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

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.

Uptake and replication in Acanthamoeba castellanii of a virulent (pVAPA-positive) strain of Rhodococcus equi and its isogenic, plasmid-cured strain

Rhodococcus equi is a soil saprophytic bacterium and intracellular pathogen that causes pneumonia in foals. Strains of R. equi that are virulent in foals contain a plasmid that encodes a virulence-associated protein A (VapA) necessary for replication in macrophages. Because other intracellular pathogens survive and replicate inside amoebae, we postulated that the VapA-bearing plasmid (pVAPA) confers a survival advantage for R. equi against environmental predators like amoebae. To test this hypothesis, we compared phagocytosis by and survival in Acanthamoeba castellanii of isogenic strains of pVAPA-positive and pVAPA-negative R. equi. Phagocytosis of the pVAPA-negative strain by A. castellanii was significantly (P < 0.0001) greater than the pVAPA-positive strain. Intracellular replication of the pVAPA-positive strain in A. castellanii was significantly (P < 0.0001) greater than the pVAPA-negative strain during both 48 h and 9 days. These results indicate that the presence of the VapA plasmid reduces uptake and aids replication of R. equi in A. castellanii.

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.”

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.

Dual Role of Mechanisms Involved in Resistance to Predation by Protozoa and Virulence to Humans

Most opportunistic pathogens transit in the environment between hosts and the environment plays a significant role in the evolution of protective traits. The coincidental evolution hypothesis suggests that virulence factors arose as a response to other selective pressures rather for virulence per se. This idea is strongly supported by the elucidation of bacterial-protozoal interactions. In response to protozoan predation, bacteria have evolved various defensive mechanisms which may also function as virulence factors. In this review, we summarize the dual role of factors involved in both grazing resistance and human pathogenesis, and compare the traits using model intracellular and extracellular pathogens. Intracellular pathogens rely on active invasion, blocking of the phagosome and lysosome fusion and resistance to phagocytic digestion to successfully invade host cells. In contrast, extracellular pathogens utilize toxin secretion and biofilm formation to avoid internalization by phagocytes. The complexity and diversity of bacterial virulence factors whose evolution is driven by protozoan predation, highlights the importance of protozoa in evolution of opportunistic pathogens.

Acid-Suppressive Therapy and Risk of Infections: Pros and Cons

This narrative review summarises the benefits, risks and appropriate use of acid-suppressing drugs (ASDs), proton pump inhibitors and histamine-2 receptor antagonists, advocating a rationale balanced and individualised approach aimed to minimise any serious adverse consequences. It focuses on current controversies on the potential of ASDs to contribute to infections-bacterial, parasitic, fungal, protozoan and viral, particularly in the elderly, comprehensively and critically discusses the growing body of observational literature linking ASD use to a variety of enteric, respiratory, skin and systemic infectious diseases and complications (Clostridium difficile diarrhoea, pneumonia, spontaneous bacterial peritonitis, septicaemia and other). The proposed pathogenic mechanisms of ASD-associated infections (related and unrelated to the inhibition of gastric acid secretion, alterations of the gut microbiome and immunity), and drug-drug interactions are also described. Both probiotics use and correcting vitamin D status may have a significant protective effect decreasing the incidence of ASD-associated infections, especially in the elderly. Despite the limitations of the existing data, the importance of individualised therapy and caution in long-term ASD use considering the balance of benefits and potential harms, factors that may predispose to and actions that may prevent/attenuate adverse effects is evident. A six-step practical algorithm for ASD therapy based on the best available evidence is presented.

Impact of Acanthamoeba Cysts on Stress Resistance of Salmonella enterica Serovar Typhimurium, Yersinia enterocolitica 4/O:3, Listeria monocytogenes 1/2a, and Escherichia coli O:26

The formation of robust resting cysts enables Acanthamoeba to resist harsh environmental conditions. This study investigated to what extent these cysts are resistant to physical and chemical stresses as applied in food industry cleaning and disinfection procedures. Moreover, it was assessed whether certain intracystic meat-borne bacterial pathogens are more stress resistant than free-living bacterial monocultures and if intracystic passage and subsequent association with trophozoites induces cross-tolerance toward other stressors. Several physical and chemical stressors (NaCl, H₂O₂, benzalkonium chloride, 55°C, heating until boiling, ethanol, dishwashing detergent, and sodium hypochlorite) frequently used in domestic and industrial food-related environments were tested against (i) Acanthamoeba castellanii cysts, (ii) single strains of bacterial monocultures, (iii) intracystic bacteria, and (iv) bacteria after intracystic passage (cyst-primed bacteria). Only heating until boiling and hypochlorite treatment were cysticidal. After boiling, no viable trophozoites could be recovered from the cysts, and hypochlorite treatment caused a 1.34- to 4.72-log₁₀ cells/ml reduction in cyst viability. All treatments were effective in reducing or even eliminating the tested bacterial monocultures, whereas bacteria residing inside cysts were more tolerant toward these stressors. All cyst-primed bacteria exhibited an increased tolerance toward subsequent H₂O₂ (>92% decrease in median log₁₀ CFU/ml reduction) and 70% ethanol (>99% decrease) treatments. Moreover, intracystic passage significantly increased the survival of Yersinia enterocolitica (74% decrease in median log₁₀ reduction), Escherichia coli (58%), and Salmonella enterica (48%) after NaCl treatment and of E. coli (96%), S. enterica (99%), and Listeria monocytogenes (99%) after sodium hypochlorite treatment compared with that of nonprimed bacteria.IMPORTANCE The results from this study demonstrated that both viable and nonviable amoebal cysts can protect internalized bacteria against stressful conditions. Moreover, cyst passage can induce cross-tolerance in bacteria, increasing their survival when exposed to selected stressors. These findings underscore the potential importance of free-living amoebae in food-related environments and their impact on the persistence of meat-borne bacterial pathogens.

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.

A role for copper in protozoan grazing - two billion years selecting for bacterial copper resistance

The Great Oxidation Event resulted in integration of soft metals in a wide range of biochemical processes including, in our opinion, killing of bacteria by protozoa. Compared to pressure from anthropologic copper contamination, little is known on impacts of protozoan predation on maintenance of copper resistance determinants in bacteria. To evaluate the role of copper and other soft metals in predatory mechanisms of protozoa, we examined survival of bacteria mutated in different transition metal efflux or uptake systems in the social amoeba Dictyostelium discoideum. Our data demonstrated a strong correlation between the presence of copper/zinc efflux as well as iron/manganese uptake, and bacterial survival in amoebae. The growth of protozoa, in turn, was dependent on bacterial copper sensitivity. The phagocytosis of bacteria induced upregulation of Dictyostelium genes encoding the copper uptake transporter p80 and a triad of Cu(I)-translocating P_IB -type ATPases. Accumulated Cu(I) in Dictyostelium was monitored using a copper biosensor bacterial strain. Altogether, our data demonstrate that Cu(I) is ultimately involved in protozoan predation of bacteria, supporting our hypothesis that protozoan grazing selected for the presence of copper resistance determinants for about two billion years.

Yersinia pestis and Yersinia pseudotuberculosis infection: a regulatory RNA perspective

Yersinia pestis, responsible for causing fulminant plague, has evolved clonally from the enteric pathogen, Y. pseudotuberculosis, which in contrast, causes a relatively benign enteric illness. An ~97% nucleotide identity over 75% of their shared protein coding genes is maintained between these two pathogens, leaving much conjecture regarding the molecular determinants responsible for producing these vastly different disease etiologies, host preferences and transmission routes. One idea is that coordinated production of distinct factors required for host adaptation and virulence in response to specific environmental cues could contribute to the distinct pathogenicity distinguishing these two species. Small non-coding RNAs that direct posttranscriptional regulation have recently been identified as key molecules that may provide such timeous expression of appropriate disease enabling factors. Here the burgeoning field of small non-coding regulatory RNAs in Yersinia pathogenesis is reviewed from the viewpoint of adaptive colonization, virulence and divergent evolution of these pathogens.

Protozoan Cysts Act as a Survival Niche and Protective Shelter for Foodborne Pathogenic Bacteria

The production of cysts, an integral part of the life cycle of many free-living protozoa, allows these organisms to survive adverse environmental conditions. Given the prevalence of free-living protozoa in food-related environments, it is hypothesized that these organisms play an important yet currently underinvestigated role in the epidemiology of foodborne pathogenic bacteria. Intracystic bacterial survival is highly relevant, as this would allow bacteria to survive the stringent cleaning and disinfection measures applied in food-related environments. The present study shows that strains of widespread and important foodborne bacteria (Salmonella enterica, Escherichia coli, Yersinia enterocolitica, and Listeria monocytogenes) survive inside cysts of the ubiquitous amoeba Acanthamoeba castellanii, even when exposed to either antibiotic treatment (100 μg/ml gentamicin) or highly acidic conditions (pH 0.2) and resume active growth in broth media following excystment. Strain- and species-specific differences in survival periods were observed, with Salmonella enterica surviving up to 3 weeks inside amoebal cysts. Up to 53% of the cysts were infected with pathogenic bacteria, which were located in the cyst cytosol. Our study suggests that the role of free-living protozoa and especially their cysts in the persistence and epidemiology of foodborne bacterial pathogens in food-related environments may be much more important than hitherto assumed.

Yersinia pseudotuberculosis IP32953 survives and replicates in trophozoites and persists in cysts of Acanthamoeba castellanii

Yersinia pseudotuberculosis is a foodborne enteric pathogen that causes a mild self-limiting diarrhea in humans. Yersinia pseudotuberculosis is able to persist in soil and water and in association with fresh produce, but the mechanism by which it persists is unknown. It has been shown that Y. pseudotuberculosis co-occurs with protozoans in these environments; therefore, this study investigates if bacterivorous free-living amoeba (FLA) are able to support persistence of Y. pseudotuberculosis. Coculture studies of Y. pseudotuberculosis and the prototype FLA, Acanthamoeba castellanii revealed that bacteria had an enhanced capacity to survive in association with amoeba and in the absence of any cytotoxic effects. Yersinia pseudotuberculosis is able to survive and replicate in trophozoites specifically localized within vacuoles, and persists within cysts over a period of at least a week. These data present the first evidence that Y. pseudotuberculosis is able to resist the bacterivorous nature of FLA and instead exhibits an enhanced ability to replicate and persist in coculture with amoeba. This study sheds light on the potential role of FLA in the ecology of Y. pseudotuberculosis which may have implications for food safety.

Transmission electron microscopy sample preparation protocols for the ultrastructural study of cysts of free-living protozoa

Cysts of free-living protozoa have an impact on the ecology and epidemiology of bacteria because they may act as a transmission vector or shelter the bacteria against hash environmental conditions. Detection and localization of intracystic bacteria and examination of the en- and excystment dynamics is a major challenge because no detailed protocols for ultrastructural analysis of cysts are currently available. Transmission electron microscopy (TEM) is ideally suited for those analyses; however, conventional TEM protocols are not satisfactory for cysts of free-living protozoa. Here we report on the design and testing of four protocols for TEM sample preparation of cysts. Two protocols, one based on chemical fixation in coated well plates and one on high-pressure freezing, were selected as the most effective for TEM-based ultrastructural studies of cysts. Our protocols will enable improved analysis of cyst structure and a better understanding of bacterial survival mechanisms in cysts.

Long-term persistence of Yersinia pseudotuberculosis in entomopathogenic nematodes

Entomopathogenic nematodes (EPNs) are small worms whose ecological behaviour consists to invade, kill insects and feed on their cadavers thanks to a species-specific symbiotic bacterium belonging to any of the genera Xenorhabdus or Photorhabdus hosted in the gastro-intestinal tract of EPNs. The symbiont provides a number of biological functions that are essential for its EPN host including the production of entomotoxins, of enzymes able to degrade the insect constitutive macromolecules and of antimicrobial compounds able to prevent the growth of competitors in the insect cadaver. The question addressed in this study was to investigate whether a mammalian pathogen taxonomically related to Xenorhabdus was able to substitute for or "hijack" the symbiotic relationship associating Xenorhabdus and Steinernema EPNs. To deal with this question, a laboratory experimental model was developed consisting in Galleria mellonella insect larvae, Steinernema EPNs with or without their natural Xenorhabdus symbiont and Yersinia pseudotuberculosis brought artificially either in the gut of EPNs or in the haemocoel of the insect larva prior to infection. The developed model demonstrated the capacity of EPNs to act as an efficient reservoir ensuring exponential multiplication, maintenance and dissemination of Y. pseudotuberculosis.