An Accurate Automatic Syringe Mechanism

Continuous culture techniques as simulators for standard cells: Jacques Monod's, Aron Novick's and Leo Szilard's quantitative approach to microbiology

Continuous culture techniques were developed in the early twentieth century to replace cumbersome studies of cell growth in batch cultures. In contrast to batch cultures, they constituted an open concept, as cells are forced to proliferate by adding new medium while cell suspension is constantly removed. During the 1940s and 1950s new devices have been designed-called "automatic syringe mechanism," "turbidostat," "chemostat," "bactogen," and "microbial auxanometer"-which allowed increasingly accurate quantitative measurements of bacterial growth. With these devices cell growth came under the external control of the experimenters and thus accessible for developing a mathematical theory of growth kinetics-developed mainly by Jacques Monod, Aron Novick and Leo Szilard in the early 1950s and still in use today. The paper explores the development of continuous culture devices and claims that these devices are simulators for standard cells following specific requirements, in particular involving mathematical constraints in the design and setting of the devices as well as experiments. These requirements have led to contemporary designs of continuous culture techniques realizing a specific event-based flow algorithm able to simulate directed evolution and produce artificial cells and microorganisms. This current development is seen as an alternative approach to today's synthetic biology.

Studies in the dynamics of disinfection: I. New data on the reaction between phenol and Bact. coli using an improved technique, together with an analysis of the distribution of resistance amongst the cells of the bacterial populations studied

1. The action of phenol onBact. colihas been studied quantitatively in detail under such conditions that unfavourable circumstances other than the presence of the germicide were as far as possible eliminated. Briefly, this involved the addition of phenol solution directly to a large volume of culture developed under rigidly standardized conditions.

2. When the mortality exceeded 95% the variation between the numbers of colonies on replicate plates, when determining the numbers of survivors, was often excessive.

3. Under the conditions used the death-rate was not constant. First, there was a phase (1) of slow but increasing death-rate which merged gradually into a phase (2) of approximately constant rate which was also the maximum for a given phenol concentration. There are indications that the death-rate may decline towards the extreme end of the disinfection process.

4. The virtual extinction time (the time taken for the survivors to fall to 1 per ml. as determined by extrapolation of the log survivors-time curve) increased as the phenol concentration decreased, as also did the duration of phase 1. The latter occupied about 60% of the virtual extinction time when the concentration was 3·48 g. phenol per 1., but with rising concentration its duration decreased at a greater rate than the virtual extinction time so that at about 7 g. per 1. phase 1 was very short though still detectable. The standard errors of the virtual extinction times varied between ± 1·5% and ± 5·43%.

5. Using the probit method it has been shown that at low concentrations of phenol the distribution of resistance ofBact. coliis approximately normal when the resistance is measured in terms of the survival times.

6. The distribution of resistance was found to be normal at all concentrations of phenol used, when that resistance was measured in units of the logarithms of the survival times, with the exception that at the lower concentrations there was a sudden change in the slope of the probit-log survival-time line at about 20% mortality. This change became less marked with rising concentration and was probably absent at 8·00 g. phenol per 1.

7. After conversion into the dosage-mortality form, the data reveal that about 70% of the cells show an approximately normal distribution of resistance when the latter is expressed in terms of the dosage survived for a fixed time, provided that the exposure time is less than 50 min.

8. The distribution tends to be normal for longer exposure times when the resistance is measured in terms of the logarithms of the dosages survived. On this basis a change of slope of the probit-log dosage line is observed as in the case when resistance is measured in terms of the logarithms of survival times.

Studies in the dynamics of disinfection: XI. The effect of lethal temperatures on standard cultures of Bact. coli. IV. An investigation of that portion of the population which survives prolonged exposure at pH 7.0

1. An attempt has been made to discover the origin and nature of the small proportion of the cells in standard cultures ofBact. coliwhich is able to withstand, apparently indefinitely, exposure to temperatures of 49, 51 and 53° C.

2. The method adopted was to allow these survivors to generate fresh cultures in the same flask merely by reducing the temperature to 35° C. and to subject the resulting cultures in turn to heat treatment. Only one process of regeneration and subsequent disinfection was carried out at 49 and 53° C., but at 51° C. the process was repeated four times. The disinfection curve was determined on each occasion and the permanent population allowed to become established.

3. Changes in the heat resistance of the cultures were judged by (a) the disinfection times (99·99 % mortality), (b) the maximum death-rates, (c) the levels of the surviving populations and (d) the variations in death-rate with time during each disinfection.

4. Marked fluctuation in numbers of viable cells was observed during each of the phases of permanent surviving population. The exact levels of survivors were therefore uncertain, but after exposure to 49° C. the mean values were not higher than when 53° C. was used, nor was there an increase in the mean numbers of survivors in the successively regenerated cultures.

5. The survivors did not grow when inoculated into either nutrient agar plates or tubes of broth held at the respective high temperatures, although they remained alive and developed readily on being transferred to 35° C.

6. The maximum death-rate decreased and the disinfection time increased markedly after one regeneration of the cultures. The increase in the latter was not wholly due to the decrease in the former, as the death-rates throughout the disinfection of the regenerated culture were lower.

7. After the second regeneration (at 51° C.) the maximum death-rate declined and the disinfection time rose still higher, but the process was reversed after the third and fourth regenerations.

8. The shape of the graph of log survivors against time altered markedly after the second regeneration at 51° C. The initial death-rate was higher but fell sharply to a minimum before rising again to the final maximum.

9. The experimental findings are discussed. An increase in the general heat resistance of these cultures may have occurred as a result of one regeneration, but the technique employed permitted the gradual alteration of the environmental conditions, and this appears to have exerted complex effects on the cultures so that the results of further regenerations are inconclusive.