Penicillin-binding protein 2 inactivation in Escherichia coli results in cell division inhibition, which is relieved by FtsZ overexpression

Bacterial cell division: Orthogonal rotation is a convergent strategy

Many rod-shaped bacteria divide at a fixed angle perpendicular to their long axis, while coccoid bacteria like Staphylococcus aureus rotate planes between generations. New research highlights the relationship between subtle asymmetry and division rotation in the understudied coccoid pathogen Neisseria gonorrhoeae.

Production of Natural Penicillin-Binding Protein 2 and Development of a Signal-Amplified Fluorescence Polarization Assay for the Determination of 28 Beta-Lactam Antibiotics in Milk

In this study, two magnetic activity-based protein profiling probes based on cephradine and amoxicillin were first synthesized that were used to produce the natural penicillin-binding protein 2 of Escherichia coli. After characterization by using LC-ESI-MS/MS, it was found that the obtained proteins by using the two probes were the same. The molecular docking for 28 beta-lactam antibiotics showed that the key amino acids were Ser330 and Ser387, the main intermolecular forces were hydrogen bond and hydrophobic interaction, and the main binding sites in their molecules were on the beta-lactam ring. Then this protein was combined with streptavidin-labeled tracer and biotinylated fluorescein isothiocyanate to establish a signal-amplified fluorescence polarization assay to determine the 28 drugs in milk. The limits of detection ranged from 0.07 to 2.21 ng/mL, and the sensitivities for the 28 drugs were improved 4 to 48-fold in comparison with the use of a fluorescein isothiocyanate-labeled fluorescent tracer. Therefore, this method could be used for rapid multiscreening of the 28 beta-lactam antibiotics in milk.

Septal wall synthesis is sufficient to change ameba-like cells into uniform oval-shaped cells in Escherichia coli L-forms

A cell wall is required to control cell shape and size to maintain growth and division. However, some bacterial species maintain their morphology and size without a cell wall, calling into question the importance of the cell wall to maintain shape and size. It has been very difficult to examine the dispensability of cell wall synthesis in rod-shaped bacteria such as Escherichia coli for maintenance of their shape and size because they lyse without cell walls under normal culture conditions. Here, we show that wall-less E. coli L-form cells, which have a heterogeneous cell morphology, can be converted to a mostly uniform oval shape solely by FtsZ-dependent division, even in the absence of cylindrical cell wall synthesis. This FtsZ-dependent control of cell shape and size in the absence of a cell wall requires at least either the Min or nucleoid occlusion systems for positioning FtsZ at mid cell division sites.

Overexpression of l,d-Transpeptidase A Induces Dispensability of Rod Complex in Escherichia coli

Antimicrobial resistance (AMR) is a significant global threat, and the presence of resistance-determinant genes is one of the major driving forces behind it. The bacterial rod complex is an essential set of proteins that is crucial for cell survival due to its role in cell wall biogenesis and shape maintenance. Therefore, these proteins offer excellent potential as drug targets; however, compensatory mutations in nontarget genes render this complex nonessential. The MreB protein of this complex is an actin homologue that rotates along the longitudinal axis of the cell to provide rod shape to the bacteria. In this study, using chemical-chemical interaction profiling and FtsZ suppression assay, we identified the MreB targeting activity of IITR07865, a previously discovered small molecule in our lab. Escherichia coli suppressors against IITR07865 revealed mutations in two cell division-associated genes, min C and pal, that have not been previously implicated in rod complex essentiality. IITR07865 resistant mutants were found to inactivate and render the rod complex nonessential, making the rod complex inhibitors ineffective. Further, through transcriptome analysis, we reveal the primary cause of resistance in suppressor strains to be the overexpression of an l, d-transpeptidase A enzyme, which is involved in peptidoglycan and Braun's lipoprotein cross-linking. Our results demonstrate a novel mechanism of resistance development in rod-shaped Gram-negative bacterial pathogen E. coli involved in UTIs where mecillinam, a clinically used antibiotic that targets rod complex, is a drug of choice.

The global RNA-RNA interactome of Klebsiella pneumoniae unveils a small RNA regulator of cell division

The ubiquitous RNA chaperone Hfq is involved in the regulation of key biological processes in many species across the bacterial kingdom. In the opportunistic human pathogen Klebsiella pneumoniae, deletion of the hfq gene affects the global transcriptome, virulence, and stress resistance; however, the ligands of the major RNA-binding protein in this species have remained elusive. In this study, we have combined transcriptomic, co-immunoprecipitation, and global RNA interactome analyses to compile an inventory of conserved and species-specific RNAs bound by Hfq and to monitor Hfq-mediated RNA-RNA interactions. In addition to dozens of RNA-RNA pairs, our study revealed an Hfq-dependent small regulatory RNA (sRNA), DinR, which is processed from the 3' terminal portion of dinI mRNA. Transcription of dinI is controlled by the master regulator of the SOS response, LexA. As DinR accumulates in K. pneumoniae in response to DNA damage, the sRNA represses translation of the ftsZ transcript by occupation of the ribosome binding site. Ectopic overexpression of DinR causes depletion of ftsZ mRNA and inhibition of cell division, while deletion of dinR antagonizes cell elongation in the presence of DNA damage. Collectively, our work highlights the important role of RNA-based gene regulation in K. pneumoniae and uncovers the central role of DinR in LexA-controlled division inhibition during the SOS response.

Genome-wide identification of genes required for alternative peptidoglycan cross-linking in Escherichia coli revealed unexpected impacts of β-lactams

The D,D-transpeptidase activity of penicillin-binding proteins (PBPs) is the well-known primary target of β-lactam antibiotics that block peptidoglycan polymerization. β-lactam-induced bacterial killing involves complex downstream responses whose causes and consequences are difficult to resolve. Here, we use the functional replacement of PBPs by a β-lactam-insensitive L,D-transpeptidase to identify genes essential to mitigate the effects of PBP inactivation by β-lactams in actively dividing bacteria. The functions of the 179 conditionally essential genes identified by this approach extend far beyond L,D-transpeptidase partners for peptidoglycan polymerization to include proteins involved in stress response and in the assembly of outer membrane polymers. The unsuspected effects of β-lactams include loss of the lipoprotein-mediated covalent bond that links the outer membrane to the peptidoglycan, destabilization of the cell envelope in spite of effective peptidoglycan cross-linking, and increased permeability of the outer membrane. The latter effect indicates that the mode of action of β-lactams involves self-promoted penetration through the outer membrane.

Increased energy demand from anabolic-catabolic processes drives β-lactam antibiotic lethality

β-Lactam antibiotics disrupt the assembly of peptidoglycan (PG) within the bacterial cell wall by inhibiting the enzymatic activity of penicillin-binding proteins (PBPs). It was recently shown that β-lactam treatment initializes a futile cycle of PG synthesis and degradation, highlighting major gaps in our understanding of the lethal effects of PBP inhibition by β-lactam antibiotics. Here, we assess the downstream metabolic consequences of treatment of Escherichia coli with the β-lactam mecillinam and show that lethality from PBP2 inhibition is a specific consequence of toxic metabolic shifts induced by energy demand from multiple catabolic and anabolic processes, including accelerated protein synthesis downstream of PG futile cycling. Resource allocation into these processes is coincident with alterations in ATP synthesis and utilization, as well as a broadly dysregulated cellular redox environment. These results indicate that the disruption of normal anabolic-catabolic homeostasis by PBP inhibition is an essential factor for β-lactam antibiotic lethality.

Disruption of the MreB Elongasome Is Overcome by Mutations in the Tricarboxylic Acid Cycle

The bacterial actin homolog, MreB, is highly conserved among rod-shaped bacteria and essential for growth under normal growth conditions. MreB directs the localization of cell wall synthesis and loss of MreB results in round cells and death. Using the MreB depolymerizing drug, A22, we show that changes to central metabolism through deletion of malate dehydrogenase from the tricarboxylic acid (TCA) cycle results in cells with an increased tolerance to A22. We hypothesize that deletion of malate dehydrogenase leads to the upregulation of gluconeogenesis resulting in an increase in cell wall precursors. Consistent with this idea, metabolite analysis revealed that malate dehydrogenase (mdh) deletion cells possess elevated levels of several glycolysis/gluconeogenesis compounds and the cell wall precursor, uridine diphosphate N-acetylglucosamine (UDP-NAG). In agreement with these results, the increased A22 resistance phenotype can be recapitulated through the addition of glucose to the media. Finally, we show that this increase in antibiotic tolerance is not specific to A22 but also applies to the cell wall-targeting antibiotic, mecillinam.

Genomic deletions in Rhodococcus based on transformation of linear heterologous DNA

Several genome engineering methods have been developed for Rhodococcus . However, they suffer from limitations such as extensive cloning, multiple steps, successful expression of heterologous genes via plasmid etc. Here, we report a rapid method for performing genomic deletions/disruptions in Rhodococcus spp. using heterologous linear DNA. The method is cost effective and less labour intensive. The applicability of the method was demonstrated by successful disruption of rodA and orphan parA. None of the disrupted genes were found to be essential for the viability of the cell. Disruption of orphan parA and rodA resulted in elongated cells and short rods, respectively. This is the first report demonstrating disruption of rodA and orphan parA genes by electroporation of heterologous linear DNA in Rhodococcus spp.

Regulation and function of class A Penicillin-binding proteins

Most bacteria surround their cell membrane with a peptidoglycan sacculus that counteracts the turgor and maintains the shape of the cell. Class A PBPs are bi-functional glycosyltransferase-transpeptidases that polymerize glycan chains and cross-link peptides. They have a major contribution to the total peptidoglycan synthesized during cell growth and cell division. In recent years it became apparent that class A PBPs participate in multiple protein? protein interactions and that some of these regulate their activities. In this opinion article, we review and discuss the role of class A PBPs in peptidoglycan growth and repair. We hypothesize that class A PBP function is essential in walled bacteria unless they have (a) SEDS protein(s) capable of replacing their function.

Avibactam Sensitizes Carbapenem-Resistant NDM-1-Producing Klebsiella pneumoniae to Innate Immune Clearance

Infections caused by New Delhi metallo-β-lactamase (NDM)–producing strains of multidrug-resistant Klebsiella pneumoniae are a global public health threat lacking reliable therapies. NDM is impervious to all existing β-lactamase inhibitor (BLI) drugs, including the non–β-lactam BLI avibactam (AVI). Though lacking direct activity against NDMs, AVI can interact with penicillin-binding protein 2 in a manner that may influence cell wall dynamics. We found that exposure of NDM-1–producing K. pneumoniae to AVI led to striking bactericidal interactions with human cathelicidin antimicrobial peptide LL-37, a frontline component of host innate immunity. Moreover, AVI markedly sensitized NDM-1–producing K. pneumoniae to killing by freshly isolated human neutrophils, platelets, and serum when complement was active. Finally, AVI monotherapy reduced lung counts of NDM-1–producing K. pneumoniae in a murine pulmonary challenge model. AVI sensitizes NDM-1–producing K. pneumoniae to innate immune clearance in ways that are not appreciated by standard antibiotic testing and that merit further study.

Upregulation of PBP1B and LpoB in cysB Mutants Confers Mecillinam (Amdinocillin) Resistance in Escherichia coli

Mecillinam (amdinocillin) is a β-lactam antibiotic that inhibits the essential penicillin-binding protein 2 (PBP2). In clinical isolates of Escherichia coli from urinary tract infections, inactivation of the cysB gene (which encodes the main regulator of cysteine biosynthesis, CysB) is the major cause of resistance. How a nonfunctional CysB protein confers resistance is unknown, however, and in this study we wanted to examine the mechanism of resistance.

Mecillinam (amdinocillin) is a β-lactam antibiotic that inhibits the essential penicillin-binding protein 2 (PBP2). In clinical isolates of Escherichia coli from urinary tract infections, inactivation of the cysB gene (which encodes the main regulator of cysteine biosynthesis, CysB) is the major cause of resistance. How a nonfunctional CysB protein confers resistance is unknown, however, and in this study we wanted to examine the mechanism of resistance. Results show that cysB mutations cause a gene regulatory response that changes the expression of ∼450 genes. Among the proteins that show increased levels are the PBP1B, LpoB, and FtsZ proteins, which are known to be involved in peptidoglycan biosynthesis. Artificial overexpression of either PBP1B or LpoB in a wild-type E. coli strain conferred mecillinam resistance; conversely, inactivation of either the mrcB gene (which encodes PBP1B) or the lpoB gene (which encodes the PBP1B activator LpoB) made cysB mutants susceptible. These results show that expression of the proteins PBP1B and LpoB is both necessary and sufficient to confer mecillinam resistance. The addition of reducing agents to a cysB mutant converted it to full susceptibility, with associated downregulation of PBP1B, LpoB, and FtsZ. We propose a model in which cysB mutants confer mecillinam resistance by inducing a response that causes upregulation of the PBP1B and LpoB proteins. The higher levels of these two proteins can then rescue cells with mecillinam-inhibited PBP2. Our results also show how resistance can be modulated by external conditions such as reducing agents.

Essential gene deletions producing gigantic bacteria

To characterize the consequences of eliminating essential functions needed for peptidoglycan synthesis, we generated deletion mutations of Acinetobacter baylyi by natural transformation and visualized the resulting microcolonies of dead cells. We found that loss of genes required for peptidoglycan precursor synthesis or polymerization led to the formation of polymorphic giant cells with diameters that could exceed ten times normal. Treatment with antibiotics targeting early or late steps of peptidoglycan synthesis also produced giant cells. The giant cells eventually lysed, although they were partially stabilized by osmotic protection. Genome-scale transposon mutant screening (Tn-seq) identified mutations that blocked or accelerated giant cell formation. Among the mutations that blocked the process were those inactivating a function predicted to cleave murein glycan chains (the MltD murein lytic transglycosylase), suggesting that giant cell formation requires MltD hydrolysis of existing peptidoglycan. Among the mutations that accelerated giant cell formation after ß-lactam treatment were those inactivating an enzyme that produces unusual 3->3 peptide cross-links in peptidoglycan (the LdtG L,D-transpeptidase). The mutations may weaken the sacculus and make it more vulnerable to further disruption. Although the study focused on A. baylyi, we found that a pathogenic relative (A. baumannii) also produced giant cells with genetic dependencies overlapping those of A. baylyi. Overall, the analysis defines a genetic pathway for giant cell formation conserved in Acinetobacter species in which independent initiating branches converge to create the unusual cells.

Although essential genes control the most basic functions of bacterial life, they are difficult to study genetically because mutants lacking the functions die. We have developed a simple procedure for creating bacteria in which different essential genes have been completely deleted, making it possible to analyze the roles of the missing functions based on the features of the dead cells that result. When genes needed for the production of the cell wall were inactivated, the bacteria formed bizarre giant cells. It was possible to identify the functions responsible for forming the giant cells, and to formulate a model for how they form. Since cell wall synthesis is one of the most important antibiotic targets, understanding how bacteria respond to its disruption may ultimately help in developing procedures to overcome antibiotic resistant bacterial infections.

Peptidoglycan

The peptidoglycan sacculus is a net-like polymer that surrounds the cytoplasmic membrane in most bacteria. It is essential to maintain the bacterial cell shape and protect from turgor. The peptidoglycan has a basic composition, common to all bacteria, with species-specific variations that can modify its biophysical properties or the pathogenicity of the bacteria. The synthesis of peptidoglycan starts in the cytoplasm and the precursor lipid II is flipped across the cytoplasmic membrane. The new peptidoglycan strands are synthesised and incorporated into the pre-existing sacculus by the coordinated activities of peptidoglycan synthases and hydrolases. In the model organism Escherichia coli there are two complexes required for the elongation and division. Each of them is regulated by different proteins from both the cytoplasmic and periplasmic sides that ensure the well-coordinated synthesis of new peptidoglycan.

Comparative proteomics analysis of Neisseria gonorrhoeae strains in response to extended-spectrum cephalosporins

Neisseria gonorrhoeae strains displaying reduced susceptibility and resistance to extended-spectrum cephalosporins (ESCs) are major public health concerns. Although resistance mechanisms of ESCs have extensively been studied, the proteome-wide investigation on the biological response to the antibiotic stress is still limited. Herein, a proteomics approach based on two-dimensional gel electrophoresis and MALDI-TOF/TOF-MS analysis was applied to investigate the global protein expression under ESC stresses of ESC-susceptible and ESC-reduced susceptible N. gonorrhoeae strains. Upon exposure to ceftriaxone, 14 and 21 proteins of ESC-susceptible and ESC-reduced susceptible strains, respectively, were shown to be differentially expressed. In the meanwhile, differential expressions of 13 and 17 proteins were detected under cefixime stress for ESC-susceptible and ESC-reduced susceptible strains, respectively. ESC antibiotics have been proven to trigger the expression of several proteins implicated in a variety of biological functions including transport system, energy metabolism, stress response and pathogenic virulence factors. Interestingly, macrophage infectivity potentiators (Ng-MIP) showed increased expression for ESC-reduced susceptible strain under ESC stress. The altered expression of Ng-MIP was found to be a unique response to ESC stresses. Our finding proposes a broad view on proteomic changes in N. gonorrhoeae in response to ESC antibiotics that provides further insights into the gonococcal antimicrobial resistance and physiological adaptation mechanism.

Absence of the KhpA and KhpB (JAG/EloR) RNA-binding proteins suppresses the requirement for PBP2b by overproduction of FtsA in Streptococcus pneumoniae D39

Suppressor mutations were isolated that obviate the requirement for essential PBP2b in peripheral elongation of peptidoglycan from the midcells of dividing Streptococcus pneumoniae D39 background cells. One suppressor was in a gene encoding a single KH-domain protein (KhpA). ΔkhpA suppresses deletions in most, but not all (mltG), genes involved in peripheral PG synthesis and in the gpsB regulatory gene. ΔkhpA mutations reduce growth rate, decrease cell size, minimally affect shape and induce expression of the WalRK cell-wall stress regulon. Reciprocal co-immunoprecipitations show that KhpA forms a complex in cells with another KH-domain protein (KhpB/JAG/EloR). ΔkhpA and ΔkhpB mutants phenocopy each other exactly, consistent with a direct interaction. RNA-immunoprecipitation showed that KhpA/KhpB bind an overlapping set of RNAs in cells. Phosphorylation of KhpB reported previously does not affect KhpB function in the D39 progenitor background. A chromosome duplication implicated FtsA overproduction in Δpbp2b suppression. We show that cellular FtsA concentration is negatively regulated by KhpA/B at the post-transcriptional level and that FtsA overproduction is necessary and sufficient for suppression of Δpbp2b. However, increased FtsA only partially accounts for the phenotypes of ΔkhpA mutants. Together, these results suggest that multimeric KhpA/B may function as a pleiotropic RNA chaperone controlling pneumococcal cell division.

Evolution of Antibiotic Resistance without Antibiotic Exposure

Antibiotic use is the main driver in the emergence of antibiotic resistance. Another unexplored possibility is that resistance evolves coincidentally in response to other selective pressures. We show that selection in the absence of antibiotics can coselect for decreased susceptibility to several antibiotics. Thus, genetic adaptation of bacteria to natural environments may drive resistance evolution by generating a pool of resistance mutations that selection could act on to enrich resistant mutants when antibiotic exposure occurs.

The mecillinam resistome reveals a role for peptidoglycan endopeptidases in stimulating cell wall synthesis in Escherichia coli

Bacterial cells are typically surrounded by an net-like macromolecule called the cell wall constructed from the heteropolymer peptidoglycan (PG). Biogenesis of this matrix is the target of penicillin and related beta-lactams. These drugs inhibit the transpeptidase activity of PG synthases called penicillin-binding proteins (PBPs), preventing the crosslinking of nascent wall material into the existing network. The beta-lactam mecillinam specifically targets the PBP2 enzyme in the cell elongation machinery of Escherichia coli. Low-throughput selections for mecillinam resistance have historically been useful in defining mechanisms involved in cell wall biogenesis and the killing activity of beta-lactam antibiotics. Here, we used transposon-sequencing (Tn-Seq) as a high-throughput method to identify nearly all mecillinam resistance loci in the E. coli genome, providing a comprehensive resource for uncovering new mechanisms underlying PG assembly and drug resistance. Induction of the stringent response or the Rcs envelope stress response has been previously implicated in mecillinam resistance. We therefore also performed the Tn-Seq analysis in mutants defective for these responses in addition to wild-type cells. Thus, the utility of the dataset was greatly enhanced by determining the stress response dependence of each resistance locus in the resistome. Reasoning that stress response-independent resistance loci are those most likely to identify direct modulators of cell wall biogenesis, we focused our downstream analysis on this subset of the resistome. Characterization of one of these alleles led to the surprising discovery that the overproduction of endopeptidase enzymes that cleave crosslinks in the cell wall promotes mecillinam resistance by stimulating PG synthesis by a subset of PBPs. Our analysis of this activation mechanism suggests that, contrary to the prevailing view in the field, PG synthases and PG cleaving enzymes need not function in multi-enzyme complexes to expand the cell wall matrix.

Penicillin and related beta-lactams are one of our oldest and most effective classes of antibiotics. These drugs target enzymes called penicillin-binding proteins (PBPs) that build the essential cell wall that surrounds bacterial cells. Beta-lactams have long been used as chemical and genetic probes to uncover the mechanisms required for proper bacterial cell wall biogenesis. In this report, we use a high-throughput genetic approach to comprehensively identify nearly all genetic loci that promote resistance to the beta-lactam mecillinam in the model organism Escherichia coli. Moreover, by performing our analysis in several different genetic backgrounds we were able to generate a rich resource that defines those alleles that promote resistance by inducing a stress response and those that are more likely to do so by directly modulating cell wall synthesis. Further characterization of one of the stress response-independent resistance loci helped us discover that enzymes that cleave crosslinks in the cell wall are capable of activating cell wall synthesis by a subset of PBPs. Our analysis of the activation mechanism challenges the prevailing view in the field that cell wall synthases and cell wall cleaving enzymes must work in multi-enzyme complexes to assemble the cell wall.

Metabolism Shapes the Cell

More than 5 decades of work support the idea that cell envelope synthesis, including the inward growth of cell division, is tightly coordinated with DNA replication and protein synthesis through central metabolism. Remarkably, no unifying model exists to account for how these fundamentally disparate processes are functionally coupled. Recent studies demonstrate that proteins involved in carbohydrate and nitrogen metabolism can moonlight as direct regulators of cell division, coordinate cell division and DNA replication, and even suppress defects in DNA replication. In this minireview, we focus on studies illustrating the intimate link between metabolism and regulation of peptidoglycan (PG) synthesis during growth and division, and we identify the following three recurring themes. (i) Nutrient availability, not growth rate, is the primary determinant of cell size. (ii) The degree of gluconeogenic flux is likely to have a profound impact on the metabolites available for cell envelope synthesis, so growth medium selection is a critical consideration when designing and interpreting experiments related to morphogenesis. (iii) Perturbations in pathways relying on commonly shared and limiting metabolites, like undecaprenyl phosphate (Und-P), can lead to pleotropic phenotypes in unrelated pathways.

(p)ppGpp and the bacterial cell cycle

Genes of the Rel/Spo homolog (RSH) superfamily synthesize and/or hydrolyse the modified nucleotides pppGpp/ ppGpp (collectively referred to as (p)ppGpp) and are prevalent across diverse bacteria and in plant chloroplasts. Bacteria accumulate (p)ppGpp in response to nutrient deprivation (generically called the stringent response) and elicit appropriate adaptive responses mainly through the regulation of transcription. Although at different concentrations (p)ppGpp affect the expression of distinct set of genes, the two well-characterized responses are reduction in expression of the protein synthesis machinery and increase in the expression of genes coding for amino acid biosynthesis. In Escherichia coli, the cellular (p)ppGpp level inversely correlates with the growth rate and increasing its concentration decreases the steady state growth rate in a defined growth medium. Since change in growth rate must be accompanied by changes in cell cycle parameters set through the activities of the DNA replication and cell division apparatus, (p)ppGpp could coordinate protein synthesis (cell mass increase) with these processes. Here we review the role of (p)ppGpp in bacterial cell cycle regulation.

Hit the right spots: cell cycle control by phosphorylated guanosines in alphaproteobacteria

The class Alphaproteobacteria includes Gram-negative free-living, symbiotic and obligate intracellular bacteria, as well as important plant, animal and human pathogens. Recent work has established the key antagonistic roles that phosphorylated guanosines, cyclic-di-GMP (c-di-GMP) and the alarmones guanosine tetraphosphate and guanosine pentaphosphate (collectively referred to as (p)ppGpp), have in the regulation of the cell cycle in these bacteria. In this Review, we discuss the insights that have been gained into the regulation of the initiation of DNA replication and cytokinesis by these second messengers, with a particular focus on the cell cycle of Caulobacter crescentus. We explore how the fluctuating levels of c-di-GMP and (p)ppGpp during the progression of the cell cycle and under conditions of stress control the synthesis and proteolysis of key regulators of the cell cycle. As these signals also promote bacterial interactions with host cells, the enzymes that control (p)ppGpp and c-di-GMP are attractive antibacterial targets.

Genomic variations leading to alterations in cell morphology of Campylobacter spp

Campylobacter jejuni, the most common cause of bacterial diarrhoeal disease, is normally helical. However, it can also adopt straight rod, elongated helical and coccoid forms. Studying how helical morphology is generated, and how it switches between its different forms, is an important objective for understanding this pathogen. Here, we aimed to determine the genetic factors involved in generating the helical shape of Campylobacter. A C. jejuni transposon (Tn) mutant library was screened for non-helical mutants with inconsistent results. Whole genome sequence variation and morphological trends within this Tn library, and in various C. jejuni wild type strains, were compared and correlated to detect genomic elements associated with helical and rod morphologies. All rod-shaped C. jejuni Tn mutants and all rod-shaped laboratory, clinical and environmental C. jejuni and Campylobacter coli contained genetic changes within the pgp1 or pgp2 genes, which encode peptidoglycan modifying enzymes. We therefore confirm the importance of Pgp1 and Pgp2 in the maintenance of helical shape and extended this to a wide range of C. jejuni and C. coli isolates. Genome sequence analysis revealed variation in the sequence and length of homopolymeric tracts found within these genes, providing a potential mechanism of phase variation of cell shape.

Factors essential for L,D-transpeptidase-mediated peptidoglycan cross-linking and β-lactam resistance in Escherichia coli

The target of β-lactam antibiotics is the D,D-transpeptidase activity of penicillin-binding proteins (PBPs) for synthesis of 4→3 cross-links in the peptidoglycan of bacterial cell walls. Unusual 3→3 cross-links formed by L,D-transpeptidases were first detected in Escherichia coli more than four decades ago, however no phenotype has previously been associated with their synthesis. Here we show that production of the L,D-transpeptidase YcbB in combination with elevated synthesis of the (p)ppGpp alarmone by RelA lead to full bypass of the D,D-transpeptidase activity of PBPs and to broad-spectrum β-lactam resistance. Production of YcbB was therefore sufficient to switch the role of (p)ppGpp from antibiotic tolerance to high-level β-lactam resistance. This observation identifies a new mode of peptidoglycan polymerization in E. coli that relies on an unexpectedly small number of enzyme activities comprising the glycosyltransferase activity of class A PBP1b and the D,D-carboxypeptidase activity of DacA in addition to the L,D-transpeptidase activity of YcbB.

DOI:http://dx.doi.org/10.7554/eLife.19469.001

Suppression of a deletion mutation in the gene encoding essential PBP2b reveals a new lytic transglycosylase involved in peripheral peptidoglycan synthesis in Streptococcus pneumoniae D39

In ellipsoid-shaped ovococcus bacteria, such as the pathogen Streptococcus pneumoniae (pneumococcus), side-wall (peripheral) peptidoglycan (PG) synthesis emanates from midcells and is catalyzed by the essential class B penicillin-binding protein PBP2b transpeptidase (TP). We report that mutations that inactivate the pneumococcal YceG-domain protein, Spd_1346 (renamed MltG), remove the requirement for PBP2b. ΔmltG mutants in unencapsulated strains accumulate inactivation mutations of class A PBP1a, which possesses TP and transglycosylase (TG) activities. The 'synthetic viable' genetic relationship between Δpbp1a and ΔmltG mutations extends to essential ΔmreCD and ΔrodZ mutations that misregulate peripheral PG synthesis. Remarkably, the single MltG(Y488D) change suppresses the requirement for PBP2b, MreCD, RodZ and RodA. Structural modeling and comparisons, catalytic-site changes and an interspecies chimera indicate that pneumococcal MltG is the functional homologue of the recently reported MltG endo-lytic transglycosylase of Escherichia coli. Depletion of pneumococcal MltG or mltG(Y488D) increases sphericity of cells, and MltG localizes with peripheral PG synthesis proteins during division. Finally, growth of Δpbp1a ΔmltG or mltG(Y488D) mutants depends on induction of expression of the WalRK TCS regulon of PG hydrolases. These results fit a model in which MltG releases anchored PG glycan strands synthesized by PBP1a for crosslinking by a PBP2b:RodA complex in peripheral PG synthesis.

Inactivation of Cell Division Protein FtsZ by SulA Makes Lon Indispensable for the Viability of a ppGpp0 Strain of Escherichia coli

The modified nucleotides (p)ppGpp play an important role in bacterial physiology. While the accumulation of the nucleotides is vital for adaptation to various kinds of stress, changes in the basal level modulates growth rate and vice versa. Studying the phenotypes unique to the strain lacking (p)ppGpp (ppGpp(0)) under overtly unstressed growth conditions may be useful to understand functions regulated by basal levels of (p)ppGpp and its physiological significance. In this study, we show that the ppGpp(0) strain, unlike the wild type, requires the Lon protease for cell division and viability in LB. Our results indicate the decrease in FtsZ concentration in the ppGpp(0) strain makes cell division vulnerable to SulA inhibition. We did not find evidence for SOS induction contributing to the cell division defect in the ppGpp(0) Δlon strain. Based on the results, we propose that basal levels of (p)ppGpp are required to sustain normal cell division in Escherichia coli during growth in rich medium and that the basal SulA level set by Lon protease is important for insulating cell division against a decrease in FtsZ concentration and conditions that can increase the susceptibility of FtsZ to SulA.

IMPORTANCE

The physiology of the stringent response has been the subject of investigation for more than 4 decades, with the majority of the work carried out using the bacterial model organism Escherichia coli. These studies have revealed that the accumulation of (p)ppGpp, the effector of the stringent response, is associated with growth retardation and changes in gene expression that vary with the intracellular concentration of (p)ppGpp. By studying a synthetic lethal phenotype, we have uncovered a function modulated by the basal levels of (p)ppGpp and studied its physiological significance. Our results show that (p)ppGpp and Lon protease contribute to the robustness of the cell division machinery in E. coli during growth in rich medium.

Filament formation of Salmonella Paratyphi A accompanied by FtsZ assembly impairment and low level ppGpp

Previously, we reported that Salmonella enterica serovar Paratyphi A strain S602 grew into multinuclear, nonseptate, and nonlethal filaments on agar plates containing nitrogenous salts. Strain S602 was more sensitive to osmotic and oxidative stress than the reference strain 3P243 of nonfilamentous Salmonella Paratyphi A. Strain S602 had an amber mutation (C154T) in rpoS. The revertant of this mutant, SR603, was repressed to form filaments under conditions with abundant nitrogenous salts. However, 3PR244, an rpoS mutant of 3P243 (C154T), did not form filaments, which implies that the rpoS mutation is not the sole cause of filamentation in strain S602. Next, we examined whether the level of guanosine 5'-diphosphate 3'-diphosphate (ppGpp) in S602 strain is involved in filament formation. The intracellular ppGpp level in filamentous cells was lower than that in nonfilamentous cells. Furthermore, cells belonging to strain RE606, a derivative of S602 where the intracellular concentration of ppGpp was increased by overexpression of the relA gene, exhibited normal Z-ring formation and cell division. In the S602 strain, the decrease in the ppGpp level induced by the presence of nitrogenous salt and the rpoS mutation led to the inhibition of Z-ring formation and the subsequent filamentation of cells.

Beta-lactam antibiotics induce a lethal malfunctioning of the bacterial cell wall synthesis machinery

Penicillin and related beta-lactams comprise one of our oldest and most widely used antibiotic therapies. These drugs have long been known to target enzymes called penicillin-binding proteins (PBPs) that build the bacterial cell wall. Investigating the downstream consequences of target inhibition and how they contribute to the lethal action of these important drugs, we demonstrate that beta-lactams do more than just inhibit the PBPs as is commonly believed. Rather, they induce a toxic malfunctioning of their target biosynthetic machinery involving a futile cycle of cell wall synthesis and degradation, thereby depleting cellular resources and bolstering their killing activity. Characterization of this mode of action additionally revealed a quality control function for enzymes that cleave bonds in the cell wall matrix. The results thus provide insight into the mechanism of cell wall assembly and suggest how best to interfere with the process for future antibiotic development.

Amdinocillin (Mecillinam) resistance mutations in clinical isolates and laboratory-selected mutants of Escherichia coli

Amdinocillin (mecillinam) is a β-lactam antibiotic that is used mainly for the treatment of uncomplicated urinary tract infections. The objectives of this study were to identify mutations that confer amdinocillin resistance on laboratory-isolated mutants and clinical isolates of Escherichia coli and to determine why amdinocillin resistance remains rare clinically even though resistance is easily selected in the laboratory. Under laboratory selection, frequencies of mutation to amdinocillin resistance varied from 8 × 10(-8) to 2 × 10(-5) per cell, depending on the concentration of amdinocillin used during selection. Several genes have been demonstrated to give amdinocillin resistance, but here eight novel genes previously unknown to be involved in amdinocillin resistance were identified. These genes encode functions involved in the respiratory chain, the ribosome, cysteine biosynthesis, tRNA synthesis, and pyrophosphate metabolism. The clinical isolates exhibited significantly greater fitness than the laboratory-isolated mutants and a different mutation spectrum. The cysB gene was mutated (inactivated) in all of the clinical isolates, in contrast to the laboratory-isolated mutants, where mainly other types of more costly mutations were found. Our results suggest that the frequency of mutation to amdinocillin resistance is high because of the large mutational target (at least 38 genes). However, the majority of these resistant mutants have a low growth rate, reducing the probability that they are stably maintained in the bladder. Inactivation of the cysB gene and a resulting loss of cysteine biosynthesis are the major mechanism of amdinocillin resistance in clinical isolates of E. coli.

Mycobacterium tuberculosis CwsA overproduction modulates cell division and cell wall synthesis

We recently showed that two small membrane proteins of Mycobacterium tuberculosis, CwsA and CrgA, interact with each other, and that loss of CwsA in M. smegmatis is associated with defects in the cell division and cell wall synthesis processes. Here we show that CwsA overproduction also affected growth, cell division and cell shape of M. smegmatis and M. tuberculosis. CwsA overproduction in M. tuberculosis led to increased sensitivity to cefsulodin, a penicillin-binding protein (PBP) 1A/1B targeting beta (β) -lactam, but was unaffected by other β-lactams and vancomycin. A M. smegmatis cwsA overexpressing strain showed bulgy cells, increased fluorescent vancomycin staining and altered localization of Wag31-mCherry fusion protein. However, the levels of phosphorylated Wag31, important for optimal peptidoglycan synthesis and growth in mycobacteria, were not affected. Interestingly, CwsA overproduction in E. coli led to the formation of large rounded cells that eventually lysed whereas the overproduction of FtsZ along with CwsA reversed this phenotype. Together, our results emphasize that optimal levels of CwsA are required for regulated cell wall synthesis, hence maintenance of cell shape, and that CwsA likely interacts with and modulates the activities of other cell wall synthetic components including PBPs.

From models to pathogens: how much have we learned about Streptococcus pneumoniae cell division?

Streptococcus pneumoniae is an oval-shaped Gram-positive coccus that lives in intimate association with its human host, both as a commensal and pathogen. The seriousness of pneumococcal infections and the spread of multi-drug resistant strains call for new lines of intervention. Bacterial cell division is an attractive target to develop antimicrobial drugs. This review discusses the recent advances in understanding S. pneumoniae growth and division, in comparison with the best studied rod-shaped models, Escherichia coli and Bacillus subtilis. To maintain their shape, these bacteria propagate by peripheral and septal peptidoglycan synthesis, involving proteins that assemble into distinct complexes called the elongasome and the divisome, respectively. Many of these proteins are conserved in S. pneumoniae, supporting the notion that the ovococcal shape is also achieved by rounds of elongation and division. Importantly, S. pneumoniae and close relatives with similar morphology differ in several aspects from the model rods. Overall, the data support a model in which a single large machinery, containing both the peripheral and septal peptidoglycan synthesis complexes, assembles at midcell and governs growth and division. The mechanisms generating the ovococcal or coccal shape in lactic-acid bacteria have likely evolved by gene reduction from a rod-shaped ancestor of the same group.

Requirement of lipid II biosynthesis for cell division in cell wall-less Wolbachia, endobacteria of arthropods and filarial nematodes

Obligate Wolbachia endobacteria have a reduced genome and retained genes are hypothesized to be crucial for survival. Although intracellular bacteria do not need a stress-bearing peptidoglycan cell wall, Wolbachia encode proteins necessary to synthesize the peptidoglycan precursor lipid II. The activity of the enzymes catalyzing the last two steps of this pathway was previously shown, and Wolbachia are sensitive to inhibition of lipid II synthesis. A puzzling characteristic of Wolbachia is the lack of genes for l-amino acid racemases essential for lipid II synthesis. Transcription analysis showed the expression of a possible alternative racemase metC, and recombinant Wolbachia MetC indeed had racemase activity that may substitute for the absent l-Ala racemase. However, enzymes needed to form mature peptidoglycan are absent and the function of Wolbachia lipid II is unknown. Inhibition of lipid II biosynthesis resulted in enlargement of Wolbachia cells and redistribution of Wolbachia peptidoglycan-associated lipoprotein, demonstrating that lipid II is required for coordinated cell division and may interact with the lipoprotein. We conclude that lipid II is essential for Wolbachia cell division and that this function is potentially conserved in the Gram-negative bacteria.

Putative mechanisms and biological role of coccoid form formation in Campylobacter jejuni

In certain conditions Campylobacter jejuni cells are capable of changing their cell shape from a typically spiral to a coccoid form (CF). By similarity to other bacteria, the latter was initially considered to be a viable but non-culturable form capable of survival in unfavourable conditions. However, subsequent studies with C. jejuni and closely related bacteria Helicobacter pylori suggested that CF represents a non-viable, degenerative form. Until now, the issue on whether the CF of C. jejuni is viable and infective is highly controversial. Despite some preliminary experiments on characterization of CF cells, neither biochemical mechanisms nor genetic determinants involved in C. jejuni cell shape changes have been characterized. In this review, we highlight known molecular mechanisms and genes involved in CF formation in other bacteria. Since orthologous genes are also present in C. jejuni, we suggest that CF formation in these bacteria is also a regulated and genetically determined process. A possible significance of CF in the lifestyle of this important bacterial pathogen is discussed.

Characterization of the elongasome core PBP2 : MreC complex of Helicobacter pylori

The definition of bacterial cell shape is a complex process requiring the participation of multiple components of an intricate macromolecular machinery. We aimed at characterizing the determinants involved in cell shape of the helical bacterium Helicobacter pylori. Using a yeast two-hybrid screen with the key cell elongation protein PBP2 as bait, we identified an interaction between PBP2 and MreC. The minimal region of MreC required for this interaction ranges from amino acids 116 to 226. Using recombinant proteins, we showed by affinity and size exclusion chromatographies and surface plasmon resonance that PBP2 and MreC form a stable complex. In vivo, the two proteins display a similar spatial localization and their complex has an apparent 1:1 stoichiometry; these results were confirmed in vitro by analytical ultracentrifugation and chemical cross-linking. Small angle X-ray scattering analyses of the PBP2 : MreC complex suggest that MreC interacts directly with the C-terminal region of PBP2. Depletion of either PBP2 or MreC leads to transition into spherical cells that lose viability. Finally, the specific expression in trans of the minimal interacting domain of MreC with PBP2 in the periplasmic space leads to cell rounding, suggesting that the PBP2/MreC complex formation in vivo is essential for cell morphology.

Compensatory evolution of pbp mutations restores the fitness cost imposed by β-lactam resistance in Streptococcus pneumoniae

The prevalence of antibiotic resistance genes in pathogenic bacteria is a major challenge to treating many infectious diseases. The spread of these genes is driven by the strong selection imposed by the use of antibacterial drugs. However, in the absence of drug selection, antibiotic resistance genes impose a fitness cost, which can be ameliorated by compensatory mutations. In Streptococcus pneumoniae, β-lactam resistance is caused by mutations in three penicillin-binding proteins, PBP1a, PBP2x, and PBP2b, all of which are implicated in cell wall synthesis and the cell division cycle. We found that the fitness cost and cell division defects conferred by pbp2b mutations (as determined by fitness competitive assays in vitro and in vivo and fluorescence microscopy) were fully compensated by the acquisition of pbp2x and pbp1a mutations, apparently by means of an increased stability and a consequent mislocalization of these protein mutants. Thus, these compensatory combinations of pbp mutant alleles resulted in an increase in the level and spectrum of β-lactam resistance. This report describes a direct correlation between antibiotic resistance increase and fitness cost compensation, both caused by the same gene mutations acquired by horizontal transfer. The clinical origin of the pbp mutations suggests that this intergenic compensatory process is involved in the persistence of β-lactam resistance among circulating strains. We propose that this compensatory mechanism is relevant for β-lactam resistance evolution in Streptococcus pneumoniae.

Dynamic polar sequestration of excess MurG may regulate enzymatic function

Advances in bacterial cell biology have demonstrated the importance of protein localization for protein function. In general, proteins are thought to localize to the sites where they are active. Here we demonstrate that in Escherichia coli, MurG, the enzyme that mediates the last step in peptidoglycan subunit biosynthesis, becomes polarly localized when expressed at high cellular concentrations. MurG only becomes polarly localized at levels that saturate MurG's cellular requirement for growth, and E. coli cells do not insert peptidoglycan at the cell poles, indicating that the polar MurG is not active. Fluorescence recovery after photobleaching (FRAP) and single-cell biochemistry experiments demonstrate that polar MurG is dynamic. Polar MurG foci are distinct from inclusion body aggregates, and polar MurG can be remobilized when MurG levels drop. These results suggest that polar MurG represents a temporary storage mechanism for excess protein that can later be remobilized into the active pool. We investigated and ruled out several candidate pathways for polar MurG localization, including peptidoglycan biosynthesis, the MreB cytoskeleton, and polar cardiolipin, as well as MurG enzymatic activity and lipid binding, suggesting that polar MurG is localized by a novel mechanism. Together, our results imply that inactive MurG is dynamically sequestered at the cell poles and that prokaryotes can thus utilize subcellular localization as a mechanism for negatively regulating enzymatic activity.

Beta-lactam antibiotics: from antibiosis to resistance and bacteriology

This review focuses on the era of antibiosis that led to a better understanding of bacterial morphology, in particular the cell wall component peptidoglycan. This is an effort to take readers on a tour de force from the concept of antibiosis, to the serendipity of antibiotics, evolution of beta-lactam development, and the molecular biology of antibiotic resistance. These areas of research have culminated in a deeper understanding of microbiology, particularly in the area of bacterial cell wall synthesis and recycling. In spite of this knowledge, which has enabled design of new even more effective therapeutics to combat bacterial infection and has provided new research tools, antibiotic resistance remains a worldwide health care problem.

Evolution of penicillin-binding protein 2 concentration and cell shape during a long-term experiment with Escherichia coli

Peptidoglycan is the major component of the bacterial cell wall and is involved in osmotic protection and in determining cell shape. Cell shape potentially influences many processes, including nutrient uptake as well as cell survival and growth. Peptidoglycan is a dynamic structure that changes during the growth cycle. Penicillin-binding proteins (PBPs) catalyze the final stages of peptidoglycan synthesis. Although PBPs are biochemically and physiologically well characterized, their broader effects, especially their effects on organismal fitness, are not well understood. In a long-term experiment, 12 populations of Escherichia coli having a common ancestor were allowed to evolve for more than 40,000 generations in a defined environment. We previously identified mutations in the pbpA operon in one-half of these populations; this operon encodes PBP2 and RodA proteins that are involved in cell wall elongation. In this study, we characterized the effects of two of these mutations on competitive fitness and other phenotypes. By constructing and performing competition experiments with strains that are isogenic except for the pbpA alleles, we showed that both mutations that evolved were beneficial in the environment used for the long-term experiment and that these mutations caused parallel phenotypic changes. In particular, they reduced the cellular concentration of PBP2, thereby generating spherical cells with an increased volume. In contrast to their fitness-enhancing effect in the environment where they evolved, both mutations decreased cellular resistance to osmotic stress. Moreover, one mutation reduced fitness during prolonged stationary phase. Therefore, alteration of the PBP2 concentration contributed to physiological trade-offs and ecological specialization during experimental evolution.

RodZ (YfgA) is required for proper assembly of the MreB actin cytoskeleton and cell shape in E. coli

The bacterial MreB actin cytoskeleton is required for cell shape maintenance in most non-spherical organisms. In rod-shaped cells such as Escherichia coli, it typically assembles along the long axis in a spiral-like configuration just underneath the cytoplasmic membrane. How this configuration is controlled and how it helps dictate cell shape is unclear. In a new genetic screen for cell shape mutants, we identified RodZ (YfgA) as an important transmembrane component of the cytoskeleton. Loss of RodZ leads to misassembly of MreB into non-spiral structures, and a consequent loss of cell shape. A juxta-membrane domain of RodZ is essential to maintain rod shape, whereas other domains on either side of the membrane have critical, but partially redundant, functions. Though one of these domains resembles a DNA-binding motif, our evidence indicates that it is primarily responsible for association of RodZ with the cytoskeleton.

Cell shape and cell-wall organization in Gram-negative bacteria

In bacterial cells, the peptidoglycan cell wall is the stress-bearing structure that dictates cell shape. Although many molecular details of the composition and assembly of cell-wall components are known, how the network of peptidoglycan subunits is organized to give the cell shape during normal growth and how it is reorganized in response to damage or environmental forces have been relatively unexplored. In this work, we introduce a quantitative physical model of the bacterial cell wall that predicts the mechanical response of cell shape to peptidoglycan damage and perturbation in the rod-shaped Gram-negative bacterium Escherichia coli. To test these predictions, we use time-lapse imaging experiments to show that damage often manifests as a bulge on the sidewall, coupled to large-scale bending of the cylindrical cell wall around the bulge. Our physical model also suggests a surprising robustness of cell shape to peptidoglycan defects, helping explain the observed porosity of the cell wall and the ability of cells to grow and maintain their shape even under conditions that limit peptide crosslinking. Finally, we show that many common bacterial cell shapes can be realized within the same model via simple spatial patterning of peptidoglycan defects, suggesting that minor patterning changes could underlie the great diversity of shapes observed in the bacterial kingdom.

Conditional lethality, division defects, membrane involution, and endocytosis in mre and mrd shape mutants of Escherichia coli

Maintenance of rod shape in Escherichia coli requires the shape proteins MreB, MreC, MreD, MrdA (PBP2), and MrdB (RodA). How loss of the Mre proteins affects E. coli viability has been unclear. We generated Mre and Mrd depletion strains under conditions that minimize selective pressure for undefined suppressors and found their phenotypes to be very similar. Cells lacking one or more of the five proteins were fully viable and propagated as small spheres under conditions of slow mass increase but formed large nondividing spheroids with noncanonical FtsZ assembly patterns at higher mass doubling rates. Extra FtsZ was sufficient to suppress lethality in each case, allowing cells to propagate as small spheres under any condition. The failure of each unsuppressed mutant to divide under nonpermissive conditions correlated with the presence of elaborate intracytoplasmic membrane-bound compartments, including vesicles/vacuoles and more-complex systems. Many, if not all, of these compartments formed by FtsZ-independent involution of the cytoplasmic membrane (CM) rather than de novo. Remarkably, while some of the compartments were still continuous with the CM and the periplasm, many were topologically separate, indicating they had been released into the cytoplasm by an endocytic-like membrane fission event. Notably, cells failed to adjust the rate of phospholipid synthesis to their new surface requirements upon depletion of MreBCD, providing a rationale for the "excess" membrane in the resulting spheroids. Both FtsZ and MinD readily assembled on intracytoplasmic membrane surfaces, and we propose that this contributes significantly to the lethal division block seen in all shape mutants under nonpermissive conditions.

DNA and origin region segregation are not affected by the transition from rod to sphere after inhibition of Escherichia coli MreB by A22

The bacterial actin homologue MreB forms a helix underneath the cytoplasmic membrane and was shown to be essential in the morphogenesis of the rod-shaped cells. Additionally, MreB was implicated to be involved in DNA segregation. However, in our hands the mreBCD deletion strain (PA340-678) grew without apparent DNA segregation defect, suggesting that the reported chromosome segregation inhibition could be caused by a temporarily effect of MreB inhibition or depletion. To assess the involvement of MreB in DNA segregation during the transition from rod to sphere, we compared the effect of A22 and the PBP2 inhibitor mecillinam on the percentage of cells with segregated nucleoids and the number of oriC foci in wild-type Escherichia coli cells. Cells became spherical in the same time window during both treatments and we could not detect any difference in the chromosome or oriC segregation between these two treatments. Additionally, flow cytometric analyses showed that A22 and mecillinam treatment gave essentially the same chromosome segregation pattern. We conclude that MreB is not directly involved in DNA segregation of E. coli.

Unstable Escherichia coli L forms revisited: growth requires peptidoglycan synthesis

Growing bacterial L forms are reputed to lack peptidoglycan, although cell division is normally inseparable from septal peptidoglycan synthesis. To explore which cell division functions L forms use, we established a protocol for quantitatively converting a culture of a wild-type Escherichia coli K-12 strain overnight to a growing L-form-like state by use of the beta-lactam cefsulodin, a specific inhibitor of penicillin-binding proteins (PBPs) 1A and 1B. In rich hypertonic medium containing cefsulodin, all cells are spherical and osmosensitive, like classical L forms. Surprisingly, however, mutant studies showed that colony formation requires d-glutamate, diaminopimelate, and MurA activity, all of which are specific to peptidoglycan synthesis. High-performance liquid chromatography analysis confirmed that these L-form-like cells contain peptidoglycan, with 7% of the normal amount. Moreover, the beta-lactam piperacillin, a specific inhibitor of the cell division protein PBP 3, rapidly blocks the cell division of these L-form-like cells. Similarly, penicillin-induced L-form-like cells, which grow only within the agar layers of rich hypertonic plates, also require d-glutamate, diaminopimelate, and MurA activity. These results strongly suggest that cefsulodin- and penicillin-induced L-form-like cells of E. coli-and possibly all L forms-have residual peptidoglycan synthesis which is essential for their growth, probably being required for cell division.

Murein (peptidoglycan) structure, architecture and biosynthesis in Escherichia coli

The periplasmic murein (peptidoglycan) sacculus is a giant macromolecule made of glycan strands cross-linked by short peptides completely surrounding the cytoplasmic membrane to protect the cell from lysis due to its internal osmotic pressure. More than 50 different muropeptides are released from the sacculus by treatment with a muramidase. Escherichia coli has six murein synthases which enlarge the sacculus by transglycosylation and transpeptidation of lipid II precursor. A set of twelve periplasmic murein hydrolases (autolysins) release murein fragments during cell growth and division. Recent data on the in vitro murein synthesis activities of the murein synthases and on the interactions between murein synthases, hydrolases and cell cycle related proteins are being summarized. There are different models for the architecture of murein and for the incorporation of new precursor into the sacculus. We present a model in which morphogenesis of the rod-shaped E. coli is driven by cytoskeleton elements competing for the control over the murein synthesis multi-enzyme complexes.

Function of penicillin-binding protein 2 in viability and morphology of Pseudomonas aeruginosa

OBJECTIVES

To investigate the function of penicillin-binding protein 2 (PBP 2) in Pseudomonas aeruginosa PAO1.

METHODS

The growth and morphology of P. aeruginosa cultured in the absence and presence of mecillinam was assessed. The gene encoding PBP 2, pbpA, was identified in the genome of P. aeruginosa PAO1 and both its full-length and an engineered truncated form were cloned and expressed in Escherichia coli. Site-directed mutagenesis was used to confirm Ser-327 as the catalytic nucleophile of its transpeptidase domain. Allelic exchange was used to construct a chromosomal mutant of pbpA in strain PAO1.

RESULTS

PAO1 grew with a spherical morphology in the presence of mecillinam at concentrations as high as 2000 mg/L. Both wild-type and truncated, soluble forms of PBP 2 were shown to bind penicillins and a competition assay demonstrated their specificity for mecillinam. The PAO1 DeltapbpA insertional mutant also grew as spheres, and complementation with a plasmid encoding active pbpA, but not with an inactive Ser-327 --> Ala derivative, restored rod-shape morphology. MIC values of a variety of beta-lactams were significantly lower for the insertional mutant compared with wild-type PAO1. The muropeptide profile of peptidoglycan from PAO1 DeltapbpA analysed by HPLC/MALDI TOF MS indicated wild-type levels of cross-linking despite the loss of PBP 2 transpeptidase activity.

CONCLUSIONS

PBP 2 in P. aeruginosa is responsible for the rod-shape morphology of the cells and contributes significantly to beta-lactam resistance. The viability of cells lacking an active PBP 2 suggests that the organization of the peptidoglycan biosynthetic machinery is different in this pathogen compared with E. coli.

Of the morphogenes that make a ring, a rod and a sphere in Escherichia coli

In 1993, William Donachie wrote "The success of molecular genetics in the study of bacterial cell division has been so great that we find ourselves, armed with much greater knowledge of detail, confronted once again with the same naive questions that we set to answer in the first place". Indeed, attempts to answer the apparently simple question of how a bacterial cell divides have led to a wealth of new knowledge, in particular over the past decade and a half. And while some questions have been answered to a great extent since the early reports of isolation of division mutants of Escherichia coli, some key pieces of the puzzle remain elusive. In addition to it being a fundamental process in bacteria that merits investigation in its own right, studying the process of cell division offers an abundance of new targets for the development of new antibacterial compounds that act directly against key division proteins and other components of the cytoskeleton, which are encoded by the morphogenes of E. coli. This review aims to present the reader with a snapshot summary of the key players in E. coli morphogenesis with emphasis on cell division and the rod to sphere transition.

Role of FtsEX in cell division of Escherichia coli: viability of ftsEX mutants is dependent on functional SufI or high osmotic strength

In Escherichia coli, at least 12 proteins, FtsZ, ZipA, FtsA, FtsE/X, FtsK, FtsQ, FtsL, FtsB, FtsW, FtsI, FtsN, and AmiC, are known to localize to the septal ring in an interdependent and sequential pathway to coordinate the septum formation at the midcell. The FtsEX complex is the latest recruit of this pathway, and unlike other division proteins, it is shown to be essential only on low-salt media. In this study, it is shown that ftsEX null mutations are not only salt remedial but also osmoremedial, which suggests that FtsEX may not be involved in salt transport as previously thought. Increased coexpression of cell division proteins FtsQ-FtsA-FtsZ or FtsN alone restored the growth defects of ftsEX mutants. ftsEX deletion exacerbated the defects of most of the mutants affected in Z ring localization and septal assembly; however, the ftsZ84 allele was a weak suppressor of ftsEX. The viability of ftsEX mutants in high-osmolarity conditions was shown to be dependent on the presence of a periplasmic protein, SufI, a substrate of twin-arginine translocase. In addition, SufI in multiple copies could substitute for the functions of FtsEX. Taken together, these results suggest that FtsE and FtsX are absolutely required for the process of cell division in conditions of low osmotic strength for the stability of the septal ring assembly and that, during high-osmolarity conditions, the FtsEX and SufI functions are redundant for this essential process.

The selective value of bacterial shape

Why do bacteria have shape? Is morphology valuable or just a trivial secondary characteristic? Why should bacteria have one shape instead of another? Three broad considerations suggest that bacterial shapes are not accidental but are biologically important: cells adopt uniform morphologies from among a wide variety of possibilities, some cells modify their shape as conditions demand, and morphology can be tracked through evolutionary lineages. All of these imply that shape is a selectable feature that aids survival. The aim of this review is to spell out the physical, environmental, and biological forces that favor different bacterial morphologies and which, therefore, contribute to natural selection. Specifically, cell shape is driven by eight general considerations: nutrient access, cell division and segregation, attachment to surfaces, passive dispersal, active motility, polar differentiation, the need to escape predators, and the advantages of cellular differentiation. Bacteria respond to these forces by performing a type of calculus, integrating over a number of environmental and behavioral factors to produce a size and shape that are optimal for the circumstances in which they live. Just as we are beginning to answer how bacteria create their shapes, it seems reasonable and essential that we expand our efforts to understand why they do so.