The "Reaction" of Bacteriologic Culture Media

It would be a large task to compile all the recorded instances in which the reaction of a medium has influenced the morphology, pigment formation, proteolytic activity, rate of fermentation, or even the mere viability of bacteria. In 1895 Fuller1 had collected "over a hundred" such cases; to-day they occur so frequently in the literature that there is no need to emphasize the importance of the subject. Adjustments in the reaction of bacteriologic culture media are generally accomplished by titrations. General use of such methods dates from the suggestions made by Schultz2 (1891), Fuller1 (1895) and others in the early nineties. Founded upon these methods are the "scales of reaction." They are eminently useful, and, now that a vast deal of laboriously-gained experience has dictated in terms of these scalies the proper reaction of important media, they are not likely to be easily displaced. But the theory of titration has undergone a fundamental change since the origination of these methods, and we are now able to gain a clearer conception of many points which were of necessity inadequately treated by the early writers. Furthermore, the physiologic importance of "titratable acidity" is dwindling before the broader conceptions of modern physiologic and physical chemistry, and it may be seriously suggested that too strong an adherence to the old titration methods is a hindrance to progress. It has long been recognized that acidity and alkalinity are best defined in terms of hydrogen ion concentration. Such a conception certainly correlates a vast number of facts. The rate of enzyme action, the stability of colloidal structures upon which cellular life depends, the solubility of many physiologically important compounds,