Viability of Listeria monocytogenes in co-culture with Acanthamoeba spp

Interaction between Acanthamoeba and Staphylococcus

Free-living amoebae of the genus Acanthamoeba are infected by various bacteria in nature, and thus bacteria can protect themselves from adverse environmental conditions. Contrary to this ameba-bacteria relationship whether Acanthamoeba has antibacterial effects on bacteria is the different aspect of the relationship between these microorganisms. In this study, we investigate various Acanthamoeba strains have antibacterial effects on various Staphylococcus strains. Three environmental Acanthamoeba strains, isolated from various aquatic environments in Turkey, and Acanthamoeba castellanii ATCC 50373 standard strains were used in the study. The antistaphylococcal effect of cell-free supernatant (CFS) obtained from these amoebae against 12 different Staphylococcus bacteria was investigated by colony counting method. In addition, the pathogenicity of the tested Acanthamoeba strains was determined using osmotolerance and thermotolerance tests. CFSs obtained from Acanthamoeba were found to have varying degrees of antistaphylococcal effects on various Staphylococcus strains (0%-100%). It was determined that the CFS of the standard Acanthamoeba strain showed 100% inhibitory effect against one clinical methicillin-resistant Staphylococcus aureus strain (M2). Also, CFS of Ugöl strain showed 99.97% inhibitory effect against one clinical methicillin-sensitive Staphylococcus epidermidis strain (L3). It was determined that all Acanthamoeba isolates had no pathogenic potential. According to the results, it has been observed that Acanthamoeba produces antibacterial substance(s) against Staphylococcus bacteria and that the ameba-bacteria relationship may also result in the detriment of the bacteria. Furthermore, the current study indicates that new and natural antimicrobial agents from Acanthamoeba can be used as an alternative to infections caused by Staphylococcus.

Environmental Free-Living Amoebae Can Predate on Diverse Antibiotic-Resistant Human Pathogens

Here, we sought to test the resistance of human pathogens to unaltered environmental free-living amoebae. Amoebae are ubiquitous eukaryotic microorganisms and important predators of bacteria. Environmental amoebae have also been proposed to serve as both potential reservoirs and training grounds for human pathogens. However, studies addressing their relationships with human pathogens often rely on a few domesticated amoebae that have been selected to feed on rich medium, thereby possibly overestimating the resistance of pathogens to these predatory phagocytes. From an open-air composting site, we recovered over 100 diverse amoebae that were able to feed on Acinetobacter baumannii and Klebsiella pneumoniae. In a standardized and quantitative assay for predation, the isolated amoebae showed a broad predation spectrum, killing clinical isolates of A. baumannii, K. pneumoniae, Pseudomonas aeruginosa, and Staphylococcus aureus. Interestingly, A. baumannii, which was previously reported to resist predation by laboratory strains of Acanthamoeba, was efficiently consumed by closely related environmental amoebae. The isolated amoebae were capable of feeding on highly virulent carbapenem-resistant or methicillin-resistant clinical isolates. In conclusion, the natural environment is a rich source of amoebae with broad-spectrum bactericidal activities, including against antibiotic-resistant isolates.

IMPORTANCE Free-living amoebae have been proposed to play an important role in hosting and disseminating various human pathogens. The resistance of human pathogens to predation by amoebae is often derived from in vitro experiments using model amoebae. Here, we sought to isolate environmental amoebae and to test their predation on diverse human pathogens, with results that challenge conclusions based on model amoebae. We found that the natural environment is a rich source of diverse amoebae with broad-spectrum predatory activities against human pathogens, including highly virulent and antibiotic-resistant clinical isolates.

Amoebae as Targets for Toxins or Effectors Secreted by Mammalian Pathogens

Numerous microorganisms, pathogenic for mammals, come from the environment where they encounter predators such as free-living amoebae (FLA). The selective pressure due to this interaction could have generated virulence traits that are deleterious for amoebae and represents a weapon against mammals. Toxins are one of these powerful tools that are essential for bacteria or fungi to survive. Which amoebae are used as a model to study the effects of toxins? What amoeba functions have been reported to be disrupted by toxins and bacterial secreted factors? Do bacteria and fungi effectors affect eukaryotic cells similarly? Here, we review some studies allowing to answer these questions, highlighting the necessity to extend investigations of microbial pathogenicity, from mammals to the environmental reservoir that are amoebae.

Pseudomonas aeruginosa FARP72 Offers Protection Against Aeromonas hydrophila Infection in Labeo rohita

Use of probiotics as the biocontrol agent for disease prevention in aquaculture is gaining importance as an alternative to the indiscriminate use of antibiotics and other chemotherapeutics. In view of this trend, the probiotic properties of a potent antagonistic bacterium, Pseudomonas aeruginosa FARP₇₂, was characterized in terms of safety, antagonistic activities, in vitro immunomodulation, and in vivo disease resistance. Immunomodulatory activity was ascertained by measuring the production of intracellular superoxide anion, nitric oxide, total leukocyte peroxidase content, and the leukocyte proliferation in head kidney leukocytes. The bacterium isolated from the skin mucus of freshwater catfish Clarias batrachus was harmless to Labeo rohita. It showed inhibitory activity against Aeromonas caviae, A. hydrophila, Edwardsiella tarda, Pseudomonas putida, and Streptococcus agalactiae as revealed by cross and parallel streaking methods. Significantly higher superoxide anion and nitric oxide production, peroxidase content, and proliferative responses of leucocytes delineated the strains' ability to interact with immune cells to activate the immune system in vitro. Significant growth inhibition of A. hydrophila from 1.55 × 10⁵ CFU/mL was observed when co-cultured with P. aeruginosa FARP₇₂ in phosphate-buffered saline (PBS) at levels ranging from 2.61 × 10⁷ to 2.61 × 10⁹ CFU/mL in 10 days. P. aeruginosa FARP₇₂ increased the survival rate of rohu fingerlings against pathogenic A. hydrophila challenge in biocontrol study in vivo as determined by cohabitation challenge. These results suggest that P. aeruginosa FARP₇₂ is a potential probiotic strain and can be used in aquaculture to improve the health status and disease resistance of fish.

Adhesion of Campylobacter jejuni Is Increased in Association with Foodborne Bacteria

The aim of this study was to evaluate Campylobacter jejuni NTCT 11168 adhesion to abiotic and biotic surfaces when grown in co-culture with Escherichia coli ATCC 11229 and/or Listeria monocytogenes 4b. Adhesion of C. jejuni to polystyrene and to Caco-2 cells and Acanthamoeba castellanii was lower for at least 3 log CFU/mL compared to E. coli and L. monocytogenes. Electron micrographs of ultrathin sections revealed interactions of C. jejuni with host cells. In co-culture with E. coli and L. monocytogenes, adhesion of C. jejuni to all tested surfaces was significantly increased for more than 1 log CFU/mL. There was 10% higher aggregation for C. jejuni than for other pathogens, and high co-aggregation of co-cultures of C. jejuni with E. coli and L. monocytogenes. These data show that C. jejuni in co-cultures with E. coli and L. monocytogenes present significantly higher risk than C. jejuni as mono-cultures, which need to be taken into account in risk evaluation. C. jejuni adhesion is a prerequisite for their colonization, biofilm formation, and further contamination of the environment. C. jejuni survival under adverse conditions as a factor in their pathogenicity and depends on their adhesion to different surfaces, not only as individual strains, but also in co-cultures with other bacteria like E. coli and L. monocytogenes.

Experimental Listeria-Tetrahymena-Amoeba food chain functioning depends on bacterial virulence traits

Background

Some pathogenic bacteria have been developing as a part of terrestrial and aquatic microbial ecosystems. Bacteria are consumed by bacteriovorous protists which are readily consumed by larger organisms. Being natural predators, protozoa are also an instrument for selection of virulence traits in bacteria. Moreover, protozoa serve as a “Trojan horse” that deliver pathogens to the human body. Here, we suggested that carnivorous amoebas feeding on smaller bacteriovorous protists might serve as “Troy” themselves when pathogens are delivered to them with their preys. A dual role might be suggested for protozoa in the development of traits required for bacterial passage along the food chain.

Results

A model food chain was developed. Pathogenic bacteria L. monocytogenes or related saprophytic bacteria L. innocua constituted the base of the food chain, bacteriovorous ciliate Tetrahymena pyriformis was an intermediate consumer, and carnivorous amoeba Amoeba proteus was a consumer of the highest order. The population of A. proteus demonstrated variations in behaviour depending on whether saprophytic or virulent Listeria was used to feed the intermediate consumer, T. pyriformis. Feeding of A. proteus with T. pyriformis that grazed on saprophytic bacteria caused prevalence of pseudopodia-possessing hungry amoebas. Statistically significant prevalence of amoebas with spherical morphology typical for fed amoebas was observed when pathogenic L. monocytogenes were included in the food chain. Moreover, consumption of tetrahymenas fed with saprophytic L. innocua improved growth of A. proteus population while L. monocytogenes-filled tetrahymenas provided negative effect. Both pathogenic and saprophytic bacteria were delivered to A. proteus alive but only L. monocytogenes multiplied within amoebas. Observed differences in A. proteus population behaviour suggested that virulent L. monocytogenes might slow down restoration of A. proteus ability to hunt again and thus restrict the size of A. proteus population. Comparison of isogenic bacterial pairs that did or did not produce the haemolysin listeriolysin O (LLO) suggested a role for LLO in passing L. monocytogenes along the food chain.

Conclusions

Our results support the idea of protozoa as a means of pathogen delivery to consumers of a higher order and demonstrated a dual role of protozoa as both a “Trojan horse” and “Troy.”

DVC-FISH to identify potentially pathogenic Legionella inside free-living amoebae from water sources

Despite all safety efforts, drinking and wastewater can still be contaminated by Legionella and free-living amoebae (FLA) since these microorganisms are capable of resisting disinfection treatments. An amoebae cyst harboring pathogenic Legionella spp. can be a transporter of this organism, protecting it and enhancing its infection abilities. Therefore, the aim of this work is to identify by DVC-FISH viable Legionella spp and Legionella pneumophila cells inside FLA from water sources in a specific and rapid way with the aim of assessing the real risk of these waters. A total of 55 water samples were processed, 30 reclaimed wastewater and 25 drinking water. FLA presence was detected in 52.7% of the total processed water samples. When DVC-FISH technique was applied, the presence of viable internalized Legionella spp. cells was identified in 69.0% of the total FLA-positive samples, concretely in 70.0% and 66.7% of wastewater and drinking water samples, respectively. L. pneumophila was simultaneously identified in 48.3% of the total FLA-positive samples, specifically in 50.0% and 44.4% of wastewater and drinking water samples, respectively. By culture, potentially pathogenic Legionella cells were recovered in 27.6% of the total FLA-positive bacteria, particularly in 35.0% and 11.1% of wastewater and drinking water samples, respectively. These findings demonstrate that FLA may promote resistance of bacteria to the performed disinfection treatments for drinking as well as for wastewater. So, in addition to the risk for the presence of pathogenic FLA in water it is necessary to take into account that these can be transporters of the pathogenic bacteria Legionella, which are able to survive inside them. The DVC-FISH method described here has been proved to be a rapid and specific tool to identify pathogenic Legionella spp. and L. penumophila viable cells harboured by FLA in these water sources, posing particular public health concern.

Shigella sonnei Does Not Use Amoebae as Protective Hosts

Shigella flexneri and Shigella sonnei bacteria cause the majority of all shigellosis cases worldwide. However, their distributions differ, with S. sonnei predominating in middle- and high-income countries and S. flexneri predominating in low-income countries. One proposed explanation for the continued range expansion of S. sonnei is that it can survive in amoebae, which could provide a protective environment for the bacteria. In this study, we demonstrate that while both S. sonnei and S. flexneri can survive coculture with the free-living amoebae Acanthamoebae castellanii, bacterial growth is predominantly extracellular. All isolates of Shigella were degraded following phagocytosis by A. castellanii, unlike those of Legionella pneumophila, which can replicate intracellularly. Our data suggest that S. sonnei is not able to use amoebae as a protective host to enhance environmental survival. Therefore, alternative explanations for S. sonnei emergence need to be considered.

IMPORTANCE The distribution of Shigella species closely mirrors a country's socioeconomic conditions. With the transition of many populous nations from low- to middle-income countries, S. sonnei infections have emerged as a major public health issue. Understanding why S. sonnei infections are resistant to improvements in living conditions is key to developing methods to reduce exposure to this pathogen. We show that free-living amoebae are not likely to be environmental hosts of S. sonnei, as all Shigella strains tested were phagocytosed and degraded by amoebae. Therefore, alternative scenarios are required to explain the emergence and persistence of S. sonnei infections.

Assessment of Listeria monocytogenes virulence in the Galleria mellonella insect larvae model

Several animal models have been used to understand the molecular basis of the pathogenicity, infectious dose and strain to strain variation of Listeria monocytogenes. The greater wax worm Galleria mellonella, as an alternative model, provides some useful advantages not available with other models and has already been described as suitable for the virulence assessment of various pathogens including L. monocytogenes. The objectives of this study are: 1) confirming the usefulness of this model with a wide panel of Listeria spp. including non-pathogenic L. innocua, L. seeligeri, L. welshimeri and animal pathogen L. ivanovii; 2) assessment of virulence of several isogenic in-frame deletion mutants in virulence and stress related genes of L. monocytogenes and 3) virulence assessment of paired food and clinical isolates of L. monocytogenes from 14 major listeriosis outbreaks occurred worldwide between 1980 and 2015. Larvae injected with different concentrations of Listeria were incubated at 37°C and monitored over seven days for time needed to kill 50% of larvae (LT50) and to determine change of bacterial population in G. mellonella, 2 and 24 hours post-inoculation. Non-pathogenic members of Listeria and L. ivanovii showed significantly (P < 0.05) higher LT50 (lower virulence) than the wild type L. monocytogenes strains. Isogenic mutants of L. monocytogenes with the deletions in prfA, plcA, hly, actA and virR genes, also showed significantly (P < 0.05) higher LT50 than the wild type strain at the inoculum of 106CFU/larva. Food isolates had significantly (P < 0.05) lower virulence than the paired clinical isolates, at all three inoculum concentrations. L. monocytogenes strains related to non-invasive (gastroenteritis) outbreaks of listeriosis showed significantly (P < 0.05) lower virulence than isolates of the same serotype obtained from outbreaks with invasive symptoms. The difference, however, was dose and strain- dependent. No significant differences in virulence were observed among the serotype tested in this study.

Detection of viable Helicobacter pylori inside free-living amoebae in wastewater and drinking water samples from Eastern Spain

Helicobacter pylori is one of the most concerning emerging waterborne pathogens. It has been suggested that it could survive in water inside free-living amoebae (FLA), but nobody has studied this relationship in the environment yet. Thus, we aimed to detect viable H. pylori cells from inside FLA in water samples. Sixty-nine wastewater and 31 drinking water samples were collected. FLA were purified and identified by PCR and sequencing. For exclusively detecting H. pylori inside FLA, samples were exposed to sodium hypochlorite and assayed by specific PMA-qPCR, DVC-FISH and culture. FLA were detected in 38.7% of drinking water and 79.7% of wastewater samples, even after disinfection. In wastewater, Acanthamoeba spp. and members of the family Vahlkampfiidae were identified. In drinking water, Acanthamoeba spp. and Echinamoeba and/or Vermamoeba were present. In 39 (58.2%) FLA-positive samples, H. pylori was detected by PMA-qPCR. After DVC-FISH, 21 (31.3%) samples harboured viable H. pylori internalized cells. H. pylori was cultured from 10 wastewater samples. To our knowledge, this is the first report that demonstrates that H. pylori can survive inside FLA in drinking water and wastewater, strongly supporting the hypothesis that FLA could play an important role in the transmission of H. pylori to humans.

Importance of amoebae as a tool to isolate amoeba-resisting microorganisms and for their ecology and evolution: the Chlamydia paradigm

Free-living amoebae are distributed worldwide and are frequently in contact with humans and animals. As cysts, they can survive in very harsh conditions and resist biocides and most disinfection procedures. Several microorganisms, called amoeba-resisting microorganisms (ARMs), have evolved to survive and multiply within these protozoa. Among them are many important pathogens, such as Legionella and Mycobacteria, and also several newly discovered Chlamydia-related bacteria, such as Parachlamydia acanthamoebae, Estrella lausannensis, Simkania negevensis or Waddlia chondrophila whose pathogenic role towards human or animal is strongly suspected. Amoebae represent an evolutionary crib for their resistant microorganisms since they can exchange genetic material with other ARMs and develop virulence traits that will be further used to infect other professional phagocytes. Moreover, amoebae constitute an ideal tool to isolate strict intracellular microorganisms from complex microbiota, since they will feed on other fast-growing bacteria, such as coliforms potentially present in the investigated samples. The paradigm that ARMs are likely resistant to macrophages, another phagocytic cell, and that they are likely virulent towards humans and animals is only partially true. Indeed, we provide examples of the Chlamydiales order that challenge this assumption and suggest that the ability to multiply in protozoa does not strictly correlate with pathogenicity and that we should rather use the ability to replicate in multiple and diverse eukaryotic cells as an indirect marker of virulence towards mammals. Thus, cell-culture-based microbial culturomics should be used in the future to try to discover new pathogenic bacterial species.

Development of a modified gentamicin protection assay to investigate the interaction between Campylobacter jejuni and Acanthamoeba castellanii ATCC 30010

Campylobacter jejuni is one of the leading causes of diarrheal illness worldwide. It is persistent in the environment and on poultry despite its microaerophilic nature and sensitivity to dessication and pH. Studies have demonstrated that C. jejuni co-incubated with Acanthamoeba spp. may be protected from harmful environmental factors. Research in this area, however has included a range of different methodologies for co-incubation, recovery of bacteria and amoebae, and verification of internalization. In this study a modified gentamicin protection assay (mGPA) was developed with a standardized co-incubation procedure. The mGPA addresses limitations of the traditional GPA by providing quantification of the rate of internalization, or lack of internalization, of C. jejuni by Acanthamoeba castellanii. The mGPA described here utilizes tubes instead of cell culture plates allowing for determination of exact numbers of A. castellanii and C. jejuni to be co-incubated prior to addition to tubes. Additionally, the mGPA allows for the incorporation of C. jejuni-only controls to determine the fate of C. jejuni throughout the assay in the absence of A. castellanii. Using the mGPA it was determined that on average 1.6×10(5) C. jejuni (or 0.006% of initial 1×10(9) inoculum) survive the assay in the absence of A. castellanii. Additionally, results obtained with the mGPA demonstrated that while co-incubation with amoebae sometimes (56% of co-incubations) provided a protective effect for C. jejuni, in other cases it did not provide any protective effect (39% of co-incubations), and in at least one case there was a statistically significant higher recovery of C. jejuni in controls when compared to C. jejuni co-incubated with amoebae. The modified gentamicin protection assay described here allows better quantification of the rate and incidence of internalization of bacteria by amoebae. Use of the standardized mGPA developed here with varying environmental parameters and/or strains of bacteria and amoebae may provide insight into factors which are involved in the initiation of internalization of bacteria by amoebae.

How the interaction of Listeria monocytogenes and Acanthamoeba spp. affects growth and distribution of the food borne pathogen

Listeria monocytogenes is a foodborne opportunistic pathogen capable to switch from an environmental saprophyte to a potentially fatal human pathogen. The fact that the pathogen maintains the genes suitable for an elaborate infectious process indicates that these genes are required to survive in the environment. However, no environmental host reservoir for L. monocytogenes has been identified so far. The similarity of free-living, bacteria-scavenging amoebae to macrophages led to the hypothesis that protozoa may represent the missing link in the ecology and pathology of L. monocytogenes. Consequently, numerous studies have been published reporting on the potential of Acanthamoeba spp. to serve as host for a variety of pathogenic bacteria. However, the data on the interaction of L. monocytogenes with Acanthamoeba spp. are inconsistent and relatively little information on the impact of this interaction on growth and distribution of the foodborne pathogen is currently available. Hence, this review focuses on the interaction of L. monocytogenes and Acanthamoeba spp. affecting survival and growth of the foodborne pathogen in natural and man-made environments, in order to highlight the potential impact of this interplay on food safety and human health.

Listeria monocytogenes, a down-to-earth pathogen

Listeria monocytogenes is the causative agent of the food-borne life threatening disease listeriosis. This pathogenic bacterium received much attention in the endeavor of deciphering the cellular mechanisms that underlie the onset of infection and its ability to adapt to the food processing environment. Although information is available on the presence of L. monocytogenes in many environmental niches including soil, water, plants, foodstuff and animals, understanding the ecology of L. monocytogenes in outdoor environments has received less attention. Soil is an environmental niche of pivotal importance in the transmission of this bacterium to plants and animals. Soil composition, microbial communities and macrofauna are extrinsic edaphic factors that direct the fate of L. monocytogenes in the soil environment. Moreover, farming practices may further affect its incidence. The genome of L. monocytogenes presents an extensive repertoire of genes encoding transport proteins and regulators, a characteristic of the genome of ubiquitous bacteria. Postgenomic analyses bring new insights in the process of soil adaptation. In the present paper focussing on soil, we review these extrinsic and intrinsic factors that drive environmental adaptation of L. monocytogenes.

Behavior of Yersinia enterocolitica in the presence of the bacterivorous Acanthamoeba castellanii

Free-living protozoa play an important role in the ecology and epidemiology of human-pathogenic bacteria. In the present study, the interaction between Yersinia enterocolitica, an important food-borne pathogen, and the free-living amoeba Acanthamoeba castellanii was studied. Several cocultivation assays were set up to assess the resistance of Y. enterocolitica to A. castellanii predation and the impact of environmental factors and bacterial strain-specific characteristics. Results showed that all Y. enterocolitica strains persist in association with A. castellanii for at least 14 days, and associations with A. castellanii enhanced survival of Yersinia under nutrient-rich conditions at 25°C and under nutrient-poor conditions at 37°C. Amoebae cultivated in the supernatant of one Yersinia strain showed temperature- and time-dependent permeabilization. Intraprotozoan survival of Y. enterocolitica depended on nutrient availability and temperature, with up to 2.8 log CFU/ml bacteria displaying intracellular survival at 7°C for at least 4 days in nutrient-rich medium. Transmission electron microscopy was performed to locate the Yersinia cells inside the amoebae. As Yersinia and Acanthamoeba share similar ecological niches, this interaction identifies a role of free-living protozoa in the ecology and epidemiology of Y. enterocolitica.

Colpoda secrete viable Listeria monocytogenes within faecal pellets

Transmission electron microscopy was used to demonstrate that co-cultures of the ciliate Colpoda RR (an environmental isolate) and Colpoda MLS-5 (a food processing environment isolate) with the pathogenic Listeria monocytogenes DRDC8 resulted in secretion of faecal pellets containing intact DRDC8 cells. A green fluorescent protein expressing variant of DRDC8 was used in co-cultures to confirm that the pellet-associated bacterial cells were L. monocytogenes. Viability was confirmed by plate counts, and assay of microbial respiratory activity-proved DRDC8 cells present within faecal pellets was metabolically active. Following treatment of faecal pellets secreted by Colpoda RR and MLS-5 with gentamycin and sodium hypochlorite (NaOCl), no loss of viability of the pellet-located DRDC8 cells was observed, indicating that faecal pellet encapsulated DRDC8 cells are resistant to biocidal agents. This work suggests that Colpoda-derived faecal pellets may provide a mechanism for transmission of L. monocytogenes and other pathogenic bacteria. Furthermore, bacteria encapsulated by faecal pellets may be resistant to disinfectants and cleaning agents used in food manufacturing and preparation facilities.

Acanthamoeba sp. promotes the survival and growth of Acinetobacter baumanii

Acinetobacter baumanii, which may be found in water, is an important emerging hospital-acquired pathogen. Free-living amoebae can be recovered from the same water networks, and it has been shown that these protozoa may support the growth of other bacteria. In this paper, we have studied potential relationships between A. baumanii and Acanthamoeba species. Two strains of A. baumanii isolated from hospital water were co-cultivated with the trophozoites or supernatants of two free-living amoebae strains: Acanthamoeba castellanii or Acanthamoeba culbertsoni. Firstly, the presence of the amoebae or their supernatants induced a major increase in A. baumanii growth, compared with controls. Secondly, A. baumanii affected only the viability of A. culbertsonii, with no effect on A. castellanii. Electron microscopy observations of the cultures investigating the bacterial location in the protozoa showed persistence of the bacteria within cyst wall even after 60 days of incubation. In our study, the survival and growth of A. baumanii could be favored by Acanthamoeba strains. Special attention should consequently be paid to the presence of free-living amoebae in hospital water systems, which can promote A. baumanii persistence.

Persistence of free-living protozoan communities across rearing cycles in commercial poultry houses

The introduction and survival of zoonotic bacterial pathogens in poultry farming have been linked to bacterial association with free-living protozoa. To date, however, no information is available on the persistence of protozoan communities in these environments across consecutive rearing cycles and how it is affected by farm- and habitat-specific characteristics and management strategies. We therefore investigated the spatial and temporal dynamics of free-living protozoa in three habitats (pipeline, water, and miscellaneous samples) in three commercial poultry houses across three rearing cycles by using the molecular fingerprinting technique denaturing gradient gel electrophoresis (DGGE). Our study provides strong evidence for the long-term (ca. 6-month) persistence of protozoa in broiler houses across consecutive rearing cycles. Various free-living protozoa (flagellates, ciliates, and amoebae), including known vectors of bacterial pathogens, were observed during the down periods in between rearing cycles. In addition, multivariate analysis and variation partitioning showed that the protozoan community structure in the broiler houses showed almost no change across rearing cycles and remained highly habitat and farm specific. Unlike in natural environments, protozoan communities inside broiler houses are therefore not seasonal. Our results imply that currently used biosecurity measures (cleaning and disinfection) applied during the down periods are not effective against many protozoans and therefore cannot prevent potential cross-contamination of bacterial pathogens via free-living protozoa between rearing cycles.

Molecular pathogenesis of Listeria monocytogenes in the alternative model host Galleria mellonella

Larvae of Galleria mellonella, the greater wax moth, provide an alternative infection model for many human pathogens as they are amenable to use at elevated incubation temperatures (37&emsp14;°C). This study and a parallel study by Mukherjee et al. [Mukherjee, K., Altincicek, B., Hain, T., Domann, E., Vilcinskas, A. & Chakraborty, T. (2010). Appl Environ Microbiol 76, 310-317] establish this insect host as an appropriate model to investigate the pathogenesis of Listeria species. In this study we show that inoculation with Listeria monocytogenes initiates a dynamic infection in G. mellonella and that production of the cytolysin listeriolysin O (LLO) is necessary for toxicity and bacterial growth. Production of LLO by the non-pathogenic species Lactococcus lactis is sufficient to induce mortality in the insect model. We employed real-time bioluminescence imaging to examine the dynamics of listerial growth and virulence gene expression in the G. mellonella model. Analysis of lux promoter fusions demonstrated significant induction of virulence gene expression upon introduction of the pathogen into insects at both 30 and 37&emsp14;°C. The host response to listerial infection was examined which demonstrated that haemocyte destruction accompanies L. monocytogenes pathogenesis and is preceded by activation of the phenoloxidase system. Furthermore, we demonstrate that Listeria innocua is pathogenic to G. mellonella through a persistence mechanism that implicates an alternative mechanism for pathogenicity in this model.

Ciliates rapidly enhance the frequency of conjugation between Escherichia coli strains through bacterial accumulation in vesicles

The mechanism underlying bacterial conjugation through protozoa was investigated. Kanamycin-resistant Escherichia coli SM10λ+ carrying pRT733 with TnphoA was used as donor bacteria and introduced by conjugation into ciprofloxacin-resistant E. coli clinical isolate recipient bacteria. Equal amounts of donor and recipient bacteria were mixed together in the presence or absence of protozoa (ciliates, free-living amoebae, myxamoebae) in Page's amoeba saline for 24 h. Transconjugants were selected with Luria broth agar containing kanamycin and ciprofloxacin. The frequency of conjugation was estimated as the number of transconjugants for each recipient. Conjugation frequency in the presence of ciliates was estimated to be approximately 10⁻⁶, but in the absence of ciliates, or in the presence of other protozoa, it was approximately 10⁻⁸. Conjugation also occurred in culture of ciliates at least 2 h after incubation. Successful conjugation was confirmed by the polymerase chain reaction. Addition of cycloheximide or latrunculin B resulted in suppression of conjugation. Heat killing the ciliates or bacteria had no effect on conjugation frequency. Co-localization of green fluorescent protein-expressing E. coli and PKH-67-vital-stained E. coli was observed in the same ciliate vesicles, suggesting that both donor and recipient bacteria had accumulated in the same vesicle. In this study, the conjugation frequency of bacteria was found to be significantly higher in vesicles purified from ciliates than those in culture suspension. We conclude that ciliates rapidly enhance the conjugation of E. coli strains through bacterial accumulation in vesicles.

Listeria monocytogenes does not survive ingestion by Acanthamoeba polyphaga

Listeria monocytogenes is a ubiquitous bacterium capable of infecting humans, particularly pregnant women and immunocompromised individuals. Although the intracellular invasion and pathogenesis of listeriosis in mammalian tissues has been well studied, little is known about the ecology of L. monocytogenes , and in particular the environmental reservoir for this bacterium has not been identified. This study used short-term co-culture at 15, 22 and 37 degrees C to examine the interaction of L. monocytogenes strains with Acanthamoeba polyphaga ACO12. Survival of L. monocytogenes cells phagocytosed by monolayers of trophozoites was assessed by culture techniques and microscopy. A. polyphaga trophozoites eliminated bacterial cells within a few hours post-phagocytosis, irrespective of the incubation temperature used. Wild-type L. monocytogenes and a phenotypic listeriolysin O mutant were unable to either multiply or survive within trophozoites. By contrast, Salmonella enterica serovar Typhimurium C5 cells used as controls were able to survive and multiply within A. polyphaga trophozoites. The data presented indicate that A. polyphaga ACO12 is unlikely to harbour L. monocytogenes, or act as an environmental reservoir for this bacterium.