16S rRNA gene high-throughput sequencing, the bacterial viability and diversity in reclaimed water distribution system and landscape lake were illustrated. The bacterial ATP contents at P1, P2, and P3 were 3.55 ± 1.79 ng/L, 3.31 ± 1.43 ng/L, and 18.97 ± 6.39 μg/L, and the intact bacterial cell concentrations were 5.91 ± 0.52 × 10⁴ cells/mL, 7.95 ± 2.58 × 10⁴ cells/mL, and 5.65 ± 2.10 × 10⁶ cells/mL, respectively. These results indicated a significant increase of bacterial viability in the landscape lake. The Shannon diversity index of 6.535, 7.05, and 6.886 at P1, P2, and P3, respectively, demonstrated no notable change of bacterial diversity from reclaimed water distribution system to landscape lake. However, the relative abundance of Pseudomonas sp. at P3 was significantly higher than that at P1. These findings indicated that viable but non-culturable (VBNC) bacteria could be revived in the landscape lake. The bacterial viability during reclaimed water reuse should deserve special attention.

Legionella pneumophila and Protozoan Hosts: Implications for the Control of Hospital and Potable Water Systems

Legionella pneumophila is an opportunistic waterborne pathogen of public health concern. It is the causative agent of Legionnaires’ disease (LD) and Pontiac fever and is ubiquitous in manufactured water systems, where protozoan hosts and complex microbial communities provide protection from disinfection procedures. This review collates the literature describing interactions between L. pneumophila and protozoan hosts in hospital and municipal potable water distribution systems. The effectiveness of currently available water disinfection protocols to control L. pneumophila and its protozoan hosts is explored. The studies identified in this systematic literature review demonstrated the failure of common disinfection procedures to achieve long term elimination of L. pneumophila and protozoan hosts from potable water. It has been demonstrated that protozoan hosts facilitate the intracellular replication and packaging of viable L. pneumophila in infectious vesicles; whereas, cyst-forming protozoans provide protection from prolonged environmental stress. Disinfection procedures and protozoan hosts also facilitate biogenesis of viable but non-culturable (VBNC) L. pneumophila which have been shown to be highly resistant to many water disinfection protocols. In conclusion, a better understanding of L. pneumophila-protozoan interactions and the structure of complex microbial biofilms is required for the improved management of L. pneumophila and the prevention of LD.

Detection and occurrence of indicator organisms and pathogens

This review paper focuses on detection and quantification techniques of indicator organisms that can be used for water quality assessment. The environmental pathogens that are critical to understand and better evaluate water quality are also discussed in this paper. Several recent studies using culture-independent methods such as microbial source tracking, pulsed field gel electrophoresis, mitochondrial DNA, and next generation sequencing to assess various environmental samples and water bodies have been reviewed. PRACTITIONER POINTS: Various waterborne pathogens and cases of outbreak occurances due to presence of pathogens are studied in this review paper. Recent studies for detecting major indicator organisms to evaluate the presence of pathogens in water bodies are reviewed. Culture-independent techniques as robust tools to detect and quantify waterborne pathogens are discussed in this review paper.

Isolation and Identification of Legionella spp. in environmental water sources based on macrophage infectivity potentiator (mip) gene sequencing in southwest Iran

Legionella species are widespread in natural water sources and man-made aqueous environments, as well as fresh-water. The present study was conducted owing to the lack of research regarding the prevalence of Legionella spp in the water sources of Ahvaz city in southwest Iran. In this study the macrophage infectivity potentiator (mip) gene sequencing was used for identification of various Legionella species isolated from different water sources. In this study, 144 water samples were collected and inoculated on the buffered charcoal-yeast extract (BCYE) agar and modified Wadowsky-Yee (MWY) medium. The DNA was extracted from positive cultures. The Legionella species were confirmed by amplifying a 654 bp fragment of the 16S rRNA gene. The mip gene of all isolates were amplified by PCR and purified for sequencing. The mip gene sequences were analyzed by jPHYDIT software version 1. The results showed a 13.9% (20/144) prevalence of Legionella spp. in water sources of Ahvaz city, southwest Iran. Analyzing of the mip gene sequences showed, out of 20 Legionella isolates, 13 isolates (54.1%) were positive for L. pneumophila, 5 isolates (20.8%) were positive for L. worsleinsis, one isolates for each one of L. dumoffi and L. fairfieldensis, (4.1%). According to our research, the occurrence of Legionella spp in water sources could be a hazard for the health systems especially in the hospitals. The regular monitoring of these water sources by health planners may therefore be useful for decreasing the risk for Legionella spp. infections.

Bactericidal efficacies of food additive grade calcium hydroxide toward Legionella pneumophila

Food additive grade calcium hydroxide (FdCa(OH)₂) in the solution of 0.17% was evaluated for its bactericidal efficacies toward Legionella pneumophila with or without sodium hypochlorite (NaOCl) at a concentration of 200 ppm total residual chlorine, at room temperature (RT) (25°C ± 2°C) and 42°C, either with or without 5% fetal bovine serum (FBS). Besides, FdCa(OH)₂ in different concentration solutions were prepared in field water samples (hot spring and bath tab water) and evaluated for their bactericidal efficacies at 42°C. FdCa(OH)₂ (0.17%) inactivated the L. pneumophila to the undetectable level (<2.6 log CFU/ml) within 5 min and 3 min, respectively, at RT and 42°C, with 5% FBS. At RT and 42°C, NaOCl inactivated L. pneumophila to the undetectable level within 5 min, without 5% FBS, but with 5% FBS, it could only inactivate this bacterium effectively (≥3 log reductions). Conversely, at RT and 42°C, the mixture of 0.17% FdCa(OH)₂ and 200 ppm NaOCl could inactivate L. pneumophila to the undetectable level, respectively, within 3 min and 1 min, even with 5% FBS, and it was elucidated that FdCa(OH)₂ has a synergistic bactericidal effect together with NaOCl. FdCa(OH)₂ 0.05% solution prepared in hot spring water could inactivate L. pneumophila to the undetectable within 3 min at 42°C. So, FdCa(OH)₂ alone could show nice bactericidal efficacy at 42°C, even with 5% FBS, as well as in field water samples.

Identification of MazF Homologue in Legionella pneumophila Which Cleaves RNA at the AACU Sequence

MazF is a sequence-specific endoribonuclease that is widely conserved in bacteria and archaea. Here, we found an MazF homologue (MazF-lp; LPO-p0114) in Legionella pneumophila. The mazF-lp gene overlaps 14 base pairs with the upstream gene mazE-lp (MazE-lp; LPO-p0115). The induction of mazF-lp caused cell growth arrest, while mazE-lp co-induction recovered cell growth in Escherichia coli. In vivo and in vitro primer extension experiments showed that MazF-lp is a sequence-specific endoribonuclease cleaving RNA at AACU. The endoribonuclease activity of purified MazF-lp was inhibited by purified MazE-lp. We found that MazE-lp and the MazEF-lp complex specifically bind to the palindromic sequence present in the 5'-untranslated region of the mazEF-lp operon. MazE-lp and MazEF-lp both likely function as a repressor for the mazEF-lp operon and for other genes, including icmR, whose gene product functions as a secretion chaperone for the IcmQ pore-forming protein, by specifically binding to the palindromic sequence in 5'-UTR of these genes.

Legionella occurrence in municipal and industrial wastewater treatment plants and risks of reclaimed wastewater reuse: Review

Wastewater treatment plants (WWTPs) have been identified as confirmed but until today underestimated sources of Legionella, playing an important role in local and community cases and outbreaks of Legionnaires' disease. In general, aerobic biological systems provide an optimum environment for the growth of Legionella due to high organic nitrogen and oxygen concentrations, ideal temperatures and the presence of protozoa. However, few studies have investigated the occurrence of Legionella in WWTPs, and many questions in regards to the interacting factors that promote the proliferation and persistence of Legionella in these treatment systems are still unanswered. This critical review summarizes the current knowledge about Legionella in municipal and industrial WWTPs, the conditions that might support their growth, as well as control strategies that have been applied. Furthermore, an overview of current quantification methods, guidelines and health risks associated with Legionella in reclaimed wastewater is also discussed in depth. A better understanding of the conditions promoting the occurrence of Legionella in WWTPs will contribute to the development of improved wastewater treatment technologies and/or innovative mitigation approaches to minimize future Legionella outbreaks.

Reduced Chlorine in Drinking Water Distribution Systems Impacts Bacterial Biodiversity in Biofilms

In drinking water distribution systems (DWDS), a disinfectant residual is usually applied to limit bacterial regrowth. However, delivering water with no or reduced chlorine residual could potentially decrease the selection for antimicrobial resistant microorganisms, favor bacterial regrowth and result in changes in bacterial populations. To evaluate the feasibility of water reduction in local DWDS while ensuring water safety, water quality was measured over 2 months in two different networks, each of them harboring sub-areas with normal and reduced chlorine. Water quality remained good in chlorine reduced samples, with limited development of total flora and absence of coliforms. Furthermore, 16S rRNA amplicon-based metagenomics was used to investigate the diversity and the composition of microbial communities in the sub-networks. Taxonomic classification of sequence reads showed a reduced bacterial diversity in sampling points with higher chlorine residuals. Chlorine disinfection created more homogeneous bacterial population, dominated by Pseudomonas, a genus that contains some major opportunistic pathogens such as P. aeruginosa. In the absence of chlorine, a larger and unknown biodiversity was unveiled, also highlighted by a decreased rate of taxonomic classification to the genus and species level. Overall, this experiment in a functional DWDS will facilitate the move toward potable water delivery systems without residual disinfectants and will improve water taste for consumers.

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.

Risk of infection from Legionella associated with spray irrigation of reclaimed water

Legionella pneumophila has been detected in reclaimed water used for spray irrigation of turfgrass in public parks and golf courses. This study determined the risks of infection from exposure to various levels of Legionella in reclaimed waters considering: the method of spray application; and the duration and frequency of exposure. Evaluation of these factors resulted in a risk of infection greater than 1:10,000 for several scenarios when the number of Legionella in the reclaimed water exceeded 1000 colony-forming units (CFU) per ml. Most current guidelines for control of Legionella in distribution systems recommend that increased monitoring or remedial action be taken when Legionella levels exceed 1000 to 10,000 CFU/ml. Based upon our risk assessment, these guidelines seem appropriate for reclaimed water systems where spray irrigation is practiced.

Health risks from exposure to Legionella in reclaimed water aerosols: Toilet flushing, spray irrigation, and cooling towers

The use of reclaimed water brings new challenges for the water industry in terms of maintaining water quality while increasing sustainability. Increased attention has been devoted to opportunistic pathogens, especially Legionella pneumophila, due to its growing importance as a portion of the waterborne disease burden in the United States. Infection occurs when a person inhales a mist containing Legionella bacteria. The top three uses for reclaimed water (cooling towers, spray irrigation, and toilet flushing) that generate aerosols were evaluated for Legionella health risks in reclaimed water using quantitative microbial risk assessment (QMRA). Risks are compared using data from nineteen United States reclaimed water utilities measured with culture-based methods, quantitative PCR (qPCR), and ethidium-monoazide-qPCR. Median toilet flushing annual infection risks exceeded 10^-4 considering multiple toilet types, while median clinical severity infection risks did not exceed this value. Sprinkler and cooling tower risks varied depending on meteorological conditions and operational characteristics such as drift eliminator performance. However, the greatest differences between risk scenarios were due to 1) the dose response model used (infection or clinical severity infection) 2) population at risk considered (residential or occupational) and 3) differences in laboratory analytical method. Theoretical setback distances necessary to achieve a median annual infection risk level of 10^-4 are proposed for spray irrigation and cooling towers. In both cooling tower and sprinkler cases, Legionella infection risks were non-trivial at potentially large setback distances, and indicate other simultaneous management practices could be needed to manage risks. The sensitivity analysis indicated that the most influential factors for variability in risks were the concentration of Legionella and aerosol partitioning and/or efficiency across all models, highlighting the importance of strategies to manage Legionella occurrence in reclaimed water.

Legionella - A threat to groundwater: Pathogen transport in recharge basin

This study elucidates the potential risk posed by Legionella during aquifer recharge practices. Experiments were conducted using pilot-scale column simulating infiltration of bacterial surrogate and pathogen, E. coli and Legionella pneumophila, under central Arizona recharge basin conditions. A column was packed with a loamy sand media collected from a recharge basin and was fitted with six sampling ports at soil depths of 15, 30, 60, 92, 122cm and acclimated for a month with tertiary treated wastewater. Transport of Legionella appeared to be delayed compared to E. coli. The breakthrough of E. coli and Legionella at 122cm depth occurred at 3 and 24h, respectively. Slow transport of Legionella is consistent with its pleomorphic nature and variation in size and shape under low nutrient conditions. Legionella persisted for a longer time in the column, but at lower concentrations. Given the novel results of this study, the transport of Legionella into groundwater aquifers can occur through engineering recharge basin conditions creating a potential public health risk.

Natural Pathogen Control Chemistry to Replace Toxic Treatment of Microbes and Biofilm in Cooling Towers

Application of toxic antibacterial agents is considered necessary to control prevalent fresh water microorganisms that grow in evaporative cooling water systems, but can adversely affect the environment and human health. However, natural antibacterial water chemistry has been applied in industrial cooling water systems for over 10 years to inhibit microorganisms with excellent results. The water chemistry method concentrates natural minerals in highly-softened water to produce elevated pH and dissolved solids, while maintaining low calcium and magnesium content. The method provides further benefits in water conservation, and generates a small volume of non-toxic natural salt concentrate for cost efficient separation and disposal if required. This report describes the antimicrobial effects of these chemistry modifications in the cooling water environment and the resultant collective inhibition of microbes, biofilm, and pathogen growth. This article also presents a novel perspective of parasitic microbiome functional relationships, including "Trojan Protozoans" and biofilms, and the function of polyvalent metal ions in the formation and inhibition of biofilms. Reducing global dependence on toxic antibacterial agents discharged to the environment is an emerging concern due to their impact on the natural microbiome, plants, animals and humans. Concurrently, scientists have concluded that discharge of antibacterial agents plays a key role in development of pathogen resistance to antimicrobials as well as antibiotics. Use of natural antibacterial chemistry can play a key role in managing the cooling water environment in a more ecologically sustainable manner.

Acanthamoeba containing endosymbiotic chlamydia isolated from hospital environments and its potential role in inflammatory exacerbation

BACKGROUND

Environmental chlamydiae belonging to the Parachlamydiaceae are obligate intracellular bacteria that infect Acanthamoeba, a free-living amoeba, and are a risk for hospital-acquired pneumonia. However, whether amoebae harboring environmental chlamydiae actually survive in hospital environments is unknown. We therefore isolated living amoebae with symbiotic chlamydiae from hospital environments.

RESULTS

One hundred smear samples were collected from Hokkaido University Hospital, Sapporo, Japan; 50 in winter (February to March, 2012) and 50 in summer (August, 2012), and used for the study. Acanthamoebae were isolated from the smear samples, and endosymbiotic chlamydial traits were assessed by infectivity, cytokine induction, and draft genomic analysis. From these, 23 amoebae were enriched on agar plates spread with heat-killed Escherichia coli. Amoeba prevalence was greater in the summer-collected samples (15/30, 50%) than those of the winter season (8/30, 26.7%), possibly indicating a seasonal variation (p = 0.096). Morphological assessment of cysts revealed 21 amoebae (21/23, 91%) to be Acanthamoeba, and cultures in PYG medium were established for 11 of these amoebae. Three amoebae contained environmental chlamydiae; however, only one amoeba (Acanthamoeba T4) with an environmental chlamydia (Protochlamydia W-9) was shown the infectious ability to Acanthamoeba C3 (reference amoebae). While Protochlamydia W-9 could infect C3 amoeba, it failed to replicate in immortal human epithelial, although exposure of HEp-2 cells to living bacteria induced the proinflammatory cytokine, IL-8. Comparative genome analysis with KEGG revealed similar genomic features compared with other Protochlamydia genomes (UWE25 and R18), except for a lack of genes encoding the type IV secretion system. Interestingly, resistance genes associated with several antibiotics and toxic compounds were identified.

CONCLUSION

These findings are the first demonstration of the distribution in a hospital of a living Acanthamoeba carrying an endosymbiotic chlamydial pathogen.

Measuring water ingestion from spray exposures

Characterisation of exposure levels is an essential requirement of health risk assessment; however for water exposures other than drinking, few quantitative exposure data exist. Thus, regulatory agencies must use estimates to formulate policy on treatment requirements for non-potable recycled water. We adapted the use of the swimming pool chemical cyanuric acid as a tracer of recreational water ingestion to permit detection of small water volumes inadvertently ingested from spray exposures. By using solutions of 700-1000 mg/L cyanuric acid in an experimental spray exposure scenario, we were able to quantify inadvertent water ingestion in almost 70% of participants undertaking a 10 min car wash activity using a high pressure spray device. Skin absorption was demonstrated to be negligible under the experimental conditions, and the measured ingestion volumes ranged from 0.06 to 3.79 mL. This method could be applied to a range of non-potable water use activities to generate exposure data for risk assessment processes. The availability of such empirical measurements will provide greater assurance to regulatory agencies and industry that potential health risks from exposure to non-potable water supplies are well understood and adequately managed to protect public health.

Legionella pneumophila: The Paradox of a Highly Sensitive Opportunistic Waterborne Pathogen Able to Persist in the Environment

Legionella pneumophila, the major causative agent of Legionnaires’ disease, is found in freshwater environments in close association with free-living amoebae and multispecies biofilms, leading to persistence, spread, biocide resistance, and elevated virulence of the bacterium. Indeed, legionellosis outbreaks are mainly due to the ability of this bacterium to colonize and persist in water facilities, despite harsh physical and chemical treatments. However, these treatments are not totally efficient and, after a lag period, L. pneumophila may be able to quickly re-colonize these systems. Several natural compounds (biosurfactants, antimicrobial peptides…) with anti-Legionella properties have recently been described in the literature, highlighting their specific activities against this pathogen. In this review, we first consider this hallmark of Legionella to resist killing, in regard to its biofilm or host-associated life style. Then, we focus more accurately on natural anti-Legionella molecules described so far, which could provide new eco-friendly and alternative ways to struggle against this important pathogen in plumbing.