Single cell cloning of free-living amoebae with isolated square agar.

An alternative amoeba cloning technique is described. The amoebic cells were scraped from the agar and inoculated in a new agar medium separated in isolated squares, preventing a cell from invading the space of another cell, increasing the reliability of cloning, and providing an efficient quality control of technique.

Extracellular amoebal-vesicles: potential transmission vehicles for respiratory viruses.

Human respiratory syncytial virus (RSV) is a major cause of acute respiratory tract infections in children and immunocompromised adults worldwide. Here we report that amoebae-release respirable-sized vesicles containing high concentrations of infectious RSV that persisted for the duration of the experiment. Given the ubiquity of amoebae in moist environments, our results suggest that extracellular amoebal-vesicles could contribute to the environmental persistence of respiratory viruses, including potential resistance to disinfection processes and thereby offering novel pathways for viral dissemination and transmission.

Growth of Stenotrophomonas maltophilia and expression of Sme efflux pumps encoding genes in the presence of supernatants from amoebal and bacterial co-cultures: towards the role of amoebal secondary metabolites.

Resistance-Nodulation-Division (RND) efflux pumps are relevant determinants of Stenotrophomonas maltophilia multidrug resistance as they can extrude a broad range of antibiotics and compounds involved in virulence and physiological functions. S. maltophilia, an environmental bacterium, was shown to be associated with amoebae and able to multiply inside them. To explore whether S. maltophilia RND efflux pumps play a role when interacting with amoebae, we evaluated the effect of amoebal culture and co-culture supernatants on the growth of S. maltophilia and the expression of sme efflux pump genes. Acanthamoeba castellanii and Willaertia magna were used as amoebal models and strain S. maltophilia BurE1 as bacterial one. Our data showed that both bacterial growth and sme gene expression were not modified by amoebal culture supernatants. On the contrary, co-culture supernatants negatively impacted the growth of BurE1 and induced the expression of three out of eight efflux pump genes, i.e. smeE, smeN and smeZ. Finally, we evidenced the production of A. castellanii secondary metabolites, putatively belonging to the diterpene family, in the amoebal supernatant and in the co-culture supernatant of A. castellanii and BurE1. Whether these compounds act directly as substrates of the efflux pumps and/or inducers of the sme genes need further investigations.

The endogenous galactofuranosidase GlfH1 hydrolyzes mycobacterial arabinogalactan.

Despite impressive progress made over the past 20 years in our understanding of mycolylarabinogalactan-peptidoglycan (mAGP) biogenesis, the mechanisms by which the tubercle bacillus Mycobacterium tuberculosis adapts its cell wall structure and composition to various environmental conditions, especially during infection, remain poorly understood. Being the central portion of the mAGP complex, arabinogalactan (AG) is believed to be the constituent of the mycobacterial cell envelope that undergoes the least structural changes, but no reports exist supporting this assumption. Herein, using recombinantly expressed mycobacterial protein, bioinformatics analyses, and kinetic and biochemical assays, we demonstrate that the AG can be remodeled by a mycobacterial endogenous enzyme. In particular, we found that the mycobacterial GlfH1 (Rv3096) protein exhibits exo-ß-d-galactofuranose hydrolase activity and is capable of hydrolyzing the galactan chain of AG by recurrent cleavage of the terminal ß-(1,5) and ß-(1,6)-Galf linkages. The characterization of this galactosidase represents a first step toward understanding the remodeling of mycobacterial AG.

Soil Testate Amoebae and Diatoms as Bioindicators of an Old Heavy Metal Contaminated Floodplain in Japan.

Soil protists are rarely included in ecotoxicological investigations, despite their fundamental role in ecological processes. Moreover, testate amoebae and diatoms contribute considerably to silicon fluxes in soils. We investigated the effects of heavy metals on testate amoebae (species and individual densities) and diatoms (individual densities) in aged soils of a floodplain (Watarase retarding basin, Japan) taking soil samples from two unpolluted reference sites and two polluted sites. The total concentrations of Cu, Pb, and Zn in soil were higher at the polluted sites as compared with the reference sites. The available concentrations of Co, Cu, and Zn in CaCl2 extracts were higher at the polluted sites but available Pb was not detectable. Testate amoeba taxonomic richness was higher in the reference sites (45/38 taxa) than in the polluted sites (36/27 taxa). The reference sites had higher diatom and amoeba densities than the polluted sites. There was a significant negative correlation between total testate amoeba density and heavy metal concentration (available Co), while significant negative correlations were found between diatom density and Co, Cu, and Zn (available and total concentration). Densities of Cyclopyxis kahli cyclostoma, Centropyxis spp., and Trinema complanatum were negatively correlated to concentrations of available heavy metals. The observed decrease in individual numbers due to heavy metal pollution resulted in a considerable decline in protozoic (testate amoebae) and protophytic (pennate diatoms) silicon pools. Our data suggest that heavy metal pollution affects biogeochemical cycling in this system.

Mycobacterium abscessus virulence traits unraveled by transcriptomic profiling in amoeba and macrophages.

Free-living amoebae are thought to represent an environmental niche in which amoeba-resistant bacteria may evolve towards pathogenicity. To get more insights into factors playing a role for adaptation to intracellular life, we characterized the transcriptomic activities of the emerging pathogen Mycobacterium abscessus in amoeba and murine macrophages (Mϕ) and compared them with the intra-amoebal transcriptome of the closely related, but less pathogenic Mycobacterium chelonae. Data on up-regulated genes in amoeba point to proteins that allow M. abscessus to resist environmental stress and induce defense mechanisms, as well as showing a switch from carbohydrate carbon sources to fatty acid metabolism. For eleven of the most upregulated genes in amoeba and/or Mϕ, we generated individual gene knock-out M. abscessus mutant strains, from which ten were found to be attenuated in amoeba and/or Mϕ in subsequence virulence analyses. Moreover, transfer of two of these genes into the genome of M. chelonae increased the intra-Mϕ survival of the recombinant strain. One knock-out mutant that had the gene encoding Eis N-acetyl transferase protein (MAB_4532c) deleted, was particularly strongly attenuated in Mϕ. Taken together, M. abscessus intra-amoeba and intra-Mϕ transcriptomes revealed the capacity of M. abscessus to adapt to an intracellular lifestyle, with amoeba largely contributing to the enhancement of M. abscessus intra-Mϕ survival.

Investigation of the Physical, Chemical Characteristics and Microbial Contamination of the Indoor Swimming Pools

Objective: The aim of this study was to investigate the physical, chemical and microbiological contamination of indoor swimming pools. Methods: Pool water specimens were collected using a plastic polypropylene sterilized bottle. The physical and chemical qualities of the waters were analyzed in terms of temperature, turbidity, pH, and free residual chlorine, with the standard methods for the examination of water. Bacteriological (routine methods) and parasitological (molecular methods) tests were carried out on pools water. Results: The mean temperature, pH, and residual chlorine of the indoor pools were 31.2 °C, 7.6 and 1.5 mg/L, respectively. Turbidity was not observed in any of the pools. The pH and temperature values were in standard ranges in 92.3% and 15.4% of the waters of swimming pools, respectively. The prevalence rates of bacterial and amoebic contaminations of the water in the swimming pools were 53.8% and 46.2%, respectively. One pool (7.7%) was contaminated with both bacteria and amoeba. Streptococcus viridans, Staphylococcus epidermidis, Pseudomonas stutzeri, Cryptosporidium and Bacillus spp. were isolated from the pool waters. Conclusion: In this study, some microorganisms were identified from the water pools. Effective management of swimming pools and proper control of the physical, chemical and microbiological property of water pools can produce the healthy recreational activity.

Co-Occurrence of Free-Living Amoeba and Legionella in Drinking Water Supply Systems.

Background and Objectives: Legionella is one of the most important water-related pathogens. Inside the water supply systems and the biofilms, Legionella interact with other bacteria and free-living amoeba (FLA). Several amoebas may serve as hosts for bacteria in aquatic systems. This study aimed to investigate the co-occurrence of Legionella spp. and FLA in drinking water supply systems. Materials and Methods: A total of 268 water samples were collected from apartment buildings, hotels, and public buildings. Detection of Legionella spp. was performed in accordance with ISO 11731:2017 standard. Three different polymerase chain reaction (PCR) protocols were used to identify FLA. Results: Occurrence of Legionella varied from an average of 12.5% in cold water samples with the most frequent occurrence observed in hot water, in areas receiving untreated groundwater, where 54.0% of the samples were Legionella positive. The occurrence of FLA was significantly higher. On average, 77.2% of samples contained at least one genus of FLA and, depending on the type of sample, the occurrence of FLA could reach 95%. In the samples collected during the study, Legionella was always isolated along with FLA, no samples containing Legionella in the absence of FLA were observed. Conclusions: The data obtained in our study can help to focus on the extensive distribution, close interaction, and long-term persistence of Legionella and FLA. Lack of Legionella risk management plans and control procedures may promote further spread of Legionella in water supply systems. In addition, the high incidence of Legionella-related FLA suggests that traditional monitoring methods may not be sufficient for Legionella control.

Amoebae Assemble Synthetic Spherical Particles To Form Reproducible Constructs.

Difflugia are testate amoebae that use particulate inorganic matter to build a protective shell (generally called a test or theca). Difflugia globulosa were grown both in culture containing only naturally occurring theca-building materials and under conditions where synthetic particles were present also. The presence of monodisperse Stöber silica microspheres of 1, 3, and 6 µm in diameter or 4 µm polystyrene spheres dramatically increased the rate of Difflugia growth, and foreign microspheres became the overwhelmingly dominant construction material. Optical and electron microscopy of the 6 µm particle studies revealed that Difflugia construct spherical vase-shaped thecae with strikingly reproducible composition, morphology, and size. Time-lapse photography revealed construction techniques and masonry skills as Difflugia herded particles together, trapped them using phagocytosis, and applied the particles with biocement from inside the developing theca. The reported observations identify taxonomy complications, biomicrofabrication possibilities, and a discrete environmental impact of synthetic particle pollutants.

Response of Anaerobic Protozoa to Oxygen Tension in Anaerobic System.

The effect of oxygen on anaerobic protozoa was studied in anaerobic batch reactors inoculated with sludge and protozoa cultures. Among the protozoa genera, Metopus, Brachonella, Plagiopyla, Trepomonas, and Vanella were more sensitive to oxygen compared to other genera. Protozoa genera Menoidium, Rhynchomonas, Cyclidium, Spathidium, and Amoeba were found to survive under aerobic conditions, and the growth rate was slightly higher or similar to anaerobic condition. O2 tension resulted in the loss of free and endosymbiotic methanogens in anaerobic system, while methanogens were observed inside the protozoan cysts. Survival of anaerobic protozoa declined considerably when the O2 tension exceeded 1% atm. sat. and showed chemosensory behavior in response to O2 exposure. Superoxide dismutase activity was detected in survived protozoa cells under O2 tension. Facultative anaerobic protozoa with SOD activity can provide a mechanism to overcome possible occurrence of oxygen toxicity in the treatment of wastewater in anaerobic reactor.

A four year survey reveals a coherent pattern between occurrence of fruit bodies and soil amoebae populations for nivicolous myxomycetes.

Among soil-inhabiting protists, myxomycetes stand out by their macroscopic fructifications which have allowed studies on their ecology and distribution for more than two hundred years. One of the most distinct ecological guilds in myxomycetes are the nivicolous or "snowbank" myxomycete species, which produce fruit bodies at the edge of melting snowbanks in spring. Relationship between the occurrence of fructifications and myxamoebae remain unknown. In this study we used modern molecular techniques, by direct DNA amplification from soil extracts (NGS metabarcoding) to compare the distribution of soil-inhabiting myxamoebae found in 2016 with fructifications from the same sites collected over the course of four years (2013, 2015-17) along an elevational transect in the northern German Alps. A coherent community composition between fructification and soil myxamoebae, though with species-specific differences in relative abundance, was revealed. Although patterns varied among species, myxamoebae were found at both low and high elevations, whereas fruit bodies were mainly found at higher elevations, likely explained by the presence of a stable and long-lasting snow cover. In addition, a year to year comparison of fructification records support the hypothesis that the abundance of fructifications strongly depends on the onset of snowfall in the previous autumn and the soil temperature regime throughout the winter.

Prevalence of free living amoeba in the domestic waters reservoirs in Sfax, Tunisia.

Free living amoebas (FLA) are opportunistic pathogen found in different water sources in the environment. The aim of this study was to investigate the prevalence of free living amoeba in different samples of domestic water reserves (DWR) in Sfax region from Tunisia. It was a prospective study dealing with 486 water samples collected from different DWR. After filtration through a cellulose acetate membrane samples were cultured on non-nutrient agar and the FLA were detected and strained with Giesma, Trichrome and red nuclear stain for morphological and morphotypic studies. FLA were found in 62% of samples. The Acanthopodial morphotype was detected in 43%, Polytactic (38%), Monotactic (28%), Fan-shaped (17%), Rugose (11%), Dactilopodial (10%) and Eruptive (9%). These results demonstrate that domestic water reserves are a significant source of the FLA and maintenance of DWR is recommended.

Enrichment of free-living amoebae in biofilms developed at upper water levels in drinking water storage towers: An inter- and intra-seasonal study.

Free-living amoebae (FLA) are ubiquitous organisms present in various natural and artificial environments, such as drinking water storage towers (DWST). Some FLA, such as Acanthamoeba sp., Naegleria fowleri, and Balamuthia mandrillaris, can cause severe infections at ocular or cerebral level in addition to being potential reservoirs of other pathogens. In this work, the abundance and diversity of FLA was evaluated in two sampling campaigns: one performed over five seasons in three DWST at three different levels (surface, middle and bottom) in water and biofilm using microscopy and PCR, and one based on the kinetics analysis in phase contrast and confocal microscopy of biofilm samples collected every two weeks during a 3-month period at the surface and at the bottom of a DWST. In the seasonal study, the FLA were detected in each DWST water in densities of ~20 to 25amoebaeL-1. A seasonal variation of amoeba distribution was observed in water samples, with maximal densities in summer at ~30amoebaeL-1 and minimal densities in winter at ~16amoebaeL-1. The FLA belonging to the genus Acanthamoeba were detected in two spring sampling campaigns, suggesting a possible seasonal appearance of this potentially pathogenic amoeba. Interestingly, a 1 log increase of amoebae density was observed in biofilm samples collected at the surface of all DWST compared to the middle and the bottom where FLA were at 0.1-0.2amoebae/cm2. In the kinetics study, an increase of amoebae density, total cell density, and biofilm thickness was observed as a function of time at the surface of the DWST, but not at the bottom. To our knowledge, this study describes for the first time a marked higher FLA density in biofilms collected at upper water levels in DWST, constituting a potential source of pathogenic micro-organisms.

Adaptation of Amoeba Plate Test To Recover Legionella Strains from Clinical Samples.

The isolation of Legionella from respiratory samples is the gold standard for diagnosis of Legionnaires' disease (LD) and enables epidemiological studies and outbreak investigations. The purpose of this work was to adapt and to evaluate the performance of an amoebic coculture procedure (the amoeba plate test [APT]) for the recovery of Legionella strains from respiratory samples, in comparison with axenic culture and liquid-based amoebic coculture (LAC). Axenic culture, LAC, and APT were prospectively performed with 133 respiratory samples from patients with LD. The sensitivities and times to results for the three techniques were compared. Using the three techniques, Legionella strains were isolated in 46.6% (n = 62) of the 133 respiratory samples. The sensitivity of axenic culture was 42.9% (n = 57), that of LAC was 30.1% (n = 40), and that of APT was 36.1% (n = 48). Seven samples were positive by axenic culture only; for those samples, there were <10 colonies in total. Five samples, all sputum samples, were positive by an amoebic procedure only (5/5 samples by APT and 2/5 samples by LAC); all had overgrowth by oropharyngeal flora with axenic culture. The combination of axenic culture with APT yielded a maximal isolation rate (i.e., 46.6%). Overall, the APT significantly reduced the median time for Legionella identification to 4 days, compared with 7 days for LAC (P < 0.0001). The results of this study support the substitution of LAC by APT, which could be implemented as a second-line technique for culture-negative samples and samples with microbial overgrowth, especially sputum samples. The findings provide a logical basis for further studies in both clinical and environmental settings.

MALDI-TOF MS analysis as a useful tool for an identification of Legionella pneumophila, a facultatively pathogenic bacterium interacting with free-living amoebae: A case study from water supply system of hospitals in Bratislava (Slovakia).

Legionellae, i.e. Legionella pneumophila, are human bacterial hydrophilic facultative pathogens causing pneumonia (Legionnaires' disease). Free-living amoebae (FLA) can serve as natural hosts and thus as reservoirs of many amoebae-resistant bacteria. An encysted amoeba can contribute to the resistance of intracellular L. pneumophila to various chemical and physical treatments. Humans can be infected by droplets containing bacteria from an environmental source or human-made devices such as shower heads, bathtubs, air-conditioning units or whirlpools. In this study, we were investigating the presence of FLA and L. pneumophila in plumbing systems of healthcare facilities in Bratislava (Slovakia) by standard diagnostic methods, while the presence of L. pneumophila was verified also by MALDI-TOF MS (matrix-assisted laser desorption/ionization time-of-flight mass spectrometry) analysis. The results showed the occurrence of L. pneumophila and FLA in 62.26% and 66.4% of samples taken from four paediatric clinics, respectively. Both standard methods and MALDI-TOF MS showed comparable results and they can be successfully applied for the identification of L. pneumophila strains in environmental samples. Our approach could be useful for further monitoring, prevention and decreasing risk of Legionella infection also in other hospitals.

Discrete Modeling of Amoeboid Locomotion and Chemotaxis in Dictyostelium discoideum by Tracking Pseudopodium Growth Direction.

Dictyostelium discoideum amoeba is a well-established model organism for studying the crawling locomotion of eukaryotic cells. These amoebae extend pseudopodium - a temporary actin-based protrusion of their body membrane to probe the medium and crawl through it. Experiments show highly-ordered patterns in the growth direction of these pseudopodia, which results in persistence cell motility. Here, we propose a discrete model for studying and investigating the cell locomotion based on the experimental evidences. According to our model, Dictyostelium selects its pseudopodium growth direction based on a second-order Markov chain process, in the absence of external cues. Consequently, compared to a random walk process, our model indicates stronger growth in the mean-square displacement of cells, which is consistent with empirical findings. In the presence of external chemical stimulants, cells tend to align with the gradient of chemoattractant molecules. To quantify this tendency, we define a coupling coefficient between the pseudopodium extension direction and the gradient of an external stimulant, which depends on the local stimulant concentration and its gradient. Additionally, we generalize the model to weak-coupling regime by utilizing perturbation methods.

An Amoebal Grazer of Cyanobacteria Requires Cobalamin Produced by Heterotrophic Bacteria.

Amoebae are unicellular eukaryotes that consume microbial prey through phagocytosis, playing a role in shaping microbial food webs. Many amoebal species can be cultivated axenically in rich media or monoxenically with a single bacterial prey species. Here, we characterize heterolobosean amoeba LPG3, a recent natural isolate, which is unable to grow on unicellular cyanobacteria, its primary food source, in the absence of a heterotrophic bacterium, a Pseudomonas species coisolate. To investigate the molecular basis of this requirement for heterotrophic bacteria, we performed a screen using the defined nonredundant transposon library of Vibrio cholerae, which implicated genes in corrinoid uptake and biosynthesis. Furthermore, cobalamin synthase deletion mutations in V. cholerae and the Pseudomonas species coisolate do not support the growth of amoeba LPG3 on cyanobacteria. While cyanobacteria are robust producers of a corrinoid variant called pseudocobalamin, this variant does not support the growth of amoeba LPG3. Instead, we show that it requires cobalamin that is produced by the Pseudomonas species coisolate. The diversity of eukaryotes utilizing corrinoids is poorly understood, and this amoebal corrinoid auxotroph serves as a model for examining predator-prey interactions and micronutrient transfer in bacterivores underpinning microbial food webs.IMPORTANCE Cyanobacteria are important primary producers in aquatic environments, where they are grazed upon by a variety of phagotrophic protists and, hence, have an impact on nutrient flux at the base of microbial food webs. Here, we characterize amoebal isolate LPG3, which consumes cyanobacteria as its primary food source but also requires heterotrophic bacteria as a source of corrinoid vitamins. Amoeba LPG3 specifically requires the corrinoid variant produced by heterotrophic bacteria and cannot grow on cyanobacteria alone, as they produce a different corrinoid variant. This same corrinoid specificity is also exhibited by other eukaryotes, including humans and algae. This amoebal model system allows us to dissect predator-prey interactions to uncover factors that may shape microbial food webs while also providing insight into corrinoid specificity in eukaryotes.

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.

Coexistence of free-living amoebae and bacteria in selected South African hospital water distribution systems.

Pathogenic free-living amoebae (FLA), such as Naegleria fowleri, Balamuthia mandrillaris and Acanthamoeba species isolated from aquatic environments have been implicated in central nervous system, eye and skin human infections. They also allow the survival, growth and transmission of bacteria such as Legionella, Mycobacteria and Vibrio species in water systems. The purpose of this study was to investigate the co-occurrence of potentially pathogenic FLA and their associated bacteria in hospital water networks in Johannesburg, South Africa. A total of 178 water (n = 95) and swab (n = 83) samples were collected from two hospital water distribution systems. FLA were isolated using the amoebal enrichment technique and identified using PCR and 18S rDNA sequencing. Amoebae potentially containing intra-amoebal bacteria were lysed and cultured on blood agar plates. Bacterial isolates were characterized using the VITEK®2 compact System. Free-living amoebae were isolated from 77 (43.3 %) of the samples. Using microscopy, PCR and 18S rRNA sequencing, Acanthamoeba spp. (T3 and T20 genotypes), Vermamoeba vermiformis and Naegleria gruberi specie were identified. The Acanthamoeba T3 and T20 genotypes have been implicated in eye and central nervous system infections. The most commonly detected bacterial species were Serratia marcescens, Stenotrophomonas maltophilia, Delftia acidovorans, Sphingomonas paucimobilis and Comamonas testosteroni. These nosocomial pathogenic bacteria are associated with systematic blood, respiratory tract, the urinary tract, surgical wounds and soft tissues infections. The detection of FLA and their associated opportunistic bacteria in the hospital water systems point out to a potential health risk to immune-compromised individuals.

Enhanced survival but not amplification of Francisella spp. in the presence of free-living amoebae.

Transmission of Francisella tularensis, the etiologic agent of tularemia, has been associated with various water sources. Survival of many waterborne pathogens within free-living amoeba (FLA) is well documented; however, the role of amoebae in the environmental persistence of F. tularensis is unclear. In this study, axenic FLA cultures of Acanthamoeba castellanii, Acanthamoeba polyphaga, and Vermamoeba vermiformis were each inoculated with virulent strains of F. tularensis (Types A and B), the attenuated live vaccine strain, and Francisella novicida. Experimental parameters included low and high multiplicity of infection and incubation temperatures of 25 and 30 °C for 0-10 days. Francisella spp. survival was enhanced by the presence of FLA; however, bacterial growth and protozoa infectivity were not observed. In contrast, co-infections of A. polyphaga and Legionella pneumophila, used as an amoeba pathogen control, resulted in bacterial proliferation, cytopathic effects, and amoebal lysis. Collectively, even though short-term incubation with FLA was beneficial, the long-term effects on Francisella survival are unknown, especially given the expenditure of available amoebal derived nutrients and the fastidious nature of Francisella spp. These factors have clear implications for the role of FLA in Francisella environmental persistence.