Interactions between Salmonella typhimurium and Acanthamoeba polyphaga, and observation of a new mode of intracellular growth within contractile vacuoles

Survival of Bordetella bronchiseptica in Acanthamoeba castellanii

The respiratory pathogenic bacterium Bordetella bronchiseptica can persistently survive in terrestrial and aquatic environments, providing a source of infection. However, the environmental lifestyle of the bacterium is poorly understood. In this study, expecting repeated encounters of the bacteria with environmental protists, we explored the interaction between B. bronchiseptica and a representative environmental amoeba, Acanthamoeba castellanii, and found that the bacteria resisted amoeba digestion and entered contractile vacuoles (CVs), which are intracellular compartments involved in osmoregulation, to escape amoeba cells. In prolonged coculture, A. castellanii supported the proliferation of B. bronchiseptica. The avirulent Bvg- phase, but not the virulent Bvg+ phase, of the bacteria was advantageous for survival in the amoebae. We further demonstrate that two Bvg+ phase-specific virulence factors, filamentous hemagglutinin and fimbriae, were targeted for predation by A. castellanii. These results are evidence that the BvgAS two-component system, the master regulator for Bvg phase conversion, plays an indispensable role in the survival of B. bronchiseptica in amoebae. IMPORTANCE The pathogenic bacterium Bordetella bronchiseptica, which causes respiratory diseases in various mammals, exhibits distinct Bvg+ and Bvg- phenotypes. The former represents the virulent phase, in which the bacteria express a set of virulence factors, while the role of the latter in the bacterial life cycle remains to be understood. In this study, we demonstrate that B. bronchiseptica in the Bvg- phase, but not the Bvg+ phase, survives and proliferates in coculture with Acanthamoeba castellanii, an environmental amoeba. Two Bvg+ phase-specific virulence factors, filamentous hemagglutinin and fimbriae, were targeted by A. castellanii predation. B. bronchiseptica turns into the Bvg- phase at temperatures in which the bacteria normally encounter these amoebae. These findings demonstrate that the Bvg- phase of B. bronchiseptica is advantageous for survival outside mammalian hosts and that the bacteria can utilize protists as transient hosts in natural environments.

Acanthamoeba, an environmental phagocyte enhancing survival and transmission of human pathogens

The opportunistic protist Acanthamoeba, which interacts with other microbes such as bacteria, fungi, and viruses, shows significant similarity in cellular and functional aspects to human macrophages. Intracellular survival of microbes in this microbivorous amoebal host may be a crucial step for initiation of infection in higher eukaryotic cells. Therefore, Acanthamoeba-microbe adaptations are considered an evolutionary model of macrophage-pathogen interactions. This paper reviews Acanthamoeba as an emerging human pathogen and different ecological interactions between Acanthamoeba and microbes that may serve as environmental training grounds and a genetic melting pot for the evolution, persistence, and transmission of potential human pathogens.

Biofilm formation of Salmonella enterica serovar Enteritidis cocultured with Acanthamoeba castellanii responds to nutrient availability

Acanthamoeba spp. and Salmonella share common habitats, and their interaction may influence the biofilm-forming ability of Salmonella. In this study, biofilm formation of Salmonella enterica serovar Enteritidis cocultured with Acanthamoeba castellanii was examined in nutrient-rich and nutrient-deficient media. Furthermore, transcript copy number of biofilm-related genes in the biofilm cells of S. Enteritidis in monoculture was compared to those in coculture with A. castellanii. Results demonstrated that the presence of A. castellanii in the culture media activates the genes involved in the biofilm formation of S. Enteritidis, regardless of the nutrient availability. However, biofilm formation of S. Enteritidis cocultured with A. castellanii was not consistent with the transcript copy number results. In nutrient-rich medium, the number of Salmonella biofilm cells and the contents of the three main components of the biofilms including eDNA, protein, and carbohydrates were higher in the presence of A. castellanii compared to monocultures. However, in nutrient-deficient medium, the number of biofilm cells, and the amount of biofilm components in coculture conditions were less than the monocultures. These results indicate that despite activation of relevant genes in both nutrient-rich and nutrient-deficient media, biofilm formation of S. Enteritidis cocultured with A. castellanii responds to nutrient availability.

Chlorine-stressed Salmonella cells are significantly more engulfed by Acanthamoeba trophozoites and have a longer intracystic survival than the non-stressed cells

In the present study, the effect of sublethal chlorine-induced oxidative stress on the subsequent interaction of Salmonella enterica serovars Enteritidis and Typhimurium with Acanthamoeba castellanii and A. polyphaga was evaluated. Sublethal chlorine concentration was determined using the lag phase extension information and used to prepare chlorine-stressed Salmonella cells. Coculture experiments of Acanthamoeba and Salmonella cells were performed in Page's amoeba saline (PAS) at 25 °C for 2 h. The results showed that the chlorine-stressed Salmonella cells were significantly more engulfed by A. castellanii and A. polyphaga trophozoites than the non-stressed cells. The uptake rates of the chlorine-stressed and non-stressed Salmonella cells were in the range of 14.17-27.34 and 6.51-11.52% for A. castellanii, and in the range of 8.32-17.76 and 2.28-6.12% for A. polyphaga trophozoites, respectively. Moreover, intracystic survival time of chlorine-stressed cells of S. Enteritidis and S. Typhimurium was significantly longer than that of non-stressed cells. While, non-stressed Salmonella cells survived within A. castellanii and A. polyphaga cysts for 13-20 and 8-15 days, chlorine-stressed cells were recovered from A. castellanii and A. polyphaga cysts after 22-32 and 15-24 days, respectively. These results underscore the importance of bacterial exposure to sublethal chlorine concentrations in their interaction with free-living amoebae, and may lead to a better understanding of the parameters affecting the persistence of Salmonella enterica serovars in food-related environments.

Cappable-Seq Reveals Specific Patterns of Metabolism and Virulence for Salmonella Typhimurium Intracellular Survival within Acanthamoeba castellanii

The bacterial pathogen Salmonella enterica, which causes enteritis, has a broad host range and extensive environmental longevity. In water and soil, Salmonella interacts with protozoa and multiplies inside their phagosomes. Although this relationship resembles that between Salmonella and mammalian phagocytes, the interaction mechanisms and bacterial genes involved are unclear. Here, we characterized global gene expression patterns of S. enterica serovar Typhimurium within Acanthamoeba castellanii at the early stage of infection by Cappable-Seq. Gene expression features of S. Typhimurium within A. castellanii were presented with downregulation of glycolysis-related, and upregulation of glyoxylate cycle-related genes. Expression of Salmonella Pathogenicity Island-1 (SPI-1), chemotaxis system, and flagellar apparatus genes was upregulated. Furthermore, expression of genes mediating oxidative stress response and iron uptake was upregulated within A. castellanii as well as within mammalian phagocytes. Hence, global S. Typhimurium gene expression patterns within A. castellanii help better understand the molecular mechanisms of Salmonella adaptation to an amoeba cell and intracellular persistence in protozoa inhabiting water and soil ecosystems.

War of the microbial world: Acanthamoeba spp. interactions with microorganisms

Acanthamoeba is known to interact with a plethora of microorganisms such as bacteria, fungi and viruses. In these interactions, the amoebae can be predatory in nature, transmission vehicle or an incubator. Amoebae consume microorganisms, especially bacteria, as food source to fulfil their nutritional needs by taking up bacteria through phagocytosis and lysing them in phagolysosomes and hence play an eminent role in the regulation of bacterial density in the nature and accountable for eradication of around 60% of the bacterial population in the environment. Acanthamoeba can also act as a “Trojan horse” for microbial transmission in the environment. Additionally, Acanthamoeba may serve as an incubator-like reservoir for microorganisms, including those that are pathogenic to humans, where the microorganisms use amoebae’s defences to resist harsh environment and evade host defences and drugs, whilst growing in numbers inside the amoebae. Furthermore, amoebae can also be used as a “genetic melting pot” where exchange of genes as well as adaptation of microorganisms, leading to higher pathogenicity, may arise. Here, we describe bacteria, fungi and viruses that are known to interact with Acanthamoeba spp.

The transcriptome of Escherichia coli O157: H7 reveals a role for oxidative stress resistance in its survival from predation by Tetrahymena

Pathogenic E. coli remains undigested upon phagocytosis by Tetrahymena and is egested from the ciliate as viable cells in its fecal pellets. Factors that are involved in the survival of Shiga toxin-producing E. coli serovar O157: H7 (EcO157) from digestion by Tetrahymena were identified by microarray analysis of its transcriptome in the protozoan phagosome. Numerous genes belonging to anaerobic metabolism and various stress responses were upregulated significantly ≥ 2-fold in EcO157 cells in the food vacuoles compared with in planktonic cells that remained uningested by the protist. Among these were the oxidative stress response genes, ahpF and katG. Fluorescence microscopy and staining with CellROX® Orange confirmed the presence of reactive oxygen species in food vacuoles containing EcO157 cells. Frequency distribution analysis of the percentage of EcO157 viable cells in Tetrahymena fecal pellets revealed that the ΔahpCF and ΔahpCFΔkatG mutants were less fit than the wild type strain and ΔkatG mutant after passage through the protist. Given the broad use of oxidants as sanitizers in the food industry, our observation of the oxidative stress response in EcO157 during its interaction with Tetrahymena emphasizes the importance of furthering our knowledge of the physiology of this human pathogen in environments relevant to its ecology and to food safety.

SopB- and SifA-dependent shaping of the Salmonella-containing vacuole proteome in the social amoeba Dictyostelium discoideum

The ability of Salmonella to survive and replicate within mammalian host cells involves the generation of a membranous compartment known as the Salmonella-containing vacuole (SCV). Salmonella employs a number of effector proteins that are injected into host cells for SCV formation using its type-3 secretion systems encoded in SPI-1 and SPI-2 (T3SS-1 and T3SS-2, respectively). Recently, we reported that S. Typhimurium requires T3SS-1 and T3SS-2 to survive in the model amoeba Dictyostelium discoideum. Despite these findings, the involved effector proteins have not been identified yet. Therefore, we evaluated the role of two major S. Typhimurium effectors SopB and SifA during D. discoideum intracellular niche formation. First, we established that S. Typhimurium resides in a vacuolar compartment within D. discoideum. Next, we isolated SCVs from amoebae infected with wild type or the ΔsopB and ΔsifA mutant strains of S. Typhimurium, and we characterised the composition of this compartment by quantitative proteomics. This comparative analysis suggests that S. Typhimurium requires SopB and SifA to modify the SCV proteome in order to generate a suitable intracellular niche in D. discoideum. Accordingly, we observed that SopB and SifA are needed for intracellular survival of S. Typhimurium in this organism. Thus, our results provide insight into the mechanisms employed by Salmonella to survive intracellularly in phagocytic amoebae.

Evidence of viable Helicobacter pylori and other bacteria of public health interest associated with free-living amoebae in lettuce samples by next generation sequencing and other molecular techniques

Vegetables are one of the sources from which Helicobacter pylori can be acquired. This bacterium infects >50% of the global population and is a recognized type I human carcinogen. H. pylori enters into the viable but non-culturable state when it is in the environment, and therefore the use of molecular techniques is much convenient for its detection. Free-living amoebae (FLA) are protozoans found in vegetables. They are transmission vehicles for amoeba-resistant bacteria, among which H. pylori is included. The aim of this study is to study the occurrence and viability of H. pylori from lettuce samples, H. pylori internalized into FLA and the microbiome of FLA isolated from these samples. Special focus was pointed to human pathogenic bacteria. H. pylori was not directly detected in any lettuce sample by means of molecular techniques and neither by culture. However, intra-amoebic H. pylori DNA was detected by means of PMA-qPCR in 55% of the samples and viable intra-amoebic H. pylori cells in 25% of the samples by means of DVC-FISH technique. When FLA microbiome was studied, 21 bacterial genera were part of FLA microbiome in all samples. Helicobacter genus was detected as part of the FLA microbiome in two samples. Other bacteria of public health interest such as Aeromonas sp., Arcobacter sp., Legionella sp., Mycobacterium sp., Pseudomonas sp. and Salmonella sp. were detected as part of FLA microbiome along the analysed samples. This study demonstrates for the first time that H. pylori is internalized as well as alive inside FLA isolated from vegetables. Moreover, this study shows that FLA promote H. pylori detection in environmental samples. In addition, as far as we are aware, this is the first study which studies the microbiome of FLA isolated from vegetables. Among the FLA microbiome, bacteria of public health interest were detected, pointing out that FLA are carriers of these pathogens which can reach humans and cause a public health concern.

Type 1 Fimbriae and Motility Play a Pivotal Role During Interactions of Salmonella typhimurium with Acanthamoeba castellanii (T4 Genotype)

Amoebic bacterial interactions are the most ancient form of host pathogen interactions. Here, we investigate the fate of Salmonella typhimurium and Acanthamoeba castellanii T4 genotype upon mutual interactions in a nutrition free environment. The role of type 1 fimbriae and motility of S. typhimurium during interactions with A. castellanii has also been investigated. Deletion of genes encoding the type 1 fimbriae subunit FimA, type 1 fimbriae tip protein FimH, chemotaxis regulatory proteins CheA and CheY and major flagella subunits FliC and FljB was performed through homologous recombination. In vitro association, invasion and survival assays of S. typhimurium wild-type and mutant strains were performed upon co-incubation of bacteria with A. castellanii trophozoites in a nutrition free environment. The deletion gene encoding type 1 fimbriae subunit FimA reduced, whereas the deletion of genes encoding flagella subunits FliC and FljB of flagella enhanced the association capability of S. typhimurium with A. castellanii. Invasion of A. castellanii by Salmonella was significantly reduced upon the loss of type 1 fimbriae subunit FimA and type 1 fimbriae tip protein FimH. Co-incubation of S. typhimurium with A. castellanii in phosphate buffered saline medium stimulated the growth of S. typhimurium wild-type and mutant strains. Viable A. castellanii trophozoites count became significantly reduced upon co-incubation with S. typhimurium within 48 h. Type 1 fimbriae play a pivotal role in the adherence of S. typhimurium to the A. castellanii cell surface. Subsequently, this interaction provides S. typhimurium an advantage in growth.

Effect of the bacterial growth phase and coculture conditions on the interaction of Acanthamoeba castellanii with Shigella dysenteriae, Shigella flexneri, and Shigella sonnei

Shigella species and Acanthamoeba castellanii share the same ecological niches, and their interaction has been addressed in a limited number of research. However, there are still uncertain aspects and discrepant findings of this interaction. In the present study, the effects of the bacterial growth phase, cocultivation temperature and the type of culture media on the interaction of A. castellanii with Shigella dysenteriae, Shigella sonnei and Shigella flexneri were evaluated. In nutrient-poor page's amoeba saline (PAS) medium, the number of recovered bacteria and the uptake rates were significantly higher in stationary phase cells than logarithmic phase cells. However, no significant differences were observed in the number of recovered bacteria and the uptake rates between logarithmic and stationary phase cells in nutrient-rich peptone-yeast extract-glucose (PYG) medium. While the number of recovered bacteria was significantly higher in nutrient-rich than nutrient-poor media, in all the three Shigella species, the bacterial uptake rates were significantly higher in nutrient-poor than nutrient-rich media at both cocultivation temperatures. In both nutrient-poor and nutrient-rich media and at both cocultivation temperatures, the number of viable Shigella species after 24 h incubation were not influenced by the presence of A. castellanii. Although Shigella species did not proliferate in A. castellanii trophozoites, a considerable number of bacteria were survived in the trophozoites up to 15 days. From the public health perspective, the results of this study are important for further understanding of the nature of the interaction of these organisms and to deal with Shigella species in the environment.

Reservoirs and Transmission Pathways of Resistant Indicator Bacteria in the Biotope Pig Stable and along the Food Chain: A Review from a One Health Perspective

The holistic approach of “One Health” includes the consideration of possible links between animals, humans, and the environment. In this review, an effort was made to highlight knowledge gaps and various factors that contribute to the transmission of antibiotic-resistant bacteria between these three reservoirs. Due to the broad scope of this topic, we focused on pig production and selected “indicator bacteria”. In this context, the role of the bacteria livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) and extended spectrum beta-lactamases carrying Escherichia coli (ESBL-E) along the pig production was particularly addressed. Hotspots of their prevalence and transmission are, for example, pig stable air for MRSA, or wastewater and manure for ESBL-E, or even humans as vectors in close contact to pigs (farmers and veterinarians). Thus, this review focuses on the biotope “stable environment” where humans and animals are both affected, but also where the end of the food chain is not neglected. We provide basic background information about antibiotics in livestock, MRSA, and ESBL-bacteria. We further present studies (predominantly European studies) in tabular form regarding the risk potentials for the transmission of resistant bacteria for humans, animals, and meat differentiated according to biotopes. However, we cannot guarantee completeness as this was only intended to give a broad superficial overview. We point out sustainable biotope approaches to try to contribute to policy management as critical assessment points in pig housing conditions, environmental care, animal health, and food product safety and quality as well as consumer acceptance have already been defined.

Presence and Persistence of Salmonella in Water: The Impact on Microbial Quality of Water and Food Safety

Salmonella ranks high among the pathogens causing foodborne disease outbreaks. According to the Centers for Disease Control and Prevention, Salmonella contributed to about 53.4% of all foodborne disease outbreaks from 2006 to 2017, and approximately 32.7% of these foodborne Salmonella outbreaks were associated with consumption of produce. Trace-back investigations have suggested that irrigation water may be a source of Salmonella contamination of produce and a vehicle for transmission. Presence and persistence of Salmonella have been reported in surface waters such as rivers, lakes, and ponds, while ground water in general offers better microbial quality for irrigation. To date, culture methods are still the gold standard for detection, isolation and identification of Salmonella in foods and water. In addition to culture, other methods for the detection of Salmonella in water include most probable number, immunoassay, and PCR. The U.S. Food and Drug Administration (FDA) issued the Produce Safety Rule (PSR) in January 2013 based on the Food Safety Modernization Act (FSMA), which calls for more efforts toward enhancing and improving approaches for the prevention of foodborne outbreaks. In the PSR, agricultural water is defined as water used for in a way that is intended to, or likely to, contact covered produce, such as spray, wash, or irrigation. In summary, Salmonella is frequently present in surface water, an important source of water for irrigation. An increasing evidence indicates irrigation water as a source (or a vehicle) for transmission of Salmonella. This pathogen can survive in aquatic environments by a number of mechanisms, including entry into the viable but nonculturable (VBNC) state and/or residing within free-living protozoa. As such, assurance of microbial quality of irrigation water is critical to curtail the produce-related foodborne outbreaks and thus enhance the food safety. In this review, we will discuss the presence and persistence of Salmonella in water and the mechanisms Salmonella uses to persist in the aquatic environment, particularly irrigation water, to better understand the impact on the microbial quality of water and food safety due to the presence of Salmonella in the water environment.

The cellulose synthase BcsA plays a role in interactions of Salmonella typhimurium with Acanthamoeba castellanii genotype T4

Pathogenic bacteria share their natural habitat with many other organisms such as animals, plants, insects, parasites and amoeba. Interactions between these organisms influence not only the life style of the host organisms, but also modulate bacterial physiology. Adaptation can include biofilm formation, capsule formation, and production of virulence factors. Although biofilm formation is a dominant mode of bacterial life in environmental settings, its role in host-pathogen interactions is not extensively studied. In this work, we investigated the role of molecular pathways involved in rdar biofilm formation in the interaction of Salmonella typhimurium with the Acanthamoeba castellanii genotype T4. Genes coding for the rdar biofilm activator CsgD, the cellulose synthase BcsA, and curli fimbriae subunits CsgBA were deleted from the genome of S. typhimurium. Assessment of interactions of wild-type and mutant strains of S. typhimurium with A. castellanii revealed that deletion of the cellulose synthase BcsA promoted association and uptake by A. castellanii, whereas the interactions with csgD and csgBA mutants were not changed. Our findings suggest that cellulose synthase BcsA inhibits the capabilities of S. typhimurium to associate with and invade into A. castellanii.

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.

Free-Living Amoebae as Hosts for and Vectors of Intracellular Microorganisms with Public Health Significance

Free-living amoebae (FLA) are parasites within both humans and animals causing a wide range of symptoms and act as hosts of, and vehicles for phylogenetically diverse microorganisms, called endocytobionts. The interaction of the FLA with sympatric microorganisms leads to an exceptional diversity within FLA. Some of these bacteria, viruses, and even eukaryotes, can live and replicate intracellularly within the FLA. This relationship provides protection to the microorganisms from external interventions and a dispersal mechanism across various habitats. Among those intracellularly-replicating or -residing organisms there are obligate and facultative pathogenic microorganisms affecting the health of humans or animals and are therefore of interest to Public Health Authorities. Mimiviruses, Pandoraviruses, and Pithoviruses are examples for interesting viral endocytobionts within FLA. Future research is expected to reveal further endocytobionts within free-living amoebae and other protozoa through co-cultivation studies, genomic, transcriptomic, and proteomic analyses.

Relevant Genes Linked to Virulence Are Required for Salmonella Typhimurium to Survive Intracellularly in the Social Amoeba Dictyostelium discoideum

The social amoeba Dictyostelium discoideum has proven to be a useful model for studying relevant aspects of the host-pathogen interaction. In this work, D. discoideum was used as a model to study the ability of Salmonella Typhimurium to survive in amoebae and to evaluate the contribution of selected genes in this process. To do this, we performed infection assays using axenic cultures of D. discoideum co-cultured with wild-type S. Typhimurium and/or defined mutant strains. Our results confirmed that wild-type S. Typhimurium is able to survive intracellularly in D. discoideum. In contrast, mutants ΔaroA and ΔwaaL are defective in intracellular survival in this amoeba. Next, we included in our study a group of mutants in genes directly linked to Salmonella virulence. Of note, mutants ΔinvA, ΔssaD, ΔclpV, and ΔphoPQ also showed an impaired ability to survive intracellularly in D. discoideum. This indicates that S. Typhimurium requires a functional biosynthetic pathway of aromatic compounds, a lipopolysaccharide containing a complete O-antigen, the type III secretion systems (T3SS) encoded in SPI-1 and SPI-2, the type VI secretion system (T6SS) encoded in SPI-6 and PhoP/PhoQ two-component system to survive in D. discoideum. To our knowledge, this is the first report on the requirement of O-antigen and T6SS in the survival of Salmonella within amoebae. In addition, mutants ΔinvA and ΔssaD were internalized in higher numbers than the wild-type strain during competitive infections, suggesting that S. Typhimurium requires the T3SS encoded in SPI-1 and SPI-2 to evade phagocytosis by D. discoideum. Altogether, these results indicate that S. Typhimurium exploits a common set of genes and molecular mechanisms to survive within amoeba and animal host cells. The use of D. discoideum as a model for host-pathogen interactions will allow us to discover the gene repertoire used by Salmonella to survive inside the amoeba and to study the cellular processes that are affected during infection.

Packaging of Campylobacter jejuni into Multilamellar Bodies by the Ciliate Tetrahymena pyriformis

Campylobacter jejuniis the leading cause of bacterial gastroenteritis worldwide. Transmission to humans occurs through consumption of contaminated food or water. The conditions affecting the persistence of C. jejuniin the environment are poorly understood. Some protozoa package and excrete bacteria into multilamellar bodies (MLBs). Packaged bacteria are protected from deleterious conditions, which increases their survival. We hypothesized that C. jejuni could be packaged under aerobic conditions by the amoeba Acanthamoeba castellanii or the ciliate Tetrahymena pyriformis, both of which are able to package other pathogenic bacteria.A. castellanii did not produce MLBs containing C. jejuni In contrast, when incubated with T. pyriformis,C. jejuni was ingested, packaged in MLBs, and then expelled into the milieu. The viability of the bacteria inside MLBs was confirmed by microscopic analyses. The kinetics of C. jejuni culturability showed that packaging increased the survival of C. jejuniup to 60 h, in contrast to the strong survival defect seen in ciliate-free culture. This study suggests that T. pyriformis may increase the risk of persistence of C. jejuniin the environment and its possible transmission between different reservoirs in food and potable water through packaging.

One Health and Food-Borne Disease: Salmonella Transmission between Humans, Animals, and Plants

There are >2,600 recognized serovars of Salmonella enterica. Many of these Salmonella serovars have a broad host range and can infect a wide variety of animals, including mammals, birds, reptiles, amphibians, fish, and insects. In addition, Salmonella can grow in plants and can survive in protozoa, soil, and water. Hence, broad-host-range Salmonella can be transmitted via feces from wild animals, farm animals, and pets or by consumption of a wide variety of common foods: poultry, beef, pork, eggs, milk, fruit, vegetables, spices, and nuts. Broad-host-range Salmonella pathogens typically cause gastroenteritis in humans. Some Salmonella serovars have a more restricted host range that is associated with changes in the virulence plasmid pSV, accumulation of pseudogenes, and chromosome rearrangements. These changes in host-restricted Salmonella alter pathogen-host interactions such that host-restricted Salmonella organisms commonly cause systemic infections and are transmitted between host populations by asymptomatic carriers. The secondary consequences of efforts to eliminate host-restricted Salmonella serovars demonstrate that basic ecological principles govern the environmental niches occupied by these pathogens, making it impossible to thwart Salmonella infections without a clear understanding of the human, animal, and environmental reservoirs of these pathogens. Thus, transmission of S. enterica provides a compelling example of the One Health paradigm because reducing human infections will require the reduction of Salmonella in animals and limitation of transmission from the environment.

An intracellular replication niche for Vibrio cholerae in the amoeba Acanthamoeba castellanii

Vibrio cholerae is a human pathogen and the causative agent of cholera. The persistence of this bacterium in aquatic environments is a key epidemiological concern, as cholera is transmitted through contaminated water. Predatory protists, such as amoebae, are major regulators of bacterial populations in such environments. Therefore, we investigated the interaction between V. cholerae and the amoeba Acanthamoeba castellanii at the single-cell level. We observed that V. cholerae can resist intracellular killing. The non-digested bacteria were either released or, alternatively, established a replication niche within the contractile vacuole of A. castellanii. V. cholerae was maintained within this compartment even upon encystment. The pathogen ultimately returned to its aquatic habitat through lysis of A. castellanii, a process that was dependent on the production of extracellular polysaccharide by the pathogen. This study reinforces the concept that V. cholerae is a facultative intracellular bacterium and describes a new host–pathogen interaction.

Draft Genome Sequence of Shewanella sp. Strain CP20

Shewanella sp. CP20 is a marine bacterium that survives ingestion by Tetrahymena pyriformis and is expelled from the protozoan within membrane-bound vacuoles, where the bacterial cells show long-term survival. Here, we report the draft genome sequence of Shewanella sp. CP20 and discuss the potential mechanisms facilitating intraprotozoan survival.

Biofilms: an advancement in our understanding of Francisella species

Our understanding of the virulence and pathogenesis of Francisella spp. has significantly advanced in recent years, including a new understanding that this organism can form biofilms. What is known so far about Francisella spp. biofilms is summarized here and future research questions are suggested. The molecular basis of biofilm production has begun to be studied, especially the role of extracellular carbohydrates and capsule, quorum sensing and two-component signaling systems. Further work has explored the contribution of amoebae, pili, outer-membrane vesicles, chitinases, and small molecules such as c-di-GMP to Francisella spp. biofilm formation. A role for Francisella spp. biofilm in feeding mosquito larvae has been suggested. As no strong role in virulence has been found yet, Francisella spp. biofilm formation is most likely a key mechanism for environmental survival and persistence. The significance and importance of Francisella spp.’s biofilm phenotype as a critical aspect of its microbial physiology is being developed. Areas for further studies include the potential role of Francisella spp. biofilms in the infection of mammalian hosts and virulence regulation.

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.

Salmonella interactions with plants and their associated microbiota

The increase in the incidence of gastroenteritis outbreaks linked to the consumption of foods of plant origin has ignited public concern and scientific interest in understanding interactions of human enteric pathogens with plants. Enteric disease caused by nontyphoidal Salmonella is a major public health burden, with the number of cases of illness linked to fresh produce, spices, and nuts surpassing those linked to foods of animal origin. Mounting evidence supports the hypothesis that colonization of plants is an important part of the life cycle of this human pathogen. Although plant responses to human pathogens are distinct from the more specific responses to phytopathogens, plants appear to recognize Salmonella, likely by detecting conserved microbial patterns, which subsequently activates basal defenses. Numerous Salmonella genes have been identified as playing a role in its colonization of plant surfaces and tissues, and in its various interactions with other members of the phyto-microbial community. Importantly, Salmonella utilizes diverse and overlapping strategies to interact with plants and their microflora, and to successfully colonize its vertebrate hosts. This review provides insight into the complex behavior of Salmonella on plants and the apparent remarkable adaptation of this human pathogen to a potentially secondary host.

Evaluation of the potential antimicrobial resistance transfer from a multi-drug resistant Escherichia coli to Salmonella in dairy calves

Previous research conducted in our laboratory found a significant prevalence of multi-drug resistant (MDR) Salmonella and MDR Escherichia coli (MDR EC) in dairy calves and suggests that the MDR EC population may be an important reservoir for resistance elements that could potentially transfer to Salmonella. Therefore, the objective of the current research was to determine if resistance transfers from MDR EC to susceptible strains of inoculated Salmonella. The experiment utilized Holstein calves (approximately 3 weeks old) naturally colonized with MDR EC and fecal culture negative for Salmonella. Fecal samples were collected for culture of Salmonella and MDR EC throughout the experiment following experimental inoculation with the susceptible Salmonella strains. Results initially suggested that resistance did transfer from the MDR E. coli to the inoculated strains of Salmonella, with these stains demonstrating resistance to multiple antibiotics following in vivo exposure to MDR EC. However, serogrouping and serotyping results from a portion of the Salmonella isolates recovered from the calves post-challenge, identified two new strains of Salmonella; therefore transfer of resistance was not demonstrated under these experimental conditions.

Amoeba-related health risk in drinking water systems: could monitoring of amoebae be a complementary approach to current quality control strategies?

Culture-based methods for fecal indicator microorganisms are the standard protocol to assess potential health risk from drinking water systems. However, these traditional fecal indicators are inappropriate surrogates for disinfection-resistant fecal pathogens and the indigenous pathogens that grow in drinking water systems. There is now a range of molecular-based methods, such as quantitative PCR, which allow detection of a variety of pathogens and alternative indicators. Hence, in addition to targeting total Escherichia coli (i.e., dead and alive) for the detection of fecal pollution, various amoebae may be suitable to indicate the potential presence of pathogenic amoeba-resisting microorganisms, such as Legionellae. Therefore, monitoring amoeba levels by quantitative PCR could be a useful tool for directly and indirectly evaluating health risk and could also be a complementary approach to current microbial quality control strategies for drinking water systems.

Survival of Campylobacter jejuni in co-culture with Acanthamoeba castellanii: role of amoeba-mediated depletion of dissolved oxygen

Campylobacter jejuni is a major cause of infectious diarrhoea worldwide but relatively little is known about its ecology. In this study, we examined its interactions with Acanthamoeba castellanii, a protozoan suspected to serve as a reservoir for bacterial pathogens. We observed rapid degradation of intracellular C.jejuni in A.castellanii 5 h post gentamicin treatment at 25°C. Conversely, we found that A.castellanii promoted the extracellular growth of C.jejuni in co-cultures at 37°C in aerobic conditions. This growth-promoting effect did not require amoebae - bacteria contact. The growth rates observed with or without contact with amoeba were similar to those observed when C.jejuni was grown in microaerophilic conditions. Preconditioned media prepared with live or dead amoebae cultivated with or without C.jejuni did not promote the growth of C.jejuni in aerobic conditions. Interestingly, the dissolved oxygen levels of co-cultures with or without amoebae - bacteria contact were much lower than those observed with culture media or with C.jejuni alone incubated in aerobic conditions, and were comparable with levels obtained after 24 h of growth of C.jejuni under microaerophilic conditions. Our studies identified the depletion of dissolved oxygen by A.castellanii as the major contributor for the observed amoeba-mediated growth enhancement.

Acanthamoeba polyphaga, a potential environmental vector for the transmission of food-borne and opportunistic pathogens

The endosymbiotic relationship could represent for many bacteria an important condition favouring their spread in the environment and in foods. For this purpose we studied the behaviour of some food-borne and opportunistic pathogens (Listeria monocytogenes, Staphylococcus aureus, Enterococcus faecalis, Salmonella enterica serovar Enteritidis, Aeromonas hydrophila, Yersinia enterocolitica) when internalized in Acanthamoeba polyphaga. Our results confirm the capability of the bacteria tested to grow within amoebal hosts. We can observe two types of interactions of the bacteria internalized in A. polyphaga. The first type, showed by Y. enterocolitica and A. hydrophila, was characterized by an early replication, probably followed by the killing and digestion of the bacteria. The second type, showed by E. faecalis and S. aureus was characterized by the persistence and grow inside the host without lysis. Lastly, when amoebae were co-cultured with L. monocytogenes and S. Enteritidis, an eclipse phase followed by an active intracellular growth was observed, suggesting a third type of predator-prey trend. The extracellular count in presence of A. polyphaga, as a result of an intracellular multiplication and subsequent release, was characterized by an increase of E. faecalis, S. aureus, L. monocytogenes and S. Enteritidis, and by a low or absent cell count for Y. enterocolitica and A. hydrophila. Our study suggests that the investigated food-borne and opportunistic pathogens are, in most cases, able to interact with A. polyphaga, to intracellularly replicate and, lastly, to be potentially spread in the environment, underlining the possible role of this protozoan in food contamination.

Increased persistence of Salmonella enterica serovar Typhi in the presence of Acanthamoeba castellanii

Salmonella enterica serovar Typhi (S. Typhi) is the etiological agent of the systemic disease typhoid fever. Transmission occurs via ingestion of contaminated food or water. S. Typhi is specific to humans, and no animal or environmental reservoirs are known. As the free-living amoeba Acanthamoeba castellanii is an environmental host for many pathogenic bacteria, this study investigates interactions between S. Typhi and A. castellanii by using cocultures. Growth of both organisms was estimated by cell count, viable count, flow cytometry, and fluorescence microscopy. Results indicate that S. Typhi can survive at least 3 weeks when grown with A. castellanii, as opposed to less than 10 days when grown as singly cultured bacteria under the same conditions. Interestingly, growth rates of amoebae after 14 days were similar in cocultures or when amoebae were singly cultured, suggesting that S. Typhi is not cytotoxic to A. castellanii. Bacteria surviving in coculture were not intracellular and did not require a physical contact with amoebae for their survival. These results suggest that S. Typhi may have a selective advantage when it is associated with A. castellanii and that amoebae may contribute to S. Typhi persistence in the environment.

Semi-automated Acanthamoeba polyphaga detection and computation of Salmonella typhimurium concentration in spatio-temporal images

Interaction between bacteria and protozoa is an increasing area of interest, however there are a few systems that allow extensive observation of the interactions. A semi-automated approach is proposed to analyse a large amount of experimental data and avoid a time demanding manual object classification. We examined a surface system consisting of non nutrient agar with a uniform bacterial lawn that extended over the agar surface, and a spatially localised central population of amoebae. Location and identification of protozoa and quantification of bacteria population are performed by the employment of image analysis techniques in a series of spatial images. The quantitative tools are based on intensity thresholding, or on probabilistic models. To accelerate organism identification, correct classification errors and attain quantitative details of all objects a custom written Graphical User Interfaces has also been developed.

Detection of Vibrio cholerae and Acanthamoeba species from same natural water samples collected from different cholera endemic areas in Sudan

Background

Vibrio cholerae O1 and V. cholerae O139 infect humans, causing the diarrheal and waterborne disease cholera, which is a worldwide health problem. V. cholerae and the free-living amoebae Acanthamoeba species are present in aquatic environments, including drinking water and it has shown that Acanthamoebae support bacterial growth and survival. Recently it has shown that Acanthamoeba species enhanced growth and survival of V. cholerae O1 and O139. Water samples from different cholera endemic areas in Sudan were collected with the aim to detect both V. cholerae and Acanthamoeba species from same natural water samples by polymerase chain reaction (PCR).

Findings

For the first time both V. cholerae and Acanthamoeba species were detected in same natural water samples collected from different cholera endemic areas in Sudan. 89% of detected V. cholerae was found with Acanthamoeba in same water samples.

Conclusions

The current findings disclose Acanthamoedae as a biological factor enhancing survival of V. cholerae in nature.

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.

Identification of protozoa in dairy lagoon wastewater that consume Escherichia coli O157:H7 preferentially

Escherichia coli O157:H7 (EcO157), an agent of life threatening hemolytic-uremic syndrome, resides in ruminants and is released in feces at numbers as high as 10 million cells/gram. EcO157 could survive in manure for as long as 21 months, but we observed a 90% decrease in cells of an outbreak strain of EcO157 within half a day in wastewater from dairy lagoons. Although chemical, environmental and biological factors may be responsible for this decrease, we observed an 11-fold increase in native protozoa when wastewater was re-inoculated with 2×10⁷ cells of EcO157/mL. These protozoa engulfed the green fluorescent protein labeled EcO157 within 2 hours after inoculation, but expelled vacuoles filled with live EcO157 cells within 3 days into surrounding wastewater, whereas other protozoa retained the EcO157-filled vacuoles for 7 days. EcO157 was not detected by confocal microscopy either inside or outside protozoa after 7 days. Mixed cultures of protozoa enriched from wastewater consumed EcO157 preferentially as compared to native aerobic bacteria, but failed to eliminate them when EcO157 cells declined to 10⁴/mL. We isolated three protozoa from mixed cultures and typed them by 18S sequencing as Vorticella microstoma, Platyophyra sp. and Colpoda aspera. While all three protozoa internalized EcO157, only Platyophyra and Colpoda acted as predators. Similar to mixed cultures, these protozoa failed to eliminate EcO157 from PBS containing no other supplemental nutrients or prey. However, spiking PBS with cereal grass medium as nutrients induced predation of EcO157 by Platyophyra sp. after 3 days or enhanced predation by Colpoda after 5 days. Therefore, attempts to enrich protozoa to decrease EcO157 from dairy lagoons, may correspond to an increase in protozoa similar to Vorticella and possibly facilitate transport of bacterial pathogens to food crops grown in proximity.

Francisella philomiragia biofilm formation and interaction with the aquatic protist Acanthamoeba castellanii

The bacterium Francisella philomiragia has been isolated from environmental samples originating from around the globe. F. philomiragia-related strains cause francisellosis of both farmed and wild fish. In addition, occasional human infections caused by F. philomiragia are found in victims of near-drowning and patients with chronic granulomatous disease. We have shown that F. philomiragia forms in vitro biofilms with increased formation at 25 °C over 37 °C conditions. We found that F. philomiragia can form a biofilm in a co-culture with live Acanthamoeba castellanii, an aquatic amoeba. Interestingly, amoeba-conditioned supernatant has an inhibitory effect on production of biofilm by F. philomiragia, whereas Francisella-conditioned supernatant has no effect on growth of amoebae. We have shown that F. philomiragia can infect A. castellanii after only 5 days of co-incubation and that it infects A. castellanii more quickly than the related species F. novicida does. Our studies point to a potentially overlooked interaction between F. philomiragia and Acanthamoeba. This relationship in the marine lifecycle of F. philomiragia may support the persistence of the bacterium in waterways and its ability to infect fish. An understanding of the persistence of this organism in aquatic systems through biofilm formation and its interaction with Acanthamoeba will be important in developing prevention strategies for this pathogen.

Salmonella transcriptional signature in Tetrahymena phagosomes and role of acid tolerance in passage through the protist

Salmonella enterica Typhimurium remains undigested in the food vacuoles of the common protist, Tetrahymena. Contrary to its interaction with Acanthamoeba spp., S. Typhimurium is not cytotoxic to Tetrahymena and is egested as viable cells in its fecal pellets. Through microarray gene expression profiling we investigated the factors in S. Typhimurium that are involved in its resistance to digestion by Tetrahymena. The transcriptome of S. Typhimurium in Tetrahymena phagosomes showed that 989 and 1282 genes were altered in expression compared with that in water and in LB culture medium, respectively. A great proportion of the upregulated genes have a role in anaerobic metabolism and the use of alternate electron acceptors. Many genes required for survival and replication within macrophages and human epithelial cells also had increased expression in Tetrahymena, including mgtC, one of the most highly induced genes in all three cells types. A ΔmgtC mutant of S. Typhimurium did not show decreased viability in Tetrahymena, but paradoxically, was egested at a higher cell density than the wild type. The expression of adiA and adiY, which are involved in arginine-dependent acid resistance, also was increased in the protozoan phagosome. A ΔadiAY mutant had lower viability after passage through Tetrahymena, and a higher proportion of S. Typhimurium wild-type cells within pellets remained viable after exposure to pH 3.4 as compared with uningested cells. Our results provide evidence that acid resistance has a role in the resistance of Salmonella to digestion by Tetrahymena and that passage through the protist confers physiological advantages relevant to its contamination cycle.

Free-living amoebae, a training field for macrophage resistance of mycobacteria

Mycobacterium species evolved from an environmental recent common ancestor by reductive evolution and lateral gene transfer. Strategies selected through evolution and developed by mycobacteria resulted in resistance to predation by environmental unicellular protists, including free-living amoebae. Indeed, mycobacteria are isolated from the same soil and water environments as are amoebae, and experimental models using Acanthamoeba spp. and Dictyostelium discoideum were exploited to analyse the mechanisms for intracellular survival. Most of these mechanisms have been further reproduced in macrophages for mycobacteria regarded as opportunistic and obligate pathogens. Amoebal cysts may protect intracellular mycobacteria against adverse conditions and may act as a vector for mycobacteria. The latter hypothesis warrants further environmental and clinical studies to better assess the role of free-living amoebae in the epidemiology of infections caused by mycobacteria.

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.

Vibrio cholerae O139 requires neither capsule nor LPS O side chain to grow inside Acanthamoeba castellanii

Vibrio cholerae, the causative agent of cholera, has the ability to grow and survive in the aquatic free-living amoeba Acanthamoeba castellanii. The aim of the present study was to examine the ability of the clinical isolate V. cholerae O139 MO10 to grow in A. castellanii and to determine the effect of the bacterial capsule and LPS O side chain on intracellular growth. Results from co-cultivation, viable counts, a gentamicin assay, electron microscopy and statistical analysis showed that the association of V. cholerae O139 MO10 with A. castellanii did not inhibit growth of the amoeba, and enhanced growth and survival of V. cholerae O139 MO10 occurred. The wild-type V. cholerae O139 MO10 and a capsule mutant or capsule/LPS double mutant grew inside A. castellanii. Neither the capsule nor the LPS O side chain of V. cholerae O139 was found to play an important role in the interaction with A. castellanii, disclosing the ability of V. cholerae to multiply and survive inside A. castellanii, as well as the role of A. castellanii as an environmental host for V. cholerae.

The Salmonella pathogenicity island 2-encoded type III secretion system is essential for the survival of Salmonella enterica serovar Typhimurium in free-living amoebae

Free-living amoebae represent a potential reservoir and predator of Salmonella enterica. Through the use of type III secretion system (T3SS) mutants and analysis of transcription of selected T3SS genes, we demonstrated that the Salmonella pathogenicity island 2 is highly induced during S. enterica serovar Typhimurium infection of Acanthamoeba polyphaga and is essential for survival within amoebae.

Microscopic and molecular studies of the diversity of free-living protozoa in meat-cutting plants

The diversity of free-living protozoa in five meat-cutting plants was determined. Light microscopy after enrichment culturing was combined with sequencing of PCR-amplified, denaturing gradient gel electrophoresis (DGGE)-separated 18S rRNA gene fragments, which was used as a fast screening method. The general results of the survey showed that a protozoan community of amoebae, ciliates, and flagellates was present in all of the plants. Protozoa were detected mainly in floor drains, in standing water on the floor, on soiled bars of cutting tables, on plastic pallets, and in out-of-use hot water knife sanitizers, but they were also detected on surfaces which come into direct contact with meat, such as conveyer belts, working surfaces of cutting tables, and needles of a meat tenderizer. After 7 days of incubation at refrigerator temperature, protozoa were detected in about one-half of the enrichment cultures. Based on microscopic observations, 61 morphospecies were found, and Bodo saltans, Bodo spp., Epistylis spp., Glaucoma scintillans, Petalomonas spp., Prodiscophrya collini, and Vannella sp. were the most frequently encountered identified organisms. Sequencing of DGGE bands resulted in identification of a total of 49 phylotypes, including representatives of the Amoebozoa, Chromalveolata, Excavata, Opisthokonta, and Rhizaria. Sequences of small heterotrophic flagellates were affiliated mainly with the Alveolata (Apicomplexa), Stramenopiles (Chrysophyceae), and Rhizaria (Cercozoa). This survey showed that there is high protozoan species richness in meat-cutting plants and that the species included species related to known hosts of food-borne pathogens.

Interactions of some common pathogenic bacteria with Acanthamoeba polyphaga

Protozoan grazing is a major trophic pathway whereby the biomass re-enters the food web. Nonetheless, not all bacteria are digested by protozoa and the number known to evade digestion, resulting in their environmental augmentation, is increasing. We investigated the interactions of Bacillus cereus, Enterococcus faecalis, Enteropathogenic Escherichia coli (EPEC), Listeria monocytogenes, Salmonella enterica serovar Typhimurium, and methicillin-sensitive Staphylococcus aureus (MSSA), with the amoeba, Acanthamoeba polyphaga. There was evidence of predation of all bacterial species except L. monocytogenes and S. aureus, where extracellular numbers were significantly higher when cultured with amoebae compared with growth in the absence of amoebae. Intracellular growth kinetic experiments and fluorescent confocal microscopy suggest that S. aureus survived and may even multiply within A. polyphaga, whereas there was no apparent intra-amoebal replication of L. monocytogenes and higher numbers were likely sustained on metabolic waste products released during coculture.