Biomedical subjects

N R Baker

Publications and source records attributed to N R Baker.

At least 37 records · Page 2Linked to original sources

PubMed1988

Photosynthesis and temperature, with particular reference to effects on quantum yield.

Previous reviews of the effects of temperature on in vivo photosynthesis have mainly concerned the effects of temperature on light saturated rates. The quantum yield of photosynthesis (phi), as a measure of light limited photosynthesis, has generally been regarded as temperature insensitive. At temperatures close to the minima and maxima at which plants can sustain photosynthetic CO2 assimilation, light may damage the photosynthetic apparatus, an effect termed photoinhibition. A constant feature of photoinhibition is a reduction in phi. In maize, chilling-dependent photoinhibition reduces both phi and the light saturated rate of CO2 assimilation (Asat) and of O2 evolution. Analysis of recovery of CO2 uptake in these leaves suggests that whilst Asat recovers in a few hours, phi may not be fully restored for days. Examination of mature crop canopies shows that only a small proportion of the leaves are likely to become light saturated and then only for part of the day. The relative significance of temperature-induced changes in Asat and phi have also been tested in canopy models of maize crop photosynthesis. These suggest that whilst changes in either parameter will have similar effects on total canopy photosynthesis on the sunniest days of the year, for an average summer's day changes in phi will be of far greater importance. Consideration is therefore given to the factors associated with thylakoid membranes that may determine temperature-induced decreases in phi. Chilling of maize leaves under high light levels reduces the quantum yield of PSII and whole chain electron transport in concert with a decrease in the capacity of isolated thylakoids to bind atrazine, which is indicative of a loss or damage to the QB protein. Besides such classical symptoms of photoinhibition of PSII, chilling also induces the accumulation of a 31 kDa polypeptide in the thylakoids of maize leaves. This polypeptide fractionates with the light-harvesting chlorophyll a/b protein complex (LHCII) and has been tentatively identified as an unprocessed precursor of CP29 since it binds chlorophyll and is immunologically related to CP29. Accumulation of the 31 kDa polypeptide is associated with a modification in the energetics of LHCII, which may result in a decrease in excitation energy from LHCII to PSII and contribute to a decrease in phi. Examination is also made of how stress-induced modifications of interactions between PSII complexes, functioning of the cyt b6/f complex, the permeability of the thylakoid membrane to protons and the activity of the coupling factor may contribute to decreases in phi.

Cold Temperature↗

PubMed1987

The effect of sublethal concentrations of aminoglycosides on adherence of Pseudomonas aeruginosa to hamster tracheal epithelium.

Perfused hamster tracheal explants were used to examine the adherence of mucoid and non-mucoid strains of Pseudomonas aeruginosa to intact tracheal epithelium when grown in 0.5 MIC of tobramycin or gentamicin. Tracheal explants were perfused for 2 h with 10(7) cfu of P. aeruginosa grown overnight in trypticase soy broth containing 0.5 MIC of tobramycin or gentamicin or without antibiotics. After infection, the explants were washed and a 4 mm section was homogenized, diluted and plated for colony counts. Mucoid strains of P. aeruginosa grown in the presence of the aminoglycosides did not produce alginate and were not as adherent as the same strains which were not grown in antibiotics. Adherence of non-mucoid strains of P. aeruginosa grown in sublethal concentrations of the aminoglycosides was not significantly reduced compared with the adherence of the same strains which were not exposed to antibiotics. These results indicate that mucoid strains of P. aeruginosa growing in the presence of sublethal concentrations of aminoglycosides do not produce alginate and may not colonize the epithelial surface.

Alginates↗

PubMed1987

The effect of sublethal levels of antibiotics on the pathogenicity of Pseudomonas aeruginosa for tracheal tissue.

Hamster tracheal organ cultures were used to evaluate the ability of two aminoglycoside and two beta-lactam antibiotics to protect the epithelium from damage due to Pseudomonas aeruginosa infection. Hamster tracheal explants were infected with strains of P. aeruginosa for 4 h and washed to remove nonadherent organisms. The explants were incubated for an additional 18 h in fresh minimal essential medium containing inhibitory or subinhibitory concentrations of antibiotics. The explants were examined by scanning electron microscopy and bacterial elastase and exotoxin A production was detected by ELISA and a western blot assay respectively. Concentrations of aminoglycosides below the MIC for the infecting strain protected the epithelium from damage and inhibited the production of exotoxin and elastase. The beta-lactam antibiotics were not protective and epithelial damage was observed at antibiotic levels equal to or higher than the MIC for that strain. The beta-lactam treated cultures continued to release elastase and exotoxin A at antibiotic concentrations equivalent to or higher than the MIC for that strain. Thus subinhibitory levels of aminoglycoside antibiotics could protect the infected epithelium from damage by inhibiting the release of toxic substances from the invading bacteria. In contrast, bacteria exposed to beta-lactam antibiotics may continue to release extracellular toxins which can damage the tissue.

ADP Ribose Transferases↗

PubMed1985

Selective inhibition of the accumulation of extracellular proteases of Pseudomonas aeruginosa by gentamicin and tobramycin.

Gentamicin and tobramycin inhibited the accumulation of extracellular proteases secreted by Pseudomonas aeruginosa. The secretion of protease was inhibited at concentrations of these drugs that were below the level required to inhibit general protein synthesis. Neither magnesium ions nor high ionic strength antagonized the ability of the aminoglycosides to block secretion of the proteases. Under these culture conditions magnesium ions were shown to antagonize the effects of the aminoglycosides on protein synthesis and aminoglycoside-mediated lysozyme lysis of P. aeruginosa. These results suggested that the drugs blocked secretion of the proteases by acting at the level of the outer membrane.

Culture Media↗

PubMed1985

Quantitation of adherence of mucoid and nonmucoid Pseudomonas aeruginosa to hamster tracheal epithelium.

Adherence of mucoid and nonmucoid isolates of Pseudomonas aeruginosa to tracheal epithelium was quantitated by using hamster tracheas mounted in a perfusion chamber. The strains of P. aeruginosa used were clinical isolates from cystic fibrosis patients and a series of laboratory strains. Aseptically excised hamster tracheas were mounted in perfusion chambers and embedded in minimal essential medium containing 1.5% agarose. The tracheas were infected with various numbers of bacteria for various periods, rinsed, homogenized, and plated on Trypticase soy agar. A 4-mm segment from each trachea was prepared for quantitation, and the other segment was prepared for examination by scanning electron microscopy. Adherence increased with time and with increasing concentrations of inoculum. Standard conditions of inoculation were set at an inoculum of 10(7) CFU/ml and a 2-h incubation. Under these conditions, the mucoid organisms adhered to the ciliated epithelium 10- to 100-fold better than did the nonmucoid organisms. Adherence of the mucoid isolates did not appear to be pilus mediated and did not involve hydrophobic interactions. The mucoid P. aeruginosa isolates could be seen adhering to the epithelium in the form of microcolonies embedded in an extracellular matrix which attaches the organisms to the cilia and to each other. The adherence may be involved in the establishment of infection of the lungs of these patients and in the inability to clear the organisms from the lungs. The model will be useful in determining the mechanism of adherence of the bacteria to the ciliated epithelium of the respiratory tract.

Adhesiveness↗

PubMed1984

Use of zinc ions to study thylakoid protein phosphorylation and the state 1-state 2 transition in vitro.

At ATP concentrations less than 0.2 millimolar, zinc ions cause a marked stimulation of endogenous protein phosphorylation in thylakoid membranes isolated from tobacco (Nicotiana tabacum L. cv Turkish Samsun), pea (Pisum sativum L. cv Feltham First) and spinach (Spinacia oleracea L. cv Northland). The greatest stimulatory effect was observed at Zn(2+) concentrations of 1 to 2 millimolar; higher concentrations were inhibitory. The stimulatory effect of Zn(2+) was independent of Mg(2+) concentration from 1 to 5 millimolar and thus does not appear to be due to the formation of a Zn(2+) -ATP complex. Phosphorylation of histones IIA, an exogenous protein substrate, was inhibited by 2 millimolar Zn(2+). At low levels of ATP, Zn(2+) not only stimulates general endogenous protein phosphorylation, but also the phosphorylation of the apoproteins of the light-harvesting chlorophyll a/b-protein complex. However, under these conditions Zn(2+) inhibits the ATP-induced quenching of photosystem II fluorescence and the increase in the ratio of photosystem I to photosystem II fluorescence which are both characteristic of the State 1-State 2 transition. These results suggest that phosphorylation of the light-harvesting chlorophyll a/b-protein complex may not directly bring about the State 1-State 2 transition.

Journal Article↗

PubMed1982

A tracheal culture model of respiratory tract infection with Pseudomonas aeruginosa.

The pathogenesis of Pseudomonas aeruginosa for the respiratory tract has been examined using hamster tracheal organ cultures. Tracheal rings prepared from male Syrian hamsters, strain LSH/LAK, were infected with P. aeruginosa for 4 h and processed at 4-h intervals for 24 h for examination by light- and electron microscopy. Tissue destruction was observed within 8 h after infection with 10(8) colony-forming units (cfu)/ml and within 12 h after infection with 10(4) or 10(6) cfu/ml. Ciliated cells that contained abnormal subcellular organelles were expelled from the epithelium. By 20 h the epithelial borders were composed primarily of nonciliated cells. Transmission- and scanning electron microscopy revealed details of the cellular destruction and attachment of P. aeruginosa to the ciliated epithelium. Pseudomonas aeruginosa causes a rapid destruction of the epithelium of hamster trachea in cultures. Hamster tracheal organ cultures have been shown to be useful in studying the pathogenesis of P. aeruginosa for the respiratory tract.

Animals↗

PubMed1982

Role of exotoxin A and proteases of Pseudomonas aeruginosa in respiratory tract infections.

Hamster tracheal organ culture has been used to study the role of exotoxin A and proteases of Pseudomonas aeruginosa in infections of the respiratory tract. Tracheal explants infected with a toxin producing, protease producing strain or a toxin deficient, protease producing strain displayed evidence of exfoliation and disorganization of the tracheal of the tracheal epithelium 12 h after initiation of infection. A toxin producing protease deficient strain caused some desquamation of cells and cellular swelling, but exfoliation was not evident. Each of the toxin producing and (or) protease producing strains inhibited protein synthesis of the explants. Purified exotoxin inhibited protein synthesis and caused some pathological changes similar to those observed with the toxin producing strain. Purified elastase from P. aeruginosa only inhibited protein synthesis at 10 microgram/mL but caused exfoliation at 0.01 microgram/mL. Alkaline protease had no detectable effect on the explants. The effects of active infection could be prevented by treatment with gentamycin. All the strains tested caused an inhibition of ciliary activity, but no correlation could be made with any of the extracellular products. Exotoxin A and elastase may be responsible for much of the destruction of respiratory tract tissue in Pseudomonas infections although other bacterial factors and host factors are of importance.

ADP Ribose Transferases↗

PubMed1981

Analysis of the slow phases of the in vivo chlorophyll fluorescence induction curve. Changes in the redox state of photosystem II electron acceptors and fluorescence emission from photosystems I and II.

An analysis of the photo-induced decline in the in vivo chlorophyll a fluorescence emission (Kautsky phenomenon) from the bean leaf is presented. The redox state of PS II electron acceptors and the fluorescence emission from PS I and PS II were monitored during quenching of fluorescence from the maximum level at P to the steady state level at T. Simultaneous measurement of the kinetics of fluorescence emission associated with PS I and PS II indicated that the ratio of P s I/PS II emission changed in an antiparallel fashion to PS II emission throughout the induction curve. Estimation of the redox state of PS II electron acceptors at given points during P to T quenching was made by exposing the leaf to additional excitation irradiation and determining the amount of variable PS II fluorescence generated. An inverse relationship was found between the proportion of PS II electron acceptors in the oxidised state and PS II fluorescence emission. The interrelationships between the redox state of PS II electron acceptors and fluorescence emission from PS I and PS II remained similar when the shape of the induction curve from P to T was modified by increasing the excitation photon flux density. The contributions of photochemical and nonphotochemical quenching to the in vivo fluorescence decline from P to T are discussed.

Chlorophyll↗

PubMed1980

Observation of two quenchers of chlorophyll fluorescence in chloroplasts at -196 degrees C.

Fluorescence induction at -196 degrees C has been monitored in chloroplasts rapidly frozen after poising at different redox potentials at room temperature. It was found that, as at room temperature, the initial level of fluorescence observed upon shutter opening (Fo), relative to the final level observed after 10 seconds of illumination (Fm) increased as the redox potential of the chloroplasts was lowered. Redox titration and revealed the presence of two quenching components with Em, 7.8 at -70 mV and -275 mV accounting for approx. 75% and 25% of the variable fluorescence (Fv). Parallel observation of fluorescence yield at room temperature similarly gave two components, with Em, 7.8 at -95 mV and -290 mV, also accounting for appox. 75% and 25%. Simultaneous measurement of fluorescence emission at -196 degrees C at 695 nm and 735 nm indicated that both emissions are quenched by the same redox components.

Chlorophyll↗

PubMed1978

Effect of High Cation Concentrations on Photosystem II Activities.

The effects of wide concentration ranges of NaCl, KCl, and MgCl(2) on ferricyanide reduction and the fluorescence induction curve of isolated spinach (Spinacia oleracea) chloroplasts were investigated. Concentrations of the monovalent salts above 100 mm and MgCl(2) above 25 mm produced a decrease in the rate of ferricyanide reduction by thylakoids uncoupled with 2.5 mm NH(4)Cl which cannot be attributed to changes in the primary photochemical capacity of photosystem II. Salt-induced decreases in the effective concentration of the secondary electron acceptor of photosystem II, plastoquinone, reduce the capacity for secondary photochemistry of photosystem II and this could contribute to the reduction in ferricyanide reduction by uncoupled thylakoids at high salinities. The rate of ferricyanide reduction by coupled thylakoids is little affected by salinity changes, indicating that the rate-limiting phosphorylation mechanism in electron flow from water to ferricyanide in coupled thylakoids is salt-tolerant, whereas the rate-limiting reaction in uncoupled ferricyanide reduction is considerably affected by salinity changes. Salt-induced changes in the fluorescence induction curve are interpreted in terms of changes in the rate constants for excitation decay by radiationless transitions, exciton transfer from photosystem II chlorophylls to other associated chlorophyll species, and photochemistry.

Journal Article↗

PubMed1977

Development of Photosystem I and Photosystem II Activities in Leaves of Light-grown Maize (Zea mays).

To compare chloroplast development in a normally grown plant with etiochloroplast development, green maize plants (Zea mays), grown under a diurnal light regime (16-hour day) were harvested 7 days after sowing and chloroplast biogenesis within the leaf tissue was examined. Determination of total chlorophyll content, ratio of chlorophyll a to chlorophyll b, and O(2)-evolving capacity were made for intact leaf tissue. Plastids at different stages of development were isolated and the electron-transporting capacities of photosystem I and photosystem II measured. Light saturation curves were produced for O(2)-evolving capacity of intact leaf tissue and for photosystem I and photosystem II activities of isolated plastids. Structural studies were also made on the developing plastids. The results indicate that the light-harvesting apparatus becomes increasingly efficient during plastid development due to an increase in the photosynthetic unit size. Photosystem I development is completed before that of photosystem II. Increases in O(2)-evolving capacity during plastid development can be correlated with increased thylakoid fusion. The pattern of photosynthetic membrane development in the light-grown maize plastids is similar to that found in greening etiochloroplasts.

Journal Article↗

PubMed1976

Development of the Primary Photochemical Apparatus of Photosynthesis during Greening of Etiolated Bean Leaves.

Seven-day-old dark-grown bean leaves were greened under continuous light. The amount of chlorophyll, the ratio of chlorophyll a to chlorophyll b, the O(2) evolving capacity and the primary photochemical activities of Photosystem I and Photosystem II were measured on the leaves after various times of greening. The primary photochemical activities were measured as the photo-oxidation of P(700), the photoreduction of C-550, and the photo-oxidation of cytochrome b(559) in intact leaves frozen to -196 C. The results indicate that the reaction centers of Photosystem I and Photosystem II begin to appear within the first few minutes and that Photosystem II reaction centers accumulate more rapidly than Photosystem I reaction centers during the first few hours of greening. The very early appearances of the primary photochemical activity of Photosystem II was also confirmed by light-induced fluorescence yield measurements at -196 C.

Journal Article↗

PubMed1974