How Teichoic Acids Could Support a Periplasm in Gram-Positive Bacteria, and Let Cell Division Cheat Turgor Pressure

Impact of high-speed nanodroplets on various pathogenic bacterial cell walls

Although the development of disinfection technologies with novel mechanisms has stagnated, we demonstrate the bactericidal effects and mechanisms of high-speed nanodroplet generation technology. The first development of this technology in 2017 gushes out a water droplet of 10 nm in size at 50 m/s; however, the target surface does not become completely wet. Nanodroplets were exposed to biofilm models of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Serratia marcescens. This phenomenon was verified when the nanodroplets collide with the surface of the bacteria at an impact pressure of ~75 MPa. S. aureus was exposed to nanodroplets for 30 seconds at 75 MPa, which exploded the bacterial body and completely sterilized. Eighteen MPa damaged the bacterial surface, causing peptidoglycan leakage. S. aureus was repaired and survives in this state. In contrast, in Gram-negative bacteria, nanodroplets with 18 MPa penetrated some biofilm-forming bacteria but did not hit all of them, and the viable count was not significantly reduced. Although all three bacterial species were completely sterilized at 75 MPa, the disinfectant effect was affected by the biomass of the biofilm formed. In summary, our findings prove that nanodroplets at 18 MPa on the bacterial surface were ineffective in killing bacteria, whereas at 75 MPa, all four bacterial species were completely sterilized. The disinfection mechanism involved a high-velocity collision of nanodroplets with the bacteria, physically destroying them. Our results showed that disinfection using this technology could be an innovative method that is completely different from existing disinfection techniques.

IMPORTANCE

Although existing disinfection techniques demonstrate bactericidal effects through chemical reactions, concerns regarding human toxicity and environmental contamination have been raised. To the best of our knowledge, this study is the first in the world to reveal that the use of this technology, with nanodroplets of less than 100 nm, can destroy and sterilize bacterial cells by colliding with biofilm-forming bacteria at 75 MPa. Furthermore, because this technology uses only water, it can solve the problems of human toxicity and environmental contamination caused by existing disinfection techniques. Because of its minimal water usage, it can be employed for sanitation worldwide without being limited to specific regions. Our report proposes an unprecedented physical disinfection approach that utilizes a high-speed nanodroplet generation technology.

Proteolytic activity of surface-exposed HtrA determines its expression level and is needed to survive acidic conditions in Clostridioides difficile

To survive in the host, pathogenic bacteria need to be able to react to the unfavorable conditions that they encounter, like low pH, elevated temperatures, antimicrobial peptides and many more. These conditions may lead to unfolding of envelope proteins and this may be lethal. One of the mechanisms through which bacteria are able to survive these conditions is through the protease/foldase activity of the high temperature requirement A (HtrA) protein. The gut pathogen Clostridioides difficile encodes one HtrA homolog that is predicted to contain a membrane anchor and a single PDZ domain. The function of HtrA in C. difficile is hitherto unknown but previous work has shown that an insertional mutant of htrA displayed elevated toxin levels, less sporulation and decreased binding to target cells. Here, we show that HtrA is membrane associated and localized on the surface of C. difficile and characterize the requirements for proteolytic activity of recombinant soluble HtrA. In addition, we show that the level of HtrA in the bacteria heavily depends on its proteolytic activity. Finally, we show that proteolytic activity of HtrA is required for survival under acidic conditions.

OpgH is an essential regulator of Caulobacter morphology

Bacterial growth and division rely on intricate regulation of morphogenetic complexes to remodel the cell envelope without compromising envelope integrity. Significant progress has been made in recent years towards understanding the regulation of cell wall metabolic enzymes. However, other cell envelope components play a role in morphogenesis as well. A primary factor required to protect envelope integrity in low osmolarity environments is OpgH, the synthase of osmoregulated periplasmic glucans (OPGs). Here, we demonstrate that OpgH is essential in the α-proteobacterium Caulobacter crescentus. Unexpectedly, depletion of OpgH or attempted complementation with a catalytically dead OpgH variant results in striking asymmetric bulging and cell lysis. These shape defects are accompanied by reduced cell wall synthesis and mislocalization of morphogenetic complexes. Interestingly, overactivation of the CenKR two-component system that has been implicated in cell envelope stress homeostasis in α-proteobacteria phenocopies the morphogenetic defects associated with OpgH depletion. Each of these perturbations leads to an increase in the levels of the elongasome protein, MreB, and decreases in the levels of divisome proteins FtsZ and MipZ as well as OpgH, itself. Constitutive production of OpgH during CenKR overactivation prevents cell bulging, but cells still exhibit morphogenetic defects. We propose that OPG depletion activates CenKR, leading to changes in the expression of cell envelope-related genes, but that OPGs also exert CenKR-independent effects on morphogenesis. Our data establish a surprising function for an OpgH homolog in morphogenesis and reveal an essential role of OpgH in maintaining cell morphology in Caulobacter.IMPORTANCEBacteria must synthesize and fortify the cell envelope in a tightly regulated manner to orchestrate growth and adaptation. Osmoregulated periplasmic glucans (OPGs) are important, but poorly understood, constituents of Gram-negative cell envelopes that contribute to envelope integrity and protect against osmotic stress. Here, we determined that the OPG synthase OpgH plays a surprising, essential role in morphogenesis in Caulobacter crescentus. Loss of OpgH causes asymmetric cell bulging and lysis via misregulation of the localization and activity of morphogenetic complexes. Overactivation of the CenKR two-component system involved in envelope homeostasis phenocopies OpgH depletion, suggesting that depletion of OpgH activates CenKR. Because cell envelope integrity is critical for bacterial survival, understanding how OpgH activity contributes to morphogenesis and maintenance of envelope integrity could aid in the development of antibiotic therapies.

The Structure and Function of the Bacterial Osmotically Inducible Protein Y

The ability to adapt to osmotically diverse and fluctuating environments is critical to the survival and resilience of bacteria that colonize the human gut and urinary tract. Environmental stress often provides cross-protection against other challenges and increases antibiotic tolerance of bacteria. Thus, it is critical to understand how E. coli and other microbes survive and adapt to stress conditions. The osmotically inducible protein Y (OsmY) is significantly upregulated in response to hypertonicity. Yet its function remains unknown for decades. We determined the solution structure and dynamics of OsmY by nuclear magnetic resonance spectroscopy, which revealed that the two Bacterial OsmY and Nodulation (BON) domains of the protein are flexibly linked under low- and high-salinity conditions. In-cell solid-state NMR further indicates that there are no gross structural changes in OsmY as a function of osmotic stress. Using cryo-electron and super-resolution fluorescence microscopy, we show that OsmY attenuates plasmolysis-induced structural changes in E. coli and improves the time to growth resumption after osmotic upshift. Structure-guided mutational and functional studies demonstrate that exposed hydrophobic residues in the BON1 domain are critical for the function of OsmY. We find no evidence for membrane interaction of the BON domains of OsmY, contrary to current assumptions. Instead, at high ionic strength, we observe an interaction with the water channel, AqpZ. Thus, OsmY does not play a simple structural role in E. coli but may influence a cascade of osmoregulatory functions of the cell.

Engineering bionanoreactor in bacteria for efficient hydrogen production

Hydrogen production through water splitting is a vital strategy for renewable and sustainable clean energy. In this study, we developed an approach integrating nanomaterial engineering and synthetic biology to establish a bionanoreactor system for efficient hydrogen production. The periplasmic space (20 to 30 nm) of an electroactive bacterium, Shewanella oneidensis MR-1, was engineered to serve as a bionanoreactor to enhance the interaction between electrons and protons, catalyzed by hydrogenases for hydrogen generation. To optimize electron transfer, we used the microbially reduced graphene oxide (rGO) to coat the electrode, which improved the electron transfer from the electrode to the cells. Native MtrCAB protein complex on S. oneidensis and self-assembled iron sulfide (FeS) nanoparticles acted in tandem to facilitate electron transfer from an electrode to the periplasm. To enhance proton transport, S. oneidensis MR-1 was engineered to express Gloeobacter rhodopsin (GR) and the light-harvesting antenna canthaxanthin. This led to efficient proton pumping when exposed to light, resulting in a 35.6% increase in the rate of hydrogen production. The overexpression of native [FeFe]-hydrogenase further improved the hydrogen production rate by 56.8%. The bionanoreactor engineered in S. oneidensis MR-1 achieved a hydrogen yield of 80.4 μmol/mg protein/day with a Faraday efficiency of 80% at a potential of -0.75 V. This periplasmic bionanoreactor combines the strengths of both nanomaterial and biological components, providing an efficient approach for microbial electrosynthesis.

Nitric Oxide-Induced Morphological Changes to Bacteria

Antimicrobial resistance poses a serious threat to global health, necessitating research for alternative approaches to treating infections. Nitric oxide (NO) is an endogenously produced molecule involved in multiple physiological processes, including the response to pathogens. Herein, we employed microscopy- and fluorescence-based techniques to investigate the effects of NO delivered from exogenous NO donors on the bacterial cell envelopes of pathogens, including resistant strains. Our goal was to assess the role of NO donor architecture (small molecules, oligosaccharides, dendrimers) on bacterial wall degradation to representative Gram-negative bacteria (Klebsiella pneumoniae, Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus, Enterococcus faecium) upon treatment. Depending on the NO donor, bactericidal NO doses spanned 1.5-5.5 mM (total NO released). Transmission electron microscopy of bacteria following NO exposure indicated extensive membrane damage to Gram-negative bacteria with warping of the cellular shape and disruption of the cell wall. Among the small-molecule NO donors, those providing a more extended release (t1/2 = 120 min) resulted in greater damage to Gram-negative bacteria. In contrast, rapid NO release (t1/2 = 24 min) altered neither the morphology nor the roughness of these bacteria. For Gram-positive bacteria, NO treatments did not result in any drastic change to cellular shape or membrane integrity, despite permeation of the cell wall as measured by depolarization assays. The use of positively charged quaternary ammonium (QA)-modified NO-releasing dendrimer proved to be the only NO donor system capable of penetrating the thick peptidoglycan layer of Gram-positive bacteria.

Molecular basis of the final step of cell division in Streptococcus pneumoniae

Bacterial cell-wall hydrolases must be tightly regulated during bacterial cell division to prevent aberrant cell lysis and to allow final separation of viable daughter cells. In a multidisciplinary work, we disclose the molecular dialogue between the cell-wall hydrolase LytB, wall teichoic acids, and the eukaryotic-like protein kinase StkP in Streptococcus pneumoniae. After characterizing the peptidoglycan recognition mode by the catalytic domain of LytB, we further demonstrate that LytB possesses a modular organization allowing the specific binding to wall teichoic acids and to the protein kinase StkP. Structural and cellular studies notably reveal that the temporal and spatial localization of LytB is governed by the interaction between specific modules of LytB and the final PASTA domain of StkP. Our data collectively provide a comprehensive understanding of how LytB performs final separation of daughter cells and highlights the regulatory role of eukaryotic-like kinases on lytic machineries in the last step of cell division in streptococci.

Insights into the Roles of Lipoteichoic Acids and MprF in Bacillus subtilis

Gram-positive bacterial cells are protected from the environment by a cell envelope that is comprised of a thick layer of peptidoglycan that maintains cell shape and teichoic acid polymers whose biological function remains unclear. In Bacillus subtilis, the loss of all class A penicillin-binding proteins (aPBPs), which function in peptidoglycan synthesis, is conditionally lethal. Here, we show that this lethality is associated with an alteration of lipoteichoic acids (LTAs) and the accumulation of the major autolysin LytE in the cell wall. Our analysis provides further evidence that the length and abundance of LTAs act to regulate the cellular level and activity of autolytic enzymes, specifically LytE. Importantly, we identify a novel function for the aminoacyl-phosphatidylglycerol synthase MprF in the modulation of LTA biosynthesis in both B. subtilis and Staphylococcus aureus. This finding has implications for our understanding of antimicrobial resistance (particularly to daptomycin) in clinically relevant bacteria and the involvement of MprF in the virulence of pathogens such as methicillin-resistant S. aureus (MRSA). IMPORTANCE In Gram-positive bacteria such as Bacillus subtilis and Staphylococcus aureus, the cell envelope is a structure that protects the cells from the environment but is also dynamic in that it must be modified in a controlled way to allow cell growth. In this study, we show that lipoteichoic acids (LTAs), which are anionic polymers attached to the membrane, have a direct role in modulating the cellular abundance of cell wall-degrading enzymes. We also find that the apparent length of the LTA is modulated by the activity of the enzyme MprF, previously implicated in modifications of the cell membrane leading to resistance to antimicrobial peptides. These findings are important contributions to our understanding of how bacteria balance cell wall synthesis and degradation to permit controlled growth and division. These results also have implications for the interpretation of antibiotic resistance, particularly for the clinical treatment of MRSA infections.

Remodeling of the Enterococcal Cell Envelope during Surface Penetration Promotes Intrinsic Resistance to Stress

Enterococcus faecalis is a normal commensal of the human gastrointestinal tract (GIT). However, upon disruption of gut homeostasis, this nonmotile bacterium can egress from its natural niche and spread to distal organs. While this translocation process can lead to life-threatening systemic infections, the underlying mechanisms remain largely unexplored. Our prior work showed that E. faecalis migration across diverse surfaces requires the formation of matrix-covered multicellular aggregates and the synthesis of exopolysaccharides, but how enterococcal cells are reprogrammed during this process is unknown. Whether surface penetration endows E. faecalis with adaptive advantages is also uncertain. Here, we report that surface penetration promotes the generation of a metabolically and phenotypically distinct E. faecalis population with an enhanced capacity to endure various forms of extracellular stress. Surface-invading enterococci demonstrated major ultrastructural alterations in their cell envelope characterized by increased membrane glycolipid content. These changes were accompanied by marked induction of specific transcriptional programs enhancing cell envelope biogenesis and glycolipid metabolism. Notably, the surface-invading population demonstrated superior tolerance to membrane-damaging antimicrobials, including daptomycin and β-defensins produced by epithelial cells. Genetic mutations impairing glycolipid biosynthesis sensitized E. faecalis to envelope stressors and reduced the ability of this bacterium to penetrate semisolid surfaces and translocate through human intestinal epithelial cell monolayers. Our study reveals that surface penetration induces distinct transcriptional, metabolic, and ultrastructural changes that equip E. faecalis with enhanced capacity to resist external stressors and thrive in its surrounding environment. IMPORTANCE Enterococcus faecalis inhabits the GIT of multiple organisms, where its establishment could be mediated by the formation of biofilm-like aggregates. In susceptible individuals, this bacterium can overgrow and breach intestinal barriers, a process that may lead to lethal systemic infections. While the formation of multicellular aggregates promotes E. faecalis migration across surfaces, little is known about the metabolic and physiological states of the enterococci encased in these surface-penetrating structures. The present study reveals that E. faecalis cells capable of migrating through semisolid surfaces genetically reprogram their metabolism toward increased cell envelope and glycolipid biogenesis, which confers superior tolerance to membrane-damaging agents. E. faecalis's success as a pathobiont depends on its antimicrobial resistance, as well as on its rapid adaptability to overcome multiple environmental challenges. Thus, targeting adaptive genetic and/or metabolic pathways induced during E. faecalis surface penetration may be useful to better confront infections by this bacterium in the clinic.

The Staphylococcus aureus cell division protein, DivIC, interacts with the cell wall and controls its biosynthesis

Bacterial cell division is a complex, dynamic process that requires multiple protein components to orchestrate its progression. Many division proteins are highly conserved across bacterial species alluding to a common, basic mechanism. Central to division is a transmembrane trimeric complex involving DivIB, DivIC and FtsL in Gram-positives. Here, we show a distinct, essential role for DivIC in division and survival of Staphylococcus aureus. DivIC spatially regulates peptidoglycan synthesis, and consequently cell wall architecture, by influencing the recruitment to the division septum of the major peptidoglycan synthetases PBP2 and FtsW. Both the function of DivIC and its recruitment to the division site depend on its extracellular domain, which interacts with the cell wall via binding to wall teichoic acids. DivIC facilitates the spatial and temporal coordination of peptidoglycan synthesis with the developing architecture of the septum during cell division. A better understanding of the cell division mechanisms in S. aureus and other pathogenic microorganisms can provide possibilities for the development of new, more effective treatments for bacterial infections.

Outer membrane vesicles as biomimetic vaccine carriers against infections and cancers

In the last decade, nanoparticle-based therapeutic modalities have emerged as promising treatment options for cancer and infectious diseases. To improve prognosis, chemotherapeutic and antimicrobial drugs must be delivered selectively to the target sites. Researchers have increasingly focused their efforts on improving drug delivery, with a particular emphasis on cancer and infectious diseases. When drugs are administered systemically, they become diluted and can diffuse to all tissues but only until the immune system intervenes and quickly removes them from circulation. To enhance and prolong the systemic circulation of drugs, nanocarriers have been explored and used; however, nanocarriers have a major drawback in that they can trigger immune responses. Numerous nanocarriers for optimal drug delivery have been developed using innovative and effective biointerface technologies. Autologous cell-derived drug carriers, such as outer membrane vesicles (OMVs), have demonstrated improved bioavailability and reduced toxicity. Thus, this study investigates the use of biomimetic OMVs as biomimetic vaccine carriers against infections and cancers to improve our understanding in the field of nanotechnology. In addition, discussion on the advantages, disadvantages, and future prospects of OMVs will also be explored. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Understanding Beta-Lactam-Induced Lysis at the Single-Cell Level

Mechanical rupture, or lysis, of the cytoplasmic membrane is a common cell death pathway in bacteria occurring in response to β-lactam antibiotics. A better understanding of the cellular design principles governing the susceptibility and response of individual cells to lysis could indicate methods of potentiating β-lactam antibiotics and clarify relevant aspects of cellular physiology. Here, we take a single-cell approach to bacterial cell lysis to examine three cellular features—turgor pressure, mechanosensitive channels, and cell shape changes—that are expected to modulate lysis. We develop a mechanical model of bacterial cell lysis and experimentally analyze the dynamics of lysis in hundreds of single Escherichia coli cells. We find that turgor pressure is the only factor, of these three cellular features, which robustly modulates lysis. We show that mechanosensitive channels do not modulate lysis due to insufficiently fast solute outflow, and that cell shape changes result in more severe cellular lesions but do not influence the dynamics of lysis. These results inform a single-cell view of bacterial cell lysis and underscore approaches of combatting antibiotic tolerance to β-lactams aimed at targeting cellular turgor.