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Biomedical subjects

M D Johnston

Publications and source records attributed to M D Johnston.

At least 19 recordsLinked to original sources
PubMed2005

Evaluation of hydrogen peroxide vapour as a method for the decontamination of surfaces contaminated with Clostridium botulinum spores.

The aim of this study was to evaluate the efficacy of hydrogen peroxide vapour (HPV) against spores of Clostridium botulinum, for use as a method for decontaminating environments where this pathogen has been handled. Spores were dried onto stainless steel slides and exposed to HPV in a sealed glovebox enclosure, transferred to a quenching agent at timed intervals during the exposure period, before survivors were cultured and enumerated. D-values were calculated from graphs of log10 survivors plotted against time and were found to range from 1.41 to 4.38 min. HPV was found to be effective at deactivating spores of toxigenic Cl. botulinum, non-toxigenic Clostridium spp. and Geobacillus stearothermophilus dried onto stainless steel surfaces. HPV could be used to decontaminate cabinets and rooms where Cl. botulinum has been handled. The cycle parameters should be based on studies carried out with relevant spores of this organism, rather than based on inactivation data for G. stearothermophilus spores, which have been used in the past as a standard biological challenge for disinfection and sterilisation procedures. HPV could provide an attractive alternative to other decontamination methods, as it was rapid, residue-free and did not give rise to the health and safety concerns associated with other gaseous decontamination systems.

Clostridium botulinum↗
PubMed2003

Red cell osmotic fragility studies in hemoglobin C-beta thalassemia: osmotically resistant microspherocytes.

Typically certain features of red cell morphology predict the results of osmotic fragility testing. Microspherocytes generally have increased and target cells decreased fragility. Blood smears in homozygous hemoglobin C disease show an interesting admixture of microspherocytes and target cells. Yet osmotic fragility studies generally show only reduced fragility and no population of fragile cells to correspond with the spherocytes. The present study demonstrates that the red cells of patients with hemoglobin C-beta thalassemia share many characteristics with hemoglobin C red cells, including the decreased osmotic fragility of all cells despite the presence of both spherocytes and target cells. These paradoxically osmotically resistant spherocytes probably arise because of cellular dehydration due to a K-Cl transport system which may be activated by binding of hemoglobin C to the red cell membrane.

Adult↗
PubMed2003

Theory of antimicrobial combinations: biocide mixtures - synergy or addition?

AIMS: To demonstrate the effect that non-linear dose responses have on the appearance of synergy in mixtures of antimicrobials. METHODS AND RESULTS: A mathematical model, which allows the prediction of the efficacy of mixtures of antimicrobials with non-linear dose responses, was produced. The efficacy of antimicrobial mixtures that would be classified as synergistic by time-kill methodology was shown to be a natural consequence of combining antimicrobials with non-linear dose responses. CONCLUSIONS: The effectiveness of admixtures of biocides and other antimicrobials with non-linear dose responses can be predicted. If the dose response (or dilution coefficient) of any biocidal component, in a mixture, is other than one, then the time-kill methodology used to ascertain the existence of synergy in antimicrobial combinations is flawed. SIGNIFICANCE AND IMPACT OF THE STUDY: The kinetic model developed allows the prediction of the efficacy of antimicrobial combinations. Combinations of known antimicrobials, which reduce the time taken to achieve a specified level of microbial inactivation, can be easily assessed once the kinetic profile of each component has been obtained. Most patented cases of antimicrobial synergy have not taken into account the possible effect of non-linear dose responses of the component materials. That much of the earlier literature can now be predicted, suggests that future cases will require more thorough proof of the alleged synergy.

Cresols↗
PubMed2003

Membrane damage to bacteria caused by single and combined biocides.

AIMS: To examine the effect on the leakage of low molecular weight cytoplasmic constituents from Staphylococcus aureus using phenolics singly and in combination, and to see if the observations could be modelled using a non-linear dose response. METHODS AND RESULTS: The rate of potassium, phosphate and adenosine triphosphate leakage was examined in the presence of chlorocresol and m-cresol. Individually, leakage was observed only at long contact times or high concentrations. Combined at these ineffective concentrations, the cytoplasmic pool of all constituents studied was released within minutes. Both chlorocresol and m-cresol were shown to have non-linear dose responses. A rate model for the combinations, which takes account of these non-linear responses, accurately predicted the observations. CONCLUSIONS: Antimicrobials, which when used alone exhibit a non-linear dose response, will also give a non-linear dose response in combination. The simple linear-additive model ignores the concept of the dilution coefficient and will always describe the phenomenon of synergy for combinations where one or more of the components has a dilution coefficient greater than one. This has been borne out by examination of the purported prime lesion of chlorocresol and m-cresol, alone and in combination. SIGNIFICANCE AND IMPACT OF THE STUDY: Studies aimed at producing synergistic mixtures of antimicrobials, which ignore the non-linear additive effect, may waste valuable research effort looking for a physiological explanation for an apparent synergy, where none, in-fact, exists. Patents granted on the basis of analyses using the linear-additive model for combinations of compounds with non-linear dose responses may no longer be supportable.

Adenosine Triphosphate↗
PubMed2002

A rapid method for assessing the suitability of quenching agents for individual biocides as well as combinations.

AIMS: To develop a novel, rapid method for testing the ability of quenching agents to neutralize disinfectants. METHODS AND RESULTS: Tests were performed to determine the suitability of different neutralizers for a range of disinfectants, using a new method based on the Bioscreen optical density analyser. Results showed that during disinfection tests, efficacy could be over-estimated due to poor, or no, neutralization of the disinfectant after a specified time of exposure to the bacteria. The failure to distinguish adequately between bacteriostatic and bactericidal effects can lead to false results during disinfectant testing. Experiments also showed that dilution of the disinfectant, following exposure to the bacteria, was not always sufficient to stop the activity of the disinfectant for chemicals with low dilution coefficients. CONCLUSIONS: The quench test proved to be very quick and easy to perform, with results being available within 18 h. Using the Bioscreen, the test is automated and determines whether dilution into a particular neutralizer is able to inactivate a disinfectant within 30 s. SIGNIFICANCE AND IMPACT OF THE STUDY: This new approach allows the efficacy of quenching agents to be determined, prior to undertaking each disinfection study, and can help in the development of more suitable quenching solutions. The test has also been used to find suitable neutralizers for mixtures of disinfectants which might be used during studies on synergistic biocide combinations.

Colony Count, Microbial↗
PubMed2002

An investigation into the changed physiological state of Vibrio bacteria as a survival mechanism in response to cold temperatures and studies on their sensitivity to heating and freezing.

AIMS: To induce pathogenic Vibrio bacteria into a changed physiological state, in response to cold temperatures in sea water, and assess their sensitivity to heating and freezing, as compared with normal cells. METHODS AND RESULTS: Cells of exponential phase Vibrio vulnificus, V. cholerae and V. parahaemolyticus were washed and inoculated into flasks of sea water, which were stored at 20 and 4 degrees C. Cells stored at 20 degrees C could be recovered after 60 d on non-selective agar (heart infusion agar; HIA) and on the selective agar (thiosulphate citrate bile salts agar) which is used in most Vibrio detection methodology. At 4 degrees C cells became non-culturable on both agars over time. The non-culturable cells appeared to be metabolically active and maintained their membrane integrity, whilst undergoing a change in morphology from rod-shaped to coccoid cells. Resuscitation was possible, in some cases, by an upshift in temperature before plating and the addition of catalase to HIA plates was found to increase recovery. Studies were carried out to assess the sensitivity of the non-culturable cells to heating and freezing compared with the normal cells. Vibrio organisms, whether culturable or in the non-culturable form, were not inactivated by freezing to -20 degrees C. Heating studies showed that V. parahaemolyticus was very heat resistant at low temperatures. However, a pasteurization regime of 2 min at 70 degrees C was found to be effective against all three strains. Experiments showed that the non-culturable cells of all three strains were similar in their heat resistance or, in some cases, were more heat sensitive than cells in the normal form. CONCLUSIONS: Cells in the changed physiological form would not be detected in fish or seafood products by the current Vibrio detection methods. Freezing had no effect in reducing cell numbers. Vibrio parahaemolyticus was very heat resistant in the low temperature pasteurization studies. The higher pasteurization regime of 70 degrees C for 2 min was effective against all three pathogens. Non-culturable cells had similar heat sensitivity or were more heat sensitive than cells in the normal state. SIGNIFICANCE OF IMPACT OF THE STUDY: The study has highlighted a need for the development of better Vibrio detection methods. The low temperature pasteurization of oysters, which has been recommended in the USA, would not be adequate against the strain of V. parahaemolyticus used in this study. Heating regimes which were found to control cells in the normal form will also be effective for the control of the cells with changed physiology.

Agar↗
PubMed2001

The effect of interfering substances on the disinfection process: a mathematical model.

AIMS: To gain a greater understanding of the effect of interfering substances on the efficacy of disinfection. METHODS AND RESULTS: Current kinetic disinfection models were augmented by a term designed to quantify the deleterious effect of soils such as milk on the disinfection process of suspended organisms. The model was based on the assumption that inactivation by added soil occurred at a much faster rate than microbial inactivation. The new model, the fat-soil model, was also able to quantify the effect of changing the initial inoculum size (1 x 10(7)-5 x 10(7) ml(-1) of Staphylococcus aureus) on the outcome of the suspension tests. Addition of catalase to the disinfection of Escherichia coli by hydrogen peroxide, resulted in changes to the shape of the log survivor/time plots. These changes were modelled on the basis of changing biocide concentration commensurate with microbial inactivation. CONCLUSIONS: The reduction in efficacy of a disinfectant in the presence of an interfering substance can be quantified through the use of adaptations to current disinfection models. SIGNIFICANCE AND IMPACT OF THE STUDY: Understanding the effect of soil on disinfection efficacy allows us to understand the limitations of disinfectants and disinfection procedures. It also gives us a mechanism with which to investigate the soil tolerance of new biocides and formulations.

Animals↗
PubMed2000

One explanation for the variability of the bacterial suspension test.

Disinfection kinetic studies of sodium dodecyl sulphate, benzalkonium chloride and sodium hypochlorite against Staphylococcus aureus revealed that when a higher inoculum level of Staph. aureus than normal was used (approximately 1 log higher), the efficacy of disinfection was severely attenuated. Kinetic analysis using the Hom model for experiments carried out on tests using 3 x 108 organisms ml-1 were unable to account for the large increase in disinfection power observed when smaller inoculum levels were used. Since the inoculum was the same in every way except for the numbers used, the large variations in the log reduction/time curves could not be explained by a variation in the resistance of the population to the biocide, as identical log reduction-time curves should have resulted. The level of disinfection achieved for a given concentration of biocide was found to be approximately linearly related to the cell number ml-1 of test solution and not to the log number. The variation observed is believed to occur due to intrinsic self-quenching of the biocide by the microbes during the course of the disinfection test. As the level of free biocide decreases, the rate of reaction decreases, giving the tails of the log reduction/time curves. Such intrinsic self-quenching could explain the large variations known to occur in the legally required disinfection suspension tests.

Bacteriological Techniques↗
PubMed2000

Disinfection kinetics: a new hypothesis and model for the tailing of log-survivor/time curves.

A new hypothesis for the understanding of chemical disinfection, which we have termed the Intrinsic Quenching hypothesis, is presented. This mechanistic treatment of disinfection kinetics is based on the hypothesis that the biocide concentration may not be in vast excess over the microbes, as is normally assumed. A mathematical model was developed and found to be useful in describing the observed kinetics of several disinfectants. The model suggested that the reason for the observation of non-linear, log-survivor curves was due to the ability of the microbes, in clean, soil-free conditions, to intrinsically quench the bulk concentration of biocide.

Disinfectants↗
PubMed2000

Identity matching of consonant-vowel-consonant words by prereaders.

Using an identity matching-to-sample procedure, normally developing prereaders who matched individual letters with high accuracy (e.g., m and s) did not show high accuracy in matching three-letter printed words that differed only in the first letter (e.g., mad and sad). Teachers and researchers should not assume that children who can discriminate individual letters can also discriminate minimally different words that contain those letters.

Child↗
PubMed1999

The affinity adsorptive recovery of an infectious herpes simplex virus vaccine.

The chromatographic purification of a recombinant Herpes Simplex Virus (type 2) from salt- and heparin-released harvests of infected complementing Vero (CR2) cells is addressed. Functionalized matrices and process operating conditions are identified that provide adequate virus titres in eluates that are significantly reduced in CR2 cell protein and DNA and possess a low level of HSV-2 protein. Virus from diluted salt-released harvests (0.14 M NaCl) was not appreciably adsorbed onto either heparin-Sepharose or Cellufine-heparin matrices but was virtually completely adsorbed onto Cellufine-sulfate and heparin-HP matrices. Virus was recovered by either a linear salt gradient elution (0.14-2 M NaCl) or by a single-step elution with 1.5 M NaCl in phosphate buffer. Recoveries of infectious virus with step elution were 21% and 89%, respectively, for these matrices. Virus from undiluted salt-released harvest (0.8 M NaCl) was substantially adsorbed onto Cellufine-sulfate gel (44% adsorption) and completely adsorbed onto heparin-HP matrices. This virus was recovered with high yield by either gradient or step elution with phosphate-buffered saline. Finally, heparin-harvested virus was fed directly to these matrices and quantitatively adsorbed. The virus could be completely recovered from the heparin-HP matrix with 1.5 M NaCl buffer to provide a purified preparation containing only 0.05 pg protein/pfu and 1.2 x 10(-4) pg DNA/pfu.

Adsorption↗
PubMed1999

A kinetic study of the effect of hydrogen peroxide and peracetic acid against Staphylococcus aureus and Pseudomonas aeruginosa using the bioscreen disinfection method.

Hydrogen peroxide and peracetic acid at pH 4 were examined against Staphylococcus aureus and Pseudomonas aeruginosa using the published 'Bioscreen' technique of biocide analysis. The data were examined using either classical Chick-Watson (CW) log-linear disinfection kinetics or the empirical, non-linear time Hom model. In some cases, modelling the data with the classical CW method gave good linear correlations, in others, however, deviations from this model were observed. In such cases the Hom model proved an adequate descriptor of the data. The Bioscreen technique therefore gives data which can be analysed using the normal mechanistic and empirical models currently available.

Disinfection↗
PubMed1998

The structure of the upper atmosphere of mars: In situ accelerometer measurements from mars global surveyor

The Mars Global Surveyor (MGS) z-axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermospheric response at mid-northern latitudes, increasing the altitude of thermospheric pressure surfaces there by as much as 8 kilometers and indicating a strong global thermospheric response to a regional dust storm. Throughout the MGS mission, thermospheric density bulges have been detected on opposite sides of the planet near 90 degreesE and 90 degreesW, in the vicinity of maximum terrain heights. This wave 2 pattern may be caused by topographically-forced planetary waves propagating up from the lower atmosphere.

Journal Article↗
PubMed1998

The primary production of an infectious recombinant Herpes Simplex Virus vaccine.

The production and extracellular release of a recombinant Herpes Simplex Virus (type 2) from monolayers of infected complementing Vero cells (CR2) are addressed. Growth and virus production conditions are identified that provide adequate virus titers with cell seeding densities and viral multiplicities of infection that could be reasonably handled in manufacturing. Harvesting by sonication of cell monolayers is shown to give the highest recovery of infectious virus (to 2.5 x 10(6) pfu/mL) but leads to process stream contamination by cellular proteins through the rupturing of cells (to 28 pg protein/pfu). By comparison, freeze-thaw cycles and osmotic rupture by hypotonic saline or glycerol shock procedures yield only low virus recovery (typically <10% of that by sonication), and are accompanied by yet higher levels of protein contamination (up to 30-fold higher pg protein/pfu). Addition of the polyanionic polymers, heparin or dextran sulphate to a harvest using either hypotonic saline, glycerol shock or isotonic phosphate buffered saline increased the yield of infectious virus in the supernatant. By contrast, addition of polycationic poly-L-lysine resulted in negligible increase in the supernatant virus titer. The highest virus titers (4.7 x 10(7) pfu/mL) were achieved following treatment of roller bottle cultured cells displaying a high cytopathic effect with heparin at 50 microg/mL for at least 3 h post harvest. This procedure also gave the lowest levels of protein contamination (<2 pg protein/pfu). The fivefold lower yield of infectious virus from cultures displaying a low cytopathic effect (<70% CPE) indicates the importance of cell physiological state at harvest.

Animals↗
PubMed1998

Disinfectant testing: use of the Bioscreen Microbiological Growth Analyser for laboratory biocide screening.

A new method is described for screening potential biocides based on the traditional suspension test using the Bioscreen optical plate reader. This new method is rapid, reproducible, quantitative and cost effective. Data obtained by this new method are not directly equivalent to the log reduction normally quoted, but give a measurement of the total effect of the biocide on the microbe population, measuring the effect of injury as well as death (non-viability). The method allows for the routine examination of disinfection kinetics, the study of which leads to greater scientific insight into disinfection than that achieved by the standard 5 min, one-point, disinfection tests currently employed.

Colony Count, Microbial↗
PubMed1998

Conditional versus trial-unique delayed matching-to-sample.

We compared performance on conditional and trial-unique delayed identity matching-to-sample procedures. In Experiment 1, participants with moderate to severe mental retardation were exposed to both procedures under a single, brief delay value. Three of 5 subjects showed higher accuracy in the trial-unique sessions. In Experiment 2, participants with mild mental retardation were exposed to delay values of 0, 2, 4, 8, and 16 seconds, randomized within each session. For 3 of 4 subjects, accuracy was highest with trial-unique, and lowest on the conditional matching-to-sample, at longer delays. Across the two studies, 6 of 9 subjects showed lower delayed matching accuracy when fewer rather than more stimuli were included in a session.

Adolescent↗
PubMed1997

Generalized identity matching of two-dimensional forms by individuals with moderate to profound mental retardation.

The classic literature suggests that individuals with MAs of less than 5 years may fail tasks that require same/different judgments. In Study 1 we used an assessment procedure that provided minimal instructional programming to determine whether 17 adults with MAs ranging from 2 years, 5 months to 4 years, 11 months would show accurate identity matching-to-sample. Stimuli were letter-like nonsense forms. Eight participants showed highly accurate matching. Eight of the 9 who failed were available for further study. Of these, 5 ultimately demonstrated highly accurate matching after training with standard fading procedures. These data suggest that a greater proportion of individuals with low MAs can exhibit generalized identity matching than previously documented in the literature.

Adolescent↗