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

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.