The Significance of the Variation in Shape of Time-Survivor Curves

Withell, E. R.

doi:10.1017/s0022172400035361

Cited by 94 publications

(58 citation statements)

References 59 publications

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“…Phelps's (1911) original suggestion was made on the assumption that the logarithm of the survivors plotted against time always gave a straight line. It is obvious from mote recent work that this is not so (Henderson Smith, 1921, 1923Withell, 1938Withell, , 1942. The most usual deviation from the straight line is a lag in the rate at the commencement of the reaction between organism and disinfectant.…”

Section: Techniquementioning

confidence: 99%

“…identical. Full details are given in a previous paper (Withell, 1942 The evaluation of bactericides nutrient agar medium was washed off with Ringer solution and approximately the same number of organisms added to each solution under test. The solutions were kept at 200 C. in a thermostatically controlled bath, with variation of + 0-10 C.…”

Section: Techniquementioning

confidence: 99%

“…From time to time samples were removed from the bactericide, and the number of viable organisms determined by roll-tube counts (Wilson, 1922;Withell, 1938). Each final dilution was inoculated into five roll tubes, and each mean count is the average of these five separate counts.…”

Section: Techniquementioning

confidence: 99%

“…A series of experiments in which the test organism was inoculated into two bactericides and the death-rate followed has been previously recorded by Withell (1942). The experiments were used in that paper to show that the logarithms of the survival times were approximately normally distributed, and the same set of experiments is used in this paper to estimate the accuracy of a series of determinations of bactericidal activity, measured by the rate of death of the organisms.…”

Section: Introductionmentioning

confidence: 99%

“…The experiments were used in that paper to show that the logarithms of the survival times were approximately normally distributed, and the same set of experiments is used in this paper to estimate the accuracy of a series of determinations of bactericidal activity, measured by the rate of death of the organisms. All 'experiment numbers' refer to a preceding paper in this series (Withell, 1942), and all counts are given in full in Appendix II to that paper. THE METHOD It is not the intention at the moment to describe in detail a new method for comparing bactericides, but to indicate how the fundamental disabilities of the present tests can be remedied in so far as they are concerned with the comparison of two bactericidal solutions.…”

Section: Introductionmentioning

confidence: 99%

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The evaluation of bactericides

Withell

1942

Epidemiol. Infect.

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The Rideal-Walker and Chick-Martin tests depend on end-point methods for comparing bactericides. End-point methods have a number of disadvantages: (1) It is difficult, if not impossible, to estimate end-points accurately. (2) The nature of the test demands a standard time. The results at any standard time will be arbitrary. If the sensitivity of the organisms varies in the two solutions tested, the coefficients obtained will vary with the time chosen as standard. It is impossible to estimate the manner in which the survivor times of the organism are distributed, by end-point methods, and therefore the standard tests in current use do not ascertain whether the result is arbitrary or not. This can only be appreciated if the tests are repeated using different times.Apart from the difficulties associated with end-points, the coefficients obtained in the present tests are calculated from one concentration of phenol and one concentration of test bactericide. This is also an arbitrary value, for the relative activity of two bactericides will depend on relative values of the concentration exponent,n, for each bactericide; if both phenol and the test bactericide have identical values ofnthe coefficients will be the same at all concentrations which are compared, but if values ofnare different then different coefficients will be obtained for every different concentration compared. As a corollary, unless the value ofnfor the test bactericide is the same as that of phenol, it is illogical to transfer the phenol coefficient obtained at one concentration to the pure substance, for this procedure implies that the phenol coefficient applies to any concentration of the test bactericide. This is only true when the concentration coefficients of phenol and test bactericide are the same. The standard methods offer no value forn.All these difficulties can be avoided if the rate of death is followed in both phenol and test bactericide, by viable counts. Two methods of interpreting the results are discussed:(1) The comparison of reaction rates, calculated by the formulaThis method is complicated by the occurrence of lag period at the start of the reaction between organism and bactericide. This lag period varies in extent in different experiments (often extending up to 50% of the deaths) and makes the interpretation of the results difficult. The lag period has been shown to be a manifestation of, the distribution of logarithmic survival times (Withell, 1942), and is a function of the standard deviation of the logarithmic survivor times (λ). This fact is taken into account in method (2).(2) The use of the time required for 50% response as a method of comparing bactericides. This time has been symbolized L.t. 50 and is analogous to the lethal dose for the 50% response (l.d. 50) widely used in biological assay. If a logarithmic time scale is used, the logarithmic survival times are found to be approximately normally distributed and consequently probit-logarithm of time lines are straight and in the case of phenol 0·5 % and para-chlor-meta-cresol 0·05% these lines are parallel. In this case a logarithmic time scale means that thecomparativetimes for all percentage responses are equal. In cases where the probit-logarithm-of-time lines are not parallel this would indicate that the standard is unsuitable for the test, forany methodsof assay based on a use of a standard to which the organism gives a different response-time relation from that which they give to the test substance or bactericide, are likely to give arbitrary results.L.T. 50 has been chosen because of the simplicity of the idea underlying it, and because a similar function, L.D. 50, has been used with success in biological assay work. There is also a solid mathematical backing for the use of L.D. 50 in assays involving biological response.It is suggested that by means of l.t. 50, phenol and the test bactericide can be compared in the following way:(a) determine the relative speeds of action of the two solutions at any convenient concentrations;(b) determine the concentration coefficient (ή) of both phenol and test bactericide by means of comparative l.t. 50 values;(c) determine the temperature coefficient for each substance by similar methods.From these observations the relative efficiency of the two bactericides can be calculated at any temperature and concentration. If two sets of figures are obtained, one with a vegetative organism, and the other with a sporing organism, a great deal of information can be summarized. It has been shown that an 0·05% solution of parachlor-meta-cresol is approximately one and a half times as efficient as an 0·5% phenol solution (against aMicrococcus). The comparative figure is about 1·0 whenBact. coliis used.

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Section: Techniquementioning

confidence: 99%

Section: Techniquementioning

confidence: 99%

Section: Techniquementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The evaluation of bactericides

Withell

1942

Epidemiol. Infect.

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Thermische Inaktivierung von Mikroorganismen: Zur Modellierung temperaturabhängiger Prozesse

Bliem¹,

Nowak²

2007

Chemie Ingenieur Technik

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Resistance of Bacterial Subpopulations to Disinfection by Chlorine Dioxide

Berg¹,

Hoff²,

Roberts³

et al. 1988

Journal AWWA

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Deviations from first-order inactivation of microorganisms are commonly observed during disinfection. These have been attributed to aggregation, decreasing concentration of disinfectant, or biological heterogeneity. The phenomenon, however, has not been properly investigated. During this study, chemostat-grown and batch-grown populations ofL.egionella pneutnophila and Escherichia coli were dosed with chlorine dioxide (ClOz) to achieve an initial concentration of 0.75 mg/L. Results indicate that a resistant fraction of cells survives in the absence of aggregation and with a constant CIOzresidual. The resistance is a phenotypic characteristic that can be manipulated by altering the antecedent growth environment. Growth rate and temperature were important determinants of the size of the recalcitrant population.

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The Significance of the Variation in Shape of Time-Survivor Curves

Cited by 94 publications

References 59 publications

The evaluation of bactericides

The evaluation of bactericides

Thermische Inaktivierung von Mikroorganismen: Zur Modellierung temperaturabhängiger Prozesse

Resistance of Bacterial Subpopulations to Disinfection by Chlorine Dioxide

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