PubMed Health⌕ Search

Biomedical subjects

A L Koch

Publications and source records attributed to A L Koch.

At least 109 records · Page 6Linked to original sources
PubMed1980

How partially inhibitory concentrations of chloramphenicol affect the growth of Escherichia coli.

In the presence of up to 6 microM chloramphenicol, balanced exponential growth of Escherichia coli occurred at a reduced rate after an adjustment period. The inhibition of ribosome function by chloramphenicol within growing cells was inferred from measurements of growth rate and cell composition during balanced growth and of pulse-labeling of cells by radioactive proline after a 10-min exposure to chloramphenicol. In each case the results were consistent with simple noncompetitive inhibition of protein synthesis, with 50% inhibition occurring at 2 microM chloramphenicol, the concentration that gave 50% binding of chloramphenicol to purified ribosomes in vitro. The differences between these results and those obtained with cell-free protein synthesizing systems were shown to be in part due to slow binding of chloramphenicol and in part due to the slow rate and lack of saturation of the cell-free protein-synthesizing systems now available. During balanced growth in concentrations of chloramphenicol 1 microM or higher, the net rate of maturation of ribosomal ribonucleic acid was also inhibited (50% at 2.8 microM). The specific growth rate during balanced growth was inhibited by 50% at 1.8 microM chloramphenicol, but at higher concentrations inhibition was greater than expected from the simple noncompetitive dose-response observed for inhibition of polypeptide synthesis. However, the inhibition of maturation of ribosomal ribonucleic acid plus the inhibition of protein synthesis quantitatively accounted for the observed inhibition of growth. Thus, we have presented for the first time an essentially complete account of the effects of partially inhibitory concentrations of an antibiotic on the growth physiology of a bacterium.

Bacterial Proteins↗
PubMed1979

Selection and recombination in populations containing tandem multiplet genes.

Computer simulation for selective conditions that may apply in nature yielded three generalizations for prokaryotic organisms with recombinant mechanisms. (1) Selective forces can suffice to maintain a tandem gene family with the nearly optimum number of genes with little variance within the population. (2) Tandem genes will occur within the population unless the population is frequently cloned or unless the function due to a single copy is capable of over-providing the needs of the organism. (3) Even when there is no selective advantage or disadvantage due to extra gene copies, the population distribution becomes more skewed with time; and organisms with only single copies of the gene comprise a progressively larger fraction of the total. This may be the case with genes that function under strong cellular regulation. Evolutionary implications of these calculations are that the occurrence of unequal recombination of tandem genes would greatly slow evolution via duplication of genetic material. This difficulty and its possible resolutions are discussed.

Animals↗
PubMed1979

Growth conditions and rifampin susceptibility.

The susceptibility of Escherichia coli to rifampin was measured during unlimited growth in rich and poor media and during chemostat growth limited by the carbon source. During batch growth at low turbidities, the susceptibility of the bacteria increased as the growth rate decreased, consistent with the longer time available for drug penetration in the poorer media. During chemostat culture, the bacteria remained highly susceptible or became genetically resistant, dependent on the manner in which the bacteria were exposed to the antibiotic. If the concentration of rifampin was abruptly raised, susceptible cells were replaced by genetically resistant cells. However, if the concentration of antibiotic was raised slowly, the genetically susceptible cells continued to grow. This difference in response of chemostat cultures according to mode of drug administration was attributed to an inducible detoxification of the drug by the bacteria, because the susceptible genotype is maintained only when the concentration of rifampin is increased gradually and when a high population of cells is maintained. Direct evidence for the inactivation of the rifampin from the bioassay of culture supernatants is presented.

Drug Resistance, Microbial↗
PubMed1978

Constancy of growth on simple and complex media.

An apparatus has been developed in which bacterial growth can be measured very precisely over short intervals of time. Its precision is presented and used to assess the constancy of growth in batch culture. Under certain conditions, i.e., Luria broth or 0.2% glucose-M9 medium at very low cell densities, the specific growth rate of Escherichia coli appeared to be constant within the measurement limits of the method. In succinate minimal medium, the growth rate increased gradually over several days and never became constant. With nutrient broth and with Luria broth, growth slowed progressively at moderate cell densities within the range considered to be in the logarithmic phase of growth. In addition, temporary slowdown in growth rate occurred in these two complex media at characteristic cell densities. These gradual increases in succinate minimal medium and temporary slowdowns in the complex media would be undetectable without precise measurements and may have been a source of variability in many bacteriological studies.

Culture Media↗
PubMed1976

Distribution of bacteria in the velocity gradient centrifuge.

Cells in different parts of the cell cycle can be separated by brief centrifugation in a density stabilized gradient: the Mitchison-Vincent technique. The position of a cell in the tube depends upon its size, shape, and density, upon the gradients of density, viscosity, and centrifugal force through which it sediments, and upon time. A program to compute the velocities and integrate the velocity profile for particles of a particular size class is presented. Because enteric bacteria are a form intermediate between right cylinders and prolate ellipsoids of revolution, the program uses values for the frictional coefficient intermediate between those calculated for ellipsoids and for cylinders. The formula f=6pietab(a/b)1/2 possesses this property and because of its simplicity greatly speeds the calculations. A second program computes the distribution of masses and then of sedimentation constants for a bacterial population, expressed either as a frequency distribution or as total mass per s-class. The effect of the known variation in cell size at division is included in these calculations, which apply to organisms undergoing balanced, asynchronous growth in which mass increase is proportional to cell size. The two programs in conjunction compute the mass or cell-number profile in an arbitrary gradient. The programs have been used to design gradients to maximize the resolution of the technique.

Bacterial Physiological Phenomena↗
PubMed1976

Inactivation of membrane transport in Escherichia coli by near-ultraviolet light.

Evidence is presented that near-ultraviolet (near-UV) light can alter galactoside transport in Escherichia coli in several independent ways. It can inactivate the permease system per se, it can interfere with metabolic energy production or transfer, and it can cause an increase in the generalized permeability of the membrane. Earlier publications suggested that near-UV destroys cofactors needed for electron transport and thus places a limitation on energy reserves. In agreement, we found that the active accumulation of [14C]thiomethyl-beta-D-galactopyranoside is decreased after irradiation by a larger factor than that due to action directly on the permease system. The effect on the latter was measured by the decrease in the rate of o-nitrophenyl-beta-D-galactopyranoside (ONPG) transport. As evidence that energy supplies for this "downhill" process did not become rate limiting after irradiation, we found that carbonylcyanide-m-chlorophenyl-hydrazone did not stimulate ONPG transport of irradiated cells. Cells genetically deficient in functional permease or cells treated with formaldehyde still transport ONPG passively, although at much lower rates. With the use of such cells, it was found that high fluences (doses) made the cells leaky. Further evidence that the permease system and the metabolic energy system can be inactivated independently is also presented. It is shown that a photoproduct from the irradiation of chloramphenicol inactivates the permease system much more efficiently than the energy system. In addition, it is shown that thio-beta-D-digalactopyranoside protects the permease system, but not the energy system, both against direct inactivation by near-UV and against photosensitized inactivation in the presence of chloramphenicol.

Carbonyl Cyanide m-Chlorophenyl Hydrazone↗
PubMed1976

Energy cost of galactoside transport to Escherichia coli.

Energy reserves of Escherichia coli can be depleted by our previously reported procedure to a level such that even the "downhill" transport of o-nitrophenyl-beta-D-galactopyranoside (ONPG) is completely dependent upon the exogenous energy supply. The ONPG concentration is high externally to the cells and is low intracellular because of the action of cytoplasmic beta-galactosidase. In the present work, depleted cell suspensions have been infused at low, steady rates with glucose and other energy sources while measurements of transport were being made. Comparing the rate of ONPG transport with the rate of introduction of glucose under conditions where the chosen glucose infusion rate limits transport, we find that 89 molecules of ONPG are transported per molecule of fully oxidized glucose. This transport yield is constant over a 6.5-fold range in rate of glucose addition. This constancy over a range of infusion rates implies that transport is the major cellular function under these special conditions. The yield value if 89 is in the agreement with the predicitions of 76 from Mitchell's chemiosmotic theory and constitutes an independent proff of its validity, since all the other proposed mechanisms of engery coupling predict much smaller yields. The lag from the start of glucose infusion into the reaction cuvette, to the extrapolated time at which a steady rate of transport and concomitant hydrolysis are achieved, is short (approximately 1 min). Similarly, the time after the infusion is stopped until the rate of transport returns to the background rate is also short. The latter implies that the energy metabolism is directed almost entirely to transport and/or other ongoing cellular processes and not to repair or renewal of an energy-independent, facilitated diffusion system.

Biological Transport, Active↗
PubMed1975

The kinetics of mycelial growth.

Based on the assumption that mycelial growth follows the logistic growth law, formulae have been developed to express the growth of fungal colonies under a variety of geometric constraints. Analysis was done of Deppe's (1973) results on surface colony growth, where the mass of the colony grew exponentially during most colonial growth, and of Trinci's (1970) results on submerged "pellet" growth, where the mass of the colony increased as the cube of time during most colony growth. In both cases, the linear dimensions of the colony were increasing linearly while the mass was changing in these quantitatively different manners. It is concluded that these disparate growth behaviours result from different habits of growth; in two-dimensional colony growth a new region of space if invaded by an amount of mycelium small in proportion to the final "carrying capacity" of the region, and in three-dimensional colony growth a region is invaded with an amount of mycelium almost equal to the region's final limiting mycelial mass. Thus, the types of growth law for colony mass which are applicable for a particular organism in a particular physical environment depend critically on the degree to which the invading hyphae initially occupy the space.

Fungi↗
PubMed1975

Effect of uncouplers on "downhill" beta-galactoside transport in energy-depleted cells of Escherichia coli.

Galactoside permease-containing cells of Escherichia coli can be depleted of energy reserves so that the "downhill" cellular hydrolysis of o-nitrophenyl-beta-d-galactopyranoside (ONPG) no longer takes place. Treatment of such energy-depleted cells with proton-conducting agents such as carbonylcyanide m-chlorophenylhydrazone results in stimulation of ONPG transport. The same agents lower transport of non-energy-depleted cells towards the same levels that result from stimulation of the energy depleted cells. Of course, these agents prevent "uphill" accumulation against a concentration gradient under all conditions. Since uncouplers allow normal and energy-depleted cells to assume the same facilitated transport capability, these results lend support to the chemiosmotic hypothesis of Mitchell that comigration of charge is necessary for the transport of neutral galactosides. Our results imply that a potential favorable to transport is maintained by metabolism in non-energy-depleted cells, whereas an unfavorable potential is developed in the initial instant of time when energy-depleted cells are given ONPG.

Biological Transport, Active↗
PubMed1975

Lag in adaptation to lactose as a probe to the timing of permease incorporation into the cell membrane.

If bacteria are incapable of forming and incorporating proteins into the cytoplasmic membranes in all phases of the cell cycle, then not all cells from an asynchronous culture should be capable of growth when switched to a new carbon and energy source whose metabolism requires new membrane function. The transfer of an inducible culture to low lactose provides such a situation since the cells cannot grow unless galactoside permease can function to concentrate the lactose internally. From such experiments, it was concluded that the Y gene product of the lac operon is synthesized, incorporated, and can start functioning in active transport, at any time throughout the bulk of the cell cycle. Not only were the lags before growth re-ensued much shorter than would be expected if the membrane transport capability could only be developed in a small portion of the cycle, but brief pulses of a gratuitous inducer shortened the lags much further. Three types of Escherichia coli ML 30 culture were studied: cells that had exhausted the limiting glucose; cells taken directly from glucose-limited chemostats; and a washed suspension of highly catabolite repressed cells from cultures grown in high levels of glucose and gluconate. The growth studies reported here were performed on-line with a minicomputer. They represent at least an order of magnitude increase in accuracy in estimating growth parameters over previous instrumentation.

Adaptation, Physiological↗