Effect of the inhibition of protein synthesis on the Escherichia coli cell envelope

Effects of antibiotics on bacterial cell morphology and their physiological origins

Characterizing the physiological response of bacterial cells to antibiotic treatment is crucial for the design of antibacterial therapies and for understanding the mechanisms of antibiotic resistance. While the effects of antibiotics are commonly characterized by their minimum inhibitory concentrations or the minimum bactericidal concentrations, the effects of antibiotics on cell morphology and physiology are less well characterized. Recent technological advances in single-cell studies of bacterial physiology have revealed how different antibiotic drugs affect the physiological state of the cell, including growth rate, cell size and shape, and macromolecular composition. Here, we review recent quantitative studies on bacterial physiology that characterize the effects of antibiotics on bacterial cell morphology and physiological parameters. In particular, we present quantitative data on how different antibiotic targets modulate cellular shape metrics including surface area, volume, surface-to-volume ratio, and the aspect ratio. Using recently developed quantitative models, we relate cell shape changes to alterations in the physiological state of the cell, characterized by changes in the rates of cell growth, protein synthesis and proteome composition. Our analysis suggests that antibiotics induce distinct morphological changes depending on their cellular targets, which may have important implications for the regulation of cellular fitness under stress.

Cell wall deficiency - an alternate bacterial lifestyle?

Historically, many species of bacteria have been reported to produce viable, cell wall deficient (CWD) variants. A variety of terms have been used to refer to CWD bacteria and a plethora of methods described in which to induce, cultivate and propagate them. In this review, we will examine the long history of scientific research on CWD bacteria examining the methods by which CWD bacteria are generated; the requirements for survival in a CWD state; the replicative processes within a CWD state; and the reversion of CWD bacteria into a walled state, or lack thereof. In doing so, we will present evidence that not all CWD variants are alike and that, at least in some cases, CWD variants arise through an adaptive lifestyle switch that enables them to live and thrive without a cell wall, often to avoid antimicrobial activity. Finally, the implications of CWD bacteria in recurring infections, tolerance to antibiotic therapy and antimicrobial resistance will be examined to illustrate the importance of greater understanding of the CWD bacteria in human health and disease.

Changes in fluidity of the E. coli outer membrane in response to temperature, divalent cations and polymyxin-B show two different mechanisms of membrane fluidity adaptation

The outer membrane (OM) is an essential component of the Gram-negative bacterial cell envelope. Restricted diffusion of integral OM proteins and lipopolysaccharide (LPS) that constitute the outer leaflet of the OM support a model in which the OM is in a semi-crystalline state. The low fluidity of the OM has been suggested to be an important property of this membrane that even contributes to cell rigidity. The LPS characteristics strongly determine the properties of the OM and the LPS layer fluidity has been measured using different techniques that require specific conditions or are technically challenging. Here, we characterize the Escherichia coli LPS fluidity by evaluating the lateral diffusion of the styryl dye FM4-64FX in fluorescence recovery after photobleaching experiments. This technique allowed us to determine the effect of different conditions and genetic backgrounds on the LPS fluidity. Our results show that a fraction of the LPS can slowly diffuse and that the fluidity of the LPS layer adapts by modifying the diffusion of the LPS and the fraction of mobile LPS molecules.

Peptide Extracts from Native Lactic Acid Bacteria Generate Ghost Cells and Spheroplasts upon Interaction with Salmonella enterica, as Promising Food Antimicrobials

Protecting foods from contamination applying peptides produced by lactic acid bacteria is a promising strategy to increase the food quality and safety. Interacting with the pathogen membranes might produce visible changes in shape or cell wall damage. Previously, we showed that the peptides produced by Lactobacillus plantarum UTNGt2, Lactobacillus plantarum UTNCys5-4, and Lactococcus lactis subsp. lactis UTNGt28 exhibit a broad spectrum of antibacterial activity against several foodborne pathogens in vitro. In this study, their possible mode of action against the commensal microorganism Salmonella enterica subsp. enterica ATCC51741 was investigated. The target membrane permeability was determined by detection of beta-galactosidase release from ONPG (o-nitro-phenyl-L-D-galactoside) substrate and changes in the whole protein profile indicating that the peptide extracts destroy the membrane integrity and may induce breaks in membrane proteins to some extent. The release of aromatic molecules such as DNA/RNA was detected after the interaction of Salmonella with the peptide extract. Transmission electronic microscopy (TEM) micrographs depicted at least four simultaneous secondary events after the peptide extract treatment underlying their antimicrobial actions, including morphological alterations of the membrane. Spheroplast and filament formation, vacuolation, and DNA relaxation were identified as the principal events from the Gt2 and Cys5-4 peptide extracts, while Gt28 induced the formation of ghost cells by release of cytoplasmic content, filaments, and separation of cell envelope layers. Gel retarding assays indicate that the Gt2 and Gt28 peptide extracts are clearly binding the Salmonella DNA, while Cys5-4 partially interacted with Salmonella genomic DNA. These results increased our knowledge about the inhibitory mechanism employed by several peptide extracts from native lactic acid bacteria against Salmonella. Further, we shall develop peptide-based formulation and evaluate their biocontrol effect in the food chains.

Fluorescent Aminoglycoside Antibiotics and Methods for Accurately Monitoring Uptake by Bacteria

Characterizing how multidrug-resistant bacteria circumvent the action of clinically used or novel antibiotics requires a detailed understanding of how the antibiotics interact with and cross bacterial membranes to accumulate in the cells and exert their action. When monitoring the interactions of drugs with bacteria, it remains challenging to differentiate functionally relevant internalized drug levels from nonspecific binding. Fluorescence is a method of choice for observing dynamics of biomolecules. In order to facilitate studies involving aminoglycoside antibiotics, we have generated fluorescently labeled aminoglycoside derivatives with uptake and bactericidal activities similar, albeit with a moderate loss, to those of the parent drug. The method combines fluorescence microscopy with fluorescence-activated cell sorting (FACS) using neomycin coupled to nonpermeable cyanine dyes. Fluorescence imaging allowed membrane-bound antibiotic to be distinguished from molecules in the cytoplasm. Patterns of uptake were assigned to different populations in the FACS analysis. Our study illustrates how fluorescent derivatives of an aminoglycoside enable a robust characterization of the three components of uptake: membrane binding, EDPI, and EDPII. Because EDPI levels are weak compared to the two other types of accumulation and critical for the action of these drugs, the three components of uptake must be taken into account separately when drawing conclusions about aminoglycoside function.

Minocycline and Silver Dual-Loaded Polyphosphoester-Based Nanoparticles for Treatment of Resistant Pseudomonas aeruginosa

Pseudomonas aeruginosa has been detected in the lungs of ∼50% of patients with cystic fibrosis (CF), including 20% of adult CF patients. The majority of these adult patients harbor multi-drug resistant (MDR) strains, limiting the available treatment options. Silver has long been used as a broad-spectrum antimicrobial agent with a low incidence of resistance. Despite low toxicity, poor availability of silver cations mandates a high dosage to effectively eradicate infections. To address this shortcoming of silver, nanoparticles have been used as delivery devices to improve treatment outcomes. Furthermore, studies have demonstrated that synergistic combinations with careful dose calibrations and efficient delivery systems result in superior antimicrobial activity while avoiding potential side effects of both therapeutics. Here 4-epi-minocycline, a metabolite of minocycline, was identified as an active antimicrobial against P. aeruginosa using a high-throughput screen. The antimicrobial activities of 4-epi-minocycline, minocycline, and silver acetate against clinical isolates of P. aeruginosa obtained from CF patients were evaluated in vitro. Next, the synergistic activity of the silver/minocycline combination against P. aeruginosa isolates was investigated using checkerboard assays and identified with end-point colony forming unit determination assays. Finally, nanoparticles coloaded with minocycline and silver were evaluated in vitro for antimicrobial activity. The results demonstrated that both silver and minocycline are potent antimicrobials alone and that the combination allows a reduced dosage of both therapeutics to achieve the same antimicrobial effect. Furthermore, the proposed synergistic silver/minocycline combination can be coloaded into nanoparticles as a next-generation antibiotic to combat the threats presented by MDR pathogens.

Morphological and ultrastructural changes in bacterial cells as an indicator of antibacterial mechanism of action

Efforts to reduce the global burden of bacterial disease and contend with escalating bacterial resistance are spurring innovation in antibacterial drug and biocide development and related technologies such as photodynamic therapy and photochemical disinfection. Elucidation of the mechanism of action of these new agents and processes can greatly facilitate their development, but it is a complex endeavour. One strategy that has been popular for many years, and which is garnering increasing interest due to recent technological advances in microscopy and a deeper understanding of the molecular events involved, is the examination of treated bacteria for changes to their morphology and ultrastructure. In this review, we take a critical look at this approach. Variables affecting antibacterial-induced alterations are discussed first. These include characteristics of the test organism (e.g. cell wall structure) and incubation conditions (e.g. growth medium osmolarity). The main body of the review then describes the different alterations that can occur. Micrographs depicting these alterations are presented, together with information on agents that induce the change, and the sequence of molecular events that lead to the change. We close by highlighting those morphological and ultrastructural changes which are consistently induced by agents sharing the same mechanism (e.g. spheroplast formation by peptidoglycan synthesis inhibitors) and explaining how changes that are induced by multiple antibacterial classes (e.g. filamentation by DNA synthesis inhibitors, FtsZ disruptors, and other types of agent) can still yield useful mechanistic information. Lastly, recommendations are made regarding future study design and execution.

Antimicrobial peptides against Pseudomonas syringae pv. actinidiae and Erwinia amylovora: Chemical synthesis, secondary structure, efficacy, and mechanistic investigations

We report on structurally modified dodecapeptide amides (KYKLFKKILKFL-NH2) and two analogs of a hexapeptide amide (WRWYCR-NH2) with antibacterial activity against the Gram negative pathogens Pseudomonas syringae pv. actinidiae (Psa) and Erwinia amylovora (Ea). Dodecapeptide minimal inhibitory concentrations (MICs) ranged from 3.2 to 15.4 µM, with the unmodified peptide being the most potent against both pathogens. The unmodified dodecapeptide also had 32-58% α-helicity in membrane mimetic environments (50% v/v trifluoroethanol and 30 mM SDS micelles). Structural modifications which included branching, acylation, and conjugation with 5-nitro-2-furaldehyde (NFA) proved detrimental to both antimicrobial activity and α-helicity. Scanning electron microscopy (SEM) revealed distinct morphological changes to bacterial cells treated with the different peptides, leading to blistering of the membrane and cell lysis. MICs of the hexapeptide amide were 3.9-7.7 µM against both pathogens. The hexapeptide acid did not show anti-bacterial activity against either pathogen. However, the NFA conjugated hexapeptide acid was more active than the parent peptide or NFA alone with MICs of 1.6-3.2 µM against the pathogens. SEM analysis revealed shriveling and collapse of bacterial cells treated with the hexapeptide, whereas shortening and compactness on exposure to streptomycin. A colorimetric assay demonstrated that the dodecapeptides were likely to act by targeting the bacterial membrane, whereas the hexapeptides, streptomycin, and NFA were not, thereby supporting the morphological changes observed during SEM. To the best of our knowledge, this appears to be the first report of antimicrobial peptide activity against Psa, a pathogen that is currently devastating the kiwifruit industry internationally.

Identification of Small-Molecule Inhibitors against Meso-2, 6-Diaminopimelate Dehydrogenase from Porphyromonas gingivalis

Species-specific antimicrobial therapy has the potential to combat the increasing threat of antibiotic resistance and alteration of the human microbiome. We therefore set out to demonstrate the beginning of a pathogen-selective drug discovery method using the periodontal pathogen Porphyromonas gingivalis as a model. Through our knowledge of metabolic networks and essential genes we identified a “druggable” essential target, meso-diaminopimelate dehydrogenase, which is found in a limited number of species. We adopted a high-throughput virtual screen method on the ZINC chemical library to select a group of potential small-molecule inhibitors. Meso-diaminopimelate dehydrogenase from P. gingivalis was first expressed and purified in Escherichia coli then characterized for enzymatic inhibitor screening studies. Several inhibitors with similar structural scaffolds containing a sulfonamide core and aromatic substituents showed dose-dependent inhibition. These compounds were further assayed showing reasonable whole-cell activity and the inhibition mechanism was determined. We conclude that the establishment of this target and screening strategy provides a model for the future development of new antimicrobials.

Mechanism of action-based classification of antibiotics using high-content bacterial image analysis

Image-based screening has become a mature field over the past decade, largely due to the detailed information that can be obtained about compound mode of action by considering the phenotypic effects of test compounds on cellular morphology. However, very few examples exist of extensions of this approach to bacterial targets. We now report the first high-throughput, high-content platform for the prediction of antibiotic modes of action using image-based screening. This approach employs a unique feature segmentation and extraction protocol to quantify key size and shape metrics of bacterial cells over a range of compound concentrations, and matches the trajectories of these metrics to those of training set compounds of known molecular target to predict the test compound's mode of action. This approach has been used to successfully predict the modes of action of a panel of known antibiotics, and has been extended to the evaluation of natural products libraries for the de novo prediction of compound function directly from primary screening data.

Morphological analysis of the antimicrobial action of nitric oxide on gram-negative pathogens using atomic force microscopy

Atomic force microscopy (AFM) was used to study the morphological changes of two gram-negative pathogens, Pseudomonas aeruginosa and Escherichia coli, after exposure to nitric oxide (NO). The time-dependent effects of NO released from a xerogel coating and the concentration-dependent effects rendered by a small molecule that releases NO in a bolus were examined and compared. Bacteria exhibited irregular and degraded exteriors. With NO-releasing surfaces, an increase in surface debris and disorganized adhesion patterns were observed compared to controls. Analysis of cell surface topography revealed that increasing membrane roughness correlated with higher doses of NO. At a lower total dose, NO delivered via a bolus resulted in greater membrane roughness than NO released from a surface via a sustained flux. At sub-inhibitory levels, treatment with amoxicillin, an antibiotic known to compromise the integrity of the cell wall, led to morphologies resembling those resulting from NO treatment. Our observations indicate that cell envelope deterioration is a visible consequence of NO-exposure for both gram-negative species studied.

Potentiation of antibacterial activity of azithromycin and other macrolides by normal human serum

The interaction of azithromycin with normal human serum was examined in relation to serum protein binding, MIC, and kinetics of killing of bacteria. While the binding of azithromycin to serum proteins is low (8.5% at a concentration of 0.01 mM in 95% serum), the presence of 40% serum during the MIC test decreased MICs by 26-fold for serum-resistant Escherichia coli and 15-fold for Staphylococcus aureus. Erythromycin had a similar but lesser effect, while roxithromycin was less active against S. aureus in the presence of serum. The rate of killing of E. coli and S. aureus by azithromycin was increased in the presence of serum. The enhancement of antibiotic activity by serum was pH independent, and heat inactivation and preabsorption with homologous bacteria failed to inhibit enhancement by serum. The macromolecular incorporation of [3H]thymidine by E. coli continuously exposed to 2 micrograms of azithromycin per ml (0.25x the MIC) and 40% serum was decreased by 80% at pH 7.8 and by 48% at pH 7.2, while azithromycin alone failed to inhibit incorporation. Inhibition of nucleic acid biosynthesis at pH 7.2 in the presence of serum was also detected with sub-MICs of erythromycin, norfloxacin, and gentamicin but not roxithromycin. A diffusible serum factor was shown to interact with azithromycin to inhibit the growth of E. coli in an agar diffusion assay to detect antibiotic-serum synergy.

Loss of pili and decreased attachment to human cells by Neisseria meningitidis and Neisseria gonorrhoeae exposed to subinhibitory concentrations of antibiotics

Recent evidence has suggested that surface structures of pathogenic bacteria, which are important in attachment to human mucosal surfaces, may be absent on bacteria grown in the presence of subinhibitory concentrations of antibiotics. We studied the effect of tetracycline and penicillin on meningococcal and gonococcal pili. Subinhibitory concentrations of tetracycline and penicillin were found to markedly reduce the number of pili per meningococcus or gonococcus and the percentage of meningococci or gonococci with pili, as determined by negative-staining electron microscopy. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of outer membrane preparations suggested that tetracycline decreased expression of pili by inhibiting synthesis of pilin subunits. In contrast, pilin subunit synthesis was unaltered by penicillin, suggesting a defect in assembly of pilin subunits or in anchoring of assembled pili. The decrease in the number of pili that occurred with subinhibitory concentrations of both tetracycline and penicillin was accompanied by a marked decrease in the ability of the organisms to attach to human cells. Gonococci or meningococci removed from the influence of subinhibitory concentrations of the antibiotics regained piliation, and attachment returned to levels near those of controls. The expression of meningococcal and gonococcal pili may be affected by factors that influence synthesis of pilin subunits or factors that interfere with the assembly and anchoring of pili in the outer membrane.

Enhanced susceptibility of Escherichia coli to intracellular killing by human polymorphonuclear leukocytes after in vitro incubation with chloramphenicol

The effect of brief exposure to chloramphenicol of a pathogenic strain of Escherichia coli on susceptibility to normal human leukocytes was examined. Leukocytes killed chloramphenicol-pretreated E. coli more efficiently than they did untreated controls. Phagocytosis of pretreated bacteria, as measured by the uptake of radiolabeled bacteria and by direct visual count of engulfed bacteria, was not significantly increased. The decrease in viability was associated with enhanced intracellular killing of phagocytosed antibiotic-damaged bacteria. Chloramphenicol pretreatment altered the frequency distribution of intracellular bacteria by decreasing the number of leukocytes containing multiple stainable bacteria. Leukocytes failed to kill chloramphenicol-pretreated E. coli in the presence of phenylbutazone, which allowed an accumulation of intracellular bacteria. These results indicate that exposure of E. coli to chloramphenicol renders the bacteria more susceptible to intracellular killing and degradation.

Influence of sublethal concentrations of antibiotics on the expression of the mannose-specific ligand of Escherichia coli

The ability of streptomycin, in subinhibitory concentrations, to differentially suppress the acquisition of the mannose-binding activity of Escherichia coli was demonstrated in several strains, but not one with a ribosomal mutation to high-level streptomycin resistance, rpsL. We also determined that the growth of bacteria in other antibiotics, notably those that interfere with protein synthesis, resulted in diminished mannose-binding activity (as measured by yeast cell agglutination), degree of piliation (as measured by electron microscopy), and adherence to human oral epithelial cells. The aminoglycoside antibiotics streptomycin, gentamicin, and neomycin had the most marked effects relative to their minimum inhibitory concentrations, followed by tetracycline. Both spectinomycin and chloramphenicol had more effect on adherence than on piliation, although spectinomycin had a more pronounced effect on mannose-binding activity than did chloramphenicol. We conclude that antibiotics, at concentrations below their minimum inhibitory concentration, may have profound effects on surface properties of bacteria that may be pertinent for their ability to colonize and infect human mucosal surfaces. The mechanism(s) may vary from one drug to another, but appear to depend on the classic actions of the antibiotics on inhibiting protein synthesis.

Biosynthesis of the outer membrane receptor for vitamin B12, E colicins, and bacteriophage BF23 by Escherichia coli: kinetics of phenotypic expression after the introduction of bfe+ and bfe alleles

The bfe locus codes for the cell surface receptor for vitamin B12, the E colicins, and bacteriophage BF23 in the Escherichia coli outer membrane. When the bfe+ allele, which is closely linked to the argH locus, was introduced into an argH bfe recipient by conjugation, arg+ recombinant cells rapidly and simultaneously acquired sensitivity to colicin E3 and phage BF23. In the reciprocal experiment introducing bfe into an argH bfe+ recipient, it was found that colicin E3-resistant, arg+ cells began to appear shortly after the arg+ recombinant population began to divide. This was far earlier than would have been predicted on the basis of 220 receptors per haploid cell. Moreover, there was a lag between the appearance of colicin resistance and the appearance of resistance to killing by phage BF23, and hence a period of time during which some arg+ recombinant cells were sensitive to the phage but resistant to the colicin. Colicin E3 added to cells during this period of time protected against phage killing, indicating that the colicin-resistant cells still had receptors capable of binding colicin on their surface. The modification of the phenotypic expression of colicin and phage resistance by inhibitors of deoxyribonucleic acid, ribonucleic acid, and protein synthesis was also investigated. The results obtained indicate that the receptor protein coded for by the bfe locus can exist on the cell surface in several different functional states.

Relation of cell growth and colicin tolerance to vitamin B12 uptake in Escherichia coli

The uptake of vitamin B12 was measured in cells of Escherichia coli whose growth had been inhibited by any of a variety of treatments. In all cases, the secondary, energy-dependent phase of B12 uptake was depressed in proportion to the decrease in growth rate, but uptake was constant in cells growing logarithmically at different rates. The depression of B12 uptake activity was independent of the site of cell metabolism affected by the inhibitor or by its effect on cell viability, and was both more rapid and of greater degree than the effects on the uptake of any of the six amino acids tested. The decline was not affected by inhibitors of either cell division or proteolysis and was manifested without any apparent decrease in the surface B12 binding activity. Transport activity was rapidly regained upon reversal of the inhibition of protein synthesis. Prompted by this response, the uptake of B12 was contrasted to the apparent uptake of the E colicins, which share the same outer membrane receptor. Sensitivity to colicin E1, measured by its inhibition of proline uptake, was not affected by growth inhibition by antibiotic treatment. Finally, there was no specific depression of B12 uptake in cells rendered colicin tolerant either by mutation or as a consequence of phage f1 infection.