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Biomedical subjects

A L Koch

Publications and source records attributed to A L Koch.

At least 19 recordsLinked to original sources
PubMed1992

Differences in the formation of poles of Enterococcus and Bacillus.

The pole of Enterococcus hirae (Streptococcus faecium) is more pointed than that of Bacillus subtilis; i.e. the pole of the former is prolate and the latter is oblate. Both species form their poles by constructing annular additions on the inside surface. In both cases, the thick septum starts to split from the outside before the septum is complete. Physiochemical considerations dictate that the peptidoglycan must be unstretched as laid down. However, it later becomes stressed and may stretch to increase its surface area or to change its shape. Our earlier analysis for B. subtilis demonstrated that, without the addition of new peptidoglycan, the nascent wall is stretched after it is externalized to 1.51 times the original area. The wall of partially formed poles that is already exteriorized continues to deform with further development. For E. hirae, Higgins & Shockman's measurements showed that the completed pole has a surface area 2.18 times larger than a completed septal disk and the wall changes shape very little after exteriorization. A model is presented here for the streptococcus in which the septal wall does not increase its surface area on exteriorization either by expansion or by murein insertion. Instead, the septal wall as it is split and exteriorized twists to become oblique, increasing the inner radius of the incomplete septum. In consequence of this rotation, extra layers of peptidoglycan are added to the inside face of the developing septum. This additional murein forms the more pointed pole shape for E. hirae. This "split-and-splay" model thus refines and extends the surface stress theory of E. hirae developed a decade ago by proposing a source of the extra wall needed for the formation of its prolate, more pointed, pole.

Animals↗
PubMed1992

Elasticity of the sacculus of Escherichia coli.

Preparations of purified peptidoglycan of Escherichia coli (i.e., sacculi) were studied by low-angle laser light scattering. Control experiments and theoretical calculations based on the Rayleigh-Gans theory showed that the mean sacculus surface area could be accurately inferred from measurements with our apparatus by using computer routines developed previously. Large changes in the mean saccular surface area resulted from alterations in the stress caused by varying the net charge on the sacculi. The net charge was affected by altering the suspending medium pH, causing carboxyl and amino groups in the peptidoglycan to gain or lose protons, or by acetylation or succinylation of the amino groups. A preponderance of either plus or minus charges caused an expansion of the mean sacculus surface area. The largest increase in area probably represents the elastic limit of the peptidoglycan and was 300% above the area of isoionic sacculi. This degree of expansion is consistent with possible conformations of the intact peptidoglycan structure without necessitating rupture of the wall fabric. Our findings concerning saccular elasticity provide support for the surface stress theory. It provides a mechanism so that bacteria can grow and divide while maintaining turgor pressure, without the necessity of having and using proteins to do the mechanical work.

Acetylation↗
PubMed1991

The first cellular bioenergetic process: primitive generation of a proton-motive force.

It is proposed that the energy-transducing system of the first cellular organism and its precursor was fueled by the oxidation of hydrogen sulfide and ferric sulfide to iron pyrites and two [H+] on the outside surface of a vesicle (the cell membrane), with the concomitant reduction of CO or CO2 on the interior. The resulting proton gradient across the cell membrane provides a proton-motive force, so that a variety of kinds of work can be done. It is envisioned as providing a selective advantage for cells capable of harvesting this potential. The proposed reactants for these reactions are consistent with the predicted composition of the Earth's early environment. Modern-day homologs of the ancestral components of the energy-transducing system are thought to be membrane-associated ferredoxins for the extracellular redox reaction, carbon monoxide dehydrogenase for the carbon fixation reaction, and ATPase for the harvesting of the proton gradient. With a source of consumable energy, the cell could drive chemical reactions and transport events in such a way as to be exploited by Darwinian evolution.

Adenosine Triphosphatases↗
PubMed1990

The relative rotation of the ends of Bacillus subtilis during growth.

Observation of long single filaments of Bacillus subtilis 168 in depression slide cultures demonstrated that one end rotated relative to the other during growth. This was observed with suspended filaments, filaments attached to glass surfaces and single stranded filaments folded back on themselves growing as a double stranded helix. This extends Mendelson's 1976 conclusion to cases with no alternative interpretation to the hypothesis that as each cell grows, the structure of the peptidoglycan changes to rotate one end relative to the other.

Bacillus subtilis↗
PubMed1990

Additional arguments for the key role of "smart" autolysins in the enlargement of the wall of gram-negative bacteria.

Because the wall of Gram-negative bacteria is thin, the mechanism for safe enlargement of the cell is subject to strong constraints. Several models for wall growth have been proposed; in the order that they have been proposed, these include: 1) an "allosteric" model in which the critical autolysin is only functional if the bond to be cleaved is near a covalently cross-linked, but unstretched oligopeptide; 2) a model in which the cell wall is thick enough to enlarge by the "inside-to-outside" mode characteristic of Gram-positive rods; 3) a "patches" model, recently proposed by Höltje, in which only parts of the cell wall are thickened at any one time; 4) a new multienzyme model in which the transpeptidase/autolysin complex cleaves one cross-linked oligopeptidoglycan chain for every two nascent chains covalently polymerized to the sacculus. These models are considered and contrasted. While none can be rigourously excluded, no. 4 is favoured. All models as applied to the Gram-negative rod-shaped bacteria require special, extraordinary features for their autolysins. These features have not been found with any other class of enzymes, but are essential to permit safe cell expansion.

Allosteric Regulation↗
PubMed1990

A proposed new system for valuing dental procedures. The relative time-cost unit.

The relative time cost unit (RTCU) is a proposed new system for valuing dental procedures that provides an alternative to traditional relative value units in fee-setting and reimbursement allowances. It incorporates personnel costs, task mixes, and task times into relative weights for dental procedures. The frequently performed procedure, "2-surface amalgam restoration," is used to illustrate how the RTCU values are derived from hospital task analysis data. The RTCU, as a data-based construct, holds appeal for restructuring fee schedules and has been used for almost a decade by insurance companies to value dental services, construct fee schedules, and evaluate reimbursement to providers.

California↗
PubMed1989

The origin of the rotation of one end of a cell relative to the other end during growth of gram-positive rods.

The Gram-positive rod wall elongates by an inside-to-outside mechanism of linking new peptidoglycan on the inside and the cracking, by autolysis, of old wall on the outside. During this process the peptidoglycan experiences stress in different directions in different levels of the wall. The stress that develops in a rod-shaped cell if the wall was uniform in physical properties throughout its thickness is twice as great in the hoop direction as in the axial direction. This leads to splitting in the direction of the longitudinal axis. However, the older, partially split, more peripheral wall is stressed in the direction of the elongating cell axis and thus favors circumferential cracks. It is suggested that these processes combine to form a system of helical cracks, grooves, or crevasses. The stable system of grooves would have the same handedness, fairly constant pitch and elongate as the cell grows. Their continuing development would result in the rotation of one end of the cell relative to the other even in cells with no spiral or apparent helical character. Such rotation has been experimentally observed with Bacillus subtilis. The proposed mechanism for rotation during growth may account, in part, for the formation of helical coils of bundles of filamentous organisms (macrofibers), the morphology of spirilla and vibroids, and for the shapes of some mutant and some antibiotic-treated organisms. Rotation due to generation of helical cracks as the result of the biophysics of the growth process as proposed here, is an alternative to the proposal by Mendelson (1976, Helical growth of Bacillus subtilis: a new model for cell growth. Proc. natn. Acad. Sci. U.S.A. 73, 1740-1744) that rotation is due to the laying down of nascent peptidoglycan in a helical pattern.

Bacillus↗
PubMed1988

Partition of autolysins between the medium, the internal part of the wall, and the surface of the wall of gram-positive rods.

Autolysins are exoenzymes formed by bacteria to aid in wall expansion and in cell division. For the Gram-positive rod, these enzymes function predominately on the periphery of the wall. As they destroy their substrate they rebind to peptidoglycan when the cell density is high. Only then is the concentration of autolysins high enough on the external surface to favor cell division and prevent the formation of chains of cells (filaments). It is suggesting that circumstance is important for the kinetics of turnover of the wall and the splitting of the septum.

Gram-Positive Bacteria↗
PubMed1988

Turgor pressure responses of a gram-negative bacterium to antibiotic treatment, measured by collapse of gas vesicles.

The internal hydrostatic pressure of Ancylobacter aquaticus was measured by collapsing the gas vesicles with an externally applied pressure. Turgor pressure was measured in conjunction with various antibiotic treatments to elucidate some aspects of the biophysics of gram-negative cell wall function. Differences in the effects of these drugs either alone or in combination with other treatments were related to known biochemical activities of these drugs. Our previous work, demonstrating a heterogeneous cellular response to beta-lactam antibodies, was confirmed and extended. Most of the cell wall growth-inhibiting antibiotics resulted in some cells (those in component I) developing a higher pressure, while the remainder (those in component II) lost turgor. Although the fraction of the cells in each component varied a little from subculture to subculture, it did not vary with time or choice of antibiotic treatment. Mecillinam gave a nearly monophasic response. All antibiotics blocking macromolecular synthesis gave monophasic curves. The 50% collapse pressure in some cases, however, was lower higher, or the same as the control.

Anti-Bacterial Agents↗
PubMed1988

Speculations on the growth strategy of prosthecate bacteria.

Appendaged bacteria with stalks that are extensions of the cell wall have had to solve the problems of growing the stalk as a tube of constant diameter and of partitioning their chromosomes into the asymmetric daughter cells. Although no experimental proof is given, it is suggested that both processes depend on the attachment of the chromosome origin and terminus to the wall at special terminal sites that contain the basal body (motor assembly) for flagellar motion.

Bacteria↗
PubMed1988

Why can't a cell grow infinitely fast?

Living cells are esoteric physiochemical systems that have evolved to survive and reproduce in their naturl environment. Under balanced conditions of growth, bacteria are probably systems as simple as any kind of free-living organism. Evolutionary forces, seemingly, should have driven prokaryotes to be very efficient. In part that is so; they make effective use of the machinery most expensive for the cell, i.e., the ribosomes and associated factors. But the evidence is that the efficiency with which they use the ribosomal machinery increases as the environment provides more favorable conditions for balanced growth. This article emphasizes the limitation to growth under optimal conditions. The role of fluctuations in the environment and the cost of accurate protein synthesis are discussed as reasons for the upper limit in obtainable specific growth rate.

Bacteria↗
PubMed1987

Variability of the turgor pressure of individual cells of the gram-negative heterotroph Ancylobacter aquaticus.

Cells of Ancylobacter aquaticus were observed under phase microscopy in a chamber to which a measured pressure could be applied. The initial collapse pressure (Ca), i.e., the lowest pressure needed to collapse the most pressure-sensitive gas vesicles, was measured for 69 cells. The cells were taken from cultures in low-density balanced exponential growth, and the experiments were performed quickly so that the bacteria were in a uniform physiological state at the time of measurement. The turgor pressure, Pt, is the difference between the pressure, C, that would cause collapse of vesicles when removed from the cell and Ca. In this paper we focus on the variability of Pt from cell to cell. Part of the observed variability of Ca was due to the variability of the collapse pressure of individual vesicles (standard deviation [SD] = 90 kPa), but because there were about 100 vesicles per cell and because a change in refracted light after the fifth vesicle (approximately) collapsed probably could be detected by the human eye, the pressure would only have an SD of 18.6 kPa due to this type of sampling error. The observed SD of Pt was 42 kPa, indicating that turgor pressure did vary considerably from cell to cell. However, the turgor pressure was independent of cell size. Statistical analysis showed that Pt would decrease 6.9 kPa over a cell cycle, but with too large an SD (19.9 kPa) to be significant. This implies that the observed change in Pt over the cell cycle is not statistically significant.

Cell Cycle↗
PubMed1987

Contraction of filaments of Escherichia coli after disruption of cell membrane by detergent.

The osmotic pressure within a living bacterium creates stresses in the peptidoglycan that stretch the sacculus. We measured the amount of stretch by monitoring the shrinkage of growing cells of Escherichia coli after removal of the osmotic pressure by disruption of the phospholipid membranes with sodium dodecyl sulfate. Because the rods of the wild type are so short, length changes of filaments of longer than 7 microns were measured on phase-contrast micrographs. The filaments were prepared by growing ftsA and ftsI strains under permissive conditions in rich medium and then shifting them to 42 degrees C for 40 to 180 min. During this time, the mutant cells became elongated but did not divide. The growing filaments were mounted on a glass surface that had been treated with poly-L-lysine or RNase. The filaments were photographed before being treated with sodium dodecyl sulfate. The filaments were rephotographed at the time when the first change in phase contrast was noted. Some filaments were also measured at 10-min time intervals from 0 to 60 min. The reduction in phase contrast signaled the leakage of solutes and the loss of turgor pressure. The average length of the filaments decreased 17%. If the circumference were stretched to the same degree, then the surface area in vivo would be 45% greater than in the relaxed state. For comparison, a fully cross-linked monolayer of E. coli peptidoglycan in its most compact conformation could stretch up to 300% in achieving the most extended conformation possible without splitting covalent bonds.

Cell Membrane↗
PubMed1987

Nephelometric determination of turgor pressure in growing gram-negative bacteria.

Gas vesicles were used as probes to measure turgor pressure in Ancylobacter aquaticus. The externally applied pressure required to collapse the vesicles in turgid cells was compared with that in cells whose turgor had been partially or totally removed by adding an impermeable solute to the external medium. Since gram-negative bacteria do not have rigid cell walls, plasmolysis is not expected to occur in the same way as it does in the cells of higher plants. Bacterial cells shrink considerably before plasmolysis occurs in hyperosmotic media. The increase in pressure required to collapse 50% of the vesicles as external osmotic pressure increases is less than predicted from the degree of osmotically inducible shrinkage seen with this organism or with another gram-negative bacterium. This feature complicates the calculation of the turgor pressure as the difference between the collapse pressure of vesicles with and without sucrose present in the medium. We propose a new model of the relationship between turgor pressure and the cell wall stress in gram-negative bacteria based on the behavior of an ideal elastic container when the pressure differential across its surface is decreased. We developed a new curve-fitting technique for evaluating bacterial turgor pressure measurements.

Ampicillin↗
PubMed1987

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.

Amidohydrolases↗