The fadL gene product of Escherichia coli is an outer membrane protein required for uptake of long-chain fatty acids and involved in sensitivity to bacteriophage T2

Shigella flexneri remodeling and consumption of host lipids during infection

IMPORTANCE

Bacterial pathogens have vastly distinct sites that they inhabit during infection. This requires adaptation due to changes in nutrient availability and antimicrobial stress. The bacterial surface is a primary barrier, and here, we show that the bacterial pathogen Shigella flexneri increases its surface decorations when it transitions to an intracellular lifestyle. We also observed changes in bacterial and host cell fatty acid homeostasis. Specifically, intracellular S. flexneri increased the expression of their fatty acid degradation pathway, while the host cell lipid pool was significantly depleted. Importantly, bacterial proliferation could be inhibited by fatty acid supplementation of host cells, thereby providing novel insights into the possible link between human malnutrition and susceptibility to S. flexneri.

Phage Therapy-Challenges, Opportunities and Future Prospects

The increasing drug resistance of bacteria to commonly used antibiotics creates the need to search for and develop alternative forms of treatment. Phage therapy fits this trend perfectly. Phages that selectively infect and kill bacteria are often the only life-saving therapeutic option. Full legalization of this treatment method could help solve the problem of multidrug-resistant infectious diseases on a global scale. The aim of this review is to present the prospects for the development of phage therapy, the ethical and legal aspects of this form of treatment given the current situation of such therapy, and the benefits of using phage products in persons for whom available therapeutic options have been exhausted or do not exist at all. In addition, the challenges faced by this form of therapy in the fight against bacterial infections are also described. More clinical studies are needed to expand knowledge about phages, their dosage, and a standardized delivery system. These activities are necessary to ensure that phage-based therapy does not take the form of an experiment but is a standard medical treatment. Bacterial viruses will probably not become a miracle cure-a panacea for infections-but they have a chance to find an important place in medicine.

Escherichia coli displays a conserved membrane proteomic response to a range of alcohols

BACKGROUND

Alcohol is a good and environment-friendly fuel that can be microbially produced, capable of eliminating many of the limitations of the present-day fossil fuels. However, the inherent toxic nature of alcohols to the microbial cells leads to end-product inhibition that limits large-scale alcohol production by fermentation. Fundamental knowledge about the stress responses of microorganisms to alcohols would greatly facilitate to improve the microbial alcohol tolerance. The current study elucidates and compares the changes in the membrane proteome of Escherichia coli in response to a range of alcohols.

RESULTS

Although alcohol toxicity increased exponentially with alcohol chain length (2-6 carbon), similar stress responses were observed in the inner and outer membrane proteome of E. coli in the presence of 2-, 4- and 6-carbon alcohols at the MIC50. This pertains to: (1) increased levels of inner membrane transporters for uptake of energy-producing metabolites, (2) reduced levels of non-essential proteins, associated with anaerobic, carbon starvation and osmotic stress, for energy conservation, (3) increased levels of murein degrading enzymes (MltA, EmtA, MliC and DigH) promoting cell elongation and 4) reduced levels of most outer membrane β-barrel proteins (LptD, FadL, LamB, TolC and BamA). Major outer membrane β-barrel protein OmpC, which is known to contribute to ethanol tolerance and membrane integrity, was notably reduced by alcohol stress. While LPS is important for OmpC trimerisation, LPS release by EDTA did not lower OmpC levels. This suggests that LPS release, which is reported under alcohol stress, does not contribute to the reduced levels of OmpC in the presence of alcohol.

CONCLUSIONS

Since alcohol primarily targets the integrity of the membrane, maintenance of outer membrane OmpC levels in the presence of alcohol might help in the survival of E. coli to higher alcohol concentrations. The study provides important information about the membrane protein responses of E. coli to a range of alcohols, which can be used to develop targeted strategies for increased microbial alcohol tolerance and hence bioalcohol production.

Shigella flexneri Adapts to Niche-Specific Stresses through Modifications in Cell Envelope Composition and Decoration

Shigella flexneri is the primary causative agent of worldwide shigellosis. As the pathogen transverses the distinct niches of the gastrointestinal tract it necessitates dynamic adaptation strategies to mitigate host antimicrobials such as dietary fatty acids (FAs) and the bile salt, deoxycholate (DOC). This study investigates the dynamics of the S. flexneri cell envelope, by interrogating adaptations following FA or DOC exposure. We deciphered the effects of FAs and DOC on bacterial membrane fatty acid and lipopolysaccharide (LPS) compositions. We identified novel LPS-based strategies by the pathogen to support resistance to these host compounds. In particular, expression of S. flexneri very-long O antigen (VL-Oag) LPS was found to play a central role in stress mitigation, as VL-Oag protects against antimicrobial FAs, but its presence rendered S. flexneri susceptible to DOC stress. Collectively, this work underpins the importance for S. flexneri to maintain appropriate regulation of cell envelope constituents, in particular VL-Oag LPS, to adequately adapt to diverse stresses during infection.

Outer Membrane Porin F in E. coli Is Critical for Effective Predation by Bdellovibrio

Bdellovibrio and like organisms (BALOs) are a unique bacterial group that live by predating on other bacteria, consuming them from within to grow and replicate before the progeny come out to complete the life cycle. The mechanisms by which these predators recognize their prey and differentiate them from nonprey bacteria, however, are still not clear. Through genetic knockout and complementation studies in different Escherichia coli strains, we found that Bdellovibrio bacteriovorus strain 109J recognizes outer membrane porin F (OmpF) on the E. coli surface and that the activity of the E. coli EnvZ-OmpR regulatory system significantly impacts predation kinetics. OmpF is not the only signal by which BALOs recognize their prey, however, as B. bacteriovorus could eventually predate on the E. coli ΔompF mutant after prolonged incubation. Furthermore, recognizing OmpF as a prey surface structure was dependent on the prey strain, as knocking out OmpF protein homologues in other prey species, including Escherichia fergusonii, Klebsiella pneumoniae, and Salmonella enterica, did not always reduce the predation rate. Consequently, although OmpF was found to be an important surface component used by Bdellovibrio to efficiently recognize and attack E. coli, future work is needed to determine what other prey surface structures are recognized by these predators. IMPORTANCE Bdellovibrio bacteriovorus and like organisms (BALOs) are Gram-negative predatory bacteria that attack other Gram-negative bacteria by penetrating their periplasm and consuming them from within to obtain the nutrients necessary for the predator's growth and replication. How these predators recognize their prey, however, has remained a mystery. Here, we show that the outer membrane porin F (OmpF) in E. coli is recognized by B. bacteriovorus strain 109J and that the loss of this protein leads to severely delayed predation. However, predation of several other prey species was not dependent on the recognition of this protein or its homologues, indicating that there are other structures recognized by the predators on the prey surface that are yet to be discovered.

Independent host- and bacterium-based determinants protect a model symbiosis from phage predation

Bacteriophages (phages) are diverse and abundant constituents of microbial communities worldwide, capable of modulating bacterial populations in diverse ways. Here, we describe the phage HNL01, which infects the marine bacterium Vibrio fischeri. We use culture-based approaches to demonstrate that mutations in the exopolysaccharide locus of V. fischeri render this bacterium resistant to infection by HNL01, highlighting the extracellular matrix as a key determinant of HNL01 infection. Additionally, using the natural symbiosis between V. fischeri and the squid Euprymna scolopes, we show that, during colonization, V. fischeri is protected from phages present in the ambient seawater. Taken together, these findings shed light on independent yet synergistic host- and bacterium-based strategies for resisting symbiosis-disrupting phage predation, and we present important implications for understanding these strategies in the context of diverse host-associated microbial ecosystems.

Diffusible Signal Factors Act through AraC-Type Transcriptional Regulators as Chemical Cues To Repress Virulence of Enteric Pathogens

Successful colonization by enteric pathogens is contingent upon effective interactions with the host and the resident microbiota. These pathogens thus respond to and integrate myriad signals to control virulence. Long-chain fatty acids repress the virulence of the important enteric pathogens Salmonella enterica and Vibrio cholerae by repressing AraC-type transcriptional regulators in pathogenicity islands. While several fatty acids are known to be repressive, we show here that cis-2-unsaturated fatty acids, a rare chemical class used as diffusible signal factors (DSFs), are highly potent inhibitors of virulence functions. We found that DSFs repressed virulence gene expression of enteric pathogens by interacting with transcriptional regulators of the AraC family. In Salmonella enterica serovar Typhimurium, DSFs repress the activity of HilD, an AraC-type activator essential to the induction of epithelial cell invasion, by both preventing its interaction with target DNA and inducing its rapid degradation by Lon protease. cis-2-Hexadecenoic acid (c2-HDA), a DSF produced by Xylella fastidiosa, was the most potent among those tested, repressing the HilD-dependent transcriptional regulator hilA and the type III secretion effector sopB >200- and 68-fold, respectively. Further, c2-HDA attenuated the transcription of the ToxT-dependent cholera toxin synthesis genes of V. cholerae c2-HDA significantly repressed invasion gene expression by Salmonella in the murine colitis model, indicating that the HilD-dependent signaling pathway functions within the complex milieu of the animal intestine. These data argue that enteric pathogens respond to DSFs as interspecies signals to identify appropriate niches in the gut for virulence activation, which could be exploited to control the virulence of enteric pathogens.

High-throughput discovery of phage receptors using transposon insertion sequencing of bacteria

As the most abundant microbes on Earth, novel bacteriophages (phages; bacteria-specific viruses) are readily isolated from environmental samples. However, it remains challenging to characterize phage-bacteria interactions, such as the host receptor(s) phages bind to initiate infection. Here, we tested whether transposon insertion sequencing (INSeq) could be used to identify bacterial genes involved in phage binding. As proof of concept, results showed that INSeq screens successfully identified genes encoding known receptors for previously characterized viruses of Escherichia coli (phages T6, T2, T4, and T7). INSeq screens were then used to identify genes involved during infection of six newly isolated coliphages. Results showed that candidate receptors could be successfully identified for the majority (five of six) of the phages; furthermore, genes encoding the phage receptor(s) were the top hit(s) in the analyses of the successful screens. INSeq screens provide a generally useful method for high-throughput discovery of phage receptors. We discuss limitations of our approach when examining uncharacterized phages, as well as usefulness of the method for exploring the evolution of broad versus narrow use of cellular receptors among phages in the biosphere.

Pleiotropic effects of rfa-gene mutations on Escherichia coli envelope properties

Mutations in the rfa operon leading to severely truncated lipopolysaccharide (LPS) structures are associated with pleiotropic effects on bacterial cells, which in turn generates a complex phenotype termed deep-rough. Literature reports distinct behavior of these mutants in terms of susceptibility to bacteriophages and to several antibacterial substances. There is so far a critical lack of understanding of such peculiar structure-reactivity relationships mainly due to a paucity of thorough biophysical and biochemical characterizations of the surfaces of these mutants. In the current study, the biophysicochemical features of the envelopes of Escherichia coli deep-rough mutants are identified from the molecular to the single cell and population levels using a suite of complementary techniques, namely microelectrophoresis, Atomic Force Microscopy (AFM) and Isobaric Tag for Relative and Absolute Quantitation (iTRAQ) for quantitative proteomics. Electrokinetic, nanomechanical and proteomic analyses evidence enhanced mutant membrane destabilization/permeability, and differentiated abundances of outer membrane proteins involved in the susceptibility phenotypes of LPS-truncated mutants towards bacteriophages, antimicrobial peptides and hydrophobic antibiotics. In particular, inner-core LPS altered mutants exhibit the most pronounced heterogeneity in the spatial distribution of their Young modulus and stiffness, which is symptomatic of deep damages on cell envelope likely to mediate phage infection process and antibiotic action.

Exogenous fatty acids alter phospholipid composition, membrane permeability, capacity for biofilm formation, and antimicrobial peptide susceptibility in Klebsiella pneumoniae

Klebsiella pneumoniae represents a major threat to human health due to a combination of its nosocomial emergence and a propensity for acquiring antibiotic resistance. Dissemination of the bacteria from its native intestinal location creates severe, complicated infections that are particularly problematic in healthcare settings. Thus, there is an urgency for identifying novel treatment regimens as the incidence of highly antibiotic‐resistant bacteria rises. Recent findings have highlighted the ability of some Gram‐negative bacteria to utilize exogenous fatty acids in ways that modify membrane phospholipids and influence virulence phenotypes, such as biofilm formation and antibiotic resistance. This study explores the ability of K. pneumoniae to assimilate and respond to exogenous fatty acids. The combination of thin‐layer chromatography liquid chromatography‐mass spectrometry confirmed adoption of numerous exogenous polyunsaturated fatty acids (PUFAs) into the phospholipid species of K. pneumoniae. Membrane permeability was variably affected as determined by two dye uptake assays. Furthermore, the availability of many PUFAs lowered the MICs to the antimicrobial peptides polymyxin B and colistin. Biofilm formation was significantly affected depending upon the supplemented fatty acid.

Exogenous polyunsaturated fatty acids (PUFAs) alter phospholipid composition, membrane permeability, biofilm formation and motility in Acinetobacter baumannii

Acinetobacter baumannii is a ubiquitous multidrug-resistant bacteria that is found on a variety of surfaces, including skin, hair and soil. During the past decade, A. baumannii has emerged as a significant cause of nosocomial infections in the United States. Recent studies have highlighted the ability of some bacteria to utilize a wide variety of fatty acids as a membrane remodelling strategy. Considering this, we hypothesized that fatty acids may have an effect on the emerging pathogen A. baumannii. Thin-layer chromatography indicated structural alterations to major phospholipids. Liquid chromatography/mass spectrometry confirmed the assimilation of numerous exogenous polyunsaturated fatty acids (PUFAs) into the phospholipid species of A. baumannii. The incorporation of fatty acids affected several bacterial phenotypes, including membrane permeability, biofilm formation, surface motility and antimicrobial peptide resistance.

Dynamics and impact of homologous recombination on the evolution of Legionella pneumophila

Legionella pneumophila is an environmental bacterium and the causative agent of Legionnaires' disease. Previous genomic studies have shown that recombination accounts for a high proportion (>96%) of diversity within several major disease-associated sequence types (STs) of L. pneumophila. This suggests that recombination represents a potentially important force shaping adaptation and virulence. Despite this, little is known about the biological effects of recombination in L. pneumophila, particularly with regards to homologous recombination (whereby genes are replaced with alternative allelic variants). Using newly available population genomic data, we have disentangled events arising from homologous and non-homologous recombination in six major disease-associated STs of L. pneumophila (subsp. pneumophila), and subsequently performed a detailed characterisation of the dynamics and impact of homologous recombination. We identified genomic "hotspots" of homologous recombination that include regions containing outer membrane proteins, the lipopolysaccharide (LPS) region and Dot/Icm effectors, which provide interesting clues to the selection pressures faced by L. pneumophila. Inference of the origin of the recombined regions showed that isolates have most frequently imported DNA from isolates belonging to their own clade, but also occasionally from other major clades of the same subspecies. This supports the hypothesis that the possibility for horizontal exchange of new adaptations between major clades of the subspecies may have been a critical factor in the recent emergence of several clinically important STs from diverse genomic backgrounds. However, acquisition of recombined regions from another subspecies, L. pneumophila subsp. fraseri, was rarely observed, suggesting the existence of a recombination barrier and/or the possibility of ongoing speciation between the two subspecies. Finally, we suggest that multi-fragment recombination may occur in L. pneumophila, whereby multiple non-contiguous segments that originate from the same molecule of donor DNA are imported into a recipient genome during a single episode of recombination.

Discovery of Novel Leptospirosis Vaccine Candidates Using Reverse and Structural Vaccinology

Leptospira spp. are diderm (two membranes) bacteria that infect mammals causing leptospirosis, a public health problem with global implications. Thousands of people die every year due to leptospirosis, especially in developing countries with tropical climates. Prophylaxis is difficult due to multiple factors, including the large number of asymptomatic hosts that transmit the bacteria, poor sanitation, increasing numbers of slum dwellers, and the lack of an effective vaccine. Several leptospiral recombinant antigens were evaluated as a replacement for the inactivated (bacterin) vaccine; however, success has been limited. A prospective vaccine candidate is likely to be a surface-related protein that can stimulate the host immune response to clear leptospires from blood and organs. In this study, a comprehensive bioinformatics approach based on reverse and structural vaccinology was applied toward the discovery of novel leptospiral vaccine candidates. The Leptospira interrogans serovar Copenhageni strain L1-130 genome was mined in silico for the enhanced identification of conserved β-barrel (βb) transmembrane proteins and outer membrane (OM) lipoproteins. Orthologs of the prospective vaccine candidates were screened in the genomes of 20 additional Leptospira spp. Three-dimensional structural models, with a high degree of confidence, were created for each of the surface-exposed proteins. Major histocompatibility complex II (MHC-II) epitopes were identified, and their locations were mapped on the structural models. A total of 18 βb transmembrane proteins and 8 OM lipoproteins were identified. These proteins were conserved among the pathogenic Leptospira spp. and were predicted to have epitopes for several variants of MHC-II receptors. A structural and functional analysis of the sequence of these surface proteins demonstrated that most βb transmembrane proteins seem to be TonB-dependent receptors associated with transportation. Other proteins identified included, e.g., TolC efflux pump proteins, a BamA-like OM component of the βb transmembrane protein assembly machinery, and the LptD-like LPS assembly protein. The structural mapping of the immunodominant epitopes identified the location of conserved, surface-exposed, immunogenic regions for each vaccine candidate. The proteins identified in this study are currently being evaluated for experimental evidence for their involvement in virulence, disease pathogenesis, and physiology, in addition to vaccine development.

Host receptors for bacteriophage adsorption

The adsorption of bacteriophages (phages) onto host cells is, in all but a few rare cases, a sine qua non condition for the onset of the infection process. Understanding the mechanisms involved and the factors affecting it is, thus, crucial for the investigation of host-phage interactions. This review provides a survey of the phage host receptors involved in recognition and adsorption and their interactions during attachment. Comprehension of the whole infection process, starting with the adsorption step, can enable and accelerate our understanding of phage ecology and the development of phage-based technologies. To assist in this effort, we have established an open-access resource--the Phage Receptor Database (PhReD)--to serve as a repository for information on known and newly identified phage receptors.

"Click Chemistry" for the Simple Determination of Fatty-Acid Uptake and Degradation: Revising the Role of Fatty-Acid Transporters

Fatty acids (FAs) have numerous functions in all living organisms, ranging from structural roles and energy production to the biosynthesis of secondary metabolites. Because of the high energy content of exogenous FAs, their acquisition is central of metabolism, and several biological systems are known, although their precise roles are not yet entirely clear. We investigated the roles of FadD (CoA ligase) and FadL (FA transporter) in different bacterial strains by using an improved version of click-chemistry-assisted labelling of azido-FAs. The high sensitivity of this method allows a direct and precise assessment of FA metabolism, and is thus far better suited than growth experiments. Our results show that although FA activation is indeed essential for FA degradation, their transport can be independent of transporters like FadL.

Key residues of S. flexneri OmpA mediate infection by bacteriophage Sf6

Many viruses, including bacteriophage, have the inherent ability to utilize several types of proteinaceous receptors as an attachment mechanism to infect cells, yet the molecular mechanisms that drive receptor binding have not been elucidated. Using bacteriophage Sf6 and its host, Shigella flexneri, we investigated how Sf6 utilizes outer membrane protein A (OmpA) for infection. Specifically, we identified that surface loops of OmpA mediate Shigella infection. We further characterized which residues in the surface loops are responsible for Sf6 binding and productive infection using a combination of in vivo and in vitro approaches including site-directed mutagenesis, phage plaque assays, circular dichroism spectroscopy, and in vitro genome ejection assays. Our data indicate that Sf6 can productively interact with other bacterial OmpAs as long as they share homology in loops 2 and 4, suggesting that these loops may determine host specificity. Our data provide a model in which Sf6 interacts with OmpA using the surface of the protein and new insights into viral attachment through binding to membrane protein receptors.

Factors other than metalloprotease are required for full virulence of French Vibrio tubiashii isolates in oyster larvae

Vibrio tubiashii is a marine pathogen isolated from larval and juvenile bivalve molluscs that causes bacillary necrosis. Recent studies demonstrated the isolation of this species in a French experimental hatchery/nursery affecting Crassostrea gigas spat in 2007. Here, using larvae of C. gigas as an interaction model, we showed that the French V. tubiashii is virulent to larvae and can cause bacillary necrosis symptoms with an LD50 of about 2.3 × 10(3) c.f.u. ml(-1) after 24 h. Moreover, complete or gel permeation HPLC fractionated extracellular products (ECPs) of this strain appeared toxic to larvae. MS-MS analysis of the different ECP fractions revealed the existence of an extracellular metalloprotease and other suspected virulence factors. This observation is also supported by the expression level of some potential virulence factors. The overall results suggest that the pathology caused by the French V. tubiashii in C. gigas oysters is caused by a group of toxic factors and not only the metalloprotease.

Elucidating the Pseudomonas aeruginosa fatty acid degradation pathway: identification of additional fatty acyl-CoA synthetase homologues

The fatty acid (FA) degradation pathway of Pseudomonas aeruginosa, an opportunistic pathogen, was recently shown to be involved in nutrient acquisition during BALB/c mouse lung infection model. The source of FA in the lung is believed to be phosphatidylcholine, the major component of lung surfactant. Previous research indicated that P. aeruginosa has more than two fatty acyl-CoA synthetase genes (fadD; PA3299 and PA3300), which are responsible for activation of FAs using ATP and coenzyme A. Through a bioinformatics approach, 11 candidate genes were identified by their homology to the Escherichia coli FadD in the present study. Four new homologues of fadD (PA1617, PA2893, PA3860, and PA3924) were functionally confirmed by their ability to complement the E. coli fadD mutant on FA-containing media. Growth phenotypes of 17 combinatorial fadD mutants on different FAs, as sole carbon sources, indicated that the four new fadD homologues are involved in FA degradation, bringing the total number of P. aeruginosa fadD genes to six. Of the four new homologues, fadD4 (PA1617) contributed the most to the degradation of different chain length FAs. Growth patterns of various fadD mutants on plant-based perfumery substances, citronellic and geranic acids, as sole carbon and energy sources indicated that fadD4 is also involved in the degradation of these plant-derived compounds. A decrease in fitness of the sextuple fadD mutant, relative to the ΔfadD1D2 mutant, was only observed during BALB/c mouse lung infection at 24 h.

Long tail fibres of the novel broad-host-range T-even bacteriophage S16 specifically recognize Salmonella OmpC

We report isolation and characterization of the novel T4-like Salmonella bacteriophage vB_SenM-S16. S16 features a T-even morphology and a highly modified 160 kbp dsDNA genome with 36.9 mol % G+C, containing 269 putative coding sequences and three tRNA genes. S16 is a virulent phage, and exhibits a maximally broad host range within the genus Salmonella, but does not infect other bacteria. Synthesis of functional S16 full-length long tail fibre (LTF) in Escherichia coli was possible by coexpression of gp37 and gp38. Surface plasmon resonance analysis revealed nanomolar equilibrium affinity of the LTF to its receptor on Salmonella cells. We show that OmpC serves as primary binding ligand, and that S16 adsorption can be transferred to E. coli by substitution of ompC with the Salmonella homologue. S16 also infects 'rough' Salmonella strains which are defective in lipopolysaccharide synthesis and/or its carbohydrate substitution, indicating that this interaction does not require an intact LPS structure. Altogether, its virulent nature, broad host range and apparent lack of host DNA transduction render S16 highly suitable for biocontrol of Salmonella in foods and animal production. The S16 LTF represents a highly specific affinity reagent useful for cell decoration and labelling, as well as bacterial immobilization and separation.

A non-invasive in vivo imaging system to study dissemination of bioluminescent Yersinia pestis CO92 in a mouse model of pneumonic plague

The gold standard in microbiology for monitoring bacterial dissemination in infected animals has always been viable plate counts. This method, despite being quantitative, requires sacrificing the infected animals. Recently, however, an alternative method of in vivo imaging of bioluminescent bacteria (IVIBB) for monitoring microbial dissemination within the host has been employed. Yersinia pestis is a Gram-negative bacterium capable of causing bubonic, septicemic, and pneumonic plague. In this study, we compared the conventional counting of bacterial colony forming units (cfu) in the various infected tissues to IVIBB in monitoring Y. pestis dissemination in a mouse model of pneumonic plague. By using a transposon mutagenesis system harboring the luciferase (luc) gene, we screened approximately 4000 clones and obtained a fully virulent, luc-positive Y. pestis CO92 (Y. pestis-luc2) reporter strain in which transposition occurred within the largest pMT1 plasmid which possesses murine toxin and capsular antigen encoding genes. The aforementioned reporter strain and the wild-type CO92 exhibited similar growth curves, formed capsule based on immunofluorescence microscopy and flow cytometry, and had a similar LD(50). Intranasal infection of mice with 15 LD(50) of CO92-luc2 resulted in animal mortality by 72 h, and an increasing number of bioluminescent bacteria were observed in various mouse organs over a 24-72 h period when whole animals were imaged. However, following levofloxacin treatment (10 mg/kg/day) for 6 days 24 h post infection, no luminescence was observed after 72 h of infection, indicating that the tested antimicrobial killed bacteria preventing their detection in host peripheral tissues. Overall, we demonstrated that IVIBB is an effective and non-invasive way of monitoring bacterial dissemination in animals following pneumonic plague having strong correlation with cfu, and our reporter CO92-luc2 strain can be employed as a useful tool to monitor the efficacy of antimicrobial countermeasures in real time.

The gp38 adhesins of the T4 superfamily: a complex modular determinant of the phage's host specificity

The tail fiber adhesins are the primary determinants of host range in the T4-type bacteriophages. Among the indispensable virion components, the sequences of the long tail fiber genes and their associated adhesins are among the most variable. The predominant form of the adhesin in the T4-type phages is not even the version of the gene encoded by T4, the archetype of the superfamily, but rather a small unrelated protein (gp38) encoded by closely related phages such as T2 and T6. This gp38 adhesin has a modular design: its N-terminal attachment domain binds at the tip of the tail fiber, whereas the C-terminal specificity domain determines its host receptor affinity. This specificity domain has a series of four hypervariable segments (HVSs) that are separated by a set of highly conserved glycine-rich motifs (GRMs) that apparently form the domain’s conserved structural core. The role of gp38’s various components was examined by a comparative analysis of a large series of gp38 adhesins from T-even superfamily phages with differing host specificities. A deletion analysis revealed that the individual HVSs and GRMs are essential to the T6 adhesin’s function and suggests that these different components all act in synergy to mediate adsorption. The evolutionary advantages of the modular design of the adhesin involving both conserved structural elements and multiple independent and easily interchanged specificity determinants are discussed.

Multiple FadD acyl-CoA synthetases contribute to differential fatty acid degradation and virulence in Pseudomonas aeruginosa

A close interconnection between nutrient metabolism and virulence factor expression contributes to the pathophysiology of Pseudomonas aeruginosa as a successful pathogen. P. aeruginosa fatty acid (FA) degradation is complicated with multiple acyl-CoA synthetase homologs (FadDs) expressed in vivo in lung tissue during cystic fibrosis infections. The promoters of two genetically linked P. aeruginosa fadD genes (fadD1 and fadD2) were mapped and northern blot analysis indicated they could exist on two different transcripts. These FadDs contain ATP/AMP signature and FA-binding motifs highly homologous to those of the Escherichia coli FadD. Upon introduction into an E. coli fadD^-/fadR^- double mutant, both P. aeruginosa fadDs functionally complemented the E. coli fadD^-/fadR^- mutant, allowing degradation of different chain-length FAs. Chromosomal mutagenesis, growth analysis, induction studies, and determination of kinetic parameters suggested that FadD1 has a substrate preference for long-chain FAs while FadD2 prefers shorter-chain FAs. When compared to the wild type strain, the fadD2 mutant exhibited decreased production of lipase, protease, rhamnolipid and phospholipase, and retardation of both swimming and swarming motilities. Interestingly, fadD1 mutant showed only increased swarming motility. Growth analysis of the fadD mutants showed noticeable deficiencies in utilizing FAs and phosphatidylcholine (major components of lung surfactant) as the sole carbon source. This defect translated into decreased in vivo fitness of P. aeruginosa in a BALB/c mouse lung infection model, supporting the role of lipids as a significant nutrient source for this bacterium in vivo.

Molecular analysis of porin gene transcription in heterogenotypic multidrug-resistant Escherichia coli isolates from scouring calves

BACKGROUND

Despite evidence that altered membrane porins may impair microbial drug uptake thereby potentially compounding efflux pump-mediated multidrug resistance, few studies have evaluated gene transcription to identify multidrug-resistance-associated porins and other potential drug targets.

METHODS

Genes that encode six membrane porins (fadL, lamB, ompC, ompF, ompW and yiaT) and two membrane proteins (tolC and ompT) were assessed by PCR and by quantitative real-time PCR (qRT-PCR) analysis of 10 multidrug-resistant (MDR) and 10 antibiotic-susceptible (AS) Escherichia coli isolates. The mean DeltaDeltaCt values for the study E. coli genes were analysed by the Wilcoxon test (P = 0.05).

RESULTS

All 20 E. coli isolates tested positive for tolC, lamB, ompC, ompF genes, while 10 MDR and 9/10 (90%) AS isolates were positive for the fadL gene. Seven out of 10 (70%) MDR and 7/10 (70%) AS isolates were positive for the yiaT gene, while 7/10 (70%) MDR and only 4/10 (40%) AS isolates were positive for the ompT gene. The mean DeltaDeltaCt values for the tolC and yiaT genes were significantly higher in MDR than in AS isolates (Wilcoxon test; P < 0.05). No significant difference was seen with respect to fadL, lamB, ompC, ompF, ompT and ompW gene transcription (Wilcoxon test; P > 0.05).

CONCLUSIONS

Findings suggest up-regulated transcription of tolC and yiaT genes in the MDR E. coli isolates. These results indirectly suggest that TolC and YiaT proteins may play some role(s) in multidrug resistance, but proteomic studies are needed before the two proteins are considered potential drug targets.

Dynamic mechanism of fatty acid transport across cellular membranes through FadL: molecular dynamics simulations

FadL is an important member of the family of fatty acid transport proteins within membranes. In this study, 11 conventional molecular dynamics (CMD) and 25 steered molecular dynamics (SMD) simulations were performed to investigate the dynamic mechanism of transport of long-chain fatty acids (LCFAs) across FadL. The CMD simulations addressed the intrinsically dynamic behavior of FadL. Both the CMD and SMD simulations revealed that a fatty acid molecule can move diffusively to a high-affinity site (HAS) from a low-affinity site (LAS). During this process, the swing motion of the L3 segment and the hydrophobic interaction between the fatty acid and FadL could play important roles. Furthermore, 22 of the SMD simulations revealed that fatty acids can pass through the gap between the hatch domain and the transmembrane domain (TMD) by different pathways. SMD simulations identified nine possible pathways for dodecanoic acid (DA) threading the barrel of FadL. The binding free energy profiles between DA and FadL along the MD trajectories indicate that all of the possible pathways are energetically favorable for the transport of fatty acids; however, one pathway (path VI) might be the most probable pathway for DA transport. The reasonability and reliability of this study were further demonstrated by correlating the MD simulation results with the available mutagenesis results. On the basis of the simulations, a mechanism for the full-length transport process of DA from the extracellular side to the periplasmic space mediated by FadL is proposed.

Two-Carbon Compounds and Fatty Acids as Carbon Sources

This review concerns the uptake and degradation of those molecules that are wholly or largely converted to acetyl-coenzyme A (CoA) in the first stage of metabolism in Escherichia coli and Salmonella enterica. These include acetate, acetoacetate, butyrate and longer fatty acids in wild type cells plus ethanol and some longer alcohols in certain mutant strains. Entering metabolism as acetyl-CoA has two important general consequences. First, generation of energy from acetyl-CoA requires operation of both the citric acid cycle and the respiratory chain to oxidize the NADH produced. Hence, acetyl-CoA serves as an energy source only during aerobic growth or during anaerobic respiration with such alternative electron acceptors as nitrate or trimethylamine oxide. In the absence of a suitable oxidant, acetyl-CoA is converted to a mixture of acetic acid and ethanol by the pathways of anaerobic fermentation. Catabolism of acetyl-CoA via the citric acid cycle releases both carbon atoms of the acetyl moiety as carbon dioxide and growth on these substrates as sole carbon source therefore requires the operation of the glyoxylate bypass to generate cell material. The pair of related two-carbon compounds, glycolate and glyoxylate are also discussed. However, despite having two carbons, these are metabolized via malate and glycerate, not via acetyl-CoA. In addition, mutants of E. coli capable of growth on ethylene glycol metabolize it via the glycolate pathway, rather than via acetyl- CoA. Propionate metabolism is also discussed because in many respects its pathway is analogous to that of acetate. The transcriptional regulation of these pathways is discussed in detail.

Long-chain Acyl-CoA Synthetase 6 Preferentially Promotes DHA Metabolism

Previously we demonstrated that supplementation with the polyunsaturated fatty acids (PUFA) arachidonic acid (AA) or docosahexaenoic acid (DHA) increased neurite outgrowth of PC12 cells during differentiation, and that overexpression of rat acyl-CoA synthetase long-chain family member 6 (Acsl6, formerly ACS2) further increased PUFA-enhanced neurite outgrowth. However, whether Acsl6 overexpression enhanced the amount of PUFA accumulated in the cells or altered the partitioning of any fatty acids into phospholipids (PLs) or triacylglycerides (TAGs) was unknown. Here we show that Acsl6 overexpression specifically promotes DHA internalization, activation to DHA-CoA, and accumulation in differentiating PC12 cells. In contrast, oleic acid (OA) and AA internalization and activation to OA-CoA and AA-CoA were increased only marginally by Acsl6 overexpression. Additionally, the level of total cellular PLs was increased in Acsl6 overexpressing cells when the medium was supplemented with AA and DHA, but not with OA. Acsl6 overexpression increased the incorporation of [(14)C]-labeled OA, AA, or DHA into PLs and TAGs. These results do not support a role for Acsl6 in the specific targeting of fatty acids into PLs or TAGs. Rather, our data support the hypothesis that Acsl6 functions primarily in DHA metabolism, and that its overexpression increases DHA and AA internalization primarily during the first 24 h of neuronal differentiation to stimulate PL synthesis and enhance neurite outgrowth.

Display of bacterial lipase on the Escherichia coli cell surface by using FadL as an anchoring motif and use of the enzyme in enantioselective biocatalysis

We have developed a novel cell surface display system by employing FadL as an anchoring motif, which is an outer membrane protein involved in long-chain fatty acid transport in Escherichia coli. A thermostable Bacillus sp. strain TG43 lipase (44.5 kDa) could be successfully displayed on the cell surface of E. coli in an active form by C-terminal deletion-fusion of lipase at the ninth external loop of FadL. The localization of the truncated FadL-lipase fusion protein on the cell surface was confirmed by confocal microscopy and Western blot analysis. Lipase activity was mainly detected with whole cells, but not with the culture supernatant, suggesting that cell lysis was not a problem. The activity of cell surface-displayed lipase was examined at different temperatures and pHs and was found to be the highest at 50 degrees C and pH 9 to 10. Cell surface-displayed lipase was quite stable, even at 60 and 70 degrees C, and retained over 90% of the full activity after incubation at 50 degrees C for a week. As a potential application, cell surface-displayed lipase was used as a whole-cell catalyst for kinetic resolution of racemic methyl mandelate. In 36 h of reaction, (S)-mandelic acid could be produced with the enantiomeric excess of 99% and the enantiomeric ratio of 292, which are remarkably higher than values obtained with crude lipase or cross-linked lipase crystal. These results suggest that FadL may be a useful anchoring motif for displaying enzymes on the cell surface of E. coli for whole-cell biocatalysis.

Acyl-CoA synthetase 2 overexpression enhances fatty acid internalization and neurite outgrowth

During neurodevelopment neurons increase phospholipid synthesis to generate additional plasma membrane that makes up the growing neurites. Compared with most cell types, neurons contain a high percentage of the polyunsaturated fatty acids (PUFAs) arachidonic acid (AA) and docosahexaenoic acid (DHA). By utilizing PC12 cell lines as a model neuronal cell line, we examined the internalization rate of AA, DHA, and non-essential oleic acid (OA), as well as their effects on neurite outgrowth. When wild type cells were differentiated, the rate of AA and DHA internalization increased 50% more than the rate of OA internalization. When media were supplemented with AA or DHA, the average neurite length was increased by approximately 40%, but supplementation with the same amount of OA had no effect. We also increased the levels of acyl-CoA synthetase-1 (ACS1) and ACS2 proteins to determine whether they contribute to PUFA internalization or neurite outgrowth. Overexpression of ACS1 increased the rate of OA internalization by 55%, and AA and DHA uptake was increased by 25%, but there was no significant change in neurite outgrowth. In ACS2-overexpressing cells, in contrast, the rate of OA internalization increased by 90%, AA by 115%, and DHA by 70%. The average aggregate neurite length in ACS2-overexpressing cells was increased by approximately 40% when the media were supplemented with PUFAs, but there was no change with OA supplementation. Taken together, these results support the hypotheses that ACSs are rate-limiting for fatty acid internalization and that ACS2 enhances neurite outgrowth by promoting PUFA internalization.

In vitro identification of two adherence factors required for in vivo virulence of Pseudomonas fluorescens

By enriching a random transposon insertion bank of Pseudomonas fluorescens for mutants affected in their adherence to the human extracellular matrix protein fibronectin, we isolated 23 adherence minus mutants. Mutants showed a defect in their ability to develop a biofilm on an abiotic surface and were impaired for virulence when tested in an in vivo virulence model in the fruit fly, Drosophila melanogaster. Molecular characterisation of these mutants showed that the transposon insertions localised to two distinct chromosomal locations, which were subsequently cloned and characterised from two mutants. A search in the databanks identified two loci in the Pseudomonas aeruginosa PAO1 genome with significant homology to the genes interrupted by the transposon insertions. Mutant IVC6 shows homology to gmd, coding for the enzyme GDP-mannose dehydratase, involved in the synthesis of A-band- O-antigen-containing lipopolysaccharide (LPS). Mutant IVG7 is significantly similar to a probable outer membrane protein of strain PAO1, with no specific function attributed thus far, yet with significant homology to Escherichia coli FadL, involved in long-chain fatty acid transport. We propose that this protein, together with LPS, is involved in the first steps of P. fluorescens adherence leading to host colonisation. Results presented here also demonstrate the pathogenic potential of P. fluorescens, assessed in an in vivo Drosophila model system, correlated with its ability to adhere to the human extracellular matrix protein, fibronectin. Correlation between the mutant phenotypes with identified virulence factors and their actual role in the virulence of P. fluorescens is discussed.

Transmembrane movement of exogenous long-chain fatty acids: proteins, enzymes, and vectorial esterification

The processes that govern the regulated transport of long-chain fatty acids across the plasma membrane are quite distinct compared to counterparts involved in the transport of hydrophilic solutes such as sugars and amino acids. These differences stem from the unique physical and chemical properties of long-chain fatty acids. To date, several distinct classes of proteins have been shown to participate in the transport of exogenous long-chain fatty acids across the membrane. More recent work is consistent with the hypothesis that in addition to the role played by proteins in this process, there is a diffusional component which must also be considered. Central to the development of this hypothesis are the appropriate experimental systems, which can be manipulated using the tools of molecular genetics. Escherichia coli and Saccharomyces cerevisiae are ideally suited as model systems to study this process in that both (i) exhibit saturable long-chain fatty acid transport at low ligand concentrations, (ii) have specific membrane-bound and membrane-associated proteins that are components of the transport apparatus, and (iii) can be easily manipulated using the tools of molecular genetics. In both systems, central players in the process of fatty acid transport are fatty acid transport proteins (FadL or Fat1p) and fatty acyl coenzyme A (CoA) synthetase (FACS; fatty acid CoA ligase [AMP forming] [EC 6.2.1.3]). FACS appears to function in concert with FadL (bacteria) or Fat1p (yeast) in the conversion of the free fatty acid to CoA thioesters concomitant with transport, thereby rendering this process unidirectional. This process of trapping transported fatty acids represents one fundamental mechanism operational in the transport of exogenous fatty acids.

Biochemical evidence against protein-mediated uptake of myristic acid in the bioluminescent marine bacterium Vibrio harveyi

The bioluminescent marine bacterium, Vibrio harveyi, can utilize exogenous myristic acid (14:0) for beta-oxidation, phospholipid and lipid A synthesis, and as an source of myristyl aldehyde for light emission in the V. harveyi dark mutant M17. A variety of genetic and biochemical strategies were employed in an attempt to isolate V. harveyi mutants defective in myristate uptake and to characterize proteins involved in this process. Although [3H]myristate uptake in a tritium suicide experiment decreased the survival of nitrosoguanidine-treated M17 cells by a factor of 10(5), none of the surviving cells characterized were defective in either incorporation of exogenous myristate into phospholipid or stimulation of light emission. These parameters were also unaffected when intact M17 cells were treated with proteases. Moreover, M17 double mutants selected on the basis of diminished luminescence response to myristate all incorporated [3H]myristate into lipids normally. Finally, no resistant colonies were obtained using the bacteriocidal fatty acid analogue, 11-bromoundecanoate, and experiments with decanoate (10:0) indicated that the V. harveyi cell envelope is very sensitive to physical disruption by fatty acids. Taken together, these results support an unfacilitated uptake of myristic acid in V. harveyi, in contrast with the regulated vectorial transport and activation of long chain fatty acids in Escherichia coli.

The amino-terminal region of the long-chain fatty acid transport protein FadL contains an externally exposed domain required for bacteriophage T2 binding

The fatty acid transport protein FadL from Escherichia coli is predicted to be rich in beta-structure and span the outer membrane multiple times to form a long-chain fatty acid specific channel. Proteolysis of FadL within whole cells, total membranes, and isolated outer membranes identified two trypsin-sensitive sites, both predicted to be in externally exposed loops of FadL. Amino acid sequence analysis of the proteolytic fragments determined that the first followed R93 and yielded a peptide beginning with 94S-L-K-A-D-N-I-A-P-T-A104 while the second followed R384 and yielded a peptide beginning with 385S-I-S-I-P-D-Q-D-R-F-W395. Proteolysis using trypsin eliminated the bacteriophage T2 binding activity associated with FadL, suggesting the T2 binding domain within FadL requires elements within one of these extracellular loops. A peptide corresponding to the amino-terminal region of FadL (FadL28-160) was purified and shown to inactivate bacteriophage T2 in a concentration-dependent manner, supporting the hypothesis that the amino-proximal extracellular loop of the protein confers T2 binding activity. Using an artificial neural network (NN) topology prediction method in combination with Gibbs motif sampling, a predicted topology of FadL within the outer membrane was developed. According to this model, FadL spans the outer membrane 20 times as antiparallel beta-strands. The 20 antiparallel beta-strands are presumed to form a beta-barrel specific for long-chain fatty acids. On the basis of our previous studies evaluating the function of FadL using site-specific mutagenesis of the fadL gene, proteolysis of FadL within outer membranes, and studies using the FadL28-160 peptide, the predicted extracellular regions between beta-strands 1 and 2 and beta-strands 3 and 4 are expected to contribute to a domain of the protein required for long-chain fatty acid and bacteriophage T2 binding. The first trypsin-sensitive site (R93) lies between predicted beta-strands 3 and 4 while the second (R384) is between beta-strands 17 and 18. The trypsin-resistant region of FadL is predicted to contain 13 antiparallel beta-strands and contribute to the long-chain fatty acid specific channel.

Multiple factors independently regulate hilA and invasion gene expression in Salmonella enterica serovar typhimurium

HilA activates the expression of Salmonella enterica serovar Typhimurium invasion genes. To learn more about regulation of hilA, we isolated Tn5 mutants exhibiting reduced hilA and/or invasion gene expression. In addition to expected mutations, we identified Tn5 insertions in pstS, fadD, flhD, flhC, and fliA. Analysis of the pstS mutant indicates that hilA and invasion genes are repressed by the response regulator PhoB in the absence of the Pst high-affinity inorganic phosphate uptake system. This system is required for negative control of the PhoR-PhoB two-component regulatory system, suggesting that hilA expression may be repressed by PhoR-PhoB under low extracellular inorganic phosphate conditions. FadD is required for uptake and degradation of long-chain fatty acids, and our analysis of the fadD mutant indicates that hilA is regulated by a FadD-dependent, FadR-independent mechanism. Thus, fatty acid derivatives may act as intracellular signals to regulate hilA expression. flhDC and fliA encode transcription factors required for flagellum production, motility, and chemotaxis. Complementation studies with flhC and fliA mutants indicate that FliZ, which is encoded in an operon with fliA, activates expression of hilA, linking regulation of hilA with motility. Finally, epistasis tests showed that PhoB, FadD, FliZ, SirA, and EnvZ act independently to regulate hilA expression and invasion. In summary, our screen has identified several distinct pathways that can modulate S. enterica serovar Typhimurium's ability to express hilA and invade host cells. Integration of signals from these different pathways may help restrict invasion gene expression during infection.

Characterization of P1-deficient isogenic mutant of Haemophilus influenzae biogroup aegyptius associated with Brazilian purpuric fever

Haemophilus influenzae biogroup aegyptius (formerly H. aegyptius) is the etiologic agent of Brazilian purpuric fever (BPF). A surface-exposed epitope on the outer membrane protein P1 is present on most strains of H. influenzae biogroup aegyptius associated with BPF but is absent in almost all non-disease associated strains. The role of the outer membrane protein P1 in the pathogenesis of this disease was evaluated by utilizing an isogenic P1-deficient mutant. We compared the ability of the wild type and P1 isogenic mutant to grow under various conditions. The P1-deficient strain grew at a similar rate to the wild type in both complex and chemically defined medium. The P1-deficient mutant also had a similar growth rate to the wild type under anaerobic conditions. Anaerobic growth, however, resulted in up-regulation of the P1 protein in the wild type strain. Three assays were used to examine the pathophysiologic role of the P1 protein in BPF: 1) serum resistance; 2) sustained bacteremia in the infant rat model; and 3) the human microvascular endothelial cell (HMEC) cytotoxicity assay. Both the mutant and wild-type strains were resistant to killing in 95% normal human serum. The P1-deficient strain was also as virulent as the wild type in both the infant rat model of bacteremia and in the HMEC-1 tissue culture model. These results demonstrate that serum resistance, sustained bacteremia in the infant rat, and cytotoxicity of HMEC cells occur in the absence of P1. The P1 protein is not essential for the pathogenic potential identified by these assays. However, these results demonstrate that an anaerobic environment is a potent physiologic regulator of P1 protein expression. The impact of anaerobiosis on protein expression and pathogenesis will require further investigations.

Long-chain fatty acid transport in bacteria and yeast. Paradigms for defining the mechanism underlying this protein-mediated process

Protein-mediated transport of exogenous long-chain fatty acids across the membrane has been defined in a number of different systems. Central to understanding the mechanism underlying this process is the development of the appropriate experimental systems which can be manipulated using the tools of molecular genetics. Escherichia coli and Saccharomyces cerevisiae are ideally suited as model systems to study this process in that both [1] exhibit saturable long-chain fatty acid transport at low ligand concentration; [2] have specific membrane-bound and membrane-associated proteins that are components of the transport apparatus; and [3] can be easily manipulated using the tools of molecular genetics. In E. coli, this process requires the outer membrane-bound fatty acid transport protein FadL and the inner membrane associated fatty acyl CoA synthetase (FACS). FadL appears to represent a substrate specific channel for long-chain fatty acids while FACS activates these compounds to CoA thioesters thereby rendering this process unidirectional. This process requires both ATP generated from either substrate-level or oxidative phosphorylation and the proton electrochemical gradient across the inner membrane. In S. cerevisiae, the process of long-chain fatty acid transport requires at least the membrane-bound protein Fat1p. Exogenously supplied fatty acids are activated by the fatty acyl CoA synthetases Faa1p and Faa4p but unlike the case in E. coli, there is not a tight linkage between transport and activation. Studies evaluating the growth parameters in the presence of long-chain fatty acids and long-chain fatty acid transport profiles of a fat1delta strain support the hypothesis that Fatlp is required for optimal levels of long-chain fatty acid transport.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Medium- and long-chain fatty acid uptake and utilization by Streptomyces coelicolor A3(2): first characterization of a gram-positive bacterial system

The first characterization of fatty acid uptake in a Gram-positive bacterium is reported. Streptomyces coelicolor A3(2) utilizes fatty acids of different chain length (C4-C18) as sole carbon and energy sources. In vivo beta-oxidation studies and the assay of two enzymes of the beta-oxidation cycle proved that fatty acid degradation is constitutive in this micro-organism. Uptake of the medium-chain fatty acid octanoate showed the characteristics of simple diffusion, whereas the uptake of palmitate, a long-chain fatty acid, occurred by both simple diffusion and active transport. After correcting for non-mediated transport, palmitate uptake measured over a wide range of concentrations followed Michaelis-Menten kinetics. The apparent Km for palmitate was 97.8 microM and the Vmax was 19.3 nmol min-1 (mg protein)-1. Competition experiments showed specificity of the mediated transport component for long-chain fatty acids (> C10). Metabolic inhibitors such as oligomycin, NaF and vanadate, and the ionophores gramicidin and carbonyl cyanide m-chlorophenylhydrazone (CCCP) inhibited palmitate uptake to different degrees, consistent with the existence of an active transport mechanism. Uptake rates measured at different pH values indicated that both the ionized and the unionized forms of octanoate crossed the cytoplasmic membrane by simple diffusion. Palmitate in its ionized form appears to be transported by an active mechanism, whereas the unionized molecule diffuses through the membrane. When present in the medium, glucose stimulated the degradation of long-chain fatty acids by increasing the rate of uptake and the level of acyl-CoA synthetase.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Osmoregulation of the fatty acid receptor gene fadL in Escherichia coli

The fadL gene of Escherichia coli codes for an outer membrane protein that is involved in the uptake of long-chain fatty acids. Uptake is regulated by environmental osmolarity, and decreases when the cells are grown under conditions of high osmolarity. A temperature-sensitive mutant that requires fatty acid for growth at 42 degrees C was unable to grow at the high temperature even in the presence of fatty acid if the medium contained 10% sucrose. Promoter activity of the fadL gene in vivo was repressed by high osmolarity in a FadR repressor null mutant. Furthermore, in vitro transcription of the fadL gene was strongly repressed by the addition of OmpR and EnvZ proteins. The results of gel retardation and DNase I protection experiments indicated that OmpR, after incubation with the protein kinase EnvZ, specifically binds to at least four sites around the fadL promoter, two upstream and two downstream from the transcriptional start site. These results suggest that transcription of the fadL gene is osmotically regulated by the OmpR-EnvZ two-component system.

Stable, conserved outer membrane epitope of strains of Haemophilus influenzae biogroup aegyptius associated with Brazilian purpuric fever

Brazilian purpuric fever is a rapidly fatal childhood disease associated with a clonal strain of Haemophilus influenzae biogroup aegyptius. We describe a conserved, surface-exposed epitope present on 95% of H. influenzae biogroup aegyptius isolates that are associated with Brazilian purpuric fever. This epitope, defined by reaction with the monoclonal antibody 8G3, is on or associated with the 48-kDa heat-modifiable P1 protein. The epitope is absent on strains of H. influenzae biogroup aegyptius that are not associated with Brazilian purpuric fever but is present on one strain of H. influenzae biotype II. None of 81 other Haemophilus strains tested reacted with 8G3. The sensitivity and specificity of the 8G3 monoclonal antibody in detecting Brazilian case-clone strains of H. influenzae biogroup aegyptius associated with Brazilian purpuric fever are 95 and 99%, respectively. Immunoelectron microscopy revealed that the epitope is surface exposed, and N-terminal amino acid sequencing of an 8G3-reactive P1 protein from a strain of H. influenzae biogroup aegyptius showed 100% correlation with the published N-terminal amino acid sequence of a P1 protein of H. influenzae type b. The virulence of the organism in an infant rat model of bacteremia was not dependent on the expression of this epitope.

Increase of phenol tolerance of Escherichia coli by alterations of the fatty acid composition of the membrane lipids

In the presence of sublethal concentrations of phenol, 4-chlorophenol, and p-cresol in the growth medium, cells of Escherichia coli modified the fatty acid composition of their lipids. The results of these changes was an increase in the degree of saturation of lipids probably in order to compensate an increase of fluidity of the membrane induced by the phenols. Supplementation of the growth medium with saturated fatty acids could also enhance the degree of lipid saturation due to the incorporation of the acyl chains in the phospholipids. At the same time the growth of cells was less inhibited than in unsupplemented cells. The increase of tolerance of cells by manipulating the lipid composition indicates that the membrane structure plays a crucial role in the mode of action of phenols.

Primary sequence of the Escherichia coli fadL gene encoding an outer membrane protein required for long-chain fatty acid transport

The fadL gene of Escherichia coli encodes an outer membrane protein (FadL) that plays a central role in the uptake of exogenous long-chain fatty acids. The nucleotide sequence of the fadL gene revealed a single open reading frame of 1,344 bp encoding a protein with 448 amino acid residues and a molecular weight of 48,831. The transcriptional start, analyzed by primer extension, was shown to be 95 bp upstream from the translational start. Apparent -10 and -35 regions were found at -12 and -37 bp upstream from the transcriptional start. Three regions with hyphenated dyad symmetry (two between the transcriptional start and the translational start and one upstream from the -10 and -35 regions) were identified that may play a role in the expression of fadL. The protein product of the fadL gene contained a signal sequence and signal peptidase I cleavage site similar to that defined for other E. coli outer membrane proteins. The N-terminal sequence of mature FadL protein was determined by automated amino acid sequencing of protein purified from the outer membrane of a strain harboring fadL under the control of a T7 RNA polymerase-responsive promoter. This amino acid sequence, Ala-Gly-Phe-Gln-Leu-Asn-Glu-Phe-Ser-Ser, verified the signal peptidase I cleavage site on pre-FadL and confirmed the N-terminal amino acid sequence of FadL predicted from the DNA sequence. Mature FadL contained 421 amino acid residues, giving a molecular weight of 45,969. The amino acid composition of FadL deduced from the DNA sequence suggested that this protein contained an abundance of hydrophobic amino acid residues and lacked cysteinyl residues. The hydrophobic amino acids within FadL were predicted to contribute to at least five regions of the protein with an overall hydrophobic character. The amino acid sequence of FadL was used to search GenBank for other proteins with amino acid sequence homology. These data demonstrated that FadL and the heat-modifiable outer membrane protein P1 of Haemophilus influenzae type b were 60.5% conserved and 42.0% identical over 438 amino acid residues.

Characterization of FadL-specific fatty acid binding in Escherichia coli

The product of the fadL gene (FadL) is a central component of the long-chain fatty acid transport system of Escherichia coli. When fatty acid activation is blocked by a mutation in the structural gene for acyl CoA synthetase (fadD) transport is inhibited allowing a FadL-specific fatty acid binding activity to be measured. This binding activity was 4- to 6-fold greater in the fadL+ fadD strain LS6928 when compared to the delta fadLfadD strain LS6929. With long-chain fatty acids, this binding activity was saturable and it was estimated that there were approx. 35,000 FadL-specific oleic acid binding sites per cell in the fadL+ strain LS6928. The FadL-specific fatty acid binding affinity was highest for oleic acid (18:1) and palmitic acid (16:0) giving apparent KD values of 2.3.10(-7) M and 8.8.10(-7) M, respectively. FadL-specific binding affinity of myristic acid (14:0) was nearly an order of magnitude less and no FadL-specific binding of decanoic acid (10:0) could be measured. Two lines of evidence suggest that FadL-fatty acid binding occurs by a hydrophobic interaction: (1) There was a preference for the long-chain substrates oleic acid and palmitic acid; and (2) oleic acid binding activity was not significantly changed over the pH range 5.0 to 8.0. The FadL-specific binding of oleic acid in the fadL+ strain LS6928 could be blocked by preincubation with antisera raised against purified FadL providing a clear correlation between the activity and identity of FadL. The binding activity associated with FadL was measured in vesicles of the outer membrane following passage over the hydrophobic resin Lipidex 1000. The KD of oleic acid binding attributable to FadL in outer membranes vesicles (6.0.10(-7) M) was in close agreement with that determined in whole cells. Overall, these studies demonstrated that FadL binds long-chain fatty acids with a relatively high affinity prior to their transport across the outer membrane.