The wall of bacteria serves the roles that mechano-proteins do in eukaryotes

A new slant to the Z ring and bacterial cell branch formation

Rod-shaped bacteria such as Escherichia coli accurately maintain their shape from generation to generation. The cytoskeletal proteins MreB and FtsZ, which respectively guide parallel growth of the sidewall and perpendicular growth of the division septum, are important to maintain a straight sidewall and uniformly rounded cell poles. FtsZ normally assembles into a ring at the cell midpoint, called the Z ring, which is oriented perpendicular to the cell's axis and is thus in perfect position to guide growth of a perpendicular septum. In this issue of Molecular Microbiology, Potluri et al. show that low molecular weight penicillin binding proteins, particularly PBP5, have a role in maintaining the perpendicular geometry of the Z ring and subsequent septum in E. coli. When these factors are absent or perturbed, division septa are readily deformed, which results in abnormal cell poles that often bifurcate over time to generate branches. The data suggest that cellular branching in E. coli is specifically induced by aberrant septation events caused by mis-oriented Z rings and not by deformation of a growing cell pole or emergence of new tips from the sidewall, which are likely mechanisms of branching in other bacterial families.

Length distribution of the peptidoglycan chains in the sacculus of Escherichia coli

The stress-bearing fabric of bacteria is made of peptidoglycan. This crosslinked fabric is formed from disaccharide pentapeptide units that are transported through the cytoplasmic membrane and then polymerized in two directions: (i) to form oligoglycan chains; and (ii) to cross link these chains by tail-to-tail bonds from the muropeptides to the protruding peptides of other chains. The distribution of the glycan chain lengths is reminiscent of the "most probable distribution of polymer chemistry. Of course, the process is more complex than solely the random addition of units to growing chains. The complexity precludes mathematical analysis, but computer modeling of the Monte Carlo type is capable of including a range of possibilities. At each time point a specified number of disaccharides are singly added to the muramic acid residue ends of existing chains chosen at random. The transfer is in exchange for the cleavage of pyrophosphate bactoprenol that transported the disaccharide pentapeptide through the membrane. The progam then selects, again at random, which chain to cleave and between which two disaccharides of the chain the cleavage event is to occur. The cleavage generates an N -acetyl 1,6 anhydro-muramic acid end and a non-reducing N -acetyl glucosamine end. The simulation can be modified so that the program does not cleave off a disaccharide next to either end of the chain. Comparisons are shown with the experimental results of Obermann & H]oltje (1994. Microbiology140, 79-87.) They obtained their data by taking the results with normal growing cells and subtracting the similar data from minicells to estimate the chain length distribution in the cylinder part of the cell. In its most basic form the computer simulation has only one fitted parameter, K, which is the number of disaccharides added to the murein for every internal cleavage event. In this form the fitting to the experimental results is poor. One possible reason for this is that the tension on the chains, and therefore the probability of being cleaved by autolysins varies with orientation of the chain on the cylinder surface. It is well known that the tension in the cylindrical wall is twice as large in the circumferential direction as in the axial one, so one class would consist of those chains aligned longitudinally, subject to lower stress, and would have a higher energy of activation for autolysis than chains aligned circumferentially. A good fit is obtained on the assumption that there are only two classes of chains; one more likely to be cleaved than the other. The key point is that only two processes: adding of disaccharide pentapeptides at random to glycan chains and cleavage between the disaccharides at random, together with the assumption that the wall is less easily hydrolysed in the axial direction is sufficient to account for the experimental distribution.

Chaperonin filaments: their formation and an evaluation of methods for studying them

Chaperonins are multisubunit protein complexes that can be isolated from cells as high-molecular-weight structures that appear as double rings in the electron microscope. We recently discovered that chaperonin double rings isolated from the hyperthermophilic archaeon Sulfolobus shibatae, when incubated at physiological temperatures in the presence of ATP and Mg2+, stacked into filaments; we hypothesized that these filaments are related to filaments seen inside S. shibatae cells and that chaperonins exist as filaments in vivo (J. D. Trent et al., 1997, Proc. Natl. Acad. Sci. USA 94, 5383-5388). This paper elucidates the conditions under which we have observed S. shibatae chaperonins to form filaments and evaluates native polyacrylamide gel electrophoresis (PAGE), TEM, spectrophotometry, and centrifugation as methods for studying these filaments. We observed that in the presence of Mg2+ combined with ATP, ADP, ATPgammaS, or GTP, native PAGE indicated that chaperonin subunits assembled into double rings and that the conformation of these double rings was effected by nucleotide binding, but we saw no indication of chaperonin filament formation. Under these same conditions, however, TEM, spectroscopy, and centrifugation methods indicated that chaperonin subunits and double rings had assembled into filaments. We determined that this discrepancy in the representation of the chaperonin structure was due to the native PAGE method itself. When we exposed chaperonin filaments to the electrophoretic field used in native PAGE, the filaments dissociated into double rings. This suggests that TEM, spectrophotometry, and centrifugation are the preferred methods for studying the higher-order structures of chaperonins, which are likely to be of biological significance.

Why do many ruminal bacteria die and lyse so quickly?

Studies using 15N have indicated that as much as 50% of the microbial mass turns over before N passes to the lower gut, and this N recycling significantly decreases the availability of microbial protein. Protozoa digest bacteria and smaller protozoa, but bacterial protein can turn over even if protozoa are not present. Fibrobacter succinogenes cultures lyse even when they are growing, and the lysis rate is independent of growth rate. When extracellular sugar is depleted, F. succinogenes secretes an extracellular proteinase that inactivates the autolysins. This method of autolytic regulation decreases the turnover of stationary cells. Bacteriophage and anaeroplasma can cause lysogeny, but, as yet, there is little proof that these processes are important determinants of bacterial turnover in vivo. Dietary manipulations (e.g., salt feeding and particle size reduction) that increase liquid and solid dilution rates can increase bacterial flow by decreasing bacterial residence time and turnover. Some dead ruminal bacteria are able to maintain their cellular integrity, and the ratio of dead to live cells in ruminal fluid may be as great as 10:1. Bacterial survival appears to be at least partially explained by the method of sugar transport. When bacteria rely solely on mechanisms of ion-coupled sugar symport, an energized membrane is necessary for the reinitiation of growth. If group translocation (phosphotransferase system) is the mechanisms of transport, uptake can be driven by phosphoenolpyruvate, and an energized membrane and the storage of intracellular reserve materials are not an absolute criteria for survival. In some cases, N deprivation accelerates death. When Prevotella ruminicola was limited for N under conditions of excess energy, methylglyoxal production caused a rapid decrease in viability. The impact of bacterial death in the rumen is not clear-cut. If the rate of fermentation is zero-order with respect to cell concentration (substrate-limited), cell death would have little impact on digestion.

Isolation of an ftsZ homolog from the archaebacterium Halobacterium salinarium: implications for the evolution of FtsZ and tubulin

We have isolated a homolog of the cell division gene ftsZ from the extremely halophilic archaebacterium Halobacterium salinarium. The predicted protein of 39 kDa is divergent relative to eubacterial homologs, with 32% identity to Escherichia coli FtsZ. No other eubacterial cell division gene homologs were found adjacent to H. salinarium ftsZ. Expression of the ftsZ gene region in H. salinarium induced significant morphological changes leading to the loss of rod shape. Phylogenetic analysis demonstrated that the H. salinarium FtsZ protein is more related to tubulins than are the FtsZ proteins of eubacteria, supporting the hypothesis that FtsZ may have evolved into eukaryotic tubulin.

Electric fields induce curved growth of Enterobacter cloacae, Escherichia coli, and Bacillus subtilis cells: implications for mechanisms of galvanotropism and bacterial growth

Directional growth in response to electric fields (galvanotropism) is known for eukaryotic cells as diverse as fibroblasts, neurons, algae, and fungal hyphae. The mechanism is not understood, but all proposals invoke actin either directly or indirectly. We applied electric fields to bacteria (which are inherently free of actin) to determine whether actin was essential for galvanotropism. Field-treated (but not control) Enterobacter cloacae and Escherichia coli cells curved rapidly toward the anode. The response was both field strength and pH dependent. The direction of curvature was reversed upon reversal of field polarity. The directional growth was not due to passive bending of the cells or to field-induced gradients of tropic substances in the medium. Field-treated Bacillus subtilis cells also curved, but the threshold was much higher than for E. cloacae or E. coli. Since the curved morphology must reflect spatial differences in the rates of cell wall synthesis and degradation, we looked for regions of active wall growth. Experiments in which the cells were decorated with latex beads revealed that the anode-facing ends of cells grew faster than the cathode-facing ends of the same cells. Inhibitors of cell wall synthesis caused spheroplasts to form on the convex regions of field-treated cells, suggesting that the initial curvature resulted from enhanced growth of cathode-facing regions. Our results indicate that an electric field modulates wall growth spatially and that the mechanism may involve differential stimulation of wall growth in both anode- and cathode-facing regions. Electric fields may therefore serve as valuable tools for studies of bacterial wall growth. Use of specific E. coli mutants may allow dissection of the galvanotropic mechanism at the molecular level.