Identification of cell wall subunits in bacillus subtilis and analysis of their segregation during growth

Analysis of morphological change mechanism of linear Arthrospira platensis based on transcriptome results

Arthrospira platensis is a kind of filament cyanobacteria, which is mainly helical with a few linear. The shape of the filaments, such as the length and the pitch, may change with the changes in the environment. Natural Arthrospira platensis FACHB793 is linear, although it has become helical due to a mutation introduced in the process of cultivation. To study the molecular mechanism responsible for the morphological changes of the filaments, two samples were isolated from a natural mutant of Arthrospira platensis FACHB793, which were helical shaped (named A793_H) and linear shaped (named A793_L). Transcriptome sequencing, GO and KEGG enrichment analysis showed that the expression of genes related to or involved in peptidoglycan biosynthesis, beta lactam resistance, photosynthetic antenna protein expression, bacterial secretion, and ABC transporter activity changed between the two samples. The expression of murE and murG in the peptidoglycan biosynthesis pathway and that of oppD in beta lactam resistance were all down-regulated in the helical filaments, which may be related to the longer cell wall and higher peptidoglycan synthesis in linear filaments than helical filaments. In helical filaments, the up-regulation of tatC gene expression in bacterial secretion may be related to the secretion of peptidoglycan degrading enzymes, which may help to change the shape from linear to helical. Moreover, apcA and cpcA in photosynthetic antenna protein expression and nrt and nirA in nitrogen metabolism were all down regulated in the helical filaments, which may be due to the deformed shape of A. platensis FACHB793, resulting in decreased photosynthetic activity in helical filaments. This research provides a foundation for elucidating the possible morphogenetic mechanism of Arthrospira platensis.

Cell wall assembly in Bacillus subtilis: how spirals and spaces challenge paradigms

Although the bacterial cell wall has been the subject of decades of investigation, recent studies continue to generate novel and controversial models of its synthesis and assembly. Here we compare and contrast the transcompartmental biosyntheses of peptidoglycan and teichoic acid in Bacillus subtilis. In addition, the current paradigms of B. subtilis wall assembly and structure are distinguished from emerging models of murein insertion and organization. We discuss evidence for the directed, cytoskeleton-dependent insertion of nascent peptidoglycan and the existence of a periplasmic compartment. Furthermore, we summarize the challenges these findings represent to the existing paradigm of murein insertion. Finally, motivated by these new developments, we discuss outstanding issues that remain to be addressed and suggest research directions that may contribute to a better understanding of cell wall assembly in B. subtilis.

Bacterial cell shape

Bacterial species have long been classified on the basis of their characteristic cell shapes. Despite intensive research, the molecular mechanisms underlying the generation and maintenance of bacterial cell shape remain largely unresolved. The field has recently taken an important step forward with the discovery that eukaryotic cytoskeletal proteins have homologues in bacteria that affect cell shape. Here, we discuss how a bacterium gains and maintains its shape, the challenges still confronting us and emerging strategies for answering difficult questions in this rapidly evolving field.

Control of cell morphogenesis in bacteria: two distinct ways to make a rod-shaped cell

Cell shape in most eubacteria is maintained by a tough external peptidoglycan cell wall. Recently, cell shape determining proteins of the MreB family were shown to form helical, actin-like cables in the cell. We used a fluorescent derivative of the antibiotic vancomycin as a probe for nascent peptidoglycan synthesis in unfixed cells of various Gram-positive bacteria. In the rod-shaped bacterium B. subtilis, synthesis of the cylindrical part of the cell wall occurs in a helical pattern governed by an MreB homolog, Mbl. However, a few rod-shaped bacteria have no MreB system. Here, a rod-like shape can be achieved by a completely different mechanism based on use of polar growth zones derived from the division machinery. These results provide insights into the diverse molecular strategies used by bacteria to control their cellular morphology, as well as suggesting ways in which these strategies may impact on growth rates and cell envelope structure.

Growth and morphogenesis in Streptomyces

In this report we propose a model of apical growth for streptomycetes. The apical tip is considered as a multilayered wall that expands by an inside-to-outside mechanism of growth. It is also assumed that each layer is made up of peptidoglycan blocks, each of them being the result of the biosynthetic activity of a wall-synthesizing unit or membrane-associated growth zone. According to our model, apical growth occurs as follows: as a consequence of the hydrostatic pressure and the cleavage of some bonds, the layers are pushed and forced to slide (one with respect to the other), migrating from the center of the tip (at the inside of the wall) towards a peripheral location (at the outside of the wall). The model also incorporates a mechanism by which apical growth can be regulated and coordinated with the replication of the chromosome.

Identification of the structural genes for N-acetylmuramoyl-L-alanine amidase and its modifier in Bacillus subtilis 168: inactivation of these genes by insertional mutagenesis has no effect on growth or cell separation

The region of the Bacillus subtilis 168 chromosome that contains the structural genes for the major vegetative cell autolysin, (N-acetyl-muramoyl-L-alanine amidase), and its modifier protein has been cloned. Insertional mutagenesis with integrative plasmids carrying small DNA fragments from this region has revealed that both genes are located on a 4 kb fragment; they are organised in one transcription unit, the modifier being transcribed first. Studies of derivatives in which either the amidase or the modifier or both proteins are inactivated have revealed that amidase-deficient strains are not affected in growth, cell separation, transformability or sporulation. Observed phenotypic differences were altered kinetics of, cell wall turn-over and a reduced rate of, autolysis of native cell wall preparations. A residual amidase activity, about 3% of that of the wild-type strain, was found in strains devoid of the major amidase. A new, distinct cell wall-bound protein, designated CWBP49', with the same molecular weight as the amidase, was identified in mutants devoid of the latter enzyme.

The functions of autolysins in the growth and division of Bacillus subtilis

Some bacteria, such as streptococci, exhibit growth from discrete and well-defined zones. In Streptococcus faecalis, growth zones can be observed in the electron microscope, and the position of the zone can be used as a marker for cell cycle events. Growth of the cell surface of Bacillus subtilis appears to be by a much different mechanism from that of streptococci. Cell elongation takes place by the insertion at many sites in the cell cylinder of peptidoglycan components. The insertion occurs on the inner face of the wall, and upon cross linking, the new wall material becomes stress bearing and older wall is pushed to the surface. When old wall reaches the surface, it becomes susceptible to excision by autolysins, resulting in wall turnover; cell elongation, due to the stretching of the cross-linked peptidoglycan, therefore, accompanies turnover and does not require a specialized growth zone.

Growth kinetics of individual Bacillus subtilis cells and correlation with nucleoid extension

The growth rate of individual cells of Bacillus subtilis (doubling time, 120 min) has been calculated by using a modification of the Collins-Richmond principle which allows the growth rate of mononucleate, binucleate, and septate cells to be calculated separately. The standard Collins-Richmond equation represents a weighted average of the growth rate calculated from these three major classes. Both approaches strongly suggest that the rate of length extension is exponential. By preparing critical-point-dried cells, in which major features of the cell such as nucleoids and cross-walls can be seen, it has also been possible to examine whether nucleoid extension is coupled to length extension. Growth rates for nucleoid movement are parallel to those of total length extension, except possibly in the case of septate cells. Furthermore, by calculating the growth rate of various portions of the cell surface, it appears likely that the limits of the site of cylindrical envelope assembly lie between the distal tips of the nucleoid; the old poles show zero growth rate. Coupling of nucleoid extension with increase of cell length is envisaged as occurring through an exponentially increasing number of DNA-surface attachment sites occupying most of the available surface.

Cell wall and DNA cosegregation in Bacillus subtilis studied by electron microscope autoradiography

Cells of a Bacillus subtilis mutant deficient in both major autolytic enzyme activities were continuously labeled in either cell wall or DNA or both cell wall and DNA. After appropriate periods of chase in minimal as well as in rich medium, thin sections of cells were autoradiographed and examined by electron microscopy. The resolution of the method was adequate to distinguish labeled DNA units from cell wall units. The latter, which could be easily identified, were shown to segregate symmetrically, suggesting a zonal mode of new wall insertion. DNA units could also be clearly recognized despite a limited fragmentation; they segregated asymmetrically with respect to the nearest septum. Analysis of cells simultaneously labeled in cell wall and DNA provided clear visual evidence of their regular but asymmetrical cosegregation, confirming a previous report obtained by light microscope autoradiography (J.-M. Schlaeppi and D. Karamata, J. Bacteriol. 152:1231-1240, 1982). In addition to labeled wall units, electron microscopy of thin sections of aligned cells has revealed fibrillar networks of wall material which are frequently associated with the cell surface. Most likely, these structures correspond to wall sloughed off by the turnover mechanism but not yet degraded to filterable or acid-soluble components.

Monovalent cations enable cell wall turnover of the turnover-deficient lyt-15 mutant of Bacillus subtilis

A lyt-15 mutant reported to be unable to turn over the cell wall exhibited the same rate of wall turnover as the standard strain if the medium contained 0.2 M NaCl, which did not affect growth. Cell wall autolysis was also optimal at 0.2 M NaCl.

Morphological and cell wall alterations in thermosensitive DNA mutants of Bacillus subtilis

Incubation of thermosensitive dna mutants of Bacillus subtilis at the nonpermissive temperature results, at the cellular level, in the appearance of swellings. The study of one particular type of swelling, named "terminal balloon", reveals that the occurrence of the latter was correlated with completion of rounds of DNA replication. Morphological and autoradiographic observations reveal that (a) cell wall consists of two layers, (b) the outer layer splits at a fixed distance from the cell pole and allows the formation of a balloon contained within a single wall layer and (c) the bulk of active synthesis of cell wall is localized in the balloon area leading to the formation of a near spherical monolayer. Implications of these results for cell wall morphogenesis are discussed.

Cell wall turnover in lyt mutants of Bacillus subtilis

One of the reported phenotypes of Bacillus subtilis strains containing the lyt mutations is a deficiency in cell wall turnover. However, it has recently been shown in our laboratory that these mutant cells can turn over their walls under certain conditions (Vitkovic et al. [2, 11] and Abstr. K3, Annu. Meet. Am. Soc. Microbiol., 1984, p. 147). I show in this paper that the long lag in wall turnover of lyt-1 (FJ3) and lyt-2 (FJ6) strains coincided with the time it took cells (which grow in long chains) to reach the end of exponential growth and to begin separating. At this time, the specific activity of N-acetylmuramyl-L-alanine amidase in the mutant strains reached the same level as in the standard strain in mid-exponential growth phase. Wall turnover was therefore probably caused by increased amidase activity. The turnover in strain lyt-15 (Nil5) was initiated by addition of NaCl and its rate reached that of the standard strain at 0.2 M NaCl. The amidase activities isolated from the lyt-15 and the standard strains varied similarly with NaCl concentration. When walls were probed with amidases and lysozyme in the presence of increasing concentrations of NaCl, the lyt-15 cell wall appeared to be very different from the walls of the standard strain or M. luteus (a standard lysozyme substrate). The results suggest that turnover deficiency in the lyt-15 strain may be due to a change in its wall resulting in a reversible resistance to turnover.

Insertion and fate of the cell wall in Bacillus subtilis

Cell wall assembly was studied in autolysin-deficient and -sufficient strains of Bacillus subtilis. Two independent probes, one for peptidoglycan and the other for surface-accessible teichoic acid, were employed to monitor cell surface changes during growth. Cell walls were specifically labeled with N-acetyl-D-[3H]glucosamine, and after growth, autoradiographs were prepared for both cell types. The locations of silver grains revealed that label was progressively lost from numerous sites on the cell cylinders, whereas label was retained on the cell poles, even after several generations. In the autolysin-deficient and chain-forming strain, it was found that the distance between densely labeled poles approximately doubled after each generation of growth. In the autolysin-sufficient strain, it was found that the numbers of labeled cell poles remained nearly constant for several generations, supporting the premise that completed septa and poles are largely conserved during growth. Fluorescein-conjugated concanavalin A was also used to determine the distribution of alpha-D-glucosylated teichoic acid on the surfaces of growing cells. Strains with temperature-sensitive phosphoglucomutase were used because in these mutants, glycosylation of cell wall teichoic acids can be controlled by temperature shifts. When the bacteria were grown at 45 degrees C, which stops the glucosylation of teichoic acid, the cells gradually lost their ability to bind concanavalin A on their cylindrical surfaces, but they retained concanavalin A-reactive sites on their poles. Discrete areas on the cylinder, defined by the binding of fluorescent concanavalin A, were absent when the synthesis of glucosylated teichoic acid was inhibited during growth for several generations at the nonpermissive temperature. When the mutant was shifted from a nonpermissive to a permissive temperature, all areas of the cylinder became able to bind the labeled concanavalin A after about one-half generation. Old cell poles were able to bind the lectin after nearly one generation at the permissive temperature, showing that new wall synthesis does occur in the cell poles, although it occurs slowly. These data, based on both qualitative and quantitative experiments, support a model for cell wall assembly in B. subtilis, in which cylinders elongate by inside-to-outside growth, with degradation of the stress-bearing old wall in wild-type organisms. Loss of wall material, by turnover, from many sites on the cylinder may be necessary for intercalation of new wall and normal length extension. Poles tend to retain their wall components during division and are turned over much more slowly.

Absence of correlation between rates of cell wall turnover and autolysis shown by Bacillus subtilis mutants

Bacillus subtilis mutants with reduced rates of cell wall autolysis reached a constant rate of wall turnover after a longer lag than the standard strain but eventually showed the same turnover rate. In reverse, a turnover-deficient mutant autolysed at a slightly higher rate than the standard strain. Consequently, there is no correlation between the rates of cell wall turnover and autolysis.

The surface stress theory of microbial morphogenesis

From the physics of the situation, one might conclude that the osmotic pressure within most prokaryotes creates a sufficiently high tension in the wall that organisms are at risk of ripping themselves apart. The Surface Stress Theory holds that they avoid this, and are able to carry out certain morphogenetic processes by linking the cleavages of appropriate bonds to enzymes that are sensitive to the stress in the bonds under attack. This tends to maintain the internal pressure and couples wall growth to cytoplasmic growth. Mechanisms with widely different geometry function for different organisms, but they have in common the requirement that new murein be covalently linked, and usually in an unextended conformation. Organisms differ in the site of wall addition and site of cleavage. In the Gram-positive Streptococcus, septum formation, and septal splitting occurs with little stretching of the unsplit septum. In Gram-positive bacilli, the cylinder grows by the inside-to-outside mechanism, and the poles appear to be formed by a split-and-stretch mechanism. Gram-negative rods, with their much thinner wall, resist a spherical shape and are capable of cell division by altering the biochemical mechanism so that initially one-third to one-fifth of the pressure-volume work required to increase the area of the side wall is needed to increase that in a developing pole. The growth of hyphae is a separate case; it requires that much less work is needed to force growth of the apex relative to the side wall. Some other bacterial shapes also can be explained by the theory. But at present, it is only a theory, although it is gradually becoming capable of accounting for current observations in detail. Its importance is that it prescribes many experiments that now need to be done.

Cosegregation of cell wall and DNA in Bacillus subtilis

Cosegregation of cell wall and DNA of a lysis-negative mutant of Bacillus subtilis was examined by continuously labeling (i) cell wall, (ii) DNA, and (iii) both cell wall and DNA. After four to five generations of chase in liquid media it was found by light microscope autoradiography that the numbers of wall segregation units per cell are 29 and 9 in rich and minimal medium, respectively. Under the same conditions the numbers of segregation units of DNA were almost 50% lower: 15 and 5, respectively. Simultaneous labeling of cell wall and DNA (iii) provided figures almost identical to those obtained for cell wall alone, (i), implying cosegregation of the two components. Statistical analysis ruled out their random distribution into daughter cells. Measurements of the positions of grain clusters at the end of the chase period along chains of cells, each derived from a single cell at the beginning of chase, show that cell wall units are localized according to a symmetrical pattern, whereas those of DNA are distributed in an asymmetrical but highly regular way. It appears that of two cell wall units of the same age one only has a strand of DNA attached to it. We present a simple diagrammatic model of cell wall organization and DNA-cell wall association which is compatible with our observations. Finally, we discuss previous experiments pertinent to cosegregation of cell wall and DNA obtained with cells grown on solid media as well as with germinating spores; an explanation for the independent segregation of cell wall and DNA observed in the latter case is advanced.

Bacterial growth and division: genes, structures, forces, and clocks

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Inversion of helix orientation in Bacillus subtilis macrofibers

The ability of helical macrofibers of Bacillus subtilis to convert from left- to right-handed structures or vice versa has been known to be controlled by the nutritional environment (N. H. Mendelson, Proc. Natl. Acad. Sci. U.S.A., 75:2478-2482, 1978). lyt mutants (Ni15, FJ3, FJ6, and FJ7) and also lyt phenocopies of wild-type strain FJ8 were able to undergo helix hand inversion as a function of temperature. The transition between right- and left-handed structures was in a very narrow range (about 2.5 degrees C) in the low to mid-40 degrees C. The helix orientation of these strains was also influenced by the concentration of divalent ions. Macrofiber handedness is governed, therefore, by at least four factors: genetic composition, temperature, and nutritional and ionic environments. Conditions normally used for growth fall, within this matrix, in the region favoring right-handed structures. Inhibition studies suggest that cell growth must occur for helix hand inversion.