Effect of salt shock on stability of lambdaimm434 lysogens

Temperate phage-antibiotic synergy across antibiotic classes reveals new mechanism for preventing lysogeny

A recent demonstration of synergy between a temperate phage and the antibiotic ciprofloxacin suggested a scalable approach to exploiting temperate phages in therapy, termed temperate phage-antibiotic synergy, which specifically interacted with the lysis-lysogeny decision. To determine whether this would hold true across antibiotics, we challenged Escherichia coli with the phage HK97 and a set of 13 antibiotics spanning seven classes. As expected, given the conserved induction pathway, we observed synergy with classes of drugs known to induce an SOS response: a sulfa drug, other quinolones, and mitomycin C. While some β-lactams exhibited synergy, this appeared to be traditional phage-antibiotic synergy, with no effect on the lysis-lysogeny decision. Curiously, we observed a potent synergy with antibiotics not known to induce the SOS response: protein synthesis inhibitors gentamicin, kanamycin, tetracycline, and azithromycin. The synergy results in an eightfold reduction in the effective minimum inhibitory concentration of gentamicin, complete eradication of the bacteria, and, when administered at sub-optimal doses, drastically decreases the frequency of lysogens emerging from the combined challenge. However, lysogens exhibit no increased sensitivity to the antibiotic; synergy was maintained in the absence of RecA; and the antibiotic reduced the initial frequency of lysogeny rather than selecting against formed lysogens. Our results confirm that SOS-inducing antibiotics broadly result in temperate-phage-specific synergy, but that other antibiotics can interact with temperate phages specifically and result in synergy. This is the first report of a means of chemically blocking entry into lysogeny, providing a new means for manipulating the key lysis-lysogeny decision.IMPORTANCEThe lysis-lysogeny decision is made by most bacterial viruses (bacteriophages, phages), determining whether to kill their host or go dormant within it. With over half of the bacteria containing phages waiting to wake, this is one of the most important behaviors in all of biology. These phages are also considered unusable for therapy because of this behavior. In this paper, we show that many antibiotics bias this behavior to "wake" the dormant phages, forcing them to kill their host, but some also prevent dormancy in the first place. These will be important tools to study this critical decision point and may enable the therapeutic use of these phages.

Differences in the Shiga Toxin (Stx) 2a Phage Regulatory Switch Region Influence Stx2 Localization and Virulence of Stx-Producing Escherichia coli in Mice

Shiga toxin (Stx)-producing Escherichia coli (STEC) is a major cause of foodborne illness globally, and infection with serotype O157:H7 is associated with increased risk of hospitalization and death in the U.S. The Stxs are encoded on a temperate bacteriophage (stx-phage), and phage induction leads to Stx expression; subtype Stx2a in particular is associated with more severe disease. Our earlier studies showed significant levels of RecA-independent Stx2 production by STEC O157:H7 strain JH2010 (stx2astx2c), even though activated RecA is the canonical trigger for stx-phage induction. This study aimed to further compare and contrast RecA-independent toxin production in Stx2-producing clinical isolates. Deletion of recA in JH2010 resulted in higher in vitro supernatant cytotoxicity compared to that from JH2016ΔrecA, and the addition of the chelator ethylenediaminetetraacetic acid (EDTA) and various metal cations to the growth medium exacerbated the difference in cytotoxicity exhibited by the two deletion strains. Both the wild-type and ΔrecA deletion strains exhibited differential cytotoxicity in the feces of infected, streptomycin (Str)-treated mice. Comparison of the stx2a-phage predicted protein sequences from JH2010 and JH2016 revealed low amino acid identity of key phage regulatory proteins that are involved in RecA-mediated stx-phage induction. Additionally, other STEC isolates containing JH2010-like and JH2016-like stx2a-phage sequences led to similar Stx2 localization, as demonstrated by JH2010ΔrecA and JH2016ΔrecA, respectively. Deletion of the stx2a-phage regulatory region in the wild-type strains prevented the differential localization of Stx2 into the culture supernatant, a finding that suggests that the stx2a-phage regulatory region is involved in the differential ΔrecA phenotypes exhibited by the two strains. We hypothesize that the amino acid differences between the JH2010 and JH2016 phage repressor proteins (CIs) lead to structural differences that are responsible for differential interaction with RecA. Overall, we discovered that non-homologous stx2a-phage regulatory proteins differentially influence RecA-independent, and possibly RecA-dependent, Stx2 production. These findings emphasize the importance of studying non-homologous regulatory elements among stx2-phages and their influence on Stx2 production and virulence of STEC isolates.

Characterization of a New Temperate Escherichia coli Phage vB_EcoP_ZX5 and Its Regulatory Protein

The study of the interaction between temperate phages and bacteria is vital to understand their role in the development of human diseases. In this study, a novel temperate Escherichia coli phage, vB_EcoP_ZX5, with a genome size of 39,565 bp, was isolated from human fecal samples. It has a short tail and belongs to the genus Uetakevirus and the family Podoviridae. Phage vB_EcoP_ZX5 encodes three lysogeny-related proteins (ORF12, ORF21, and ORF4) and can be integrated into the 3'-end of guaA of its host E. coli YO1 for stable transmission to offspring bacteria. Phage vB_EcoP_ZX5 in lysogenized E. coli YO1+ was induced spontaneously, with a free phage titer of 10⁷ PFU/mL. The integration of vB_EcoP_ZX5 had no significant effect on growth, biofilm, environmental stress response, antibiotic sensitivity, adherence to HeLa cells, and virulence of E. coli YO1. The ORF4 anti-repressor, ORF12 integrase, and ORF21 repressors that affect the lytic-lysogenic cycle of vB_EcoP_ZX5 were verified by protein overexpression. We could tell from changes of the number of total phages and the transcription level of phage genes that repressor protein is the key determinant of lytic-to-lysogenic conversion, and anti-repressor protein promotes the conversion from lysogenic cycle to lytic cycle.

Reduced bacterial mortality and enhanced viral productivity during sinking in the ocean

Particle sinking is an important process in the ocean, influencing the biogeochemical cycle and driving the long-term preservation of carbon into the deep sea via the biological pump. However, as an important component of marine ecosystems, the role of viruses during sinking is still poorly understood. Therefore, we performed a series of transplantation experiments in the South China Sea to simulate environmental changes during sinking and investigate their effects on viral eco-dynamics and life strategy. Our study demonstrated increased viral production but decreased virus-mediated bacterial mortality after transplantation. A larger burst size and switch from the lysogenic to lytic strategy were shown to contribute to enhanced viral productivity. We provide experimental evidence that surface viral ecological characteristics changed dramatically after transplantation into deep-sea waters, indicating a potential importance of viruses during vertical sinking in the ocean. This effect probably provides positive feedback on the efficiency of the biological pump.

Everybody loves cheese: crosslink between persistence and virulence of Shiga-toxin Escherichia coli

General cheese manufacturing involves high temperatures, fermentation and ripening steps that function as hurdles to microbial growth. On the other hand, the application of several different formulations and manufacturing techniques may create a bacterial protective environment. In cheese, the persistent behavior of Shiga toxin-producing Escherichia coli (STEC) relies on complex mechanisms that enable bacteria to respond to stressful conditions found in cheese matrix. In this review, we discuss how STEC manages to survive to high and low temperatures, hyperosmotic conditions, exposure to weak organic acids, and pH decreasing related to cheese manufacturing, the cheese matrix itself and storage. Moreover, we discuss how these stress responses interact with each other by enhancing adaptation and consequently, the persistence of STEC in cheese. Further, we show how virulence genes eae and tir are affected by stress response mechanisms, increasing either cell adherence or virulence factors production, which leads to a selection of more resistant and virulent pathogens in the cheese industry, leading to a public health issue.

Uneven host cell growth causes lysogenic virus induction in the Baltic Sea

In the Baltic Sea redoxcline, lysogenic viruses infecting prokaryotes have rarely been detected using the commonly used inducing agent mitomycin C. However, it is well known that not all viruses are induceable by mitomycin C and growing evidence suggests that changes in trophic conditions may trigger the induction of lysogenic viruses. We hypothesized that using antibiotics to simulate a strong change in trophic conditions for antibiotica-resistant cells due to reduced competition for resources might lead to the induction of lysogenic viruses into the lytic cycle within these cells. This hypothesis was tested by incubating prokaryotes obtained throughout the Baltic Sea redoxcline in seawater with substantially reduced numbers of viruses. We used a mixture of the protein synthesis-inhibiting antibiotics streptomycin and erythromycin to induce the desired changes in trophic conditions for resistant cells and at the same time ensuring that no progeny viruses were formed in sensitive cells. No inducible lysogenic viruses could be detected in incubations amended with mitomycin C. Yet, the presence of streptomycin and erythromycin increased virus-induced mortality of prokaryotes by 56–930% compared to controls, resulting in the induction of lysogenic viruses equivalent to 2–14% of in situ prokaryotic abundance. The results indicate the existence of a previously unrecognized induction mechanism for lysogenic viruses in the Baltic Sea redoxcline, as the mode of action distinctly differs between the used antibiotics (no virus production within affected cells) and mitomycin C (lysogenic viruses are produced within affected cells). Obtaining accurate experimental data on levels of lysogeny in prokaryotic host cells remains challenging, as relying on mitomycin C alone may severely underestimate lysogeny.

Non-pathogenic Escherichia coli Enhance Stx2a Production of E. coli O157:H7 Through Both bamA-Dependent and Independent Mechanisms

Intestinal colonization by the foodborne pathogen Escherichia coli O157:H7 leads to serious disease symptoms, including hemolytic uremic syndrome (HUS) and hemorrhagic colitis (HC). Synthesis of one or more Shiga toxins (Stx) is essential for HUS and HC development. The genes encoding Stx, including Stx2a, are found within a lambdoid prophage integrated in the E. coli O157:H7 chromosome. Enhanced Stx2a expression was reported when specific non-pathogenic E. coli strains were co-cultured with E. coli O157:H7, and it was hypothesized that this phenotype required the non-pathogenic E. coli to be sensitive to stx-converting phage infection. We tested this hypothesis by generating phage resistant non-pathogenic E. coli strains where bamA (an essential gene and Stx phage receptor) was replaced with an ortholog from other species. Such heterologous gene replacement abolished the ability of the laboratory strain E. coli C600 to enhance toxin production when co-cultured with E. coli O157:H7 strain PA2, which belongs to the hypervirulent clade 8. The extracellular loops of BamA (loop 4, 6, 7) were further shown to be important for infection by stx2a-converting phages. However, similar gene replacement in another commensal E. coli, designated 1.1954, revealed a bamA-independent mechanism for toxin amplification. Toxin enhancement by 1.1954 was not the result of phage infection through an alternative receptor (LamB or FadL), lysogen formation by stx2a-converting phages, or the production of a secreted molecule. Collectively, these data suggest that non-pathogenic E. coli can enhance toxin production through at least two mechanisms.

Molecular Mechanisms Governing "Hair-Trigger" Induction of Shiga Toxin-Encoding Prophages

Shiga toxin (Stx)-encoding E. coli (STEC) strains are responsible for sporadic outbreaks of food poisoning dating to 1982, when the first STEC strain, E. coli O157:H7, was isolated. Regardless of STEC serotype, the primary symptoms of STEC infections are caused by Stx that is synthesized from genes resident on lambdoid prophage present in STEC. Despite similar etiology, the severity of STEC-mediated disease varies by outbreak. However, it is unclear what modulates the severity of STEC-mediated disease. Stx production and release is controlled by lytic growth of the Stx-encoding bacteriophage, which in turn, is controlled by the phage repressor. Here, we confirm our earlier suggestion that the higher spontaneous induction frequency of Stx-encoding prophage is a consequence, in part, of lower intracellular repressor levels in STEC strains versus non-STEC strains. We also show that this lowered intracellular repressor concentration is a consequence of the utilization of alternative binding/regulatory strategies by the phage repressor. We suggest that a higher spontaneous induction frequency would lead to increased virulence.

Cheating, facilitation and cooperation regulate the effectiveness of phage-encoded exotoxins as antipredator molecules

Temperate phage encoded Shiga toxin (Stx) kills the bacterivorous predator, Tetrahymena thermophila, providing Stx⁺Escherichia coli with a survival advantage over Stx⁻ cells. Although bacterial death accompanies Stx release, since bacteria grow clonally the fitness benefits of predator killing accrue to the kin of the sacrificed organism, meaning Stx‐mediated protist killing is a form of self‐destructive cooperation. We show here that the fitness benefits of Stx production are not restricted to the kin of the phage‐encoding bacteria. Instead, nearby “free loading” bacteria, irrespective of their genotype, also reap the benefit of Stx‐mediated predator killing. This finding indicates that the phage‐borne Stx exotoxin behaves as a public good. Stx is encoded by a mobile phage. We find that Stx‐encoding phage can use susceptible bacteria in the population as surrogates to enhance toxin and phage production. Moreover, our findings also demonstrate that engulfment and concentration of Stx‐encoding and susceptible Stx⁻ bacteria in the Tetrahymena phagosome enhances the transfer of Stx‐encoding temperate phage from the host to the susceptible bacteria. This transfer increases the population of cooperating bacteria within the community. Since these bacteria now encode Stx, the predation‐stimulated increase in phage transfer increases the population of toxin encoding bacteria in the environment.

Mixing alters the lytic activity of viruses in the dark ocean

In aquatic habitats, viral lysis of prokaryotic cells lowers the overall efficiency of the microbial loop, by which dissolved organic carbon is transfered to higher trophic levels. Mixing of water masses in the dark ocean occurs on a global scale and may have far reaching consequences for the different prokaryotic and virus communities found in these waters by altering the environmental conditions these communities experience. We hypothesize that mixing of deep ocean water masses enhances the lytic activity of viruses infecting prokaryotes. To address this hypothesis, major deep-sea water masses of the Atlantic Ocean such as North Atlantic Deep Water, Mediterranean Sea Overflow Water, Antarctic Intermediate Water, and Antarctic Bottom Water were sampled at five locations. Prokaryotic cells from these samples were collected by filtration and subsequently incubated in virus-reduced water from either the same (control) or a different water mass (transplantation treatment). Additionally, mixtures of prokaryotes obtained from two different water masses were incubated in a mixture of virus-reduced water from the same water masses (control) or in virus-reduced water from the source water masses separately (mixing treatments). Pronounced differences in productivity-related parameters (prokaryotic leucine incorporation, prokaryotic and viral abundance) between water masses caused strong changes in viral lysis of prokaryotes. Often, mixing of water masses increased viral lysis of prokaryotes, indicating that lysogenic viruses were induced into the lytic cycle. Mixing-induced changes in viral lysis had a strong effect on the community composition of prokaryotes and viruses. Our data show that mixing of deep-sea water masses alters levels of viral lysis of prokaryotes and in many cases weakens the efficiency of the microbial loop by enhancing the recycling of organic carbon in the deep ocean.

Vibrational Relaxation of the Backbone and Base Modes in LacDNA Complexes by UV Resonance Raman Spectroscopy

Vibrational band shape analysis through time correlation function concept is widely used to obtain experimental information on the molecular dynamics of medium-size molecules in different environments. Interesting details are revealed by extending this technique to biomolecules such as functional groups of the nucleic acids in media approaching the physiological conditions. In this work a study into the UV resonance Raman (UVRR) vibrational half bandwidths of functional groups in LacDNA, upon lowering the pH (pH 6.4, pH 3.45) and in the presence of Mn²⁺ and Ca²⁺ ions, respectively, was of interest. The corresponding global relaxation times have been derived. Also, the 793 cm^-1 UVRR band, corresponding to ν (backbone O-P-O, dT) oscillator of LacDNA in aqueous solutions, was selected for band shape-analysis. Vibrational relaxation appears as the dominant relaxation process for this mode, with vibrational dephasing being the most efficient for this oscillator. Current theories developed for vibrational dephasing have been applied to this profile, and relevant relaxation parameters have been obtained and discussed. To our knowledge this is the first study on DNA oligomers vibrational band shape analysis through time correlation function concept.

Tetrahymena phagocytic vesicles as ecological micro-niches of phage transfer

The microbial communities in natural environments such as soil, pond water, or animal rumens are composed of a diverse mixture of bacteria and protozoa including ciliates or flagellates. In such microbiomes, a major source of bacterial mortality is grazing by phagocytic protists. Many protists are omnivorous heterotrophs, feeding on a range of different bacterial species. Due to this indiscriminate feeding, different bacterial species can assemble together in the same phagocytic vesicles where they can potentially exchange genetic material. Here we show that Tetrahymena thermophila imports and accumulates phage donor and recipient bacterial strains in its phagocytic vesicles and that under laboratory conditions the ingested bacteria remain viable for ≥2 h. Prophages in the ingested bacteria induce immediately after ingestion, and the released phages are concentrated in the phagocytic vesicles of the ciliate. These phages retain their ability to infect phage-susceptible bacterial strains. As a consequence of being confined within the phagosome, the frequency of lysogen formation in these vesicles increases 6-fold as compared with the bulk solution. Collectively, these observations suggest that T. thermophila aids in dissemination of bacteriophages by accumulating susceptible bacteria and phages in their phagocytic vesicles.

Mechanisms that Determine the Differential Stability of Stx⁺ and Stx(-) Lysogens

Phages 933W, BAA2326, 434, and λ are evolutionarily-related temperate lambdoid phages that infect Escherichia coli. Although these are highly-similar phages, BAA2326 and 933W naturally encode Shiga toxin 2 (Stx⁺), but phage 434 and λ do not (Stx⁻). Previous reports suggest that the 933W Stx⁺ prophage forms less stable lysogens in E. coli than does the Stx⁻ prophages λ, P22, and 434. The higher spontaneous induction frequency of the Stx⁺ prophage may be correlated with both virulence and dispersion of the Stx2-encoding phage. Here, we examined the hypothesis that lysogen instability is a common feature of Stx⁺ prophages. We found in both the absence and presence of prophage inducers (DNA damaging agents, salts), the Stx⁺ prophages induce at higher frequencies than do Stx⁻ prophages. The observed instability of Stx⁺ prophages does not appear to be the result of any differences in phage development properties between Stx⁺ and Stx⁻ phages. Our results indicate that differential stability of Stx⁺ and Stx⁻ prophages results from both RecA-dependent and RecA-independent effects on the intracellular concentration of the respective cI repressors.

Computational models of populations of bacteria and lytic phage

The use of phages to control and reduce numbers of unwanted bacteria can be traced back to the early 1900s, when phages were explored as a tool to treat infections before the wide scale use of antibiotics. Recently, phage therapy has received renewed interest as a method to treat multiresistant bacteria. Phages are also widely used in the food industry to prevent the growth of certain bacteria in foods, and are currently being explored as a tool for use in bioremediation and wastewater treatment. Despite the large body of biological research on phages, relatively little attention has been given to computational modeling of the population dynamics of phage and bacterial interactions. The earliest model was described by Campbell in the 1960s. Subsequent modifications to this model include partial or complete resistance, multiple phage binding sites, and spatial heterogeneity. This review provides a general introduction to modeling of the population dynamics of bacteria and phage. The review introduces the basic model and relevant concepts and evaluates more complex variations of the basic model published to date, including a model of disease epidemics caused by infectious bacteria. Finally, the shortcomings and potential ways to improve the models are discussed.

Sodium and Potassium Interactions with Nucleic Acids

Metal ions are essential cofactors for the structure and functions of nucleic acids. Yet, the early discovery in the 70s of the crucial role of Mg(2+) in stabilizing tRNA structures has occulted for a long time the importance of monovalent cations. Renewed interest in these ions was brought in the late 90s by the discovery of specific potassium metal ions in the core of a group I intron. Their importance in nucleic acid folding and catalytic activity is now well established. However, detection of K(+) and Na(+) ions is notoriously problematic and the question about their specificity is recurrent. Here we review the different methods that can be used to detect K(+) and Na(+) ions in nucleic acid structures such as X-ray crystallography, nuclear magnetic resonance or molecular dynamics simulations. We also discuss specific versus non-specific binding to different structures through various examples.

Ionic switch controls the DNA state in phage λ

We have recently found that DNA packaged in phage λ undergoes a disordering transition triggered by temperature, which results in increased genome mobility. This solid-to-fluid like DNA transition markedly increases the number of infectious λ particles facilitating infection. However, the structural transition strongly depends on temperature and ionic conditions in the surrounding medium. Using titration microcalorimetry combined with solution X-ray scattering, we mapped both energetic and structural changes associated with transition of the encapsidated λ-DNA. Packaged DNA needs to reach a critical stress level in order for transition to occur. We varied the stress on DNA in the capsid by changing the temperature, packaged DNA length and ionic conditions. We found striking evidence that the intracapsid DNA transition is 'switched on' at the ionic conditions mimicking those in vivo and also at the physiologic temperature of infection at 37°C. This ion regulated on-off switch of packaged DNA mobility in turn affects viral replication. These results suggest a remarkable adaptation of phage λ to the environment of its host bacteria in the human gut. The metastable DNA state in the capsid provides a new paradigm for the physical evolution of viruses.

Listeria phage and phage tail induction triggered by components of bacterial growth media (phosphate, LiCl, nalidixic acid, and acriflavine)

The detection of Listeria monocytogenes from food is currently carried out using a double enrichment. For the ISO methodology, this double enrichment is performed using half-Fraser and Fraser broths, in which the overgrowth of L. innocua can occur in samples where both species are present. In this study, we analyzed the induction of phages and phage tails of Listeria spp. in these media and in two brain heart infusion (BHI) broths (BHIM [bioMérieux] and BHIK [Biokar]) to identify putative effectors. It appears that Na2HPO4 at concentrations ranging from 1 to 40 g/liter with an initial pH of 7.5 can induce phage or phage tail production of Listeria spp., especially with 10 g/liter of Na2HPO4 and a pH of 7.5, conditions present in half-Fraser and Fraser broths. Exposure to LiCl in BHIM (18 to 21 g/liter) can also induce phage and phage tail release, but in half-Fraser and Fraser broths, the concentration of LiCl is much lower (3 g/liter). Low phage titers were induced by acriflavine and/or nalidixic acid. We also show that the production of phages and phage tails can occur in half-Fraser and Fraser broths. This study points out that induction of phages and phage tails could be triggered by compounds present in enrichment media. This could lead to a false-negative result for the detection of L. monocytogenes in food products.

PhoB activates Escherichia coli O157:H7 virulence factors in response to inorganic phosphate limitation

Enterohemorrhagic Escherichia coli (EHEC), an emerging food- and water-borne hazard, is highly pathogenic to humans. In the environment, EHEC must survive phosphate (Pi) limitation. The response to such Pi starvation is an induction of the Pho regulon including the Pst system that senses Pi variation. The interplay between the virulence of EHEC, Pho-Pst system and environmental Pi remains unknown. To understand the effects of Pi deprivation on the molecular mechanisms involved in EHEC survival and virulence under Pho regulon control, we undertook transcriptome profiling of the EDL933 wild-type strain grown under high Pi and low Pi conditions and its isogenic ΔphoB mutant grown in low Pi conditions. The differentially expressed genes included 1067 Pi-dependent genes and 603 PhoB-dependent genes. Of these 131 genes were both Pi and PhoB-dependent. Differentially expressed genes that were selected included those involved in Pi homeostasis, cellular metabolism, acid stress, oxidative stress and RpoS-dependent stress responses. Differentially expressed virulence systems included the locus of enterocyte effacement (LEE) encoding the type-3 secretion system (T3SS) and its effectors, as well as BP-933W prophage encoded Shiga toxin 2 genes. Moreover, PhoB directly regulated LEE and stx2 gene expression through binding to specific Pho boxes. However, in Pi-rich medium, constitutive activation of the Pho regulon decreased LEE gene expression and reduced adherence to HeLa cells. Together, these findings reveal that EHEC has evolved a sophisticated response to Pi limitation involving multiple biochemical strategies that contribute to its ability to respond to variations in environmental Pi and to coordinating the virulence response.

Advances toward the elucidation of hypertonic saline effects on Pseudomonas aeruginosa from cystic fibrosis patients

OBJECTIVES

Nebulized hypertonic saline (HTS) has beneficial effects including reducing pulmonary exacerbations in Cystic Fibrosis (CF) patients. Several mechanisms may explain these effects but antimicrobial activity of NaCl remains largely unexplored. We aimed to measure the antimicrobial effect of NaCl on Pseudomonas aeruginosa isolated from the respiratory tract in CF patients.

METHODS

NaCl minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) were determined for strains characterized for mucoidy, antimicrobial resistance, and ability to form biofilm using 0,9% to 15% NaCl solutions. NaCl effects on biofilm formation, preformed biofilm, and mobility were evaluated. Kinetics of antimicrobial effects was studied.

RESULTS

The growth of all isolates (n = 85) from 34 patients was inhibited by 6% NaCl solution. A 10% concentration had a bactericidal activity on 90% of the isolates. Mucoid and multidrug resistant (MDR) isolates displayed lower MICs compared to non-mucoid and to non-MDR isolates, respectively. Time-kill kinetics showed that NaCl exhibited a rapid, dose and growth phase dependent bactericidal effect. Three percent or more of NaCl inhibited biofilm formation for 69% of strongly adherent isolates. A dose-dependent decrease of preformed biofilm viability and an inhibitory activity on bacterial motility were observed.

CONCLUSIONS

NaCl inhibited the growth of all isolates and killed 38% of tested isolates within concentration range currently used in therapeutics. Our results suggest that anti-pseudomonal activity is another mechanism of action of HTS to add to those already established. Clinical trials are needed to compare diverse HTS conditions of use (rhythm, dose and mode of delivery) to obtain efficient and optimized anti-P. aeruginosa effects. More generally, NaCl effect on other opportunistic pathogens as well as on global microbiotae recovered during polymicrobial diseases warrants further investigations.

Survival of Shiga toxin-producing Escherichia coli and Stx bacteriophages in moisture enhanced beef

Moisture enhancement of meat through injection is a technology to improve the sensory properties and the weight of meat. However, the technology may increase the risk of food borne infections. Shiga toxin-producing Escherichia coli (STEC) or bacteriophages carrying cytotoxin genes (Shiga toxin genes, stx), which is normally only present on the surface of intact beef, may be transferred to the inner parts of the muscle during the injection process. Pathogens and bacteriophages surviving the storage period may not be eliminated in the cooking process since many consumers prefer undercooked beef. Measures to increase the microbial food safety of moisture enhanced beef may include sterilization or washing of the outer surface of the meat before injection, avoiding recycling of marinade and addition of antimicrobial agents to the marinade. This paper reviews the literature regarding microbial safety of moisture enhanced beef with special emphasis on STEC and Stx bacteriophages. Also, results from a European Union research project, ProSafeBeef (Food-CT-16 2006-36241) are presented.

The Trojan Horse of the microbiological arms race: phage-encoded toxins as a defence against eukaryotic predators

Phage-encoded Shiga toxin (Stx) acts as a bacterial defence against the eukaryotic predator Tetrahymena. To function as an effective bacterial anti-predator defence, Stx must kill a broad spectrum of predators. Consistent with that assertion, we show here that bacterially encoded Stx efficiently kills the bacteriovore Acanthamoeba castellanii in co-culture. We also show that, in addition to Stx, the phage-encoded exotoxin, diphtheria toxin (Dtx) expressed by Corynebacterium diphtheriae also can function as part of an anti-predator strategy; it kills Acanthamoeba in co-culture. Interestingly, only exotoxins produced by bacteria internalized by the Acanthamoeba predator are cytolethal; the presence of purified Dtx or Stx in culture medium has no effect on predator viability. This finding is consistent with our results indicating that intoxication of Acanthamoeba by these exotoxins does not require a receptor. Thus bacteria, in the disguise of a food source, function as a 'Trojan Horse', carrying genes encoding an exotoxin into target organisms. This 'Trojan Horse' mechanism of exotoxin delivery into predator cells allows intoxication of predators that lack a cell surface receptor for the particular toxin, allowing bacteria-bearing exotoxins to kill a broader spectrum of predators, increasing the fitness of the otherwise 'defenceless' prey bacteria.

Linking the lytic and lysogenic bacteriophage cycles to environmental conditions, host physiology and their variability in coastal lagoons

Changes in environmental conditions and prokaryote physiology can strongly affect the dynamics of both the lysogenic and lytic bacteriophage replication cycles in aquatic systems. However, it remains unclear whether it is the nature, amplitude or frequency of these changes that alter the phage replication cycles. We performed an annual survey of three Mediterranean lagoons with contrasting levels of chlorophyll a concentration and salinity to explore how these cues and their variability influence either replication cycle. The lytic cycle was always detected and showed seasonal patterns, whereas the lysogenic cycle was often undetected and highly variable. The lytic cycle was influenced by environmental and prokaryotic physiological cues, increasing with concentrations of dissolved organic carbon, chlorophyll a, and the proportion of respiring cells, and decreasing with the proportion of damaged cells. In contrast, lysogeny was not explained by the magnitude of any environmental or physiological parameter, but increased with the amplitude of change in prokaryote physiology. Our study suggests that both cycles are regulated by distinct factors: the lytic cycle is dependent on environmental parameters and host physiology, while lysogeny is dependent on the variability of prokaryote physiology. This could lead to the contrasting patterns observed between both cycles in aquatic systems.

Sea ice microorganisms: environmental constraints and extracellular responses

Inherent to sea ice, like other high latitude environments, is the strong seasonality driven by changes in insolation throughout the year. Sea-ice organisms are exposed to shifting, sometimes limiting, conditions of temperature and salinity. An array of adaptations to survive these and other challenges has been acquired by those organisms that inhabit the ice. One key adaptive response is the production of extracellular polymeric substances (EPS), which play multiple roles in the entrapment, retention and survival of microorganisms in sea ice. In this concept paper we consider two main areas of sea-ice microbiology: the physico-chemical properties that define sea ice as a microbial habitat, imparting particular advantages and limits; and extracellular responses elicited in microbial inhabitants as they exploit or survive these conditions. Emphasis is placed on protective strategies used in the face of fluctuating and extreme environmental conditions in sea ice. Gaps in knowledge and testable hypotheses are identified for future research.

Lysogenic Streptococcus suis isolate SS2-4 containing prophage SMP showed increased mortality in zebra fish compared to the wild-type isolate

Streptococcus suis (S. suis) infection is considered to be a major problem in the swine industry worldwide. Based on the capsular type, 33 serotypes of S. suis have been described, with serotype 2 (SS2) being the most frequently isolated from diseased piglets. Little is known, however, about the pathogenesis and virulence factors of S. suis. Research on bacteriophages highlights a new area in S. suis research. A S. suis serotype 2 bacteriophage, designated SMP, has been previously isolated in our laboratory. Here, we selected a lysogenic isolate in which the SMP phage was integrated into the chromosome of strain SS2-4. Compared to the wild-type isolate, the lysogenic strain showed increased mortality in zebra fish. Moreover the sensitivity of the lysogenic strain to lysozyme was seven times higher than that of the wild-type.

Bacteriophage 434 Hex protein prevents recA-mediated repressor autocleavage

In a λ^imm434 lysogen, two proteins are expressed from the integrated prophage. Both are encoded by the same mRNA whose transcription initiates at the P_RM promoter. One protein is the 434 repressor, needed for the establishment and maintenance of lysogeny. The other is Hex which is translated from an open reading frame that apparently partially overlaps the 434 repressor coding region. In the wild type host, disruption of the gene encoding Hex destabilizes λ^imm434 lysogens. However, the hex mutation has no effect on lysogen stability in a recA⁻ host. These observations suggest that Hex functions by modulating the ability of RecA to stimulate 434 repressor autocleavage. We tested this hypothesis by identifying and purifying Hex to determine if this protein inhibited RecA‑stimulated autocleavage of 434 repressor in vitro. Our results show that in vitro a fragment of Hex prevents RecA-stimulated autocleavage of 434 repressor, as well as the repressors of the closely related phage P22. Surprisingly, Hex does not prevent RecA‑stimulated autocleavage of phage lambda repressor, nor the E. coli LexA repressor.

Entry and killing of Tetrahymena thermophila by bacterially produced Shiga toxin

Phage-encoded Shiga toxin (Stx) acts as a bacterial defense against the eukaryotic predator Tetrahymena thermophila. It is unknown how Stx enters Tetrahymena protozoa or how it kills them. Tetrahymena protozoa are phagocytotic; hence, Stx could gain entry to the cytoplasm through the oral apparatus or via endocytosis. We find that Stx2 can kill T. thermophila protozoa that lack an oral apparatus, indicating that Stx2 can enter these cells via endocytosis. As opposed to the lack of effect on mammalian phagocytes, Stx2 produced by bacteria encapsulated within phagocytotic vesicles is also capable of killing Tetrahymena. Addition of an excess of the carbohydrate binding subunits of Stx2 (StxB) and/or ricin (ricin B) blocks Stx2 cytotoxicity. Thus, regardless of whether Stx2 enters the cytoplasm by endocytosis or from the phagocytotic vesicle, this transport is mediated by a putative glycoconjugate receptor. Bacteriophage-mediated lysis of Stx-encoding bacteria is necessary for Stx toxicity in Tetrahymena; i.e., toxin released as a consequence of digestion of bacteria by Tetrahymena is harmless to the cell. This finding provides a rationale as to why the genes encoding Stx are found almost exclusively on bacteriophages; Stx must be released from the bacteria prior to the digestion of the cell, or it will not be able to exert its cytotoxic effect. It also suggests a reason why other bacterial exotoxins are also found only on temperate bacteriophages. Incubation of Tetrahymena with purified Stx2 decreases total protein synthesis. This finding indicates that, similar to mammalian cells, Stx2 kills Tetrahymena by inactivating its ribosomes.

Tetrahymena is a bacterial predator and a model for mammalian phagocytosis and intracellular vesicular trafficking. Phage-encoded exotoxins apparently have evolved for the purpose of bacterial antipredator defense. These exotoxins kill mammalian cells by inactivating universally conserved factors and/or pathways. Tetrahymena and susceptible mammalian cells are killed when exposed to bacteriophage-encoded Shiga toxin (Stx). Stx toxicity in mammalian cells requires Stx binding to the globotriaosyl ceramide (Gb3) receptor, followed by receptor-mediated endocytosis (RME). We show that, similar to mammalian cells, internalized Stx inhibits protein synthesis in Tetrahymena. Although Tetrahymena lacks Gb3, our results suggest that the cytotoxic effect of Stx on Tetrahymena is apparently mediated by a receptor, thereby arguing for the existence of RME in Tetrahymena. As opposed to the case with mammalian phagocytes, Stx produced by bacteria inside Tetrahymena is cytotoxic, suggesting that these cells may represent a “missing link” between unicellular eukaryotic bacterial predators and phagocytotic mammalian cells.

Characterizing RecA-independent induction of Shiga toxin2-encoding phages by EDTA treatment

BACKGROUND

The bacteriophage life cycle has an important role in Shiga toxin (Stx) expression. The induction of Shiga toxin-encoding phages (Stx phages) increases toxin production as a result of replication of the phage genome, and phage lysis of the host cell also provides a means of Stx toxin to exit the cell. Previous studies suggested that prophage induction might also occur in the absence of SOS response, independently of RecA.

METHODOLOGY/PRINCIPAL FINDINGS

The influence of EDTA on RecA-independent Stx2 phage induction was assessed, in laboratory lysogens and in EHEC strains carrying Stx2 phages in their genome, by Real-Time PCR. RecA-independent mechanisms described for phage λ induction (RcsA and DsrA) were not involved in Stx2 phage induction. In addition, mutations in the pathway for the stress response of the bacterial envelope to EDTA did not contribute to Stx2 phage induction. The effect of EDTA on Stx phage induction is due to its chelating properties, which was also confirmed by the use of citrate, another chelating agent. Our results indicate that EDTA affects Stx2 phage induction by disruption of the bacterial outer membrane due to chelation of Mg(2+). In all the conditions evaluated, the pH value had a decisive role in Stx2 phage induction.

CONCLUSIONS/SIGNIFICANCE

Chelating agents, such as EDTA and citrate, induce Stx phages, which raises concerns due to their frequent use in food and pharmaceutical products. This study contributes to our understanding of the phenomenon of induction and release of Stx phages as an important factor in the pathogenicity of Shiga toxin-producing Escherichia coli (STEC) and in the emergence of new pathogenic strains.

Phage λ--new insights into regulatory circuits

Bacteriophage λ, rediscovered in the early 1950s, has served as a model in molecular biology studies for decades. Although currently more complex organisms and more complicated biological systems can be studied, this phage is still an excellent model to investigate principles of biological processes occurring at the molecular level. In fact, very few other biological models provide possibilities to examine regulations of biological mechanisms as detailed as performed with λ. In this chapter, recent advances in our understanding of mechanisms of bacteriophage λ development are summarized and discussed. Particularly, studies on (i) phage DNA injection, (ii) molecular bases of the lysis-versus-lysogenization decision and the lysogenization process itself, (iii) prophage maintenance and induction, (iv), λ DNA replication, (v) phage-encoded recombination systems, (vi) transcription antitermination, (vii) formation of the virion structure, and (viii) lysis of the host cell, as published during several past years, will be presented.

Bacteriophages carrying Shiga toxin genes: genomic variations, detection and potential treatment of pathogenic bacteria

Although most Escherichia coli strains occur in the mammalian intestine as commensals, some of them, including enterohemorrhagic E. coli (EHEC), are capable of causing disease in humans. The most notorious virulence factors of EHEC are Shiga toxins, encoded by genes located on genomes of lambdoid prophages. Production and release of these toxins is strongly stimulated after the induction of these prophages. Many antibiotics used to treat bacterial infections stimulate induction of Shiga toxin-converting prophages, enhancing severity of the disease symptoms. Hence, treatment with antibiotics is not recommended if infection with EHEC is confirmed or even suspected. In this light, rapid detection of EHEC is crucial, and understanding the mechanisms of prophage induction and phage development in the human intestine is important to facilitate development of procedures preventing or alleviating Shiga toxin-caused diseases.

Viral distribution and life strategies in the Bach Dang Estuary, Vietnam

Although the structure and dynamics of planktonic viruses in freshwater and seawater environments are relatively well documented, little is known about the occurrence and activity of these viruses in estuaries, especially in the tropics. Viral abundance, life strategies, and morphotype distribution were examined in the Bach Dang Estuary (Vietnam) during the dry season in 2009. The abundance of both viruses and their prokaryotic hosts decreased significantly from upstream to downstream, probably as the result of nutrient dilution and osmotic stress faced by the freshwater communities. The antibiotic mitomycin-C revealed that the fraction of lysogenic cells was substantially higher in the lower seawater part of the estuary (max 27.1%) than in the upper freshwater area where no inducible lysogens were observed. The question of whether there is a massive, continuous induction of marine lysogens caused by the mixing with freshwater is considered. Conversely, the production of lytic viruses declined as salinity increased, indicating a spatial succession of viral life strategies in this tropical estuary. Icosahedral tailless viruses with capsids smaller than 60 nm dominated the viral assemblage throughout the estuary (63.0% to 72.1% of the total viral counts), and their distribution was positively correlated with that of viral lytic production. Interestingly, the gamma-proteobacteria explained a significant portion of the variance in the <60 nm and 60 to 90 nm tailless viruses (92% and 80%, respectively), and in the Myoviridae (73%). Also, 60% of the variance of the tailless larger viruses (>90 nm) was explained by the beta-proteobacteria. Overall, these results support the view that the environment, through selection mechanisms, probably shapes the structure of the prokaryotic community. This might be in turn a source of selection for the virioplankton community via specific affiliation favoring particular morphotypes and life strategies.

Within-host competition determines reproductive success of temperate bacteriophages

Within-host competition between parasites is frequently invoked as a major force for parasite evolution, yet quantitative studies on its extent in an organismal group are lacking. Temperate bacteriophages are diverse and abundant parasites of bacteria, distinguished by their ability to enter a facultative dormant state in their host. Bacteria can accumulate multiple phages that may eventually abandon dormancy in response to host stress. Host resources are then converted into phage particles, whose release requires cell death. To study within-host competition between phages, I used the bacterium Escherichia coli and 11 lambdoid phages to construct single and double lysogens. Lysogenic bacterial cultures were then induced and time to host cell lysis and productivity of phages was measured. In double lysogens, this revealed strong competitive interactions as in all cases productivity of at least one phage declined. The outcome of within-host competition was often asymmetrical, and phages were found to vary hierarchically in within-host competitive ability. In double infections, the phage with the shorter lysis time determined the timing of cell lysis, which was associated with a competitive advantage when time differences were large. The results emphasize that within-host competition greatly affects phage fitness and that multiple infections should be considered an integral part of bacteriophage ecology.

Disentangling the relative influence of bacterioplankton phylogeny and metabolism on lysogeny in reservoirs and lagoons

Previous studies indicate that lysogeny is preponderant when environmental conditions are challenging for the bacterial communities and when their metabolism is reduced. Furthermore, it appears that lysogeny is more frequent within certain bacterial phylogenetic groups. In this comparative study from 10 freshwater reservoirs and 10 coastal lagoons, we aim to disentangle the influence of these different factors. In eight reservoirs and four lagoons, lysogeny was detected by induction assays with mitomycin C, and induction significantly modified the bacterial community composition (BCC), whereas community composition remained constant in ecosystems in which lysogeny was not observed. Among the phylogenetic groups studied, the most abundant ones were Bacteroidetes and α-proteobacteria in lagoons, and β-proteobacteria and Bacteroidetes in reservoirs. These dominant groups comprised the highest proportions of inducible lysogens. In order to unravel the effects of bacterial metabolism from phylogeny on lysogeny, we measured bacterial community physiology and the specific activities of selected phylogenetic groups. The proportion of inducible lysogens within the α- and the β-proteobacteria decreased with increasing group-specific metabolism in lagoons and reservoirs, respectively. In contrast, this relationship was not observed for the other lysogen-containing groups. Hence, both host physiology and phylogeny are critical for the establishment of lysogeny. This study illustrates the importance of lysogeny among the most abundant phylogenetic groups, and further suggests its strong structuring impact on BCC.

Lytic failure in cross-inoculation assays between phages and prokaryotes from three aquatic sites of contrasting salinity

Little is known about the ability of phages to successfully colonize contrasting aquatic niches. We conducted experimental cross-infections between viruses and prokaryotes from three tropical sites of West Africa, with distinct salinities: a freshwater reservoir, a marine coastal station and a hypersaline lake. A cellular poison-based method (potassium cyanide) revealed that the addition of native viruses (regardless of the water type) consistently stimulated viral production. Conversely, in all incubations conducted with allochtonous (non-native) viruses, their overall production was not promoted, which suggests a lytic failure. Prokaryotic heterotrophic production increased in fresh and marine water supplemented with native viruses, but not in the hypersaline water. These results point to the role of the viral shunt in low-salinity environments, where the release of bioavailable lysis products might be of high nutritional value for the noninfected prokaryotes. In contrast, in hypersaline water where glycerol is a major carbon and energy source for the heterotrophic community, dissolved organic matter (DOM) of lytic origin may represent a less important DOM source for prokaryotes. Finally, our results suggest that cosmopolitan phages capable of moving between biomes are probably rare in aquatic habitats, supporting the common idea that most wild phages are relatively limited in their host range.

Hydrogen peroxide-mediated induction of the Shiga toxin-converting lambdoid prophage ST2-8624 in Escherichia coli O157:H7

Shiga toxin-producing Escherichia coli (STEC) may cause bloody diarrhea and hemorrhagic colitis, with sometimes severe complications. Because genes coding for Shiga toxins are located on lambdoid prophages, effective toxin production occurs only after prophage induction. However, although agents that effectively induce prophage lambda (a paradigm of the family of lambdoid phages) under laboratory conditions, such as UV irradiation or DNA replication inhibitors, are well known, it is unlikely that such factors are present in human intestine infected with STEC. In this report, we demonstrate that induction of a Shiga toxin-converting prophage in its host (E. coli O157:H7) occurs not only in the presence of DNA-interfering antibiotics (mitomycin C and norfloxacin) but also under conditions of oxidative stress [following treatment with hydrogen peroxide (H(2)O(2))]. Under these conditions, we observed not only effective prophage induction but also expression of the reporter gene (replacing the original stx2 gene). In the light of previously published reports, indicating that oxidative stress conditions might occur during colonization of human intestine by enteric bacteria, and that neutrophil-produced H(2)O(2) can increase production of the Shiga toxin in a clinical isolate of STEC, these results suggest that oxidative stress may be one of the agents responsible for stimulating the pathogenicity determinants of STEC, leading to induction of Shiga toxin-converting prophages in these bacteria.

Acyl-homoserine lactones can induce virus production in lysogenic bacteria: an alternative paradigm for prophage induction

Prophage typically are induced to a lytic cycle under stressful environmental conditions or when the host's survival is threatened. However, stress-independent, spontaneous induction also occurs in nature and may be cell density dependent, but the in vivo signal(s) that can trigger induction is unknown. In the present study, we report that acyl-homoserine lactones (AHL), the essential signaling molecules of quorum sensing in many gram-negative bacteria, can trigger phage production in soil and groundwater bacteria. This phenomenon also was operative in a lambda lysogen of Escherichia coli. In model coculture systems, we monitored the real-time AHL production from Pseudomonas aeruginosa PAO1 using an AHL bioluminescent sensor and demonstrated that lambda-prophage induction in E. coli was correlated with AHL production. As a working model in E. coli, we show that the induction responses of lambda with AHL remained unaffected when recA was deleted, suggesting that this mechanism does not involve an SOS response. In the same lambda lysogen we also demonstrated that sdiA, the AHL receptor, and rcsA, a positive transcriptional regulator of exopolysaccharide synthesis, are involved in the AHL-mediated induction process. These findings relate viral reproduction to chemical signals associated with high host cell abundance, suggesting an alternative paradigm for prophage induction.

Differential efficiency of induction of various lambdoid prophages responsible for production of Shiga toxins in response to different induction agents

Shiga toxin-producing Escherichia coli (STEC) is a group of pathogenic strains responsible for bloody diarrhea and hemorrhagic colitis, with often severe complications. Shiga toxins are the main factors causing the phathogenicity of STEC. Production of these toxins depends on the presence of stx1 and stx2 genes, which are located on lambdoid prophages, and their expression is stimulated upon prophage induction. Therefore, a transition of the phage genome from the prophage state to an extrachromosomal genetic element, and its further propagation, is crucial for the pathogenic effects. However, our knowledge on specific conditions for induction of these prophages in bacteria occurring in human intestine is very limited. In this report we present results of our studies on five different phages, originally occurring in STEC strains, in comparison to bacteriophage lambda. We found that efficiencies of induction of prophages and their further development vary considerably in response to different induction agents. Moreover, efficiency of progeny phage production might be modulated by other factors, like temperature or bacterial growth rate. Therefore, it is likely that pathogenicity of different STEC strains may be significantly different under specific conditions in their natural habitats.