Reports of the special Committee upon Pathological Methods. V, The reaction of media.

  • Great Britain. Medical Research Committee. Special Committee upon Pathological Methods.
Date:
1919
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Licence: Public Domain Mark

Credit: Reports of the special Committee upon Pathological Methods. V, The reaction of media. Source: Wellcome Collection.

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    MopErN work in bacteriology has shown that among the factors influencing growth the reaction of the medium is an important one. Many observers have pointed out that although some bacteria will grow in media having a wide range of reaction, there are certain reactions at which optimum growth takes place. On the other hand, there are certain bacteria which require a more limited range, so that to obtain the best possible growth the reaction of the medium employed must be accurately defined within these limits. Until a few years ago, the reaction of a medium was expressed in terms of the amount of a standard normal or decinormal acid which would be necessary to bring the medium to neutrality, using litmus paper or phenolphthalein solution as an indicator. This method served a very useful purpose in adjusting media of similar composition to a more or less arbitrary standard which trial had shown to be generally suitable for growth. Later experience has demonstrated that these titration tests do not give the true reaction when carried out in media of different com- positions, and, what is more disadvantageous, tests carried out by individual workers, under what are presumed to be identical conditions, lead to widely divergent results. This is shown in a table given by Clark in which eight careful observers, using the same apparatus and the same medium, obtained results varying between 0-7 cc. and 5-le.c. of a S NaOH solution in titrating a 5 per cent. peptone solution. It is clear that the reactions of these neutralized peptones cannot have been the same, and that errors of this size may be serious. If under these carefully controlled conditions the variations were so great, it can be safely assumed that workers in different laboratories would encounter even wider discrepancies when attempting a similar series of experiments. The causes of the errors in titration are partly due to the personal equation of the manipulator, his colour sense, &c., but for the most part to the fact that the end point is not sharp. Near the end point of the titration there is a zone where the addition of acid or alkali does not produce a striking change in the colour of the indicator (see Buffers, p. 7). The true reaction of any fluid is determined by the amount of hydrogen or hydroxyl ions which are present in it. For theoretical and practical reasons which are not for the moment important, the reaction of a fluid is therefore expressed by the concentration of hydrogen ions which it contains. Using this standard to check the results of the older titration method an A 2 3 Wt. 5978, 116/236, 2000, 8/19. 0.U.P.