Morphology of an Escherichia coli mutant with a temperature-dependent round cell shape

β-Lactam Resistance in ESKAPE Pathogens Mediated Through Modifications in Penicillin-Binding Proteins: An Overview

Bacteria acquire β-lactam resistance through a multitude of mechanisms among which production of β-lactamases (enzymes that hydrolyze β-lactams) is the most common, especially in Gram-negatives. Structural changes in the high-molecular-weight, essential penicillin-binding proteins (PBPs) are widespread in Gram-positives and increasingly reported in Gram-negatives. PBP-mediated resistance is largely achieved by accumulation of mutation(s) resulting in reduced binding affinities of β-lactams. Herein, we discuss PBP-mediated resistance among ESKAPE pathogens that cause diverse hospital- and community-acquired infections globally.

Sensitivity Analysis of Different Shapes of a Plastic Optical Fiber-Based Immunosensor for Escherichia coli: Simulation and Experimental Results

Conventional pathogen detection methods require trained personnel, specialized laboratories and can take days to provide a result. Thus, portable biosensors with rapid detection response are vital for the current needs for in-loco quality assays. In this work the authors analyze the characteristics of an immunosensor based on the evanescent field in plastic optical fibers with macro curvature by comparing experimental with simulated results. The work studies different shapes of evanescent-wave based fiber optic sensors, adopting a computational modeling to evaluate the probes with the best sensitivity. The simulation showed that for a U-Shaped sensor, the best results can be achieved with a sensor of 980 µm diameter by 5.0 mm in curvature for refractive index sensing, whereas the meander-shaped sensor with 250 μm in diameter with radius of curvature of 1.5 mm, showed better sensitivity for either bacteria and refractive index (RI) sensing. Then, an immunosensor was developed, firstly to measure refractive index and after that, functionalized to detect Escherichia coli. Based on the results with the simulation, we conducted studies with a real sensor for RI measurements and for Escherichia coli detection aiming to establish the best diameter and curvature radius in order to obtain an optimized sensor. On comparing the experimental results with predictions made from the modelling, good agreements were obtained. The simulations performed allowed the evaluation of new geometric configurations of biosensors that can be easily constructed and that promise improved sensitivity.

Short FtsZ filaments can drive asymmetric cell envelope constriction at the onset of bacterial cytokinesis

FtsZ, the bacterial homologue of eukaryotic tubulin, plays a central role in cell division in nearly all bacteria and many archaea. It forms filaments under the cytoplasmic membrane at the division site where, together with other proteins it recruits, it drives peptidoglycan synthesis and constricts the cell. Despite extensive study, the arrangement of FtsZ filaments and their role in division continue to be debated. Here, we apply electron cryotomography to image the native structure of intact dividing cells and show that constriction in a variety of Gram-negative bacterial cells, including Proteus mirabilis and Caulobacter crescentus, initiates asymmetrically, accompanied by asymmetric peptidoglycan incorporation and short FtsZ-like filament formation. These results show that a complete ring of FtsZ is not required for constriction and lead us to propose a model for FtsZ-driven division in which short dynamic FtsZ filaments can drive initial peptidoglycan synthesis and envelope constriction at the onset of cytokinesis, later increasing in length and number to encircle the division plane and complete constriction.

Dead-end intermediates in the enterobacterial common antigen pathway induce morphological defects in Escherichia coli by competing for undecaprenyl phosphate

Bacterial morphology is determined primarily by the architecture of the peptidoglycan (PG) cell wall, a mesh-like layer that encases the cell. To identify novel mechanisms that create or maintain cell shape in Escherichia coli, we used flow cytometry to screen a transposon insertion library and identified a wecE mutant that altered cell shape, causing cells to filament and swell. WecE is a sugar aminotransferase involved in the biosynthesis of enterobacterial common antigen (ECA), a non-essential outer membrane glycolipid of the Enterobacteriaceae. Loss of wecE interrupts biosynthesis of ECA and causes the accumulation of the undecaprenyl pyrophosphate-linked intermediate ECA-lipid II. The wecE shape defects were reversed by: (i) preventing initiation of ECA biosynthesis, (ii) increasing the synthesis of the lipid carrier undecaprenyl phosphate (Und-P), (iii) diverting Und-P to PG synthesis or (iv) promoting Und-P recycling. The results argue that the buildup of ECA-lipid II sequesters part of the pool of Und-P, which, in turn, adversely affects PG synthesis. The data strongly suggest there is competition for a common pool of Und-P, whose proper distribution to alternate metabolic pathways is required to maintain normal cell shape in E. coli.

De novo morphogenesis in L-forms via geometric control of cell growth

In virtually all bacteria, the cell wall is crucial for mechanical integrity and for determining cell shape. Escherichia coli's rod-like shape is maintained via the spatiotemporal patterning of cell-wall synthesis by the actin homologue MreB. Here, we transiently inhibited cell-wall synthesis in E. coli to generate cell-wall-deficient, spherical L-forms, and found that they robustly reverted to a rod-like shape within several generations after inhibition cessation. The chemical composition of the cell wall remained essentially unchanged during this process, as indicated by liquid chromatography. Throughout reversion, MreB localized to inwardly curved regions of the cell, and fluorescent cell wall labelling revealed that MreB targets synthesis to those regions. When exposed to the MreB inhibitor A22, reverting cells regrew a cell wall but failed to recover a rod-like shape. Our results suggest that MreB provides the geometric measure that allows E. coli to actively establish and regulate its morphology.

Chlamydia co-opts the rod shape-determining proteins MreB and Pbp2 for cell division

Chlamydiae are obligate intracellular bacterial pathogens that have extensively reduced their genome in adapting to the intracellular environment. The chlamydial genome contains only three annotated cell division genes and lacks ftsZ. How this obligate intracellular pathogen divides is uncharacterized. Chlamydiae contain two high-molecular-weight (HMW) penicillin binding proteins (Pbp) implicated in peptidoglycan synthesis, Pbp2 and Pbp3/FtsI. We show here, using HMW Pbp-specific penicillin derivatives, that both Pbp2 and Pbp3 are essential for chlamydial cell division. Ultrastructural analyses of antibiotic-treated cultures revealed distinct phenotypes: Pbp2 inhibition induced internal cell bodies within a single outer membrane whereas Pbp3 inhibition induced elongated phenotypes with little internal division. Each HMW Pbp interacts with the Chlamydia cell division protein FtsK. Chlamydiae are coccoid yet contain MreB, a rod shape-determining protein linked to Pbp2 in bacilli. Using MreB-specific antibiotics, we show that MreB is essential for chlamydial growth and division. Importantly, co-treatment with MreB-specific and Pbp-specific antibiotics resulted in the MreB-inhibited phenotype, placing MreB upstream of Pbp function in chlamydial cell division. Finally, we showed that MreB also interacts with FtsK. We propose that, in Chlamydia, MreB acts as a central co-ordinator at the division site to substitute for the lack of FtsZ in this bacterium.

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.

MreB drives de novo rod morphogenesis in Caulobacter crescentus via remodeling of the cell wall

MreB, the bacterial actin-like cytoskeleton, is required for the rod morphology of many bacterial species. Disruption of MreB function results in loss of rod morphology and cell rounding. Here, we show that the widely used MreB inhibitor A22 causes MreB-independent growth inhibition that varies with the drug concentration, culture medium conditions, and bacterial species tested. MP265, an A22 structural analog, is less toxic than A22 for growth yet equally efficient for disrupting the MreB cytoskeleton. The action of A22 and MP265 is enhanced by basic pH of the culture medium. Using this knowledge and the rapid reversibility of drug action, we examined the restoration of rod shape in lemon-shaped Caulobacter crescentus cells pretreated with MP265 or A22 under nontoxic conditions. We found that reversible restoration of MreB function after drug removal causes extensive morphological changes including a remarkable cell thinning accompanied with elongation, cell branching, and shedding of outer membrane vesicles. We also thoroughly characterized the composition of C. crescentus peptidoglycan by high-performance liquid chromatography and mass spectrometry and showed that MreB disruption and recovery of rod shape following restoration of MreB function are accompanied by considerable changes in composition. Our results provide insight into MreB function in peptidoglycan remodeling and rod shape morphogenesis and suggest that MreB promotes the transglycosylase activity of penicillin-binding proteins.

Activity-based protein profiling: from enzyme chemistry to proteomic chemistry

Genome sequencing projects have provided researchers with a complete inventory of the predicted proteins produced by eukaryotic and prokaryotic organisms. Assignment of functions to these proteins represents one of the principal challenges for the field of proteomics. Activity-based protein profiling (ABPP) has emerged as a powerful chemical proteomic strategy to characterize enzyme function directly in native biological systems on a global scale. Here, we review the basic technology of ABPP, the enzyme classes addressable by this method, and the biological discoveries attributable to its application.

Cell size and nucleoid organization of engineered Escherichia coli cells with a reduced genome

The minimization of a genome is necessary to identify experimentally the minimal gene set that contains only those genes that are essential and sufficient to sustain a functioning cell. Recent developments in genetic techniques have made it possible to generate bacteria with a markedly reduced genome. We developed a simple system for formation of markerless chromosomal deletions, and constructed and characterized a series of large-scale chromosomal deletion mutants of Escherichia coli that lack between 2.4 and 29.7% of the parental chromosome. Combining deletion mutations changes cell length and width, and the mutant cells with larger deletions were even longer and wider than the parental cells. The nucleoid organization of the mutants is also changed: the nucleoids occur as multiple small nucleoids and are localized peripherally near the envelope. Inhibition of translation causes them to condense into one or two packed nucleoids, suggesting that the coupling of transcription and translation of membrane proteins peripherally localizes chromosomes. Because these phenotypes are similar to those of spherical cells, those may be a consequence of the morphological change. Based on the nucleoid localization observed with these mutants, we discuss the cellular nucleoid dynamics.

Identification of Streptococcus thermophilus CNRZ368 genes involved in defense against superoxide stress

To better understand the defense mechanism of Streptococcus thermophilus against superoxide stress, molecular analysis of 10 menadione-sensitive mutants, obtained by insertional mutagenesis, was undertaken. This analysis allowed the identification of 10 genes that, with respect to their putative functions, were classified into five categories: (i) those involved in cell wall metabolism, (ii) those involved in exopolysaccharide translocation, (iii) those involved in RNA modification, (iv) those involved in iron homeostasis, and (v) those whose functions are still unknown. The behavior of the 10 menadione-sensitive mutants exposed to heat shock was investigated. Data from these experiments allowed us to distinguish genes whose action might be specific to oxidative stress defense (tgt, ossF, and ossG) from those whose action may be generalized to other stressful conditions (mreD, rodA, pbp2b, cpsX, and iscU). Among the mutants, two harbored an independently inserted copy of pGh9:ISS1 in two loci close to each other. More precisely, these two loci are homologous to the sufD and iscU genes, which are involved in the biosynthesis of iron-sulfur clusters. This region, called the suf region, was further characterized in S. thermophilus CNRZ368 by sequencing and by construction of DeltasufD and iscU(97) nonpolar mutants. The streptonigrin sensitivity levels of both mutants suggest that these two genes are involved in iron metabolism.

Constitutive septal murein synthesis in Escherichia coli with impaired activity of the morphogenetic proteins RodA and penicillin-binding protein 2

The pattern of peptidoglycan (murein) segregation in cells of Escherichia coli with impaired activity of the morphogenetic proteins penicillin-binding protein 2 and RodA has been investigated by the D-cysteine-biotin immunolabeling technique (M. A. de Pedro, J. C. Quintela, J.-V. Höltje, and H. Schwarz, J. Bacteriol. 179:2823-2834, 1997). Inactivation of these proteins either by amdinocillin treatment or by mutations in the corresponding genes, pbpA and rodA, respectively, leads to the generation of round, osmotically stable cells. In normal rod-shaped cells, new murein precursors are incorporated all over the lateral wall in a diffuse manner, being mixed up homogeneously with preexisting material, except during septation, when strictly localized murein synthesis occurs. In contrast, in rounded cells, incorporation of new precursors is apparently a zonal process, localized at positions at which division had previously taken place. Consequently, there is no mixing of new and old murein. Old murein is preserved for long periods of time in large, well-defined areas. We propose that the observed patterns are the result of a failure to switch off septal murein synthesis at the end of septation events. Furthermore, the segregation results confirm that round cells of rodA mutants do divide in alternate, perpendicular planes as previously proposed (K. J. Begg and W. D. Donachie, J. Bacteriol. 180:2564-2567, 1998).

Visualizing multiple constrictions in spheroidal Escherichia coli cells

An Escherichia coli cell grows by elongation and divides in a perpendicular plane. Alternating planes of successive divisions in three dimensions can only be ascertained when multiple constrictions exist simultaneously in large, spheroidal cells (with extended constriction process), if the division signals are enhanced. Large, spheroidal cells are obtained by a brief mecillinam treatment, and more frequent divisions are achieved by manipulating the rate of chromosome replication without affecting cell mass growth rate. Such a procedure has recently been performed by thymine-limitation of E. coli K12 strain CR34 (Zaritsky et al., Microbiology 145 (1999), 1052-1022). Enhancing the replication rate in cells with multi-forked replicating chromosomes (by addition of deoxyguanosine) shortens the intervals between successive terminations and thus triggers divisions more frequently. Monoclonal antibodies against FtsZ were used to visualize the rings of secondary constrictions, but apparent shortage of FtsZ to complete rings over wide cells allowed assembly of arcs only. The arcs observed were not parallel nor perpendicular; the tilted constriction planes are consistent with our 3-D 'nucleoid segregation'model for division under conditions which relieve the cylindrical constraint for nucleoid segregation by the bacillari peptidoglycan sacculus (Woldringh et al. , J. Bacteriol. 176 (1994) 6030-6038). The shortage in FtsZ may explain the longer time required to complete the division process in wide cells with long circumferences, observed during thymine step-up. Overexpression of fusion protein FtsZ-GFP on a multi-copy plasmid should circumvent the shortage.

Division planes alternate in spherical cells of Escherichia coli

In the spherical cells of Escherichia coli rodA mutants, division is initiated at a single point, from which a furrow extends progressively around the cell. Using "giant" rodA ftsA cells, we confirmed that each new division furrow is initiated at the midpoint of the previous division plane and runs perpendicular to it.

Control of cell shape and elongation by the rodA gene in Bacillus subtilis

The Escherichia coli rodA and ftsW genes and the spoVE gene of Bacillus subtilis encode membrane proteins that control peptidoglycan synthesis during cellular elongation, division and sporulation respectively. While rodA and ftsW are essential genes in E. coli, the B. subtilis spoVE gene is dispensable for growth and is only required for the synthesis of the spore cortex peptidoglycan. In this work, we report on the characterization of a B. subtilis gene, designated rodA, encoding a homologue of E. coli RodA. We found that the growth of a B. subtilis strain carrying a fusion of rodA to the IPTG-inducible Pspac promoter is inducer dependent. Limiting concentrations of inducer caused the formation of spherical cells, which eventually lysed. An increase in the level of IPTG induced a sphere-to-short rod transition that re-established viability. Higher levels of inducer restored normal cell length. Staining of the septal or polar cap peptidoglycan by a fluorescent lectin was unaffected during growth of the mutant under restrictive conditions. Our results suggest that rodA functions in maintaining the rod shape of the cell and that this function is essential for viability. In addition, RodA has an irreplaceable role in the extension of the lateral walls of the cell. Electron microscopy observations support these conclusions. The ultrastructural analysis further suggests that the growth arrest that accompanies loss of the rod shape is caused by the cell's inability to construct a division septum capable of spanning the enlarged cell. RodA is similar over its entire length to members of a large protein family (SEDS, for shape, elongation, division and sporulation). Members of the SEDS family are probably present in all eubacteria that synthesize peptidoglycan as part of their cell envelope.

Bacterial cell division

Bacteria usually divide by building a central septum across the middle of the cell. This review focuses on recent results indicating that the tubulin-like FtsZ protein plays a central role in cytokinesis as a major component of a contractile cytoskeleton. Assembly of this cytoskeletal element abutting the membrane is a key point for regulation. The characterization of FtsZ homologues in Mycoplasmas, Archaea, and chloroplasts implies that the constriction mechanism is conserved and that FtsZ can constrict in the absence of peptidoglycan synthesis. In most Eubacteria, the internal cytoskeleton must also regulate synthesis of septal peptidoglycan. The Escherichia coli septum-specific penicillin-binding protein 3 (PBP3) forms a complex with other enzymes involved in murein metabolism, suggesting a centrally located transmembrane complex capable of splicing multiple new strands of peptidoglycan into the cell wall. Important questions remain about the spatial and temporal control of bacterial division.

Division pattern of a round mutant of Escherichia coli

A round mutant of Escherichia coli, when grown in Methocel medium, forms chains of cells and does not form tetrads. This implies that successive division planes of the round mutant are parallel rather than perpendicular. These results differ from a previous proposal that division planes in this round mutant are perpendicular to the prior division plane (W. D. Donachie, S. Addinall, and K. Begg, Bioessays 17:569-576, 1995).

Two polypeptide products of the Escherichia coli cell division gene ftsW and a possible role for FtsW in FtsZ function

Two new mutations in the cell division gene ftsW have been isolated and characterized. The ftsW263(Ts) mutation results in a block to division at the initiation stage, similar to that previously observed with the ftsW201(Ts) mutation. The ftsW1640(Ts) mutation, however, causes a block to division at a later stage. The ftsW201 and ftsW263 mutants were shown to be phenotypically sensitive to the genetic background and growth conditions and are possibly relA dependent. Immunofluorescence microscopy showed that the FtsZ protein can localize to presumptive division sites in strains carrying ftsW(Ts) mutations at the nonpermissive temperature, suggesting that FtsW is unlikely to be specifically required for the localization of FtsZ to the division site. Examination of the localization of FtsZ in an ftsW rodA double mutant (lemon-shaped cells) revealed several classes of cells ranging from a common class where an FtsZ ring structure is absent to a class where FtsZ forms a complete ring at the midpoint of a lemon-shaped cell, suggesting a role for FtsW in the establishment of a stable FtsZ-based septal structure. We further demonstrate that two FtsW peptides, FtsWL (large) and FtsWS (small), can be identified and that the expression of ftsWS is sufficient for complementation of ftsW(Ts) mutations.

Cell shape and chromosome partition in prokaryotes or, why E. coli is rod-shaped and haploid

In the rod-shaped cells of E. coli, chromosome segregation takes place immediately after replication has been completed. A septum then forms between the two sister chromosomes. In the absence of certain membrane proteins, cells grow instead as large, multichromosomal spheres that divide successively in planes that are at right angles to one another. Although multichromosomal, the spherical cells cannot be maintained as heterozygotes. These observations imply that, in these mutants, each individual chromosome gives rise to a separate clone of descendant cells. This suggests a model in which sites for cell division form between pairs of sister chromosomes at the time of segregation, but are not used in spherical cells until further rounds of replication have taken place, thus ensuring clonal ('hierarchical') segregation of chromosomes into progeny cells. The role of the morphogenetic membrane proteins is to convert the basically spherical cell into a cylinder that is able to divide as soon as replication and segregation have been completed, and thus to maximise the number of viable cells per genome.

Identification of FtsW and characterization of a new ftsW division mutant of Escherichia coli

The product of the ftsW gene has been identified as a polypeptide that, like the related RodA protein, shows anomalous mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. FtsW is produced at low levels that can be increased by altering the translation initiation region of the mRNA. Overproduction of FtsW strongly inhibits cell growth. A new mutant allele, ftsW201, causes a temperature-dependent block in the initiation stage of cell division which is similar to the division block in ftsZ mutants. The block in initiation of division in the ftsW201 allele is shown to be independent of FtsZ or the FtsZ inhibitor, SulA. In addition, the ftsW201 mutant is hypersensitive to overproduction of the division initiation protein FtsZ at the permissive temperature. Our results suggest a role for FtsW in an early stage of division which may involve an interaction with FtsZ.

Increased susceptibility to beta-lactam antibiotics and decreased porin content caused by envB mutations of Salmonella typhimurium

Isogenic derivatives carrying envB6, envB9, or envB+ alleles were obtained from a strain of Salmonella typhimurium that was partially resistant to mecillinam, a beta-lactam antibiotic specific for penicillin-binding protein 2 (PBP 2). Testing of the isogenic strains with several antibacterial agents demonstrated that envB mutations either increased resistance (mecillinam) or did not affect the response (imipemen) to beta-lactams that act primarily on PBP 2, while susceptibilities to beta-lactams that act on PBP 1B, PBP 3, or both were increased. Furthermore, the susceptibilities of envB strains to hydrophobic compounds such as rifampin, novobiocin, or chloramphenicol were not modified, even though their susceptibilities to deoxycholate and crystal violet were enhanced. Outer cell membranes of envB mutants presented a 50% reduction in protein content compared with that of the isogenic envB+ strains, and OmpF and OmpD porins were particularly affected by the reduction. No alteration in the amount or pattern of periplasmic proteins was noticed, and lipopolysaccharides from envB mutants appeared to be normal by sodium dodecyl sulfate-urea-polyacrylamide gel electrophoresis. By using derivatives that produced a plasmid-encoded beta-lactamase, it was demonstrated that envB cells are slightly less permeable to cephalothin than envB+ bacteria are. It is concluded that the high susceptibility of envB mutants to beta-lactams is due to the increased effectiveness of the antibiotics on PBP 1B, PBP 3, or both.

Bacterial cell shape regulation: testing of additional predictions unique to the two-competing-sites model for peptidoglycan assembly and isolation of conditional rod-shaped mutants from some wild-type cocci

The two-competing-sites model for peptidoglycan assembly for bacterial cell shape regulation suggests that in rods, bacterial cell shape depends on the balance between two reactions (sites), one responsible for lateral wall elongation and the other responsible for septum formation. The two reactions compete with each other so that no lateral wall can be formed during septum formation and vice versa. When the site for lateral wall elongation overcomes that for septum formation, long rods or filaments are formed and cell division may be blocked. When the reaction leading to septum formation is hyperactive compared with the other, coccobacilli or cocci are formed. Other bacteria carry only one site for peptidoglycan assembly and can grow only as cocci. The two-competing-sites model predicts that two different types of cocci exist (among both morphology mutants and wild-type strains); one carries only the site for septum formation, whereas the other also carries the site for lateral wall elongation, the former site predominating over the latter. As a consequence of the inhibition (by antibiotics or by mutations) of septum formation in wild-type cocci of various species and in coccoid morphology mutants, some cocci are expected to undergo transition to rod shape and others are not. We have evaluated these predictions and show that they are in agreement. In fact, we found that among wild-type cocci belonging to 13 species, those of 6 species formed rods, whereas the remaining organisms maintained their coccal shape when septa were inhibited by antibiotics. Some coccoid morphology mutants of rod-shaped bacteria underwent coccus-to-rod transition after septum inhibition by antibiotics, whereas others maintained their coccal shape. When a mutation that causes septum inhibition was expressed in a morphology mutant of Klebsiella pneumoniae grown as a coccus, transition to rod shape was observed. A total of 914 mutants unable to form colonies at 42 degrees C were isolated from the coccoid species mentioned above. Between 75 and 95% of the mutants isolated from the species that formed rods when septum formation was inhibited by antibiotics but none of those isolated from the others underwent coccus-to-rod transition upon incubation at the nonpermissive temperature.

Cell length, nucleoid separation, and cell division of rod-shaped and spherical cells of Escherichia coli

By comparing the dimensions and DNA contents of normal rod-shaped Escherichia coli with those of mutants that grow as spheres or ellipsoids, we have determined that two parameters remain unchanged: the DNA/mass ratio and the average cell length (diameter, for spherical cells). In consequence, the average volumes and DNA contents of the spherical mutant cells are about four to six times greater than those of rod-shaped cells growing at a similar rate. In addition, it was found that cells of both rod and sphere forms had approximately the same number of nucleoids (as seen when the DNA was condensed after inhibition of protein synthesis). The nucleoids of the spherical cells therefore consist of four to six completed chromosomes each (polytene nucleoids). We suggest that the attainment of a minimum cell length is necessary for nucleoid separation after chromosome replication and that such a separation is itself a prerequisite for septum formation.

Effects of temperature inactivation of penicillin-binding protein 2 on envelope growth in Escherichia coli

The transition from rod-shaped to spheroidal cells was studied in a temperature-sensitive strain (SP45) of Escherichia coli K12, carrying a mutation (pbpA) in the gene coding for penicillin-binding protein 2 (PBP-2). This transition imposed by the restrictive temperature was associated with reduction of peptidoglycan/surface area and of cellular osmotic stability. Addition of nalidixic acid (20 micrograms/ml) at the temperature shift from 30 to 42 degrees C resulted in lysis of some cells and appearance of spheroidal bulges along the cylinders in other cells, consistent with the hypothesis of envelope weakening due to inactivation of PBP-2.

Streptococcus faecium mutants that are temperature sensitive for cell growth and show alterations in penicillin-binding proteins

The penicillin-binding proteins (PBPs) of 209 cell division (or growth) temperature-sensitive mutants of Streptococcus faecium were analyzed in this study. A total of nine strains showed either constitutive or temperature-sensitive conditional damage in the PBPs. Analysis of these nine strains yielded the following results: one carried a PBP 1 constitutively showing a lower molecular weight; one constitutively lacked PBP 2; two lacked PBP 3 at 42 degrees C, but not at 30 degrees C; one was normal at 30 degrees C but at 42 degrees C lacked PBP 3 and overproduced PBP 5; two were normal at 42 degrees C and lacked PBP 5 at 30 degrees C; one constitutively lacked PBP 5; and one carried a PBP 6 constitutively split in two bands. The mutant lacking PBP 3 and overproducing PBP 5 continued to grow at 42 degrees C for 150 min and then lysed. Revertants selected for growth capability at 42 degrees C from the mutants altered in PBPs 5 and 6 maintained the same PBP alterations, while those isolated from the strains with altered PBP 1 or lacking PBP 2 or PBP 3 showed a normal PBP pattern. Penicillin-resistant derivatives were isolated at 30 degrees C from the mutants lacking PBP 2 and from that lacking PBP 3. All these derivatives continued to show the same PBP damage as the parents, but overproduced PBP 5 and grew at 42 degrees C. These findings indicate that high-molecular-weight, but not low-molecular-weight, PBPs are essential for cell growth in S. faecium. This is in complete agreement with previous findings obtained with a different experimental system. On the basis of both previous and present data it is suggested that PBPs 1, 2, and 3 appear necessary for cell growth at optimal temperature (and at maximal rate), but not for cell growth at a submaximal one (or at a reduced rate), and an overproduced PBP 5 is capable of taking over the function of PBPs 1, 2, and 3.

Interaction between membrane proteins PBP3 and rodA is required for normal cell shape and division in Escherichia coli

In Escherichia coli, the products of several genes are required for septation, and the products of several others are required for the maintenance of the rod shape of the cells. We show here that the combination of certain mutations in a division gene (ftsI) with a specific mutation in one of the shape genes (rodA) could produce cells with normal shape and division, although separately these mutations led to a loss of the capacity to divide (ftsI) or to form normal rod-shaped cells (rodA). In contrast, combinations between other mutant alleles of these genes produced double mutants which had lost the capacity both to divide and to form rod-shaped cells. The mutual phenotypic correction observed within particular pairs of mutant genes suggests that the normal morphogenetic cycle of growth and division may require direct interaction between the two membrane proteins which are the products of these genes.

Staphylococci, in vitro and in vivo

Strains of Staphylococcus aureus were grown in broth and by the membrane technique; both drug-free media and media containing cloxacillin were used. The staphylococci grown in broth containing cloxacillin showed one thick cross wall and were larger than those grown in drug-free broth: 1.6 micron in diameter as opposed to 0.9 micron. The staphylococci grown on membranes placed on agar containing cloxacillin were 2-3 microns in diameter and contained three or more cross walls. Mice were infected intraperitoneally with staphylococci. After treatment with cloxacillin, the peritoneal fluid and spleens contained staphylococci that were 2-3 microns in diameter with three or more cross walls. A staphylococcal endocarditis was induced in rabbits that were then treated with cloxacillin. The staphylococci in the vegetation of the treated rabbits were 2-3 microns in diameter and contained multiple cross walls. Large staphylococci with multiple cross walls were observed in specimens from patients with respiratory infections treated with beta-lactam antibiotics. It appears, therefore, that the ultrastructure of staphylococci in vivo is comparable to that of staphylococci grown on a solid support medium such as a membrane, and different from that of staphylococci grown in a liquid medium.

Cell shape and division in Escherichia coli: experiments with shape and division mutants

Double mutants which carry mutations in genes (rodA, pbpA) required for cell elongation (i.e., maintenance of rod shape) in combination with mutations in genes (ftsA, ftsI, ftsQ, or ftsZ) required for septation were constructed. Such mutants were able to grow for about two mass doublings at a normal rate at the restrictive temperature (42 degrees C). The morphology of the cells formed under these conditions was interpreted by assuming the existence of a generalized system for peptidoglycan growth together with two additional systems which modify the shape of the growing peptidoglycan layer. The results also showed that different fts genes probably control different stages in septation. ftsZ (sulB or sfiB) appears to be required for the earliest step in septation, ftsQ and ftsI (pbpB or sep) are required for a later step or steps, and ftsA is required only for the latest stages in septation.

Inhibition of lateral wall elongation by mecillinam stimulates cell division in certain cell division conditional mutants of Escherichia coli

The effect of mecillinam, a beta-lactam antibiotic that specifically binds penicillin-binding protein 2 of Escherichia coli, causes transition from rod to coccal shape, and inhibits cell division in sensitive cells, has been tested on three different E. coli temperature-sensitive cell division mutants. At the nonpermissive temperature, the antibiotic allows an increase in cell number for strains BUG6 and AX655 but not for AX621. In strain AX655, the cell division stimulation was observed only if the antibiotic was added immediately after shifting to the nonpermissive temperature, whereas in BUG6, the rise in cell number was observed also when mecillinam was added after 90 min of incubation at the nonpermissive temperature. In all cases, cell division began occurring 30 min after addition of the antibiotic. Mecillinam had no effect on division of dnaA, dnaB temperature-sensitive mutants or on division of BUG6 derivatives made resistant to this antibiotic. Other beta-lactam antibiotics such as penicillin, ampicillin, cephalexin, and piperacillin and non beta-lactam antibiotics such as fosfomycin, teichomycin, and vancomycin that inhibit cell wall synthesis did not show any effect on cell division for any of the mutants. The response of the three cell division mutants to mecillinam is interpreted in terms of a recently proposed model for shape regulation in bacteria.

Effects of mecillinam and cefoxitin on growth, macromolecular synthesis, and penicillin-binding proteins in a variety of streptococci

Although some strains of streptococci seem to be virtually inert to mecillinam, the growth of other strains, notably certain viridans streptococci (Streptococcus mutans and Streptococcus sanguis) was inhibited by relatively low concentrations of the drug. Inhibition of the synthesis of peptidoglycan, RNA, protein, and DNA in two tolerant strains, S. mutans FA-1 and GS-5, was studied over a wide range of concentrations of mecillinam, benzylpenicillin, and cefoxitin. The responses of both strains to all three beta-lactams were very similar; that is, synthesis of insoluble peptidoglycan was most susceptible. Inhibition of peptidoglycan synthesis was followed rapidly and sequentially by substantial but less severe inhibitions of RNA and protein synthesis. Significant inhibition of DNA synthesis was not observed. Binding studies with [14C]benzylpenicillin alone or after preexposure of membrane preparations to benzylpenicillin, mecillinam, or cefoxitin suggest that reasonably selective binding of a beta-lactam antibiotic to one or two of the major penicillin-binding proteins (PBP 1 or PBP 4) of S. mutans GS-5 and FA-1 may be the initial step in the series of events that results in the inhibition of growth and in the inhibition of insoluble peptidoglycan assembly and of RNA and protein synthesis.

Labeling pattern of major penicillin-binding proteins of Escherichia coli during the division cycle

Escherichia coli cells were synchronized by the elutriation technique. The pattern of penicillin-binding proteins (PBPs) in synchronously growing cells was determined with an iodinated derivative of ampicillin in intact cells as well as in isolated membranes. This was done under nonsaturating conditions as well as under conditions in which the PBPs were saturated with [125I]ampicillin. No evidence was found for fluctuations in the PBP pattern: the PBPs seem to be present in a constant ratio throughout the division cycle. The E. coli cells exert their control on shape maintenance and cell wall growth apparently not on the level of concentration of PBPs in the cell but rather on activation of existing components.

Novel peptidoglycans in Caulobacter and Asticcacaulis spp

Peptidoglycan sacculi free of poly-beta-hydroxybutyric acid were prepared from whole cells of four species of Caulobacter and two species of Asticcacaluis and from morphological mutants of Caulobacter crescentus and Caulobacter leidyi. Acid hydrolysates of the sacculi were analyzed quantitatively, and each of the hydrolysates was found to contain significant amounts of only five ninhydrin-reactive compounds: alanine, glutamic acid, alpha , omega-diaminopimelic acid, muramic acid, and glucosamine. Four types of peptidoglycans were distinguishable on the basis of the molar ratios among these five compounds. The respective ratios were as follows: in C. leidyi, 2:1:1:1:0.8; in Asticcacaulis biprosthecum, 1.7:1.6:1.1:0.7; in the cells of the remaining species, 2:1:1:1.2:0.8; and in stalks shed by the abscission mutant 2NY66, 2:1:1:1:1.67. Thus, in addition to some species differences among these caulobacters, it was found that the peptidoglycan sacculus of the stalked C. crescentus cell is chemically differentiated; the cellular peptidoglycan is richer in muramic acid than is the peptidoglycan of typical gram-negative bacteria, and the peptidoglycan of the stalk is correspondingly rich in glucosamine. Empirical formulas for the repeating units of the peptidoglycans have been inferred on the basis of the molar ratios of their amino components.

Murein synthesis and beta-lactam antibiotic susceptibility during rod-to-sphere transition in a pbpA(Ts) mutant of Escherichia coli

The conditional morphology mutant of Escherichia coli SP45 grows as a rod at 30 degrees C and assumes a spherical shape after 90 min of incubation at 42 degrees C. The rod-to-sphere morphological transition has been found to be associated with the disappearance of penicillin-binding protein 2 (PBP-2), the progressive reduction (as much as 50%) of murein synthesis, as measured both in intact cells and ether-permeabilized bacteria, and alterations in the structure of the cell envelope, including detachment of the outer membrane from the underlying structures. The detachment was initially localized at the poles of the cells and then spread over the entire surface. Shape transition was also linked to increased susceptibility to beta-lactam antibiotics which preferentially bound to PBP-1A (cephalothin, cephaloridine) or to PBP-3 (furazlocillin, piperacillin). Treatment with beta-lactams possessing a high affinity for PBP-1A, although inducing a low degree of peptidoglycan synthesis inhibition (5 to 10%), was associated with a marked loss of cell viability and massive lysis. On the other hand, the simultaneous absence of PBP-2 and inhibition of PBP-3 causes a significant reduction of peptidoglycan synthesis, yet only slightly affected cell viability. Whereas PBP-1A inhibition during shape transition had no effect on morphology, addition of antibiotics binding to PBP-3 30 min after the temperature shift-up caused formation of elongated cells with a centrally located bulge, not observed in similarly treated cells grown at 30 degrees C. Inhibition of PBP-3 in round cells 90 min after temperature shift caused formation of giant cells, indicating complete loss of elongation ability. The different effects of the simultaneous inhibition of two PBPs, combining mutational loss with specific binding in vivo of another PBP by beta-lactams, provide new insight into the role of these proteins and the killing mechanisms of this class of antibiotics.

Topographical distribution of penicillin-binding proteins in the Escherichia coli membrane

The penicillin-binding proteins (PBPs) found in the membranes of Escherichia coli X925 minicells (primarily cell ends or septa) were compared with those found in rod-shaped cells (primarily sidewalls) in an effort to determine whether certain PBPs are unevenly distributed over the bacterial cell membrane. The seven major PBPs of E. coli were all present in minicell membranes. PBP 1B was altered in minicells, however, appearing as two bands on sodium dodecyl sulfate-polyacrylamide gels rather than the usual three. PBP 2, which is needed for longitudinal growth of the cell but not for septum formation, was significantly reduced in minicell membranes. This observation is consistent with the fact that minicells contain very little sidewall material and raises the possibility that the specialized function of PBP 2 may be determined or regulated by its uneven topographical distribution in the membrane. None of the PBPs appeared to be selectively enriched in minicell membranes.

Purification of penicillin-binding protein 2 of Escherichia coli

Penicillin-binding protein 2 (PBP-2) of Escherichia coli K-12 was purified by covalent affinity chromatography using 6-aminopenicillanic acid covalently coupled to carboxymethyl-Sepharose (6-APA-CM-Sepharose). Purification of PBP-2 was accomplished by prebinding the methoxy cephalosporin, cefoxitin, to the Triton X-100-solubilized PBPs of E. coli and then incubating the PBPs with 6-APA-CM-Sepharose. Cefoxitin readily binds to all the E. coli PBPs except PBP-2 and, thus, in the presence of cefoxitin, only PBP-2 could bind to the 6-APA-CM-Sepharose. The purification of a mixture of all of the PBPs of E. coli by affinity chromatography is also described.

Effects of furazlocillin, a beta-lactam antibiotic which binds selectively to penicillin-binding protein 3, on Escherichia coli mutants deficient in other penicillin-binding proteins

Furazlocillin binds selectively to penicillin-binding protein 3 (PBP-3), prevents septation of Escherichia coli, and allows the cells to form long filaments without lysis. The effect of furazlocillin on the morphology, autolysis, and murein synthesis of E. coli mutants deficient in either PBP-1A, PBP-1Bs, or PBP-2 was studied. The results reveal that PBP-1A and PBP-1Bs functions are not equivalent since furazlocillin affects the morphology, autolysis, and murein synthesis of PBP1A- mutants quite differently from that of PBP-1Bs mutants. Different "PBP-2-" mutants were found to respond to furazlocillin in dramatically different ways: strain LS-1 cells formed elongated rods with a central bulge which eventually lysed, whereas SP6 cells formed stable "barbells" in which the two daughter cells were well separated but remained connected by a thick central region.

Defective and plaque-forming lambda transducing bacteriophage carrying penicillin-binding protein-cell shape genes: genetic and physical mapping and identification of gene products from the lip-dacA-rodA-pbpA-leuS region of the Escherichia coli chromosome

A series of defective lambda transducing phage carrying genes from the lip-leuS region of the Escherichia coli chromosome (min 14 on the current linkage map) has been isolated. The phage defined the gene order as lac---lip-dacA-rodA-pbpA-leuS---gal. These included the structural genes for penicillin-binding protein 2 (pbpA) and penicillin-binding protein 5 (dacA) as well as a previously unidentified cell shape gene that we have called rodA. rodA mutants were spherical and very similar to pbpA mutants but were distinguishable from them in that they had no defects in the activity of penicillin-binding protein 2. The separation into two groups of spherical mutants with mutations that mapped close to lip was confirmed by complementation analysis. The genes dacA, rodA, and pbpA lie within a 12-kilobase region, and represent a cluster of genes involved in cell shape determination and peptidoglycan synthesis. A restriction map of the lip-leuS region was established, and restriction fragments were cloned from defective transducing phage into appropriate lambda vectors to generate plaque-forming phage that carried genes from this region. Analysis of the proteins synthesized from lambda transducing phage in ultraviolet light-irradiated cells of E. coli resulted in the identification of the leuS, pbpA, dacA, and lip gene products, but the product of the rodA gene was not identified. The nine proteins that were synthesized from the lip-leuS region accounted for 57% of its coding capacity. Phage derivatives were constructed that allowed about 50-fold amplification of the levels of penicillin-binding proteins 2 and 5 in the cytoplasmic membrane.

Cluster of mrdA and mrdB genes responsible for the rod shape and mecillinam sensitivity of Escherichia coli

Two closely linked genes, mrdA and mrdB, located at ca. 14.2 min on the Escherichia coli chromosomal linkage map, seen to be responsible for the normal rod shape and mecillinam sensitivity of E. coli. The product of mrdA was concluded to be penicillin-binding protein 2, because mrdA mutations caused formation of thermosensitive penicillin-binding protein 2. The product of the mrdB gene is unknown. At 42 degrees, C, mutation in either of these genes caused formation of spherical cells and mecillinam resistance. Both mutations was recessive, and complementation, as detected in +-/-+ meroheterodiploids having the wild-type phenotype, provided strong evidence that the two mutations are in different complementation groups. P1 transduction suggested that the most plausible gene order is leuS-mrdA-mrdB-lip. The rodA mutation reported previously seems to be similar to the mrdB murations, but the identities of the two have not yet been proven.

Temperature-sensitive 6-aminopenicillanic acid-resistant mutants of Escherichia coli

Wild-type Escherichia coli cells became spherical in the presence of low concentrations of 6-aminopenicillanic acid (APA). Higher concentrations of APA caused inhibition of cell division (filamentation) and lysis. Spontaneous temperature-sensitive APA-resistant mutants were isolated and characterized. These mutants also possessed increased resistance to mecillinam but not to benzylpenicillin, ampicillin, and cephaloridine. They formed round or pleomorphic cells at the nonpermissive temperature and possessed thermolabile penicillin-binding protein 2 activity. Temperature-resistant revertants of these mutants had normal rod-shaped morphology, normal levels of sensitivity to APA and mecillinam, and thermostable penicillin-binding protein 2 activity. The mutation conferring APA resistance cotransduced with lip and mapped, therefore, close to 14 min on the E. coli linkage map. APA appears to be more specific than mecillinam as a selective agent for the isolation of penicillin-binding protein 2 mutants.