Soft Materials that Intercept, Respond to, and Sequester Bacterial Siderophores.

We report the design and characterization of Fe-containing soft materials that respond to, interface with, and/or sequester Fe-chelating 'siderophores' that bacteria use to scavenge for iron and regulate iron homeostasis. We demonstrate that metal-organic network coatings fabricated by crosslinking tannic acid with iron(III) are stable in bacterial growth media, but erode upon exposure to biologically relevant concentrations of enterobactin and deferoxamine B, two siderophores produced by Gram-negative and Gram-positive bacteria, respectively. Our results are consistent with changes in network stability triggered by the extraction of iron(III) and reveal rates of siderophore-induced disassembly to depend upon both siderophore concentration and affinity for iron(III). These coatings also disassemble when incubated in the presence of cultures of wild-type Escherichia coli. Assays using genetically modified strains of E. coli reveal the erosion of these materials by live cultures to be promoted by secretion of enterobactin and not from other factors resulting from bacterial growth and metabolism. This stimuli-responsive behavior can also be exploited to design coatings that release the Fe-chelating antibiotic ciprofloxacin into bacterial cultures. Finally, we report the discovery of Fe-containing polymer hydrogels that avidly sequester and scavenge enterobactin from surrounding media. The materials reported here are (i) capable of interfacing or interfering with mechanisms that bacteria use to maintain iron homeostasis, either by yielding iron to or by sequestering iron-scavenging agents from bacteria, and can (ii) respond dynamically to or report on the presence of populations of iron-scavenging bacteria. Our results thus provide new tools that could prove useful for microbiological research and enable new stimuli-responsive strategies for interfacing with or controlling the behaviors of communities of iron-scavenging bacteria.

Enterococcus NlpC/p60 Peptidoglycan Hydrolase SagA Localizes to Sites of Cell Division and Requires Only a Catalytic Dyad for Protease Activity.

Peptidoglycan is a vital component of the bacterial cell wall, and its dynamic remodeling by NlpC/p60 hydrolases is crucial for proper cell division and survival. Beyond these essential functions, we previously discovered that Enterococcus species express and secrete the NlpC/p60 hydrolase-secreted antigen A (SagA), whose catalytic activity can modulate host immune responses in animal models. However, the localization and peptidoglycan hydrolase activity of SagA in Enterococcus was still unclear. In this study, we show that SagA contributes to a triseptal structure in dividing cells of enterococci and localizes to sites of cell division through its N-terminal coiled-coil domain. Using molecular modeling and site-directed mutagenesis, we identify amino acid residues within the SagA-NlpC/p60 domain that are crucial for catalytic activity and potential substrate binding. Notably, these studies revealed that SagA may function via a catalytic Cys-His dyad instead of the predicted Cys-His-His triad, which is conserved in SagA orthologs from other Enterococcus species. Our results provide key additional insight into peptidoglycan remodeling in Enterococcus by SagA NlpC/p60 hydrolases.

Bacterial Swarming Reduces Proteus mirabilis and Vibrio parahaemolyticus Cell Stiffness and Increases ß-Lactam Susceptibility.

Swarmer cells of the Gram-negative uropathogenic bacteria Proteus mirabilis and Vibrio parahaemolyticus become long (>10 to 100 µm) and multinucleate during their growth and motility on polymer surfaces. We demonstrated that the increasing cell length is accompanied by a large increase in flexibility. Using a microfluidic assay to measure single-cell mechanics, we identified large differences in the swarmer cell stiffness (bending rigidity) of P. mirabilis (5.5 × 10-22 N m2) and V. parahaemolyticus (1.0 × 10-22 N m2) compared to vegetative cells (1.4 × 10-20 N m2 and 2.2 × 10-22 N m2, respectively). The reduction in bending rigidity (∼2-fold to ∼26-fold) was accompanied by a decrease in the average polysaccharide strand length of the peptidoglycan layer of the cell wall from 28 to 30 disaccharides to 19 to 22 disaccharides. Atomic force microscopy revealed a reduction in P. mirabilis peptidoglycan thickness from 1.5 nm (vegetative cells) to 1.0 nm (swarmer cells), and electron cryotomography indicated changes in swarmer cell wall morphology. P. mirabilis and V. parahaemolyticus swarmer cells became increasingly sensitive to osmotic pressure and susceptible to cell wall-modifying antibiotics (compared to vegetative cells)-they were ∼30% more likely to die after 3 h of treatment with MICs of the ß-lactams cephalexin and penicillin G. The adaptive cost of "swarming" was offset by the increase in cell susceptibility to physical and chemical changes in their environment, thereby suggesting the development of new chemotherapies for bacteria that leverage swarming for the colonization of hosts and for survival.IMPORTANCEProteus mirabilis and Vibrio parahaemolyticus are bacteria that infect humans. To adapt to environmental changes, these bacteria alter their cell morphology and move collectively to access new sources of nutrients in a process referred to as "swarming." We found that changes in the composition and thickness of the peptidoglycan layer of the cell wall make swarmer cells of P. mirabilis and V. parahaemolyticus more flexible (i.e., reduce cell stiffness) and that they become more sensitive to osmotic pressure and cell wall-targeting antibiotics (e.g., ß-lactams). These results highlight the importance of assessing the extracellular environment in determining antibiotic doses and the use of ß-lactam antibiotics for treating infections caused by swarmer cells of P. mirabilis and V. parahaemolyticus.

Cardiolipin Alters Rhodobacter sphaeroides Cell Shape by Affecting Peptidoglycan Precursor Biosynthesis.

Cardiolipin (CL) is an anionic phospholipid that plays an important role in regulating protein biochemistry in bacteria and mitochondria. Deleting the CL synthase gene (Δcls) in Rhodobacter sphaeroides depletes CL and decreases cell length by 20%. Using a chemical biology approach, we found that a CL deficiency does not impair the function of the cell wall elongasome in R. sphaeroides; instead, biosynthesis of the peptidoglycan (PG) precursor lipid II is decreased. Treating R. sphaeroides cells with fosfomycin and d-cycloserine inhibits lipid II biosynthesis and creates phenotypes in cell shape, PG composition, and spatial PG assembly that are strikingly similar to those seen with R. sphaeroides Δcls cells, suggesting that CL deficiency alters the elongation of R. sphaeroides cells by reducing lipid II biosynthesis. We found that MurG-a glycosyltransferase that performs the last step of lipid II biosynthesis-interacts with anionic phospholipids in native (i.e., R. sphaeroides) and artificial membranes. Lipid II production decreases 25% in R. sphaeroides Δcls cells compared to wild-type cells, and overexpression of MurG in R. sphaeroides Δcls cells restores their rod shape, indicating that CL deficiency decreases MurG activity and alters cell shape. The R. sphaeroides Δcls mutant is more sensitive than the wild-type strain to antibiotics targeting PG synthesis, including fosfomycin, d-cycloserine, S-(3,4-dichlorobenzyl)isothiourea (A22), mecillinam, and ampicillin, suggesting that CL biosynthesis may be a potential target for combination chemotherapies that block the bacterial cell wall.IMPORTANCE The phospholipid composition of the cell membrane influences the spatial and temporal biochemistry of cells. We studied molecular mechanisms connecting membrane composition to cell morphology in the model bacterium Rhodobacter sphaeroides The peptidoglycan (PG) layer of the cell wall is a dominant component of cell mechanical properties; consequently, it has been an important antibiotic target. We found that the anionic phospholipid cardiolipin (CL) plays a role in determination of the shape of R. sphaeroides cells by affecting PG precursor biosynthesis. Removing CL in R. sphaeroides alters cell morphology and increases its sensitivity to antibiotics targeting proteins synthesizing PG. These studies provide a connection to spatial biochemical control in mitochondria, which contain an inner membrane with topological features in common with R. sphaeroides.

Organization and function of anionic phospholipids in bacteria.

In addition to playing a central role as a permeability barrier for controlling the diffusion of molecules and ions in and out of bacterial cells, phospholipid (PL) membranes regulate the spatial and temporal position and function of membrane proteins that play an essential role in a variety of cellular functions. Based on the very large number of membrane-associated proteins encoded in genomes, an understanding of the role of PLs may be central to understanding bacterial cell biology. This area of microbiology has received considerable attention over the past two decades, and the local enrichment of anionic PLs has emerged as a candidate mechanism for biomolecular organization in bacterial cells. In this review, we summarize the current understanding of anionic PLs in bacteria, including their biosynthesis, subcellular localization, and physiological relevance, discuss evidence and mechanisms for enriching anionic PLs in membranes, and conclude with an assessment of future directions for this area of bacterial biochemistry, biophysics, and cell biology.

A Cardiolipin-Deficient Mutant of Rhodobacter sphaeroides Has an Altered Cell Shape and Is Impaired in Biofilm Formation.

UNLABELLED: Cell shape has been suggested to play an important role in the regulation of bacterial attachment to surfaces and the formation of communities associated with surfaces. We found that a cardiolipin synthase (Δcls) mutant of the rod-shaped bacterium Rhodobacter sphaeroides--in which synthesis of the anionic, highly curved phospholipid cardiolipin (CL) is reduced by 90%--produces ellipsoid-shaped cells that are impaired in biofilm formation. Reducing the concentration of CL did not cause significant defects in R. sphaeroides cell growth, swimming motility, lipopolysaccharide and exopolysaccharide production, surface adhesion protein expression, and membrane permeability. Complementation of the CL-deficient mutant by ectopically expressing CL synthase restored cells to their rod shape and increased biofilm formation. Treating R. sphaeroides cells with a low concentration (10 µg/ml) of the small-molecule MreB inhibitor S-(3,4-dichlorobenzyl)isothiourea produced ellipsoid-shaped cells that had no obvious growth defect yet reduced R. sphaeroides biofilm formation. This study demonstrates that CL plays a role in R. sphaeroides cell shape determination, biofilm formation, and the ability of the bacterium to adapt to its environment. IMPORTANCE: Membrane composition plays a fundamental role in the adaptation of many bacteria to environmental stress. In this study, we build a new connection between the anionic phospholipid cardiolipin (CL) and cellular adaptation in Rhodobacter sphaeroides. We demonstrate that CL plays a role in the regulation of R. sphaeroides morphology and is important for the ability of this bacterium to form biofilms. This study correlates CL concentration, cell shape, and biofilm formation and provides the first example of how membrane composition in bacteria alters cell morphology and influences adaptation. This study also provides insight into the potential of phospholipid biosynthesis as a target for new chemical strategies designed to alter or prevent biofilm formation.

Polar localization of Escherichia coli chemoreceptors requires an intact Tol-Pal complex.

Subcellular biomolecular localization is critical for the metabolic and structural properties of the cell. The functional implications of the spatiotemporal distribution of protein complexes during the bacterial cell cycle have long been acknowledged; however, the molecular mechanisms for generating and maintaining their dynamic localization in bacteria are not completely understood. Here we demonstrate that the trans-envelope Tol-Pal complex, a widely conserved component of the cell envelope of Gram-negative bacteria, is required to maintain the polar positioning of chemoreceptor clusters in Escherichia coli. Localization of the chemoreceptors was independent of phospholipid composition of the membrane and the curvature of the cell wall. Instead, our data indicate that chemoreceptors interact with components of the Tol-Pal complex and that this interaction is required to polarly localize chemoreceptor clusters. We found that disruption of the Tol-Pal complex perturbs the polar localization of chemoreceptors, alters cell motility, and affects chemotaxis. We propose that the E. coli Tol-Pal complex restricts mobility of the chemoreceptor clusters at the cell poles and may be involved in regulatory mechanisms that co-ordinate cell division and segregation of the chemosensory machinery.

Biomolecular recognition with a sensitivity-enhanced nanowire transistor biosensor.

In applications of silicon nanowire field-effect transistors (SiNW-FETs) as biosensors, the SiNW-FETs conventionally are all area modified (AAM), with receptors covering not only the minute SiNW surface area but also the relatively large surrounding substrate area. In this study, using a bottom-up technique, we successfully fabricated selective surface modified (SSM) SiNW-FETs with the receptors on the SiNW sensing surface only. In this approach, the strategy was to modify the SiNWs with a chemical linker of 3-aminopropyltrimethoxysilane (APTMS) prior to photolithographic fabrication of the device. The APTMS molecules modifying the SiNWs survived the harsh photolithographic processes, including coating with photoresist, washing with organic solvent, and thermal annealing. These SSM SiNW-FETs also exhibited desirable electrical characteristics such as ohmic contact and high transconductance. Using the biotin-avidin binding system, we showed that the faster response time and smaller sample requirements of the SSM SiNW-FETs, relative to the conventional AAM SiNW-FETs, clearly show that restricting the surface modification of the SiNW-FETs substantially improves their detection sensitivity. Detection with a SSM boronic acid-modified SiNW-FET of the dopamine released under high-K(+) buffer stimulation from living PC12 cells also demonstrates that SiNW-FETs can serve as highly sensitive biosensors for biomedical diagnosis. In binding affinity measurements with SiNW-FETs, the dissociation constants (Kd) of the biotin-avidin and dopamine-boronic acid complexes were determined to be 15 ± 1 fM and 33 ± 8 fM, respectively.

Improved silicon nanowire field-effect transistors for fast protein-protein interaction screening.

Understanding how proteins interact with each other is the basis for studying the biological mechanisms behind various physiological activities. Silicon nanowire field-effect transistors (SiNW-FETs) are sensitive sensors used to detect biomolecular interactions in real-time. However, the majority of the applications that use SiNW-FETs are for known interactions between different molecules. To explore the capability of SiNW-FETs as fast screening devices to identify unknown interacting molecules, we applied mass spectrometry (MS) to analyze molecules reversibly bound to the SiNW-FETs. Calmodulin (CaM) is a Ca(2+)-sensing protein that is ubiquitously expressed in cells and its interaction with target molecules is Ca(2+)-dependent. By modifying the SiNW-FET surface with glutathione, glutathione S-transferase (GST)-tagged CaM binds reversibly to the SiNW-FET. We first verified the Ca(2+)-dependent interaction between GST-CaM and purified troponin I, which is involved in muscle contraction, through the conductance changes of the SiNW-FET. Furthermore, the cell lysate containing overexpressed Ca(2+)/CaM-dependent protein kinase IIα induced a conductance change in the GST-CaM-modified SiNW-FET. The bound proteins were eluted and subsequently identified by MS as CaM and kinase. In another example, candidate proteins from neuronal cell lysates interacting with calneuron I (CalnI), a CaM-like protein, were captured with a GST-CalnI-modified SiNW-FET. The proteins that interacted with CalnI were eluted and verified by MS. The Ca(2+)-dependent interaction between GST-CalnI and one of the candidates, heat shock protein 70, was re-confirmed via the SiNW-FET measurement. Our results demonstrate the effectiveness of combining MS with SiNW-FETs to quickly screen interacting molecules from cell lysates.

Caveolin-1 regulates γ-secretase-mediated AßPP processing by modulating spatial distribution of γ-secretase in membrane.

Amyloidogenic processing of amyloid-ß precursor protein (AßPP) is associated with cholesterol- and sphingolipid-rich lipid rafts. Caveolin-1, a raft-residing protein, has been implicated in the pathogenesis of Alzheimer's disease. To determine the role of caveolin-1 in governing γ-secretase-mediated AßPP proteolysis, cellular γ-secretase activity was assessed in response to alteration in caveolin-1 expression. We demonstrated that suppression of caveolin-1 expression by RNA interference resulted in a significant increase in γ-secretase-mediated proteolysis of AßPP, generation of amyloid-ß, and cleavage of Notch. Overexpression of caveolin-1 attenuated γ-secretase-mediated proteolysis of AßPP and Notch, substantiating the negative regulation of γ-secretase by caveolin-1. Furthermore, we found that cells deficient in caveolin-1 exhibited significantly increased co-localization of γ-secretase with clathrin-coated non-caveolar endocytic vesicles, demonstrating that the partitioning of γ-secretase between caveolar and non-caveolar membranes can be modulated by caveolin-1. Our data also showed that JNK activation is essential for caveolin-1-mediated regulation of γ-secretase. Together, our results strongly suggest that caveolin-1 is an important regulator of γ-secretase activity.

Direct MinE-membrane interaction contributes to the proper localization of MinDE in E. coli.

Dynamic oscillation of the Min system in Escherichia coli determines the placement of the division plane at the midcell. In addition to stimulating MinD ATPase activity, we report here that MinE can directly interact with the membrane and this interaction contributes to the proper MinDE localization and dynamics. The N-terminal domain of MinE is involved in direct contact between MinE and the membranes that may subsequently be stabilized by the C-terminal domain of MinE. In an in vitro system, MinE caused liposome deformation into membrane tubules, a property similar to that previously reported for MinD. We isolated a mutant MinE containing residue substitutions in R10, K11 and K12 that was fully capable of stimulating MinD ATPase activity, but was deficient in membrane binding. Importantly, this mutant was unable to support normal MinDE localization and oscillation, suggesting that direct MinE interaction with the membrane is critical for the dynamic behavior of the Min system.