Iron control of the Vibrio fischeri luminescence system in Escherichia coli

Fragmentation of polypropylene into microplastics promoted by photo-aging; release of metals, toxicity and inhibition of biodegradability

The widespread presence of microplastics (MP) in water represents an environmental problem, not only because of the harmful effects of their size and potential to vector other pollutants, but also because of the release of additives, degradation products and residues contained in the polymer matrix. The latter includes metallic catalysts, which are often overlooked. This study focuses on the photo-aging of polypropylene (PP) and the resulting structural changes that promote its fragmentation microplastics (PP-MPs) and release of metals, as well as the resulting toxicity of leachates and their potential to inhibit biodegradation of organics in water. The pristine, photo-aged and waste PP are ground under the same regime to assess susceptibility to fragmentation. Obtained PP-MPs are submitted to leaching tests; the release of organics and metals is monitored by Total Organic Carbon (TOC) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis, respectively. The leachates are assessed for their toxicity against Vibrio fischeri, Daphnia magna and Pseudokirchneriella subcapitata and their influence on the biodegradability of the glucose solution. Photo-aging induced changes in the crystallinity and morphology of the PP and manifested in the abundance of smaller MPs, as revealed by the particle size distribution. In the case of pristine PP, all particles were > 100 μm in size, while aged PP yielded significant mass fraction of MPs <100 μm. The toxicity of leachates from aged PP-MPs is higher than that of pristine and exhibits a positive correlation with portion of metals released. The biodegradability of glucose is strongly inhibited by PP-MPs leachates containing a mixture of metals in trace concentrations.

Inhibition of marine Vibrio sp. by pyoverdine from Pseudomonas aeruginosa PA1

Siderophores are low-molecular-weight chemicals that are secreted by many microorganisms to chelate iron from the external environment in order to facilitate their growth and diverse metabolisms. In this study, a fluorescent siderophore, pyoverdine, secreted by Pseudomonas aeruginosa PA1 was purified by affinity chromatography using Cu-sepharose. Pyoverdine was determined to have a molecular mass of 1333.54 Da, as determined by MALDI-TOF/TOF, and belong to type I pyoverdine, as determined by PCR analysis of its corresponding outer membrane ferri-pyoverdine receptor. Pyoverdine showed different degrees of inhibitory effects on the growth of marine Vibrio sp. strains. It was also shown that the biofilm developed by Vibrio parahaemolyticus WzW1 and Wz2121 and Vibrio cyclitrophicus HS12 was significantly reduced, alone with the repressed growth in the presence of pyoverdine. Siderophore production was determined in the strains of Vibrio sp. in response to the pyoverdine-induced iron-limited conditions. The siderophore production of most Vibrio sp. was up-regulated, with the exception of the bacteria that produced little siderophore. Furthermore, Apostichopus japonicus cultured in pyoverdine pretreated seawater showed a relative percent of survival of 89% when they were challenged by Vibrio splendidus. Our results demonstrated that pyoverdine may be a promising agent that could be potentially applied to treat vibriosis.

The iron-dependent regulator fur controls pheromone signaling systems and luminescence in the squid symbiont Vibrio fischeri ES114

Bacteria often use pheromones to coordinate group behaviors in specific environments. While high cell density is required for pheromones to achieve stimulatory levels, environmental cues can also influence pheromone accumulation and signaling. For the squid symbiont Vibrio fischeri ES114, bioluminescence requires pheromone-mediated regulation, and this signaling is induced in the host to a greater extent than in culture, even at an equivalent cell density. Our goal is to better understand this environment-specific control over pheromone signaling and bioluminescence. Previous work with V. fischeri MJ1 showed that iron limitation induces luminescence, and we recently found that ES114 encounters a low-iron environment in its host. Here we show that ES114 induces luminescence at lower cell density and achieves brighter luminescence in low-iron media. This iron-dependent effect on luminescence required ferric uptake regulator (Fur), which we propose influences two pheromone signaling master regulators, LitR and LuxR. Genetic and bioinformatic analyses suggested that under low-iron conditions, Fur-mediated repression of litR is relieved, enabling more LitR to perform its established role as an activator of luxR. Interestingly, Fur may similarly control the LitR homolog SmcR of Vibrio vulnificus. These results reveal an intriguing regulatory link between low-iron conditions, which are often encountered in host tissues, and pheromone-dependent master regulators.

The haem-uptake gene cluster in Vibrio fischeri is regulated by Fur and contributes to symbiotic colonization

Although it is accepted that bacteria-colonizing host tissues are commonly faced with iron-limiting conditions and that pathogenic bacteria often utilize iron from host-derived haem-based compounds, the mechanisms of iron acquisition by beneficial symbiotic bacteria are less clear. The bacterium Vibrio fischeri mutualistically colonizes the light organ of the squid Euprymna scolopes. Genome sequence analysis of V. fischeri revealed a putative haem-uptake gene cluster, and through mutant analysis we confirmed this cluster is important for haemin use by V. fischeri in culture. LacZ reporter assays demonstrated Fur-dependent transcriptional regulation of cluster promoter activity in culture. GFP-based reporter assays revealed that gene cluster promoter activity is induced in symbiotic V. fischeri as early as 14 h post inoculation, although colonization assays with the haem uptake mutant suggested an inability to uptake haem does not begin to limit colonization until later stages of the symbiosis. Our data indicate that the squid light organ is a low iron environment and that haem-based sources of iron are used by symbiotic V. fischeri cells. These findings provide important additional information on the availability of iron during symbiotic colonization of E. scolopes by V. fischeri, as well as the role of haem uptake in non-pathogenic host-microbe interactions.

Analysing traces of autoinducer-2 requires standardization of the Vibrio harveyi bioassay

Autoinducer-2 (furanosyl borate diester) is a biologically active compound whose role as a universal bacterial signalling molecule is currently under intense investigation. Because of its instability and the low concentrations of it found in biological samples, its detection relies at present on a bioassay that measures the difference in the timing of the luminescence of the Vibrio harveyi BB170 sensor strain with and without externally added AI-2. Here we systematically investigated which parameters affected the fold induction values of luminescence obtained in the bioassay and developed a modified protocol. Our experiments showed that growth and luminescence of V. harveyi BB170 are strongly influenced by trace elements. In particular, addition of Fe(3+) within a certain concentration range to the growth medium of the preinoculum culture improved the reproducibility and reduced the variance of the bioassay. In contrast, trace elements and vitamins introduced directly into the bioassay caused inhibitory effects. The initial density and luminescence of the sensor strain are very important and the values required for these parameters were defined. Borate interferes with the detection of AI-2 by giving false positive results. The response of V. harveyi BB170 to chemically synthesized AI-2 in the bioassay is nonlinear except over a very small concentration range; it is maximum over three orders of magnitude and shows inhibition above 35 microM. Based on the modified protocol, we were able to detect AI-2 in the absence of inhibitors with maximum fold induction values for the positive control (chemically synthesized AI-2) of >120 with a standard deviation of approximately 30% in a reliable and reproducible way.

Salmonella stress management and its relevance to behaviour during intestinal colonisation and infection

The enteric pathogen Salmonella enterica is exposed to a number of stressful environments during its life cycle within and outside its various hosts. During intestinal colonisation Salmonella is successively exposed to acid pH in the stomach, to the detergent-like activity of bile, to decreasing oxygen supply, to the presence of multiple metabolites produced by the normal gut microflora and finally it is exposed to cationic antimicrobial peptides present on the surface of epithelial cells. There are four major regulators controlling relevant stress responses in Salmonella, namely RpoS, PhoPQ, Fur and OmpR/EnvZ. Except for Fur, inactivation of genes encoding the other stress regulators results in attenuated virulence and such mutants can therefore be considered as vaccine candidates. In contrast, a decrease in oxygen supply monitored by Fnr and ArcAB, or oxidative stress controlled by OxyR and SoxRS is not regarded as a stress associated with host colonisation since inactivation of either of these systems does not result in reductions in colonisation. The role of quorum-sensing through luxS and sdiA is also considered as a regulator of virulence and colonisation.

Regulation of the cytotoxic enterotoxin gene in Aeromonas hydrophila: characterization of an iron uptake regulator

The cytotoxic enterotoxin Act from a diarrheal isolate, SSU, of Aeromonas hydrophila is aerolysin related and crucial to the pathogenesis of Aeromonas infections. To elucidate the role of environmental signals which influence the expression of the cytotoxic enterotoxin gene (act), a portion of the act gene, including the putative promoter region, was fused in frame to a truncated alkaline phosphatase gene (phoA) of Escherichia coli. The act::phoA reporter gene was then introduced into the chromosome of A. hydrophila by using the suicide vector pJQ200SK, allowing the fusion protein to be secreted out into the culture medium. Western blot analysis demonstrated the presence of a correctly size 110-kDa fusion protein in the culture supernatant, which reacted with both anti-Act and anti-alkaline phosphatase antibodies. Based on alkaline phosphatase (PhoA) activity in the culture supernatant, we demonstrated that calcium significantly increased the activity of the act promoter but that glucose and iron repressed its activity in a dose-dependent fashion. The act promoter exhibited optimal activity at pH 7.0 and at 37 degrees C, and maximal PhoA activity was noted when the culture was aerated. Using a Vibrio cholerae iron uptake regulator gene (fur) as a probe, a 2.6-kb SalI/HindIII DNA fragment from an A. hydrophila chromosome was cloned and sequenced. The DNA sequence revealed a 429-bp open reading frame that exhibited 69% homology at the DNA level with the fur gene and 79% homology at the amino acid level with the iron uptake regulator (Fur) protein of V. cholerae. Complementation experiments demonstrated that the A. hydrophila fur gene could restore iron regulation in an E. coli fur-minus mutant. Using the suicide vector pDMS197, we generated a fur isogenic mutant of wild-type A. hydrophila SSU. Northern blot analysis data indicated that the repression in the transcription of the act gene by iron was relieved in the fur isogenic mutant. Further, iron regulation in the fur isogenic mutant of A. hydrophila could be restored by complementation. These results are important in understanding the regulation of the act gene under in vivo conditions.

Mapping stress-induced changes in autoinducer AI-2 production in chemostat-cultivated Escherichia coli K-12

Numerous gram-negative bacteria employ a cell-to-cell signaling mechanism, termed quorum sensing, for controlling gene expression in response to population density. Recently, this phenomenon has been discovered in Escherichia coli, and while pathogenic E. coli utilize quorum sensing to regulate pathogenesis (i.e., expression of virulence genes), the role of quorum sensing in nonpathogenic E. coli is less clear, and in particular, there is no information regarding the role of quorum sensing during the overexpression of recombinant proteins. The production of autoinducer AI-2, a signaling molecule employed by E. coli for intercellular communication, was studied in E. coli W3110 chemostat cultures using a Vibrio harveyi AI-2 reporter assay (M. G. Surrette and B. L. Bassler, Proc. Natl. Acad. Sci. USA 95:7046-7050, 1998). Chemostat cultures enabled a study of AI-2 regulation through steady-state and transient responses to a variety of environmental stimuli. Results demonstrated that AI-2 levels increased with the steady-state culture growth rate. In addition, AI-2 increased following pulsed addition of glucose, Fe(III), NaCl, and dithiothreitol and decreased following aerobiosis, amino acid starvation, and isopropyl-beta-D-thiogalactopyranoside-induced expression of human interleukin-2 (hIL-2). In general, the AI-2 responses to several perturbations were indicative of a shift in metabolic activity or state of the cells induced by the individual stress. Because of our interest in the expression of heterologous proteins in E. coli, the transcription of four quorum-regulated genes and 20 stress genes was mapped during the transient response to induced expression of hIL-2. Significant regulatory overlap was revealed among several stress and starvation genes and known quorum-sensing genes.

Cloning and genetic analysis of the Vibrio vulnificus fur gene and construction of a fur mutant by in vivo marker exchange

Vibrio vulnificus infections have been associated with iron overload and preexisting liver disease. Iron may play a major role in the pathogenesis of V. vulnificus infections. Many virulence genes, as well as genes involved in the transport of iron by bacteria, are regulated by iron, with increased expression under low-iron conditions. In Escherichia coli and Vibrio cholerae, transcriptional regulation by iron depends on the fur gene. We utilized Southern hybridization under low- and high-stringency conditions with both E. coli and V. cholerae fur gene probes to demonstrate that there are fur-homologous sequences in the DNAs of V. vulnificus, Vibrio fischeri, and Aeromonas sp. but not in the DNAs of the other bacterial species tested. We developed a restriction map and cloned the fur-homologous sequence from V. vulnificus. The hybridizing clone of V. vulnificus chromosomal DNA complemented a V. cholerae fur mutant. DNA sequence analysis confirmed the presence of a 149-amino-acid open reading frame that was 77% homologous to E. coli Fur and 93% homologous to V. cholerae Fur. Primer extension localized a single promoter for the V. vulnificus fur gene. Northern (RNA) blot analysis and beta-galactosidase assays of an operon fusion to lacZ suggested that there was not significant regulation of transcription of V. vulnificus fur by iron or the E. coli Fur protein. We used marker exchange to construct a V. vulnificus fur deletion mutant and confirmed its phenotype by observing overexpression of iron-regulated outer membrane proteins on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The fur deletion mutant of V. vulnificus will be helpful in future studies of the role of iron in V. vulnificus pathogenesis.

Evidence that GroEL, not sigma 32, is involved in transcriptional regulation of the Vibrio fischeri luminescence genes in Escherichia coli

In Escherichia coli, transcription of the inducible Vibrio fischeri luminescence operon, luxICDABE, has been reported to require sigma 32, the product of rpoH. Consistent with previous studies, we report that an E. coli delta rpoH mutant, KY1601 containing luxICDABE and luxR, which codes for the activator of luxICDABE transcription on a plasmid (pJE202), was weakly luminescent. Transformation of this E. coli strain with a plasmid containing rpoH under the control of the tac promoter resulted in high levels of cellular luminescence. However, the level of expression of the pJE202 luxICDABE was also high in E. coli 1603, a delta rpoH mutant with a second-site mutation that resulted in sigma 32-independent overexpression of the groE operon. Apparently, sigma 32 is not directly required for the transcription of luxICDABE in E. coli but is required for sufficient expression of groE, which is in turn required for the transcription of luxICDABE. This conclusion is supported by the finding that E. coli groE mutants containing pJE202 were weakly luminescent. In the E. coli delta rpoH mutant KY1601, the sigma 32 requirement for the transcription of luxICDABE was partially compensated for by the addition of saturating concentrations of the inducer to the culture medium and largely compensated for when cells were transformed with a luxR overexpression vector. These data support the hypothesis that sigma 32 is not required for transcription of luxICDABE. Rather, it appears that the products of groE are required for the folding of LuxR into an active protein, like they are for the folding of several other proteins.

Mechanism for iron control of the Vibrio fischeri luminescence system: involvement of cyclic AMP and cyclic AMP receptor protein and modulation of DNA level

Iron controls luminescence in Vibrio fischeri by an indirect but undefined mechanism. To gain insight into that mechanism, the involvement of cyclic AMP (cAMP) and cAMP receptor protein (CRP) and of modulation of DNA levels in iron control of luminescence were examined in V. fischeri and in Escherichia coli containing the cloned V. fischeri lux genes on plasmids. For V. fischeri and E. coli adenylate cyclase (cya) and CRP (crp) mutants containing intact lux genes (luxR luxICDABEG), presence of the iron chelator ethylenediamine-di(o-hydroxyphenyl acetic acid) (EDDHA) increased expression of the luminescence system like in the parent strains only in the cya mutants in the presence of added cAMP. In the E. coli strains containing a plasmid with a Mu dl(lacZ) fusion in luxR, levels of beta-galactosidase activity (expression from the luxR promoter) and luciferase activity (expression from the lux operon promoter) were both 2-3-fold higher in the presence of EDDHA in the parent strain, and for the mutants this response to EDDHA was observed only in the cya mutant in the presence of added cAMP. Therefore, cAMP and CRP are required for the iron restriction effect on luminescence, and their involvement in iron control apparently is distinct from the known differential control of transcription from the luxR and luxICDABEG promoters by cAMP-CRP. Furthermore, plasmid and chromosomal DNA levels were higher in E. coli and V. fischeri in the presence of EDDHA. The higher DNA levels correlated with an increase in expression of chromosomally encoded beta-galactosidase in E. coli and with a higher level of autoinducer in cultures of V. fischeri. These results implicate cAMP-CRP and modulation of DNA levels in the mechanism of iron control of the V. fischeri luminescence system.