The Exocytoskeleton

Mirubactin C rescues the lethal effect of cell wall biosynthesis mutations in Bacillus subtilis

Growth of most rod-shaped bacteria is accompanied by the insertion of new peptidoglycan into the cylindrical cell wall. This insertion, which helps maintain and determine the shape of the cell, is guided by a protein machine called the rod complex or elongasome. Although most of the proteins in this complex are essential under normal growth conditions, cell viability can be rescued, for reasons that are not understood, by the presence of a high (mM) Mg2+ concentration. We screened for natural product compounds that could rescue the growth of mutants affected in rod-complex function. By screening > 2,000 extracts from a diverse collection of actinobacteria, we identified a compound, mirubactin C, related to the known iron siderophore mirubactin A, which rescued growth in the low micromolar range, and this activity was confirmed using synthetic mirubactin C. The compound also displayed toxicity at higher concentrations, and this effect appears related to iron homeostasis. However, several lines of evidence suggest that the mirubactin C rescuing activity is not due simply to iron sequestration. The results support an emerging view that the functions of bacterial siderophores extend well beyond simply iron binding and uptake.

A mechanistic perspective on targeting bacterial drug resistance with nanoparticles

Bacterial infections are an important cause of mortality worldwide owing to the prevalence of drug resistant bacteria. Bacteria develop resistance against antimicrobial drugs by several mechanisms such as enzyme inactivation, reduced cell permeability, modifying target site or enzyme, enhanced efflux because of high expression of efflux pumps, biofilm formation or drug-resistance gene expression. New and alternative ways such as nanoparticle (NP) applications are being established to overcome the growing multidrug-resistance in bacteria. NPs have unique antimicrobial characteristics that make them appropriate for medical application to overcome antibiotic resistance. The proposed antibacterial mechanisms of NPs are cell membrane damage, changing cell wall penetration, reactive oxygen species (ROS) production, effect on DNA and proteins, and impact on biofilm formation. The present review mainly focuses on discussing various mechanisms of bacterial drug resistance and the applications of NPs as alternative antibacterial systems. Combination therapy of NPs and antibiotics as a novel approach in medicine towards antimicrobial resistance is also discussed.

Unusual features and molecular pathways of Staphylococcus aureus L-form bacteria

Staphylococcus aureus can be converted to cell wall-deficient L-form bacteria in specific environment which is associated with recurrent and persistent infections. The biophysical properties and molecular basis involved in S. aureus L-form formation are poorly understood. Here, S. aureus unstable L-form model was established not only in Newman strain, but also in ATCC 25923 and five different antibiotic-resistant clinical strains, and the morphology and mechanical properties of Newman strain L-forms were characterized by using atomic force microscopy. Meanwhile, zeta potential, growth and proliferation properties, and hemolysis of L-forms were determined. Gene expression changes involved in transition from S. aureus wild type into L-forms were identified. Our studies showed that L-form S. aureus presented pleomorphism, rough surface, and higher elasticity modulus. L-forms were characterized by less surface charge and had higher hemolysis than the walled form. The S. aureus L-form "fried egg" colony was derived from a single bacterium rather than from aggregation of different bacterial cells. Transcriptomics analysis revealed that several pathways involved in energy metabolism, stress response, protein synthesis, RNA metabolism, and virulence were involved in L-form formation in S. aureus. Our results shed new light on the biological properties and mechanisms underlying L-form formation in S. aureus. These findings will not only be useful for understanding the unique properties and mechanisms of L-form bacteria, but also provide therapeutic targets for developing more effective treatments for S. aureus L-forms.

L-form bacteria, chronic diseases and the origins of life

The peptidoglycan cell wall is widely conserved across the bacterial domain, suggesting that it appeared early in the evolution of bacteria. It is normally essential but under certain conditions wall-deficient or ‘L-form’ bacteria can be isolated. In Bacillus subtilis this normally requires two genetic changes. The first, exemplified by mutations shutting down wall precursor synthesis, works by increasing membrane synthesis. This promotes the unusual form of proliferation used by L-forms, involving a range of relatively disorganized membrane blebbing or vesiculation events. The secondary class of mutations probably work by relieving oxidative stress that L-forms may incur due to their unbalanced metabolism. Repression or inhibition of cell wall precursor synthesis can stimulate the L-form transition in a wide range of bacteria, of both Gram-positive and -negative lineages. L-forms are completely resistant to most antibiotics working specifically on cell wall synthesis, such as penicillins and cephalosporins, consistent with the many reports of their involvement in various chronic diseases. They are potentially important in biotechnology, because lack of a wall can be advantageous in a range of production or strain improvement applications. Finally, L-forms provide an interesting model system for studying early steps in the evolution of cellular life.

This article is part of the themed issue ‘The new bacteriology’.

Variations in the Nanomechanical Properties of Virulent and Avirulent Listeria monocytogenes

Atomic force microscopy (AFM) was used to quantify both the nanomechanical properties of pathogenic (ATCC 51776 & EGDe) and non-pathogenic (ATCC 15313 & HCC25) Listeria monocytogenes strains and the conformational properties of their surface biopolymers. The nanomechanical properties of the various L. monocytogenes strains were quantified in terms of Young's moduli of cells. To estimate Young's moduli, the classic Hertz model of contact mechanics and a modified version of it that takes into account substrate effects were used to fit the AFM nanoindentation-force measurements collected while pushing onto the bacterial surface biopolymer brush. When compared, the classic Hertz model always predicted higher Young's moduli values of bacterial cell elasticity compared to the modified Hertz model. On average, the modified Hertz model showed that virulent strains are approximately twice as rigid (88.1 ± 14.5 KPa) as the avirulent strains (47.3 ± 7.6 kPa). To quantify the conformational properties of L. monocytogenes' strains surface biopolymers, two models were used. First, the entropic-based, statistical mechanical, random walk formulation, the wormlike chain (WLC) model was used to estimate the elastic properties of the bacterial surface molecules. The WLC model results indicated that the virulent strains are characterized by a more flexible surface biopolymers as indicated by shorter persistence lengths (L(p) = 0.21 ± 0.08 nm) compared to the avirulent strains (L(p) = 0.24 ± 0.14 nm). Second, a steric model developed to describe the repulsive forces measured between the AFM tip and bacterial surface biopolymers indicated that the virulent strains are characterized by crowded and longer biopolymer brushes compared to those of the avirulent strains. Finally, scaling relationships developed for grafted polyelectrolyte brushes indicated L. monocytogenes strains' biopolymer brushes are charged. Collectively, our data indicate that the conformational properties of the bacterial surface biopolymers and their surface densities play an important role in controlling the overall bacterial cell elasticity.

Distinct and essential morphogenic functions for wall- and lipo-teichoic acids in Bacillus subtilis

Teichoic acids (TAs) are anionic polymers that constitute a major component of the cell wall in most Gram-positive bacteria. Despite decades of study, their function has remained unclear. TAs are covalently linked either to the cell wall peptidoglycan (wall TA (WTA)) or to the membrane (lipo-TA (LTA)). We have characterized the key enzyme of LTA synthesis in Bacillus subtilis, LTA synthase (LtaS). We show that LTA is needed for divalent cation homoeostasis and that its absence has severe effects on cell morphogenesis and cell division. Inactivation of both LTA and WTA is lethal and comparison of the individual mutants suggests that they have differentiated roles in elongation (WTA) and division (LTA). B. subtilis has four ltaS paralogues and we show how their roles are partially differentiated. Two paralogues have a redundant role in LTA synthesis during sporulation and their absence gives a novel absolute block in sporulation. The crystal structure of the extracytoplasmic part of LtaS, solved at 2.4-A resolution, reveals a phosphorylated threonine residue, which provides clues about the catalytic mechanism and identifies the active site of the enzyme.

Structural co-evolution of viruses and cells in the primordial world

Viruses and cells co-evolve due to the parasitic nature of viruses. Yet there are no models suggesting how the unicellular organisms and their viruses might co-evolve structurally. Here, in this study, we plunge into this unexplored field from a wide perspective and try to describe some of the intriguing ways in which viruses may have shaped the cellular life forms on the ancient Earth. At first we propose a scenario where viruses act as a driving force in the emergence of bacterial cell walls by providing favorable intermediates for the otherwise improbable steps in the cell wall generation. We also discuss the role of viruses in the evolution of cell surface components such as receptors and second membranes. Finally we focus on hypothetical proto-viruses, the selfish abusers of the RNA-world, in explaining some of the very early stages in the origin and evolution of life. Proto-viruses may be responsible for creating the first true cells in order to support their selfish needs. In this model we also suggest a logical pathway to explaining the emergence of modern viruses.

Intracellular cytoskeletal elements and cytoskeletons in bacteria

Within a short period of time after the discovery of bacterial cytoskletons, major progress had been made in areas such as general spatial layout of cytoskeletons, their involvement in a variety of cellfunctions (shape control, cell division, chromosome segregation, cell motility). This progress was achieved by application of advanced investigation techniques. Homologs of eukaryotic actin, tubulin, and intermediate filaments were found in bacteria; cytoskeletal proteins not closely or not at all related to any of these major cytoskeletal proteins were discovered in a number of bacteria such as Mycoplasmas, Spiroplasmas, Spirochetes, Treponema, Caulobacter. A structural role for bacterial elongation factor Tu was indicated. On the basis of this new thinking, new approaches in biotechnology and new drugs are on the way.