Polyamines as constituents of the outer membranes of Escherichia coli and Salmonella typhimurium

Transcriptional response of the xerotolerant Arthrobacter sp. Helios strain to PEG-induced drought stress

A new bacterial strain has been isolated from the microbiome of solar panels and classified as Arthrobacter sp. Helios according to its 16S rDNA, positioning it in the “Arthrobacter citreus group.” The isolated strain is highly tolerant to desiccation, UV radiation and to the presence of metals and metalloids, while it is motile and capable of growing in a variety of carbon sources. These characteristics, together with observation that Arthrobacter sp. Helios seems to be permanently prepared to handle the desiccation stress, make it very versatile and give it a great potential to use it as a biotechnological chassis. The new strain genome has been sequenced and its analysis revealed that it is extremely well poised to respond to environmental stresses. We have analyzed the transcriptional response of this strain to PEG6000-mediated arid stress to investigate the desiccation resistance mechanism. Most of the induced genes participate in cellular homeostasis such as ion and osmolyte transport and iron scavenging. Moreover, the greatest induction has been found in a gene cluster responsible for biogenic amine catabolism, suggesting their involvement in the desiccation resistance mechanism in this bacterium.

Dickeya Manipulates Multiple Quorum Sensing Systems to Control Virulence and Collective Behaviors

Soft rot Pectobacteriaceae (SRP), typical of Pectobacterium and Dickeya, are a class of Gram-negative bacterial pathogens that cause devastating diseases on a wide range of crops and ornamental plants worldwide. Quorum sensing (QS) is a cell-cell communication mechanism regulating the expression of specific genes by releasing QS signal molecules associated with cell density, in most cases, involving in the vital process of virulence and infection. In recent years, several types of QS systems have been uncovered in Dickeya pathogens to control diverse biological behaviors, especially bacterial pathogenicity and transkingdom interactions. This review depicts an integral QS regulation network of Dickeya, elaborates in detail the regulation of specific QS system on different biological functions of the pathogens and hosts, aiming at providing a systematic overview of Dickeya pathogenicity and interactions with hosts, and, finally, expects the future prospective of effectively controlling the bacterial soft rot disease caused by Dickeya by quenching the key QS signal.

Interdisciplinary insights into the link between gut microbiome and gastric carcinogenesis-what is currently known?

Currently, gastric cancer is one of the leading death-related cancer globally. The etiopathogenesis of gastric cancer is multifactorial and includes among others dysbiotic alterations of gastric microbiota. Molecular techniques revealed that stomach is not a sterile organ and it is resides with ecosystem of microbes. Due to the fact that the role of Helicobacter pylori infection in development of gastric cancer is established and well-studied, this paper is mainly focused on the role of other bacterial as well as viral and fungal gut microbiota imbalance in gastric carcinogenesis. Notably, not only the composition of gastric microbiota may play an important role in development of gastric cancer, but also its activity. Microbial metabolites, such as short-chain fatty acids, polyamines, N-nitroso compounds, and lactate, may significantly affect gastric carcinogenesis. Therefore, this paper discussed aforementioned aspects with the interdisciplinary insights (regarding also immunological point of view) into the association between gut microbiome and gastric carcinogenesis based on up-to-date studies.

Prediction of Burkholderia pseudomallei DsbA substrates identifies potential virulence factors and vaccine targets

Identification of bacterial virulence factors is critical for understanding disease pathogenesis, drug discovery and vaccine development. In this study we used two approaches to predict virulence factors of Burkholderia pseudomallei, the Gram-negative bacterium that causes melioidosis. B. pseudomallei is naturally antibiotic resistant and there are no clinically available melioidosis vaccines. To identify B. pseudomallei protein targets for drug discovery and vaccine development, we chose to search for substrates of the B. pseudomallei periplasmic disulfide bond forming protein A (DsbA). DsbA introduces disulfide bonds into extra-cytoplasmic proteins and is essential for virulence in many Gram-negative organism, including B. pseudomallei. The first approach to identify B. pseudomallei DsbA virulence factor substrates was a large-scale genomic analysis of 511 unique B. pseudomallei disease-associated strains. This yielded 4,496 core gene products, of which we hypothesise 263 are DsbA substrates. Manual curation and database screening of the 263 mature proteins yielded 81 associated with disease pathogenesis or virulence. These were screened for structural homologues to predict potential B-cell epitopes. In the second approach, we searched the B. pseudomallei genome for homologues of the more than 90 known DsbA substrates in other bacteria. Using this approach, we identified 15 putative B. pseudomallei DsbA virulence factor substrates, with two of these previously identified in the genomic approach, bringing the total number of putative DsbA virulence factor substrates to 94. The two putative B. pseudomallei virulence factors identified by both methods are homologues of PenI family β-lactamase and a molecular chaperone. These two proteins could serve as high priority targets for future B. pseudomallei virulence factor characterization.

Dissecting the Molecular Mechanism of Colistin Resistance in mcr-1 Bacteria

Colistin or polymyxin B is the last resort antibiotic to treat infections of multidrug-resistant Gram-negative bacteria by disrupting their outer membranes. The recent emergence of Gram-negative bacteria that demonstrate colistin resistance, particularly plasmid-mediated mobile colistin resistance (mcr), poses a big challenge to the treatment of multidrug resistance infections. Using molecular dynamics simulations, we explore the mechanism of colistin resistance in a model lipid A bilayer mimicking the Gram-negative mcr-1 bacterial outer membrane. The simulation results reveal that the outer membrane of normal Gram-negative bacteria is stabilized by salt bridges between positively charged divalent ions and negatively charged phosphate groups of the membranes. In the presence of positively charged polymyxin B, these salt bridges are disrupted, and calcium is released into the aqueous phase, resulting in membrane disruption. In contrast, the lipid A in the outer membrane of mcr-1 bacteria has a novel modification, this being a covalently attached phosphoethanolamine group. This group enables the formation of a large number of hydrogen bonds between the amine and phosphate groups, resulting in an electrostatic net on the membrane. This extensive noncovalent electrostatic cross-linking between the lipid molecules collectively enhances the membrane stability and results in resistance to the action of cationic peptides such as polymyxin B. The simulation results shed new atomistic insights for understanding the mechanistic basis of colistin resistance and provide clues for the design of new membrane disruptors and permeabilizers to treat mcr-1 infections.

The Evolutionary Conservation of Escherichia coli Drug Efflux Pumps Supports Physiological Functions

Bacteria harness an impressive repertoire of resistance mechanisms to evade the inhibitory action of antibiotics. One such mechanism involves efflux pump-mediated extrusion of drugs from the bacterial cell, which significantly contributes to multidrug resistance. Intriguingly, most drug efflux pumps are chromosomally encoded components of the intrinsic antibiotic resistome. In addition, in terms of xenobiotic detoxification, bacterial efflux systems often exhibit significant levels of functional redundancy. Efflux pumps are also considered to be highly conserved; however, the extent of conservation in many bacterial species has not been reported and the majority of genes that encode efflux pumps appear to be dispensable for growth. These observations, in combination with an increasing body of experimental evidence, imply alternative roles in bacterial physiology. Indeed, the ability of efflux pumps to facilitate antibiotic resistance could be a fortuitous by-product of ancient physiological functions. Using Escherichia coli as a model organism, we here evaluated the evolutionary conservation of drug efflux pumps and we provide phylogenetic analysis of the major efflux families. We show the E. coli drug efflux system has remained relatively stable and the majority (∼80%) of pumps are encoded in the core genome. This analysis further supports the importance of drug efflux pumps in E. coli physiology. In this review, we also provide an update on the roles of drug efflux pumps in the detoxification of endogenously synthesized substrates and pH homeostasis. Overall, gaining insight into drug efflux pump conservation, common evolutionary ancestors, and physiological functions could enable strategies to combat these intrinsic and ancient elements.

Mechanism of polyamine induced colistin resistance through electrostatic networks on bacterial outer membranes

Naturally occurring linear polyamines are known to enable bacteria to be resistant to cationic membrane active peptides. To understand this protective mechanism, molecular dynamics simulations are employed to probe their effect on a model bacterial outer membrane. Being protonated at physiological pH, the amine groups of the polyamine engage in favorable electrostatic interactions with the negatively charged phosphate groups of the membrane. Additionally, the amine groups form large number of hydrogen bonds with the phosphate groups. At high concentrations, these hydrogen bonds and the electrostatic network can non-covalently crosslink the lipid A molecules, resulting in stabilization of the outer membrane against membrane active antibiotics such as colistin and polymyxin B. Moreover, large polyamine molecules (e.g., spermidine) have a stronger stabilization effect than small polyamine molecules (e.g., ethylene diamine). The atomistic insights provide useful guidance for the design of next generation membrane active amine-rich antibiotics, especially to tackle the growing threat of multi-drug resistance of Gram negative bacteria.

Pulmonary Surfactant Promotes Virulence Gene Expression and Biofilm Formation in Klebsiella pneumoniae

The interactions between Klebsiella pneumoniae and the host environment at the site of infection are largely unknown. Pulmonary surfactant serves as an initial point of contact for inhaled bacteria entering the lung and is thought to contain molecular cues that aid colonization and pathogenesis. To gain insight into this ecological transition, we characterized the transcriptional response of K. pneumoniae MGH 78578 to purified pulmonary surfactant. This work revealed changes within the K. pneumoniae transcriptome that likely contribute to host colonization, adaptation, and virulence in vivo Notable transcripts expressed under these conditions include genes involved in capsule synthesis, lipopolysaccharide modification, antibiotic resistance, biofilm formation, and metabolism. In addition, we tested the contributions of other surfactant-induced transcripts to K. pneumoniae survival using engineered isogenic KPPR1 deletion strains in a murine model of acute pneumonia. In these infection studies, we identified the MdtJI polyamine efflux pump and the ProU glycine betaine ABC transporter to be significant mediators of K. pneumoniae survival within the lung and confirmed previous evidence for the importance of de novo leucine synthesis to bacterial survival during infection. Finally, we determined that pulmonary surfactant promoted type 3 fimbria-mediated biofilm formation in K. pneumoniae and identified two surfactant constituents, phosphatidylcholine and cholesterol, that drive this response. This study provides novel insight into the interactions occurring between K. pneumoniae and the host at an important infection site and demonstrates the utility of purified lung surfactant preparations for dissecting host-lung pathogen interactions in vitro.

The Role of Cadaverine Synthesis on Pneumococcal Capsule and Protein Expression

Invasive infections caused by Streptococcus pneumoniae, a commensal in the nasopharynx, pose significant risk to human health. Limited serotype coverage by the available polysaccharide-based conjugate vaccines coupled with increasing incidence of antibiotic resistance complicates therapeutic strategies. Bacterial physiology and metabolism that allows pathogens to adapt to the host are a promising avenue for the discovery of novel therapeutics. Intracellular polyamine concentrations are tightly regulated by biosynthesis, transport and degradation. We previously reported that deletion of cadA, a gene that encodes for lysine decarboxylase, an enzyme that catalyzes cadaverine synthesis results in an attenuated phenotype. Here, we report the impact of cadA deletion on pneumococcal capsule and protein expression. Our data show that genes for polyamine biosynthesis and transport are downregulated in ∆cadA. Immunoblot assays show reduced capsule in ∆cadA. Reduced capsule synthesis could be due to reduced transcription and availability of precursors for synthesis. The capsule is the predominant virulence factor in pneumococci and is critical for evading opsonophagocytosis and its loss in ∆cadA could explain the reported attenuation in vivo. Results from this study show that capsule synthesis in pneumococci is regulated by polyamine metabolism, which can be targeted for developing novel therapies.

speG Is Required for Intracellular Replication of Salmonella in Various Human Cells and Affects Its Polyamine Metabolism and Global Transcriptomes

The speG gene has been reported to regulate polyamine metabolism in Escherichia coli and Shigella, but its role in Salmonella remains unknown. Our preliminary studies have revealed that speG widely affects the transcriptomes of infected in vitro M and Caco-2 cells and that it is required for the intracellular replication of Salmonella enterica serovar Typhimurium (S. Typhimurium) in HeLa cells. In this study, we demonstrated that speG plays a time-dependent and cell type-independent role in the intracellular replication of S. Typhimurium. Moreover, high-performance liquid chromatography (HPLC) of four major polyamines demonstrated putrescine, spermine, and cadaverine as the leading polyamines in S. Typhimurium. The deletion of speG significantly increased the levels of the three polyamines in intracellular S. Typhimurium, suggesting the inhibitory effect of speG on the biosynthesis of these polyamines. The deletion of speG was associated with elevated levels of these polyamines in the attenuated intracellular replication of S. Typhimurium in host cells. This result was subsequently validated by the dose-dependent suppression of intracellular proliferation after the addition of the polyamines. Furthermore, our RNA transcriptome analysis of S. Typhimurium SL1344 and its speG mutant outside and inside Caco-2 cells revealed that speG regulates the genes associated with flagellar biosynthesis, fimbrial expression, and functions of types III and I secretion systems. speG also affects the expression of genes that have been rarely reported to correlate with polyamine metabolism in Salmonella, including those associated with the periplasmic nitrate reductase system, glucarate metabolism, the phosphotransferase system, cytochromes, and the succinate reductase complex in S. Typhimurium in the mid-log growth phase, as well as those in the ilv-leu and histidine biosynthesis operons of intracellular S. Typhimurium after invasion in Caco-2 cells. In the present study, we characterized the phenotypes and transcriptome effects of speG in S. Typhimurium and reviewed the relevant literature to facilitate a more comprehensive understanding of the potential role of speG in the polyamine metabolism and virulence regulation of Salmonella.

Spermine inhibits Vibrio cholerae biofilm formation through the NspS-MbaA polyamine signaling system

The aquatic bacterium and human intestinal pathogen, Vibrio cholerae, senses and responds to a variety of environment-specific cues to regulate biofilm formation. Specifically, the polyamines norspermidine and spermidine enhance and repress V. cholerae biofilm formation, respectively. These effects are relevant for understanding V. cholerae pathogenicity and are mediated through the periplasmic binding protein NspS and the transmembrane bis-(3'-5') cyclic diguanosine monophosphate (c-di-GMP) phosphodiesterase MbaA. However, the levels of spermidine required to inhibit biofilm formation through this pathway are unlikely to be encountered by V. cholerae in aquatic reservoirs or within the human host during infection. We therefore hypothesized that other polyamines in the gastrointestinal tract may control V. cholerae biofilm formation at physiological levels. The tetramine spermine has been reported to be present at nearly 50 μm concentrations in the intestinal lumen. Here, we report that spermine acts as an exogenous cue that inhibits V. cholerae biofilm formation through the NspS-MbaA signaling system. We found that this effect probably occurs through a direct interaction of spermine with NspS, as purified NspS protein could bind spermine in vitro Spermine also inhibited biofilm formation by altering the transcription of the vps genes involved in biofilm matrix production. Global c-di-GMP levels were unaffected by spermine supplementation, suggesting that biofilm formation may be regulated by variations in local rather than global c-di-GMP pools. Finally, we propose a model illustrating how the NspS-MbaA signaling system may communicate exogenous polyamine content to the cell to control biofilm formation in the aquatic environment and within the human intestine.

Polyphosphate kinases modulate Campylobacter jejuni outer membrane constituents and alter its capacity to invade and survive in intestinal epithelial cells in vitro

Campylobacter jejuni is the most prevalent cause of bacterial gastroenteritis worldwide. Polyphosphate kinases 1 and 2 (PPK1 and PPK2) regulate several cellular processes, including the biosynthesis of the bacterial cell wall. Despite their importance, whether PPK1 and PPK2 modulate the composition of C. jejuni outer membrane constituents (OMCs) and consequently impact its interaction with host cells remains unknown. Our comparative analysis between C. jejuni wild type, Δppk1, and Δppk2 strains showed qualitative and quantitative differences in the total OMC composition among these strains. Importantly, these OMC variations observed on the C. jejuni polyphosphate kinase mutants are directly related to their capacity to invade, survive, and alter the immune response of intestinal epithelial cells in vitro. Specifically, sub-fractionation of the C. jejuni OMC indicated that OMC proteins are uniquely associated with bacterial invasion, whereas C. jejuni OMC proteins, lipids, and lipoglycans are all associated with C. jejuni intracellular survival. This study provides new insights regarding the function of polyphosphate kinases and their role in C. jejuni infection.

Exposure of Salmonella enterica Serovar Typhimurium to Three Humectants Used in the Food Industry Induces Different Osmoadaptation Systems

Common salt (NaCl) is frequently used by the food industry to add flavor and to act as a humectant in order to reduce the water content of a food product. The improved health awareness of consumers is leading to a demand for food products with reduced salt content; thus, manufacturers require alternative water activity-reducing agents which elicit the same general effects as NaCl. Two examples include KCl and glycerol. These agents lower the water activity of a food matrix and also contribute to limit the growth of the microbiota, including foodborne pathogens. Little is currently known about how foodborne pathogens respond to these water activity-lowering agents. Here we examined the response of Salmonella enterica serovar Typhimurium 4/74 to NaCl, KCl, and glycerol at three time points, using a constant water activity level, compared with the response of a control inoculum. All conditions induced the upregulation of gluconate metabolic genes after 6 h of exposure. Bacteria exposed to NaCl and KCl demonstrated the upregulation of the osmoprotective transporter mechanisms encoded by the proP, proU, and osmU (STM1491 to STM1494) genes. Glycerol exposure elicited the downregulation of these osmoadaptive mechanisms but stimulated an increase in lipopolysaccharide and membrane protein-associated genes after 1 h. The most extensive changes in gene expression occurred following exposure to KCl. Because many of these genes were of unknown function, further characterization may identify KCl-specific adaptive processes that are not stimulated by NaCl. This study shows that the response of S. Typhimurium to different humectants does not simply reflect reduced water activity and likely involves systems that are linked to specific humectants.

Spermidine biosynthesis and transport modulate pneumococcal autolysis

Polyamines are small cationic molecules that have far-reaching roles in biology. In the case of pathogenic bacteria, these functions include those central to their pathogenesis. Streptococcus pneumoniae is a major bacterial pathogen, causing a diverse range of diseases that account for significant morbidity and mortality worldwide. In this work, we characterize the polyamine biosynthetic pathway of S. pneumoniae, demonstrating that this organism produces spermidine from arginine. The synthesis of spermidine was found to be nonessential for growth in a polyamine-free chemically defined medium. However, mutant strains lacking the ability to synthesize or transport spermidine displayed a significant delay in the onset of autolysis. We provide evidence for a model in which spermidine modulates the activity of the major autolysin LytA in the pneumococcal cell wall compartment via interactions with negatively charged molecules, such as teichoic acids.

Surface-localized spermidine protects the Pseudomonas aeruginosa outer membrane from antibiotic treatment and oxidative stress

Extracellular DNA acts as a cation chelator and induces the expression of antibiotic resistance genes regulated by Mg(2+) levels. Here we report the characterization of novel DNA-induced genes in Pseudomonas aeruginosa that are annotated as homologs of the spermidine synthesis genes speD (PA4773) and speE (PA4774). The addition of sublethal concentrations of DNA and membrane-damaging antibiotics induced expression of the genes PA4773 to PA4775, as shown using transcriptional lux fusions and quantitative RT-PCR. Exogenous polyamine addition prevented DNA- and peptide-mediated gene induction. Mutation of PA4774 resulted in an increased outer membrane (OM) susceptibility phenotype upon polymyxin B, CP10A, and gentamicin treatment. When the membrane-localized fluorescent probe C(11)-BODIPY(581/591) was used as an indicator of peroxidation of membrane lipids, the PA4774::lux mutant demonstrated an increased susceptibility to oxidative membrane damage from H(2)O(2) treatment. Addition of exogenous polyamines protected the membranes of the PA4774::lux mutant from polymyxin B and H(2)O(2) treatment. Polyamines from the outer surface were isolated and shown to contain putrescine and spermidine by using high-performance liquid chromatography and mass spectrometry. The PA4774::lux mutant did not produce spermidine on the cell surface, but genetic complementation restored surface spermidine production as well as the antibiotic and oxidative stress resistance phenotypes of the membrane. We have identified new functions for spermidine on the cell surface and propose that polyamines are produced under Mg(2+)-limiting conditions as an organic polycation to bind lipopolysaccharide (LPS) and to stabilize and protect the outer membrane against antibiotic and oxidative damage.

Polyamines in bacteria: pleiotropic effects yet specific mechanisms

Extensive data in a wide range of organisms point to the importance of polyamine homeostasis for growth. The two most common polyamines found in bacteria are putrescine and spermidine. The investigation of polyamine function in bacteria has revealed that they are involved in a number of functions other than growth, which include incorporation into the cell wall and biosynthesis of siderophores. They are also important in acid resistance and can act as a free radical ion scavenger. More recently it has been suggested that polyamines play a potential role in signaling cellular differentiation in Proteus mirabilis. Polyamines have also been shown to be essential in biofilm formation in Yersinia pestis. The pleiotropic nature of polyamines has made their investigation difficult, particularly in discerning any specific effect from more global growth effects. Here we describe key developments in the investigation of the function of polyamines in bacteria that have revealed new roles for polyamines distinct from growth. We describe the bacterial genes necessary for biosynthesis and transport, with a focus on Y. pestis. Finally we review a novel role for polyamines in the regulation of biofilm development in Y. pestis and provide evidence that the investigation of polyamines in Y. pestis may provide a model for understanding the mechanism through which polyamines regulate biofilm formation.

Polyamines are essential for the formation of plague biofilm

We provide the first evidence for a link between polyamines and biofilm levels in Yersinia pestis, the causative agent of plague. Polyamine-deficient mutants of Y. pestis were generated with a single deletion in speA or speC and a double deletion mutant. The genes speA and speC code for the biosynthetic enzymes arginine decarboxylase and ornithine decarboxylase, respectively. The level of the polyamine putrescine compared to the parental speA+ speC+ strain (KIM6+) was depleted progressively, with the highest levels found in the Y. pestis DeltaspeC mutant (55% reduction), followed by the DeltaspeA mutant (95% reduction) and the DeltaspeA DeltaspeC mutant (>99% reduction). Spermidine, on the other hand, remained constant in the single mutants but was undetected in the double mutant. The growth rates of mutants with single deletions were not altered, while the DeltaspeA DeltaspeC mutant grew at 65% of the exponential growth rate of the speA+ speC+ strain. Biofilm levels were assayed by three independent measures: Congo red binding, crystal violet staining, and confocal laser scanning microscopy. The level of biofilm correlated to the level of putrescine as measured by high-performance liquid chromatography-mass spectrometry and as observed in a chemical complementation curve. Complementation of the DeltaspeA DeltaspeC mutant with speA showed nearly full recovery of biofilm to levels observed in the speA+ speC+ strain. Chemical complementation of the double mutant and recovery of the biofilm defect were only observed with the polyamine putrescine.

Identification of genes associated with survival of Salmonella enterica serovar Enteritidis in chicken egg albumen

Salmonella enterica consists of over 2,000 serovars that are major causes of morbidity and mortality associated with contaminated food. Despite similarities among serovars of Salmonella enterica, many demonstrate unique host specificities, epidemiological characteristics, and clinical manifestations. One of the unique epidemiological characteristics of the serovar Enteritidis is that it is the only bacterium routinely transmitted to humans through intact chicken eggs. Therefore, Salmonella enterica serovar Enteritidis must be able to persist inside chicken eggs to be transmitted to humans, and its survival in egg is important for its transmission to the human population. The ability of Salmonella enterica serovar Enteritidis to survive in and transmit through eggs may have contributed to its drastically increased prevalence in the 1980s and 1990s. In the present study, using transposon-mediated mutagenesis, we have identified genes important for the association of Salmonella enterica serovar Enteritidis with chicken eggs. Our results indicate that genes involved in cell wall structural and functional integrity, and nucleic acid and amino acid metabolism are important for Salmonella enterica serovar Enteritidis to persist in egg albumen. Two regions unique to Salmonella enterica serovar Enteritidis were also identified, one of which enhanced the survival of a Salmonella enterica serovar Typhimurium isolate in egg albumen. The implication of our results to the serovar specificity of Salmonella enterica is also explored in the present study.

Evidence that putrescine acts as an extracellular signal required for swarming in Proteus mirabilis

In a search for Proteus mirabilis genes that were regulated by cell-to-cell signalling, a lacZ fusion (cmr437::mini-Tn5lacZ) was identified that was repressed 10-fold by a self-produced extracellular signal from wild-type cells. However, the cmr437::mini-Tn5lacZ insertion itself led to a marked reduction in this extracellular repressing signal. The cmr437::mini-Tn5lacZ insertion was mapped to a speA homologue in P. mirabilis. Sequence analysis indicated that a speB homologue was encoded downstream of speA. Products of the SpeA and SpeB enzymes (agmatine and putrescine) were tested for repression of cmr437::lacZ. Agmatine did not have repressing activity. However, putrescine was an effective repressing molecule at concentrations down to 30 microM. A second prominent phenotype of the cmr437 (speA)::mini-Tn5lacZ insertion was a severe defect in swarming motility. This swarming defect was also observed in a strain containing a disruption of the downstream speB gene. Differentiation of the speB mutant to swarmer cells was delayed by two hours relative to wild-type cells. Furthermore, the speB mutant was unable to migrate effectively across agar surfaces and formed very closely spaced swarming rings. Exogenous putrescine restored both the normal timing of swarmer cell differentiation and the ability to migrate to speB mutants.

Increased uptake of putrescine in the rhizosphere inhibits competitive root colonization by Pseudomonas fluorescens strain WCS365

Sequence analysis of the chromosomal Tn5lacZ flanking regions of the Pseudomonas fluorescens WCS365 competitive root colonization mutant PCL1206 showed that the Tn5lacZ is inserted between genes homologous to bioA and potF. The latter gene is the first gene of the potF1F2GHI operon, which codes for a putrescine transport system in Escherichia coli. The position of the Tn5lacZ suggests an effect on the expression of the pot operon. A mutation in the potF1 gene as constructed in PCL1270, however, had no effect on competitive root colonization. The rate of uptake of [1,4-14C]putrescine by cells of mutant PCL1206 appeared to be increased, whereas cells of strain PCL1270 were strongly impaired in the uptake of putrescine. Dansylation of tomato root exudate and subsequent thin-layer chromatography showed the presence of a component with the same Rf value as dansyl-putrescine, which was identified as dansyl-putrescine by mass spectrometric analyses. Other polyamines such as spermine and spermidine were not detected in the root exudate. Growth of mutant strains, either alone or in competition with the wild type, was tested in media containing putrescine, spermine, or spermidine as the sole nitrogen source. The results show that mutant PCL1206 is strongly impaired in growth on putrescine and slightly impaired on spermine and spermidine. The presence of the polyamines had a similar effect on the growth rate of strain PCL1270 in the presence of putrescine but a less severe effect in the presence of spermine and spermidine. We conclude that an increased rate of putrescine uptake has a bacteriostatic effect on Pseudomonas spp. cells. We have shown that putrescine is an important tomato root exudate component and that root-colonizing pseudomonads must carefully regulate their rate of uptake because increased uptake causes a decreased growth rate and, therefore, a decreased competitive colonization ability.

Molecular basis for the polyamine-ompF porin interactions: inhibitor and mutant studies

By testing the sensitivity of Escherichia coli OmpF porin to various natural and synthetic polyamines of different lengths, charge and other molecular characteristics, we were able to identify the molecular properties required for compounds to act as inhibitors of OmpF in the nanomolar range. Inhibitors require at least two amine groups to be effective. For diamines, the optimum length of the hydrocarbon spacer was found to be of eight to ten methylene groups. Triamine molecules based on a 12-carbon motif were found to be more effective that spermidine, an eight-carbon trivalent derivative. But differences in inhibition efficiencies were also found for trivalent compounds depending on the relative position of the internal secondary amine group with respect to the terminal groups. Finally, quaternary ammonium derivatives had no effect, suggesting that the nature of the terminal amine is important for the interaction. From these observations, we deduce that inhibition efficiency in the nanomolar range requires a 12-carbon chain triamine with terminal primary amine groups and replacement of the eighth methylene by a secondary amine. The need for this type of molecular architecture suggests that inhibition is governed by interactions between specific amine groups and protein residues, and that this is not simply due to the accumulation of charges into the pore. Together with previous observations from site-directed mutagenesis studies and inspection of the crystal structure of OmpF, these results allowed us to propose three residues (D113, D121 and Y294) as putative sites of interaction between the channel and spermine. Alanine substitution at each of these three residues resulted in a loss of inhibition by spermine, while mutations of only D113 and D121 affected inhibition by spermidine. Based on these observations, we suggest a model for the molecular determinants involved in the porin-polyamine interaction.

Inducing effect of diamines on transcription of the cephamycin C genes from the lat and pcbAB promoters in Nocardia lactamdurans

The diamines putrescine, cadaverine, and diaminopropane stimulate cephamycin biosynthesis in Nocardia lactamdurans, in shake flasks and fermentors, without altering cell growth. Intracellular levels of the P7 protein (a component of the methoxylation system involved in cephamycin biosynthesis) were increased by diaminopropane, as shown by immunoblotting studies. Lysine-6-aminotransferase and piperideine-6-carboxylate dehydrogenase activities involved in biosynthesis of the alpha-aminoadipic acid precursor were also greatly stimulated. The diamine stimulatory effect is exerted at the transcriptional level, as shown by low-resolution S1 protection studies. The transcript corresponding to the pcbAB gene and to a lesser extent also the lat transcript were significantly increased in diaminopropane-supplemented cultures, whereas transcription from the cefD promoter was not affected. Coupling of the lat and pcbAB promoters to the reporter xylE gene showed that expression from the lat and pcbAB promoters was increased by addition of diaminopropane in Streptomyces lividans. Intracellular accumulation of diamines in Nocardia may be a signal to trigger antibiotic production.

Excretion of endogenous cadaverine leads to a decrease in porin-mediated outer membrane permeability

The permeability of the outer membrane of Escherichia coli to hydrophilic compounds is controlled by porin channels. Electrophysiological experiments showed that polyamines inhibit ionic flux through cationic porins when applied to either side of the membrane. Externally added polyamines, such as cadaverine, decrease porin-mediated fluxes of beta-lactam antibiotics in live cells. Here we tested the effects of endogenously expressed cadaverine on the rate of permeation of cephaloridine through porins, by manipulating in a pH-independent way the expression of the cadBA operon, which encodes proteins involved in the decarboxylation of lysine to cadaverine and in cadaverine excretion. We report that increased levels of excreted cadaverine correlate with a decreased outer membrane permeability to cephaloridine, without any change in porin expression. Cadaverine appears to promote a sustained inhibition of porins, since the effect remains even after removal of the exogenously added or excreted polyamine. The cadaverine-induced inhibition is sufficient to provide cells with some resistance to ampicillin but not to hydrophobic antibiotics. Finally, the mere expression of cadC, in the absence of cadaverine production, leads to a reduction in the amounts of OmpF and OmpC proteins, which suggests a novel mechanism for the environmental control of porin expression. The results presented here support the notion that polyamines can act as endogenous modulators of outer membrane permeability, possibly as part of an adaptive response to acidic conditions.

Complex inhibition of OmpF and OmpC bacterial porins by polyamines

The effects of four polyamines (putrescine, cadaverine, spermidine, and spermine) on the activity of bacterial porins OmpC and OmpF were investigated by electrophysiology. Membrane vesicles made from the outer membrane of Escherichia coli strains expressing only OmpC or OmpF were reconstituted into liposomes probed by patch clamp. The channel activity was recorded in control solutions and in the presence of increasing concentrations of a specific polyamine. In all cases, concentration- and voltage-dependent inhibitory effects were observed. They include both the suppression of channel openings and the enhancement of channel closures as well as the promotion of blocked or inactivated states. OmpF and OmpC, although highly homologous, have distinct sensitivities to modulation, especially by spermine. This compound inhibits OmpF in the nanomolar range, which is in agreement with its potency on eukaryotic channels. Putrescine was the least effective (upper millimolar range) and also had inhibitory effects qualitatively distinct from those exerted by the other polyamines. The compounds appear to bind to at least two distinct binding sites, one of which resides within the pore. The potencies to this site are lower when the polyamines are applied from the extracellular side than from the periplasmic side, suggesting an asymmetric binding site.

Function and modulation of bacterial porins: insights from electrophysiology

Electrophysiological techniques provide a wealth of information regarding the molecular mechanisms that underlie the function and modulation of ion channels. They have revealed that bacterial porins do not behave as static, permanently open pores but display a much more complex and dynamic behavior than anticipated from non-electrophysiological studies. The channels switch between short-lived open and closed conformations (gating activity), and can also remain in an inactivated, non-ion conducting state for prolonged periods of time. Thus the role of porins is not limited to that of a molecular filter, but is extended to the control of outer membrane permeability through the regulation of their activity. Electrophysiological studies have indeed demonstrated that both gating and inactivation are modulated by a variety of physical and chemical parameters and are highly cooperative phenomena, often involving numerous channels working in concert. Cooperativity acts as an amplification mechanism that grants a large population of porins, such as found in the outer membrane, with sensitivity to modulation by external or internal factors. By conferring permeability properties to the outer membrane, porins play a crucial role in the bacterium's antibiotic susceptibility and survival in various environmental conditions. The detailed information that electrophysiology only can provide on porin function and modulation promises to yield a more accurate description of how porin properties can be used by cells to adapt to a changing environment, and to offer mechanisms that might optimize the drug sensitivity of the microorganism.

Disruption of polyamine modulation by a single amino acid substitution on the L3 loop of the OmpC porin channel

Structural studies have demonstrated that the extracellular L3 loop of porin constricts the channel and suggest that this loop might be involved in channel selectivity and gating. We previously showed that positively charged polyamines can induce changes in porin gating kinetics by stabilization of closed states. Here we report the effects of the mutation of two different aspartate residues of Escherichia coli OmpC porin on the polyamine sensitivity of the channel. Aspartate 105 or aspartate 118 on the L3 loop was replaced by glutamine by site-directed mutagenesis. The gating activity of the wild-type and mutant channels were studied by patch-clamp of liposomes containing reconstituted outer membrane fractions, in the absence or the presence of either polyamine spermine or cadaverine. Porin channels with a D118Q mutation, at the root of L3, still showed some, albeit milder, sensitivity to polyamine modulation. On the other hand, the D105Q mutation, at the tip of L3, abolished the increase in closing frequency which is typically observed in the presence of polyamines. We conclude that aspartate 105 primarily, but not aspartate 118, plays an important role in mediating the polyamine-induced changes in gating kinetics that result in the inhibition of the OmpC channel.

The lipid A biosynthesis deficiency of the Escherichia coli antibiotic-supersensitive mutant LH530 is suppressed by a novel locus, ORF195

A new mutant of Escherichia coli K-12 supersensitive to both hydrophobic and large hydrophilic antibiotics was isolated and characterized. The mutant grew well at 28 degrees C, poorly at 37 degrees C, and did not grow at 42 degrees C. The rate of its lipid A biosynthesis was reduced as compared to that of the parent strain. This deficiency was rescued by a novel locus, ORF195, the function of which has not been elucidated. ORF195 is located in the 76 min region in the E. coli chromosome and encodes a hypothetical 21.8 kDa protein with no signal sequence. ORF195 isolated from the mutant strain had an identical sequence to the wild-type allele, indicating a suppressor function of the gene product.

Polyamines decrease Escherichia coli outer membrane permeability

The permeability of the outer membranes of gram-negative bacteria to hydrophilic compounds is mostly due to the presence of porin channels. We tested the effects of four polyamines (putrescine, cadaverine, spermidine, and spermine) on two processes known to depend on intact porin function: fluxes of beta-lactam antibiotics in live cells and chemotaxis. In both cases, inhibition was observed. Measurements of the rate of permeation of cephaloridine and of chemotaxis in swarm plates and capillary assays were used to determine the concentration dependence of this modulation. The effective concentration ranges depended on the nature of the polyamine and varied from submillimolar for spermine to tens of millimolar for cadaverine. Both OmpC and OmpF porins were inhibited, although the effects on OmpC appeared to be milder. These results are in agreement with our observations that polyamines inhibit porin-mediated ion fluxes in electrophysiological experiments, and they suggest that a low-affinity polyamine binding site might exist in these porins. These results reveal the potential use of porins as targets for blocking agents and suggest that polyamines may act as endogenous modulators of outer membrane permeability.

Influence of pH on bacterial gene expression

Bacteria respond to changes in internal and external pH by adjusting the activity and synthesis of proteins associated with many different processes, including proton translocation, amino acid degradation, adaptation to acidic or basic conditions and virulence. While, for many of these examples, the physiological and biological consequence of the pH-induced response is clear, the mechanism by which the transcription/translation machinery is signalled is not. These examples are discussed along with several others in which the function of the gene or protein remains a mystery.