The inner membrane protein YhdP modulates the rate of anterograde phospholipid flow in Escherichia coli

Community-specific cell death sustains bacterial expansion under phosphorus starvation

Colony expansion is important for establishing territories. It is unclear to what extent bacteria can maintain colony expansion under nutrient limitation. Here, we found that Escherichia coli biofilms could maintain steady expansion for an extended period of time under severe phosphorus limitation. The expansion was supported by reactive-oxygen-species-mediated cell death within the biofilm. The cell death was spatially separated from the region of growth, resulting in cross-regional recycling of phosphorus from the lysed bacteria. The increase in cell death and the steady growth after phosphorus removal was community specific and was not observed in planktonic bacteria. Lastly, phosphorus had a unique role in the cell-death-mediated nutrient recycling, as the phenomenon described above was not observed under carbon or nitrogen starvation. Our work reveals how bacterial communities use spatially coordinated metabolism to cope with phosphorus limitation, which promotes robust expansion of the bacteria in fluctuating environments.

Terrabacteria: redefining bacterial envelope diversity, biogenesis and evolution

The bacterial envelope is one of the oldest and most essential cellular components and has been traditionally divided into Gram-positive (monoderm) and Gram-negative (diderm). Recent landmark studies have challenged a major paradigm in microbiology by inferring that the last bacterial common ancestor had a diderm envelope and that the outer membrane (OM) was lost repeatedly in evolution to give rise to monoderms. Intriguingly, OM losses appear to have occurred exclusively in the Terrabacteria, one of the two major clades of bacteria. In this Review, we present current knowledge about the Terrabacteria. We describe their diversity and phylogeny and then highlight the vast phenotypic diversity of the Terrabacteria cell envelopes, which display large deviations from the textbook examples of diderms and monoderms, challenging the classical Gram-positive-Gram-negative divide. We highlight the striking differences in the systems involved in OM biogenesis in Terrabacteria with respect to the classical diderm experimental models and how they provide novel insights into the diversity and biogenesis of the bacterial cell envelope. We also discuss the potential evolutionary steps that might have led to the multiple losses of the OM and speculate on how the very first OM might have emerged before the last bacterial common ancestor.

Phospholipid Transport Across the Bacterial Periplasm Through the Envelope-spanning Bridge YhdP

The outer membrane of Gram-negative bacteria provides a formidable barrier, essential for both pathogenesis and antimicrobial resistance. Biogenesis of this complex structure necessitates the transport of phospholipids across the cell envelope. Recently, YhdP was implicated as a major protagonist in the trafficking of inner membrane phospholipids to the outer membrane; however the molecular mechanism of YhdP mediated transport remains elusive. Here, utilising AlphaFold, we observe YhdP to form an elongated assembly of 60 β-strands that curve to form a continuous hydrophobic groove. This architecture is consistent with our negative stain electron microscopy data which reveals YhdP to be approximately 250 Å in length and thus sufficient to span the bacterial cell envelope. Furthermore, molecular dynamics simulations and bacterial growth assays indicate essential helical regions at the N- and C-termini of YhdP, that may embed into the inner and outer membranes respectively, reinforcing its envelope spanning nature. Our in vivo crosslinking data reveal phosphate-containing substrates captured along the length of the YhdP groove, providing direct evidence that YhdP interacts with a phosphate-containing substrate, which we propose to be phospholipids. This finding is congruent with our molecular dynamics simulations which demonstrate the propensity for inner membrane lipids to spontaneously enter the groove of YhdP. Collectively, our results support a model in which YhdP bridges the cell envelope, providing a hydrophobic environment for the transport of phospholipids to the outer membrane.

How Bacteria Establish and Maintain Outer Membrane Lipid Asymmetry

Gram-negative bacteria build an asymmetric outer membrane (OM), with lipopolysaccharides (LPS) and phospholipids (PLs) occupying the outer and inner leaflets, respectively. This distinct lipid arrangement is widely conserved within the Bacteria domain and confers strong protection against physical and chemical insults. The OM is physically separated from the inner membrane and the cytoplasm, where most cellular resources are located; therefore, the cell faces unique challenges in the assembly and maintenance of this asymmetric bilayer. Here, we present a framework for how gram-negative bacteria initially establish and continuously maintain OM lipid asymmetry, discussing the state-of-the-art knowledge of specialized lipid transport machines that place LPS and PLs directly into their corresponding leaflets in the OM, prevent excess PL accumulation and mislocalization, and correct any lipid asymmetry defects. We critically assess current studies, or the lack thereof, and highlight important future directions for research on OM lipid transport, homeostasis, and asymmetry.

Breaking Barriers: Exploiting Envelope Biogenesis and Stress Responses to Develop Novel Antimicrobial Strategies in Gram-Negative Bacteria

Antimicrobial resistance (AMR) has emerged as a global health threat, necessitating immediate actions to develop novel antimicrobial strategies and enforce strong stewardship of existing antibiotics to manage the emergence of drug-resistant strains. This issue is particularly concerning when it comes to Gram-negative bacteria, which possess an almost impenetrable outer membrane (OM) that acts as a formidable barrier to existing antimicrobial compounds. This OM is an asymmetric structure, composed of various components that confer stability, fluidity, and integrity to the bacterial cell. The maintenance and restoration of membrane integrity are regulated by envelope stress response systems (ESRs), which monitor its assembly and detect damages caused by external insults. Bacterial communities encounter a wide range of environmental niches to which they must respond and adapt for survival, sustenance, and virulence. ESRs play crucial roles in coordinating the expression of virulence factors, adaptive physiological behaviors, and antibiotic resistance determinants. Given their role in regulating bacterial cell physiology and maintaining membrane homeostasis, ESRs present promising targets for drug development. Considering numerous studies highlighting the involvement of ESRs in virulence, antibiotic resistance, and alternative resistance mechanisms in pathogens, this review aims to present these systems as potential drug targets, thereby encouraging further research in this direction.

The translocation assembly module (TAM) catalyzes the assembly of bacterial outer membrane proteins in vitro

The translocation and assembly module (TAM) has been proposed to play a crucial role in the assembly of a small subset of outer membrane proteins (OMPs) in Proteobacteria based on experiments conducted in vivo using tamA and tamB mutant strains and in vitro using biophysical methods. TAM consists of an OMP (TamA) and a periplasmic protein that is anchored to the inner membrane by a single α helix (TamB). Here we examine the function of the purified E. coli complex in vitro after reconstituting it into proteoliposomes. We find that TAM catalyzes the assembly of four model OMPs nearly as well as the β-barrel assembly machine (BAM), a universal heterooligomer that contains a TamA homolog (BamA) and that catalyzes the assembly of almost all E. coli OMPs. Consistent with previous results, both TamA and TamB are required for significant TAM activity. Our study provides direct evidence that TAM can function as an independent OMP insertase and describes a new method to gain insights into TAM function.

The translocation assembly module (TAM) catalyzes the assembly of bacterial outer membrane proteins in vitro

The bacterial translocation assembly module (TAM) contains an outer membrane protein (OMP) (TamA) and an elongated periplasmic protein that is anchored to the inner membrane by a single α helix (TamB). TAM has been proposed to play a critical role in the assembly of a small subset of OMPs produced by Proteobacteria based on experiments conducted in vivo using tamA and/or tamB deletion or mutant strains and in vitro using biophysical methods. Recent genetic experiments, however, have strongly suggested that TAM promotes phospholipid homeostasis. To test the idea that TAM catalyzes OMP assembly directly, we examined the function of the purified E. coli complex in vitro after reconstituting it into proteoliposomes. Remarkably, we find that TAM catalyzes the assembly of four model OMPs nearly as well as the β-barrel assembly machinery (BAM), a universal heterooligomer that contains a TamA homolog (BamA) and that catalyzes the assembly of almost all E. coli OMPs. Consistent with previous results, both TamA and TamB are required for significant TAM activity. Our results provide strong evidence that although their peripheral subunits are unrelated, both BAM and TAM function as independent OMP insertases. Furthermore, our study describes a new method to gain insights into TAM function.

Genetic evidence for functional diversification of gram-negative intermembrane phospholipid transporters

The outer membrane of gram-negative bacteria is a barrier to chemical and physical stress. Phospholipid transport between the inner and outer membranes has been an area of intense investigation and, in E. coli K-12, it has recently been shown to be mediated by YhdP, TamB, and YdbH, which are suggested to provide hydrophobic channels for phospholipid diffusion, with YhdP and TamB playing the major roles. However, YhdP and TamB have different phenotypes suggesting distinct functions. It remains unclear whether these functions are related to phospholipid metabolism. We investigated a synthetic cold sensitivity caused by deletion of fadR, a transcriptional regulator controlling fatty acid degradation and unsaturated fatty acid production, and yhdP, but not by ΔtamB ΔfadR or ΔydbH ΔfadR. Deletion of tamB recuses the ΔyhdP ΔfadR cold sensitivity further demonstrating the phenotype is related to functional diversification between these genes. The ΔyhdP ΔfadR strain shows a greater increase in cardiolipin upon transfer to the non-permissive temperature and genetically lowering cardiolipin levels can suppress cold sensitivity. These data also reveal a qualitative difference between cardiolipin synthases in E. coli, as deletion of clsA and clsC suppresses cold sensitivity but deletion of clsB does not. Moreover, increased fatty acid saturation is necessary for cold sensitivity and lowering this level genetically or through supplementation of oleic acid suppresses the cold sensitivity of the ΔyhdP ΔfadR strain. Together, our data clearly demonstrate that the diversification of function between YhdP and TamB is related to phospholipid metabolism. Although indirect regulatory effects are possible, we favor the parsimonious hypothesis that YhdP and TamB have differential phospholipid-substrate transport preferences. Thus, our data provide a potential mechanism for independent control of the phospholipid composition of the inner and outer membranes in response to changing conditions based on regulation of abundance or activity of YhdP and TamB.

Co-ordinated assembly of the multilayered cell envelope of Gram-negative bacteria

Bacteria surround themselves with complex cell envelopes to maintain their integrity and protect against external insults. The envelope of Gram-negative organisms is multilayered, with two membranes sandwiching the periplasmic space that contains the peptidoglycan cell wall. Understanding how this complicated surface architecture is assembled during cell growth and division is a major fundamental problem in microbiology. Additionally, because the envelope is an important antibiotic target and determinant of intrinsic antibiotic resistance, understanding the mechanisms governing its assembly is relevant to therapeutic development. In the last several decades, most of the factors required to build the Gram-negative envelope have been identified. However, surprisingly, little is known about how the biogenesis of the different cell surface layers is co-ordinated. Here, we provide an overview of recent work that is beginning to uncover the links connecting the different envelope biosynthetic pathways and assembly machines to ensure uniform envelope growth.

Lipid osmosis, membrane tension, and other mechanochemical driving forces of lipid flow

Nonvesicular lipid transport among different membranes or membrane domains plays crucial roles in lipid homeostasis and organelle biogenesis. However, the forces that drive such lipid transport are not well understood. We propose that lipids tend to flow towards the membrane area with a higher membrane protein density in a process termed lipid osmosis. This process lowers the membrane tension in the area, resulting in a membrane tension difference called osmotic membrane tension. We examine the thermodynamic basis and experimental evidence of lipid osmosis and osmotic membrane tension. We predict that lipid osmosis can drive bulk lipid flows between different membrane regions through lipid transfer proteins, scramblases, or similar barriers that selectively pass lipids but not membrane proteins. We also speculate on the biological functions of lipid osmosis. Finally, we explore other driving forces for lipid transfer and describe potential methods and systems to further test our theory.

Genetic evidence for functional diversification of gram-negative intermembrane phospholipid transporters

The outer membrane of Gram-negative bacteria is a barrier to chemical and physical stress. Phospholipid transport between the inner and outer membranes has been an area of intense investigation and, in E. coli K-12, it has recently been shown to be mediated by YhdP, TamB, and YdbH, which are suggested to provide hydrophobic channels for phospholipid diffusion, with YhdP and TamB playing the major roles. However, YhdP and TamB have different phenotypes suggesting distinct functions. We investigated these functions using synthetic cold sensitivity (at 30 °C) caused by deletion of yhdP and fadR, a transcriptional regulator controlling fatty acid degradation and unsaturated fatty acid production, but not by ΔtamB ΔfadR or ΔydbH ΔfadR,. Deletion of tamB suppresses the ΔyhdP ΔfadR cold sensitivity suggesting this phenotype is related to phospholipid transport. The ΔyhdP ΔfadR strain shows a greater increase in cardiolipin upon transfer to the non-permissive temperature and genetically lowering cardiolipin levels can suppress cold sensitivity. These data also reveal a qualitative difference between cardiolipin synthases in E. coli, as deletion of clsA and clsC suppresses cold sensitivity but deletion of clsB does not despite lower cardiolipin levels. In addition to increased cardiolipin, increased fatty acid saturation is necessary for cold sensitivity and lowering this level genetically or through supplementation of oleic acid suppresses the cold sensitivity of the ΔyhdP ΔfadR strain. Although indirect effects are possible, we favor the parsimonious hypothesis that YhdP and TamB have differential substrate transport preferences, most likely with YhdP preferentially transporting more saturated phospholipids and TamB preferentially transporting more unsaturated phospholipids. We envision cardiolipin contributing to this transport preference by sterically clogging TamB-mediated transport of saturated phospholipids. Thus, our data provide a potential mechanism for independent control of the phospholipid composition of the inner and outer membranes in response to changing conditions.

YdbH and YnbE form an intermembrane bridge to maintain lipid homeostasis in the outer membrane of Escherichia coli

The outer membrane (OM) of didermic gram-negative bacteria is essential for growth, maintenance of cellular integrity, and innate resistance to many antimicrobials. Its asymmetric lipid distribution, with phospholipids in the inner leaflet and lipopolysaccharides (LPS) in the outer leaflet, is required for these functions. Lpt proteins form a transenvelope bridge that transports newly synthesized LPS from the inner membrane (IM) to OM, but how the bulk of phospholipids are transported between these membranes is poorly understood. Recently, three members of the AsmA-like protein family, TamB, YhdP, and YdbH, were shown to be functionally redundant and were proposed to transport phospholipids between IM and OM in Escherichia coli. These proteins belong to the repeating β-groove superfamily, which includes eukaryotic lipid-transfer proteins that mediate phospholipid transport between organelles at contact sites. Here, we show that the IM-anchored YdbH protein interacts with the OM lipoprotein YnbE to form a functional protein bridge between the IM and OM in E. coli. Based on AlphaFold-Multimer predictions, genetic data, and in vivo site-directed cross-linking, we propose that YnbE interacts with YdbH through β-strand augmentation to extend the continuous hydrophobic β-groove of YdbH that is thought to shield acyl chains of phospholipids as they travel through the aqueous intermembrane periplasmic compartment. Our data also suggest that the periplasmic protein YdbL prevents extensive amyloid-like multimerization of YnbE in cells. We, therefore, propose that YdbL has a chaperone-like function that prevents uncontrolled runaway multimerization of YnbE to ensure the proper formation of the YdbH-YnbE intermembrane bridge.

Lipid osmosis, membrane tension, and other mechanochemical driving forces of lipid flow

Nonvesicular lipid transport among different membranes or membrane domains plays crucial roles in lipid homeostasis and organelle biogenesis. However, the forces that drive such lipid transport are not well understood. We propose that lipids tend to flow towards the membrane area with a higher membrane protein density in a process termed lipid osmosis. This process lowers the membrane tension in the area, resulting in a membrane tension difference called osmotic membrane tension. We examine the thermodynamic basis and experimental evidence of lipid osmosis and osmotic membrane tension. We predict that lipid osmosis can drive bulk lipid flows between different membrane regions through lipid transfer proteins, scramblases, or other similar barriers that selectively pass lipids but not membrane proteins. We also speculate on the biological functions of lipid osmosis. Finally, we explore other driving forces for lipid transfer and describe potential methods and systems to further test our theory.

New insights into the concentration-dependent regulation of membrane biofouling formation via continuous nanoplastics stimulation

The release of nanoplastics (NPs) into the environment is growing due to the extensive use of plastic products. Numerous studies have confirmed the negative effects of NPs on microorganisms, which poses uncertainties concerning their impact on nanofiltration (NF) membrane biofouling. This study investigated the initial cell adhesion process, NF membrane biofouling kinetic processes and bacterial responses of Pseudomonas aeruginosa (P. aeruginosa) exposed to varied NPs concentrations (0-50 mg·L-1). Transcriptome analysis demonstrated that low concentration of NPs (0.1 mg·L-1) promoted bacterial quorum sensing, energy metabolism, exopolysaccharide biosynthesis and bacterial secretion systems. Correspondingly, the polysaccharide content increased remarkably to 2.77 times the unexposed control, which served as a protective barrier for bacteria to avoid the impact of NPs-induced stress. Suppressed homologous recombination, microbial metabolic potentials and flagellar assembly were detected in bacteria exposed to a high concentration (50 mg·L-1) of NPs, mainly due to the triggered reactive oxygen species (ROS) generation, genomic DNA damage, and decreased energy production. Overall, enhanced formation of the extracellular polymeric substances (EPS) and aggravated membrane flux decline were observed when NPs interacted with the membrane surface by cell secretions (low NPs levels) or cell lysis (high NPs levels). These findings shed light on understanding the microbial metabolism mechanism and membrane biofouling propensity with NPs stress at both the molecular and gene levels.

Genetic interaction mapping reveals functional relationships between peptidoglycan endopeptidases and carboxypeptidases

Peptidoglycan (PG) is the main component of the bacterial cell wall; it maintains cell shape while protecting the cell from internal osmotic pressure and external environmental challenges. PG synthesis is essential for bacterial growth and survival, and a series of PG modifications are required to allow expansion of the sacculus. Endopeptidases (EPs), for example, cleave the crosslinks between adjacent PG strands to allow the incorporation of newly synthesized PG. EPs are collectively essential for bacterial growth and must likely be carefully regulated to prevent sacculus degradation and cell death. However, EP regulation mechanisms are poorly understood. Here, we used TnSeq to uncover novel EP regulators in Vibrio cholerae. This screen revealed that the carboxypeptidase DacA1 (PBP5) alleviates EP toxicity. dacA1 is essential for viability on LB medium, and this essentiality was suppressed by EP overexpression, revealing that EP toxicity both mitigates, and is mitigated by, a defect in dacA1. A subsequent suppressor screen to restore viability of ΔdacA1 in LB medium identified hypomorphic mutants in the PG synthesis pathway, as well as mutations that promote EP activation. Our data thus reveal a more complex role of DacA1 in maintaining PG homeostasis than previously assumed.

The TAM, a Translocation and Assembly Module for protein assembly and potential conduit for phospholipid transfer

The assembly of β-barrel proteins into the bacterial outer membrane is an essential process enabling the colonization of new environmental niches. The TAM was discovered as a module of the β-barrel protein assembly machinery; it is a heterodimeric complex composed of an outer membrane protein (TamA) bound to an inner membrane protein (TamB). The TAM spans the periplasm, providing a scaffold through the peptidoglycan layer and catalyzing the translocation and assembly of β-barrel proteins into the outer membrane. Recently, studies on another membrane protein (YhdP) have suggested that TamB might play a role in phospholipid transport to the outer membrane. Here we review and re-evaluate the literature covering the experimental studies on the TAM over the past decade, to reconcile what appear to be conflicting claims on the function of the TAM.

mSphere of Influence: The discovery of a missing link in bacterial cell envelope biogenesis

Gregory Harrison is a bacteriologist researching essential pathways in bacteria as potential therapeutic targets. In this mSphere of Influence article, he reflects on a series of studies that employ complementary genetic approaches to define the crucial role of AsmA-family proteins in transporting phospholipids between the inner and outer membranes of Gram-negative bacteria. The authors of these three studies identify this family of lipid transporters through the means of bacterial genetics, answering a long-standing question in bacterial physiology, and serving as a reminder that a well-designed genetic strategy can go a long way in uncovering new biology.

Redundant essentiality of AsmA-like proteins in Pseudomonas aeruginosa

The outer membrane (OM) is an essential structure of Gram-negative bacteria that provides mechanical strength and protection from large and/or hydrophobic toxic molecules, including many antibiotics. The OM is composed of glycerophospholipids (GPLs) and lipopolysaccharide (LPS) in the inner and outer leaflets, respectively, and hosts integral β-barrel proteins and lipoproteins. While the systems responsible for translocation and insertion of LPS and OM proteins have been elucidated, the mechanism(s) mediating transport of GPLs from the inner membrane to the OM has remained elusive for decades. Very recently, studies performed in Escherichia coli proposed a role in this process for AsmA-like proteins that are predicted to share structural features with eukaryotic lipid transporters. In this study, we provide the first systematic investigation of AsmA-like proteins in a bacterium other than E. coli, the opportunistic human pathogen Pseudomonas aeruginosa. Bioinformatic analyses revealed that P. aeruginosa possesses seven AsmA-like proteins. Deletion of asmA-like genes in many different combinations, coupled with conditional mutagenesis, revealed that four AsmA-like proteins are redundantly essential for growth and OM integrity in P. aeruginosa, including a novel AsmA-like protein (PA4735) that is not present in E. coli. Cells depleted of AsmA-like proteins showed severe defects in the OM permeability barrier that were partially rescued by lowering the synthesis or transport of LPS. Since fine balancing of GPL and LPS levels is crucial for OM integrity, this evidence supports the role of AsmA-like proteins in GPL transport toward the OM.

IMPORTANCE

Given the importance of the outer membrane (OM) for viability and antibiotic resistance in Gram-negative bacteria, in the last decades, several studies have focused on the characterization of the systems involved in OM biogenesis, which have also been explored as targets for antibacterial drug development. However, the mechanism mediating translocation of glycerophospholipids (GPLs) to the OM remained unknown until recent studies provided evidence that AsmA-like proteins could be responsible for this process. Here, we demonstrate for the first time that AsmA-like proteins are essential and redundant for growth and OM integrity in a Gram-negative bacterium other than the model organism Escherichia coli and demonstrate that the human pathogen Pseudomonas aeruginosa has an additional essential AsmA-like protein that is not present in E. coli, thus expanding the range of AsmA-like proteins that play key functions in Gram-negative bacteria.

Molecular mechanism of phospholipid transport at the bacterial outer membrane interface

The outer membrane (OM) of Gram-negative bacteria is an asymmetric lipid bilayer with outer leaflet lipopolysaccharides and inner leaflet phospholipids (PLs). This unique lipid asymmetry renders the OM impermeable to external insults, including antibiotics and bile salts. To maintain this barrier, the OmpC-Mla system removes mislocalized PLs from the OM outer leaflet, and transports them to the inner membrane (IM); in the first step, the OmpC-MlaA complex transfers PLs to the periplasmic chaperone MlaC, but mechanistic details are lacking. Here, we biochemically and structurally characterize the MlaA-MlaC transient complex. We map the interaction surfaces between MlaA and MlaC in Escherichia coli, and show that electrostatic interactions are important for MlaC recruitment to the OM. We further demonstrate that interactions with MlaC modulate conformational states in MlaA. Finally, we solve a 2.9-Å cryo-EM structure of a disulfide-trapped OmpC-MlaA-MlaC complex in nanodiscs, reinforcing the mechanism of MlaC recruitment, and highlighting membrane thinning as a plausible strategy for directing lipids for transport. Our work offers critical insights into retrograde PL transport by the OmpC-Mla system in maintaining OM lipid asymmetry.

The Mla system of diderm Firmicute Veillonella parvula reveals an ancestral transenvelope bridge for phospholipid trafficking

E. coli and most other diderm bacteria (those with two membranes) have an inner membrane enriched in glycerophospholipids (GPLs) and an asymmetric outer membrane (OM) containing GPLs in its inner leaflet and primarily lipopolysaccharides in its outer leaflet. In E. coli, this lipid asymmetry is maintained by the Mla system which consists of six proteins: the OM lipoprotein MlaA extracts GPLs from the outer leaflet, and the periplasmic chaperone MlaC transfers them across the periplasm to the inner membrane complex MlaBDEF. However, GPL trafficking still remains poorly understood, and has only been studied in a handful of model species. Here, we investigate GPL trafficking in Veillonella parvula, a diderm Firmicute with an Mla system that lacks MlaA and MlaC, but contains an elongated MlaD. V. parvula mla mutants display phenotypes characteristic of disrupted lipid asymmetry which can be suppressed by mutations in tamB, supporting that these two systems have opposite GPL trafficking functions across diverse bacterial lineages. Structural modelling and subcellular localisation assays suggest that V. parvula MlaD forms a transenvelope bridge, comprising a typical inner membrane-localised MCE domain and, in addition, an outer membrane ß-barrel. Phylogenomic analyses indicate that this elongated MlaD type is widely distributed across diderm bacteria and likely forms part of the ancestral functional core of the Mla system, which would be composed of MlaEFD only.

TamAB is regulated by PhoPQ and functions in outer membrane homeostasis during Salmonella pathogenesis

Salmonella survive and replicate in macrophages, which normally kill bacteria by exposing them to a variety of harsh conditions and antimicrobial effectors, many of which target the bacterial cell envelope. The PhoPQ two-component system responds to the phagosome environment and induces factors that protect the outer membrane, allowing adaptation and growth in the macrophage. We show that PhoPQ induces the transcription of the tamAB operon both in vitro and in macrophages. The TamA protein is structurally similar to BamA, an essential protein in the Bam complex that assembles β-barrel proteins in the outer membrane, while TamB is an AsmA-family protein implicated in lipid transport between the inner and outer membranes. We show that the Bam machinery is stressed in vitro under low Mg2+, low pH conditions that mimic the phagosome. Not surprisingly, mutations affecting Bam function confer significant virulence defects. Although loss of TamAB alone confers no virulence defect, a tamAB deletion confers a synthetic phenotype in bam mutant backgrounds in animals and macrophages, and in vitro upon treatment with vancomycin or sodium dodecyl sulfate. Mutations affecting YhdP, which functions in partial redundancy with TamB, also confer synthetic phenotypes with bam mutations in the animal, but this interaction is not evident in vitro. Thus, in the harsh phagocytic environment of the macrophage, the outer membrane Bam machinery is compromised, and the TamAB system, and perhaps other PhoPQ-regulated factors, is induced to compensate. It is most likely that TamAB and other systems assist the Bam complex indirectly by affecting outer membrane properties. IMPORTANCE The TamAB system has been implicated in both outer membrane protein localization and phospholipid transport between the inner and outer membranes. We show that the β-barrel protein assembly complex, Bam, is stressed under conditions thought to mimic the macrophage phagosome. TamAB expression is controlled by the PhoPQ two-component system and induced in macrophages. This system somehow compensates for the Bam complex as evidenced by the fact that mutations affecting the two systems confer synthetic phenotypes in animals, macrophages, and in vitro in the presence of vancomycin or SDS. This study has implications concerning the role of TamAB in outer membrane homeostasis. It also contributes to our understanding of the systems necessary for Salmonella to adapt and reproduce within the macrophage phagosome.

RBG Motif Bridge-Like Lipid Transport Proteins: Structure, Functions, and Open Questions

The life of eukaryotic cells requires the transport of lipids between membranes, which are separated by the aqueous environment of the cytosol. Vesicle-mediated traffic along the secretory and endocytic pathways and lipid transfer proteins (LTPs) cooperate in this transport. Until recently, known LTPs were shown to carry one or a few lipids at a time and were thought to mediate transport by shuttle-like mechanisms. Over the last few years, a new family of LTPs has been discovered that is defined by a repeating β-groove (RBG) rod-like structure with a hydrophobic channel running along their entire length. This structure and the localization of these proteins at membrane contact sites suggest a bridge-like mechanism of lipid transport. Mutations in some of these proteins result in neurodegenerative and developmental disorders. Here we review the known properties and well-established or putative physiological roles of these proteins, and we highlight the many questions that remain open about their functions.

Phospholipid transport to the bacterial outer membrane through an envelope-spanning bridge

The outer membrane of Gram-negative bacteria provides a formidable barrier, essential for both pathogenesis and antimicrobial resistance. Biogenesis of the outer membrane requires the transport of phospholipids across the cell envelope. Recently, YhdP was implicated as a major protagonist in the transport of phospholipids from the inner membrane to the outer membrane however the molecular mechanism of YhdP mediated transport remains elusive. Here, utilising AlphaFold, we observe YhdP to form an elongated assembly of 60 β strands that curve to form a continuous hydrophobic groove. This architecture is consistent with our negative stain electron microscopy data which reveals YhdP to be approximately 250 Å in length and thus sufficient to span the bacterial cell envelope. Furthermore, molecular dynamics simulations and in vivo bacterial growth assays indicate essential helical regions at the N- and C-termini of YhdP, that may embed into the inner and outer membranes respectively, reinforcing its envelope spanning nature. Our in vivo crosslinking data reveal phosphate-containing substrates captured along the length of the YhdP groove, providing direct evidence that YhdP transports phospholipids. This finding is congruent with our molecular dynamics simulations which demonstrate the propensity for inner membrane lipids to spontaneously enter the groove of YhdP. Collectively, our results support a model in which YhdP bridges the cell envelope, providing a hydrophobic environment for the transport of phospholipids to the outer membrane.

Mechanism of outer membrane destabilization by global reduction of protein content

The outer membrane (OM) of Gram-negative bacteria such as Escherichia coli is an asymmetric bilayer with the glycolipid lipopolysaccharide (LPS) in the outer leaflet and glycerophospholipids in the inner. Nearly all integral OM proteins (OMPs) have a characteristic β-barrel fold and are assembled in the OM by the BAM complex, which contains one essential β-barrel protein (BamA), one essential lipoprotein (BamD), and three non-essential lipoproteins (BamBCE). A gain-of-function mutation in bamA enables survival in the absence of BamD, showing that the essential function of this protein is regulatory. Here, we demonstrate that the global reduction in OMPs caused by BamD loss weakens the OM, altering cell shape and causing OM rupture in spent medium. To fill the void created by OMP loss, phospholipids (PLs) flip into the outer leaflet. Under these conditions, mechanisms that remove PLs from the outer leaflet create tension between the OM leaflets, which contributes to membrane rupture. Rupture is prevented by suppressor mutations that release the tension by halting PL removal from the outer leaflet. However, these suppressors do not restore OM stiffness or normal cell shape, revealing a possible connection between OM stiffness and cell shape.

Novel role for peptidoglycan carboxypeptidases in maintaining the balance between bacterial cell wall synthesis and degradation

Peptidoglycan (PG) is the main component of the bacterial cell wall; it maintains cell shape while protecting the cell from internal osmotic pressure and external environmental challenges. PG synthesis is essential for bacterial growth and survival, and a series of PG modifications are required to allow expansion of the sacculus. Endopeptidases (EPs), for example, cleave the crosslinks between adjacent PG strands to allow the incorporation of newly synthesized PG. EPs are collectively essential for bacterial growth and must likely be carefully regulated to prevent sacculus degradation and cell death. However, EP regulation mechanisms are poorly understood. Here, we used TnSeq to uncover novel EP regulation factors in Vibrio cholerae. This screen revealed that the carboxypeptidase DacA1 (PBP5) alleviates EP toxicity. dacA1 is essential for viability on LB medium, and this essentiality was suppressed by EP overexpression, revealing that EP toxicity both mitigates, and is mitigated by, a defect in dacA1. A subsequent suppressor screen to restore viability of ΔdacA1 in LB medium was answered by hypomorphic mutants in the PG synthesis pathway, as well as mutations that promote PG degradation. Our data thus reveal a key role of DacA1 in maintaining the balance between PG synthesis and degradation.

The DedA superfamily member PetA is required for the transbilayer distribution of phosphatidylethanolamine in bacterial membranes

The sorting of phospholipids between the inner and outer leaflets of the membrane bilayer is a fundamental problem in all organisms. Despite years of investigation, most of the enzymes that catalyze phospholipid reorientation in bacteria remain unknown. Studies from almost half a century ago in Bacillus subtilis and Bacillus megaterium revealed that newly synthesized phosphatidylethanolamine (PE) is rapidly translocated to the outer leaflet of the bilayer [Rothman & Kennedy, Proc. Natl. Acad. Sci. U.S.A. 74, 1821-1825 (1977)] but the identity of the putative PE flippase has eluded discovery. Recently, members of the DedA superfamily have been implicated in flipping the bacterial lipid carrier undecaprenyl phosphate and in scrambling eukaryotic phospholipids in vitro. Here, using the antimicrobial peptide duramycin that targets outward-facing PE, we show that Bacillus subtilis cells lacking the DedA paralog PetA (formerly YbfM) have increased resistance to duramycin. Sensitivity to duramycin is restored by expression of B. subtilis PetA or homologs from other bacteria. Analysis of duramycin-mediated killing upon induction of PE synthesis indicates that PetA is required for efficient PE transport. Finally, using fluorescently labeled duramycin we demonstrate that cells lacking PetA have reduced PE in their outer leaflet compared to wildtype. We conclude that PetA is the long-sought PE transporter. These data combined with bioinformatic analysis of other DedA paralogs argue that the primary role of DedA superfamily members is transporting distinct lipids across the membrane bilayer.

Envelope-Stress Sensing Mechanism of Rcs and Cpx Signaling Pathways in Gram-Negative Bacteria

The global public health burden of bacterial antimicrobial resistance (AMR) is intensified by Gram-negative bacteria, which have an additional membrane, the outer membrane (OM), outside of the peptidoglycan (PG) cell wall. Bacterial two-component systems (TCSs) aid in maintaining envelope integrity through a phosphorylation cascade by controlling gene expression through sensor kinases and response regulators. In Escherichia coli, the major TCSs defending cells from envelope stress and adaptation are Rcs and Cpx, which are aided by OM lipoproteins RcsF and NlpE as sensors, respectively. In this review, we focus on these two OM sensors. β-Barrel assembly machinery (BAM) inserts transmembrane OM proteins (OMPs) into the OM. BAM co-assembles RcsF, the Rcs sensor, with OMPs, forming the RcsF-OMP complex. Researchers have presented two models for stress sensing in the Rcs pathway. The first model suggests that LPS perturbation stress disassembles the RcsF-OMP complex, freeing RcsF to activate Rcs. The second model proposes that BAM cannot assemble RcsF into OMPs when the OM or PG is under specific stresses, and thus, the unassembled RcsF activates Rcs. These two models may not be mutually exclusive. Here, we evaluate these two models critically in order to elucidate the stress sensing mechanism. NlpE, the Cpx sensor, has an N-terminal (NTD) and a C-terminal domain (CTD). A defect in lipoprotein trafficking results in NlpE retention in the inner membrane, provoking the Cpx response. Signaling requires the NlpE NTD, but not the NlpE CTD; however, OM-anchored NlpE senses adherence to a hydrophobic surface, with the NlpE CTD playing a key role in this function.

Mechanism of outer membrane destabilization by global reduction of protein content

The outer membrane (OM) of Gram-negative bacteria such as Escherichia coli is an asymmetric bilayer with the glycolipid lipopolysaccharide (LPS) in the outer leaflet and glycerophospholipids in the inner. Nearly all integral OM proteins (OMPs) have a characteristic β-barrel fold and are assembled in the OM by the BAM complex, which contains one essential β-barrel protein (BamA), one essential lipoprotein (BamD), and three non-essential lipoproteins (BamBCE). A gain-of-function mutation in bamA enables survival in the absence of BamD, showing that the essential function of this protein is regulatory. We demonstrate that the global reduction in OMPs caused by BamD loss weakens the OM, altering cell shape and causing OM rupture in spent medium. To fill the void created by OMP loss, PLs flip into the outer leaflet. Under these conditions, mechanisms that remove PLs from the outer leaflet create tension between the OM leaflets, which contributes to membrane rupture. Rupture is prevented by suppressor mutations that release the tension by halting PL removal from the outer leaflet. However, these suppressors do not restore OM stiffness or normal cell shape, revealing a possible connection between OM stiffness and cell shape.

Significance Statement

The outer membrane (OM) is a selective permeability barrier that contributes to the intrinsic antibiotic resistance of Gram-negative bacteria. Biophysical characterization of the roles of the component proteins, lipopolysaccharides, and phospholipids is limited by both the essentiality of the OM and its asymmetrical organization. In this study, we dramatically change OM physiology by limiting the protein content, which requires phospholipid localization to the outer leaflet and thus disrupts OM asymmetry. By characterizing the perturbed OM of various mutants, we provide novel insight into the links among OM composition, OM stiffness, and cell shape regulation. These findings deepen our understanding of bacterial cell envelope biology and provide a platform for further interrogation of OM properties.

Teasing apart the evolution of lipoprotein trafficking in gram-negative bacteria reveals a bifunctional LolA

The outer membrane (OM) is the defining feature of gram-negative bacteria and is an essential organelle. Accordingly, OM assembly pathways and their essential protein components are conserved throughout all gram-negative species. Lipoprotein trafficking lies at the heart of OM assembly since it supplies several different biogenesis machines with essential lipoproteins. The Escherichia coli Lol trafficking pathway relies on an inner membrane LolCDE transporter that transfers newly made lipoproteins to the chaperone LolA, which rapidly traffics lipoproteins across the periplasm to LolB for insertion into the OM. Strikingly, many gram-negative species (like Caulobacter vibrioides) do not produce LolB, yet essential lipoproteins are still trafficked to the OM. How the final step of trafficking occurs in these organisms has remained a long-standing mystery. We demonstrate that LolA from C. vibrioides can complement the deletion of both LolA and LolB in E. coli, revealing that this protein possesses both chaperone and insertion activities. Moreover, we define the region of C. vibrioides LolA that is responsible for its bifunctionality. This knowledge enabled us to convert E. coli LolA into a similarly bifunctional protein, capable of chaperone and insertion activities. We propose that a bifunctional LolA eliminates the need for LolB. Our findings provide an explanation for why some gram-negative species have retained an essential LolA yet completely lack a dedicated LolB protein.

Cracking outer membrane biogenesis

The outer membrane is a distinguishing feature of the Gram-negative envelope. It lies on the external face of the peptidoglycan sacculus and forms a robust permeability barrier that protects extracytoplasmic structures from environmental insults. Overcoming the barrier imposed by the outer membrane presents a significant hurdle towards developing novel antibiotics that are effective against Gram-negative bacteria. As the outer membrane is an essential component of the cell, proteins involved in its biogenesis are themselves promising antibiotic targets. Here, we summarize key findings that have built our understanding of the outer membrane. Foundational studies describing the discovery and composition of the outer membrane as well as the pathways involved in its construction are discussed.

Bacterial AsmA-Like Proteins: Bridging the Gap in Intermembrane Phospholipid Transport

In eukaryotic cells, nonvesicular lipid transport between organelles is mediated by lipid-transfer proteins. Recently, a new class of these lipid transporters has been described to facilitate the bulk of inter-organelle lipid transport at contact sites by forming bridge-like structures with a hydrophobic groove through which lipids travel. Because their predicted structure is composed of repeating β-groove (RBG) domains, they have been named the RBG protein superfamily. Early studies on RBG proteins VPS13 and ATG2 recognized the resemblance of their predicted structures to that of the bacterial Lpt system, which transports newly synthesized lipopolysaccharides (LPS) between the inner and the outer membranes (IMs and OMs) of Gram-negative bacteria. In these didermic bacteria, the IMs and OMs are separated by an aqueous periplasmic compartment that is traversed by a bridge-like structure built with β-jelly roll domains from several Lpt proteins that provides a hydrophobic groove for LPS molecules to travel across the periplasm. Despite structural and functional similarities between RBG proteins and the Lpt system, the bacterial AsmA-like protein family has recently emerged as the likely ancestor of RBG proteins and long sought-after transporters that facilitate the transfer of phospholipids from the IM to the OM. Here, we review our current understanding of the structure and function of bacterial AsmA-like proteins, mainly focusing on recent studies that have led to the proposal that AsmA-like proteins mediate the bulk of phospholipid transfer between the IMs and OMs.

Cellular memory of rapid growth is sensitive to nutrient depletion during starvation

Bacteria frequently encounter nutrient fluctuations in natural environments, yet we understand little about their ability to maintain physiological memory of previous food sources. Starvation is a particularly acute case, in which cells must balance adaptation to stresses with limited nutrient supply. Here, we show that Escherichia coli cells immediately accelerate and decelerate in growth upon transitions from spent to fresh media and vice versa, respectively, and memory of rapid growth can be maintained for many hours under constant flow of spent medium. However, after transient exposure of stationary-phase cells to fresh medium, subsequent aerobic incubation in increasingly spent medium led to lysis and limited growth when rejuvenated in fresh medium. Growth defects were avoided by incubation in anaerobic spent medium or water, suggesting that defects were caused by respiration during the process of nutrient depletion in spent medium. These findings highlight the importance of respiration for stationary phase survival and underscore the broad range of starvation outcomes depending on environmental history.

Lipid Transport Across Bacterial Membranes

The movement of lipids within and between membranes in bacteria is essential for building and maintaining the bacterial cell envelope. Moving lipids to their final destination is often energetically unfavorable and does not readily occur spontaneously. Bacteria have evolved several protein-mediated transport systems that bind specific lipid substrates and catalyze the transport of lipids across membranes and from one membrane to another. Specific protein flippases act in translocating lipids across the plasma membrane, overcoming the obstacle of moving relatively large and chemically diverse lipids between leaflets of the bilayer. Active transporters found in double-membraned bacteria have evolved sophisticated mechanisms to traffic lipids between the two membranes, including assembling to form large, multiprotein complexes that resemble bridges, shuttles, and tunnels, shielding lipids from the hydrophilic environment of the periplasm during transport. In this review, we explore our current understanding of the mechanisms thought to drive bacterial lipid transport.

Structure and mechanism of the bacterial lipid ABC transporter, MlaFEDB

The cell envelope of Gram-negative bacteria is composed of an inner membrane, outer membane, and an intervening periplasmic space. How the outer membrane lipids are trafficked and assembled there, and how the asymmetry of the outer membrane is maintained is an area of intense research. The Mla system has been implicated in the maintenance of lipid asymmetry in the outer membrane, and is generally thought to drive the removal of mislocalized phospholipids from the outer membrane and their retrograde transport to the inner membrane. At the heart of the Mla pathway is a structurally unique ABC transporter complex in the inner membrane, called MlaFEDB. Recently, an explosion of cryo-EM studies has begun to shed light on the structure and lipid translocation mechanism of MlaFEDB, with many parallels to other ABC transporter families, including human ABCA and ABCG, as well as bacterial lipopolysaccharide and O-antigen transporters. Here we synthesize information from all available structures, and propose a model for lipid trafficking across the cell envelope by MlaFEDB.

Applying fluorescent dye assays to discriminate Escherichia coli chlorhexidine resistance phenotypes from porin and mlaA deletions and efflux pumps

Bacterial resistance to the antiseptic chlorhexidine (CHX), is a growing problem, recently shown to be caused by deleterious mutations to the phospholipid transport system component (mlaA) as well as efflux pump overexpression. Comparisons of CHX resistance mechanisms, such as porin deletions (ompCF), and over-expressed efflux pumps (acrB, qacE, aceI), are lacking and may be distinguishable using antiseptic rapid fluorescent dye testing assays. Using E. coli K-12 CHX adapted isolates (CHXR1), gene deletion mutants, and over-expressed transformants the phenotypes of these CHX resistance genes were compared using antimicrobial susceptibility tests (AST), rapid fluorescent propidium iodide dye-based membrane integrity assays (RFDMIA), and scanning electron microscopy (SEM). AST findings showed CHXR1, ΔacrB, ΔompCF, and transformants pCA24N-aceI and pCA24N-mlaA conferred greater (two to fourfold) MIC changes when compared to matched controls. Examination of these mutants/transformants using CHX RFDMIA showed that porin dual-deletions (ΔompCF) and mlaA alterations (ΔmlaA; pCA24N-mlaA, CHXR1) were distinguishable from controls. Results for over-expressed (pMS119EH-aceI) and deleted (ΔacrB) efflux pump RFDMIA could not be distinguished with propidium iodide, only with ethidium bromide, suggesting propidium iodide is better suited for detecting porin and mlaA associated CHX resistance mechanisms. SEM of CHXR1 and unadapted E. coli cells exposed to increasing CHX concentrations revealed that CHX does not visibly damage cell envelope integrity at any tested concentration but did identify elongated CHXR1 cells. ΔmlaA confers similar levels of CHX resistance as efflux overexpression and porin deletions, however, only outer membrane-altering porin and mlaA deletions can be reliably distinguished using RFDMIA.

Of zones, bridges and chaperones - phospholipid transport in bacterial outer membrane assembly and homeostasis

The outer membrane (OM) is a formidable permeability barrier that protects Gram-negative bacteria from detergents and antibiotics. It possesses exquisite lipid asymmetry, requiring the placement and retention of lipopolysaccharides (LPS) in the outer leaflet, and phospholipids (PLs) in the inner leaflet. To establish OM lipid asymmetry, LPS are transported from the inner membrane (IM) directly to the outer leaflet of the OM. In contrast, mechanisms for PL trafficking across the cell envelope are much less understood. In this review, we summarize and discuss recent advances in our understanding of PL transport, making parallel comparisons to well-established pathways for OM lipoprotein (Lol) and LPS (Lpt). Insights into putative PL transport systems highlight possible connections back to the 'Bayer bridges', adhesion zones between the IM and the OM that had been observed more than 50 years ago, and proposed as passages for export of OM components, including LPS and PLs.

Physical properties of the bacterial outer membrane

It has long been appreciated that the Gram-negative outer membrane acts as a permeability barrier, but recent studies have uncovered a more expansive and versatile role for the outer membrane in cellular physiology and viability. Owing to recent developments in microfluidics and microscopy, the structural, rheological and mechanical properties of the outer membrane are becoming apparent across multiple scales. In this Review, we discuss experimental and computational studies that have revealed key molecular factors and interactions that give rise to the spatial organization, limited diffusivity and stress-bearing capacity of the outer membrane. These physical properties suggest broad connections between cellular structure and physiology, and we explore future prospects for further elucidation of the implications of outer membrane construction for cellular fitness and survival.

Absence of YhdP, TamB, and YdbH leads to defects in glycerophospholipid transport and cell morphology in Gram-negative bacteria

The outer membrane (OM) of Gram-negative bacteria provides the cell with a formidable barrier that excludes external threats. The two major constituents of this asymmetric barrier are lipopolysaccharide (LPS) found in the outer leaflet, and glycerophospholipids (GPLs) in the inner leaflet. Maintaining the asymmetric nature and balance of LPS to GPLs in the OM is critical for bacterial viability. The biosynthetic pathways of LPS and GPLs are well characterized, but unlike LPS transport, how GPLs are translocated to the OM remains enigmatic. Understanding this aspect of cell envelope biology could provide a foundation for new antibacterial therapies. Here, we report that YhdP and its homologues, TamB and YdbH, members of the “AsmA-like” family, are critical for OM integrity and necessary for proper GPL transport to the OM. The absence of the two largest AsmA-like proteins (YhdP and TamB) leads to cell lysis and antibiotic sensitivity, phenotypes that are rescued by reducing LPS synthesis. We also find that yhdP, tamB double mutants shed excess LPS through outer membrane vesicles, presumably to maintain OM homeostasis when normal anterograde GPL transport is disrupted. Moreover, a yhdP, tamB, ydbH triple mutant is synthetically lethal, but if GPL transport is partially restored by overexpression of YhdP, the cell shape adjusts to accommodate increased membrane content as the cell accumulates GPLs in the IM. Our results therefore suggest a model in which “AsmA-like” proteins transport GPLs to the OM, and when hindered, changes in cell shape and shedding of excess LPS aids in maintaining OM asymmetry.

Much like armor, the OM of Gram-negative bacteria serves as the cell’s first line of defense against harsh environments and toxic molecules. The two major components of the OM are LPS and GPLs. To offer effective protection, the cell must maintain the appropriate balance of LPS and GPLs at the bacterial surface. Here we report that members of the “AsmA-like” family, YhdP, TamB, and YdbH contribute to the OM barrier by aiding in GPL transport and are critical for antibiotic resistance.

YhdP, TamB, and YdbH Are Redundant but Essential for Growth and Lipid Homeostasis of the Gram-Negative Outer Membrane

The bacterial cell envelope is the first line of defense and point of contact with the environment and other organisms. Envelope biogenesis is therefore crucial for the survival and physiology of bacteria and is often targeted by antimicrobials. Gram-negative bacteria have a multilayered envelope delimited by an inner and outer membrane (IM and OM, respectively). The OM is a barrier against many antimicrobials because of its asymmetric lipid structure, with phospholipids composing the inner leaflet and lipopolysaccharides (LPS) the outer leaflet. Since lipid synthesis occurs at the IM, phospholipids and LPS are transported across the cell envelope and asymmetrically assembled at the OM during growth. How phospholipids are transported to the OM remains unknown. Recently, the Escherichia coli protein YhdP has been proposed to participate in this process through an unknown mechanism. YhdP belongs to the AsmA-like clan and contains domains homologous to those found in lipid transporters. Here, we used genetics to investigate the six members of the AsmA-like clan of proteins in E. coli. Our data show that YhdP and its paralogs TamB and YdbH are redundant, but not equivalent, in performing an essential function in the cell envelope. Among the AsmA-like paralogs, only the combined loss of YhdP, TamB, and YdbH is lethal, and any of these three proteins is sufficient for growth. We also show that these proteins are required for OM lipid homeostasis and propose that they are the long-sought-after phospholipid transporters that are required for OM biogenesis.

The Transporter-Mediated Cellular Uptake and Efflux of Pharmaceutical Drugs and Biotechnology Products: How and Why Phospholipid Bilayer Transport Is Negligible in Real Biomembranes

Over the years, my colleagues and I have come to realise that the likelihood of pharmaceutical drugs being able to diffuse through whatever unhindered phospholipid bilayer may exist in intact biological membranes in vivo is vanishingly low. This is because (i) most real biomembranes are mostly protein, not lipid, (ii) unlike purely lipid bilayers that can form transient aqueous channels, the high concentrations of proteins serve to stop such activity, (iii) natural evolution long ago selected against transport methods that just let any undesirable products enter a cell, (iv) transporters have now been identified for all kinds of molecules (even water) that were once thought not to require them, (v) many experiments show a massive variation in the uptake of drugs between different cells, tissues, and organisms, that cannot be explained if lipid bilayer transport is significant or if efflux were the only differentiator, and (vi) many experiments that manipulate the expression level of individual transporters as an independent variable demonstrate their role in drug and nutrient uptake (including in cytotoxicity or adverse drug reactions). This makes such transporters valuable both as a means of targeting drugs (not least anti-infectives) to selected cells or tissues and also as drug targets. The same considerations apply to the exploitation of substrate uptake and product efflux transporters in biotechnology. We are also beginning to recognise that transporters are more promiscuous, and antiporter activity is much more widespread, than had been realised, and that such processes are adaptive (i.e., were selected by natural evolution). The purpose of the present review is to summarise the above, and to rehearse and update readers on recent developments. These developments lead us to retain and indeed to strengthen our contention that for transmembrane pharmaceutical drug transport "phospholipid bilayer transport is negligible".

Eugene P. Kennedy's Legacy: Defining Bacterial Phospholipid Pathways and Function

In the 1950's and 1960's Eugene P. Kennedy laid out the blueprint for phospholipid biosynthesis in somatic cells and Escherichia coli, which have been coined the Kennedy Pathways for phospholipid biosynthesis. His research group continued to make seminal contributions in the area of phospholipids until his retirement in the early 1990's. During these years he mentored many young scientists that continued to build on his early discoveries and who also mentored additional scientists that continue to make important contributions in areas related to phospholipids and membrane biogenesis. This review will focus on the initial E. coli Kennedy Pathways and how his early contributions have laid the foundation for our current understanding of bacterial phospholipid genetics, biochemistry and function as carried on by his scientific progeny and others who have been inspired to study microbial phospholipids.

Transport of lipopolysaccharides and phospholipids to the outer membrane

Cells must build and maintain at least one membrane that surrounds essential cellular components and provides structural integrity. Gram-negative bacteria possess an inner membrane, which separates the aqueous cytoplasmic and periplasmic compartments, and an outer membrane, which surrounds the periplasm. The outer membrane is an asymmetric bilayer with phospholipids in its inner leaflet and lipopolysaccharides in its outer leaflet. This structure provides cellular integrity and prevents the entry of many toxic compounds into the cell. Constructing the outer membrane is challenging, since its lipid constituents must be synthesized within the inner membrane, transported across the periplasm, and ultimately assembled into an asymmetric structure. This review highlights major recent advances in our understanding of the mechanism and structure of the intermembrane, multi-protein machine that transports lipopolysaccharide across the cell envelope. Although our understanding of phospholipid transport is very limited, we also provide a brief update on this topic.

Structural insights into outer membrane asymmetry maintenance in Gram-negative bacteria by MlaFEDB

The highly asymmetric outer membrane of Gram-negative bacteria functions in the defense against cytotoxic substances, such as antibiotics. The Mla pathway maintains outer membrane lipid asymmetry by transporting phospholipids between the inner and outer membranes. It comprises six Mla proteins, MlaFEDBCA, including the ABC transporter MlaFEDB, which functions via an unknown mechanism. Here we determine cryo-EM structures of Escherichia coli MlaFEDB in an apo state and bound to phospholipid, ADP or AMP-PNP to a resolution of 3.3-4.1 Å and establish a proteoliposome-based transport system that includes MlaFEDB, MlaC and MlaA-OmpF to monitor the transport direction of phospholipids. In vitro transport assays and in vivo membrane permeability assays combined with mutagenesis identify functional residues that not only recognize and transport phospholipids but also regulate the activity and structural stability of the MlaFEDB complex. Our results provide mechanistic insights into the Mla pathway, which could aid antimicrobial drug development.