Mode Of Experimenting

In order to test the effect of a drug on the movements of bacteria already developed, a drop of the solution containing bacteria may be mixed, under the microscope, with a drop of the solution of a drug in the way already described at page 63, and the strength of solution necessary to destroy their movements estimated in the same manner.

In order to combine experiments on the movements, and on the reproduction, so as to ascertain whether the bacteria which have been rendered motionless by heat or drugs are really dead, or are only torpid, the covering-glass in the experiment just described is taken up with a pair of sterilised forceps, and dropped into some sterilised Cohn's solution (vide p. 72). It is then put along with the standard solution into a warm chamber, and left for a day or two. If the bacteria have been destroyed, it will remain clear like the standard solution, but if they have only become torpid, it will be more or less opalescent or milky.

In performing this experiment, great care must be taken that the solution of the drug has been sterilised by boiling; and that the covering-glass, glass slide, all the instruments, and indeed everything used in the experiments, have been also thoroughly sterilised by heating.

A temperature of 66° to 70° C. usually arrests the movements of bacteria, and if continued for an hour destroys adult organisms, though not the spores. A temperature of 100° C. usually destroys the spores as well, but this is not always the case.

If the bacteria are moist, this temperature generally kills them, but not if they happen to be dry, and a much higher tern1 Fodor, op. cit. p. 147.

perature is then required. They may become dry, before being killed, by a little solution containing them having flowed or spurted into the higher part of the tube or flask, where the water evaporates and leaves them dry before the temperature has been sufficiently raised to destroy them.

The bacteria grown in different fluids are not all equally sensitive to drugs.

The most destructive substances to bacteria are corrosive sublimate, chlorine, bromine, and iodine. Quinine and the other cinchona alkaloids also destroy bacteria, their power diminishing in the following order : - quinine, quinidine, cinchonidine, and lastly cinchonine.

Bebeerine is nearly as powerful, and potassium picrate is even superior to quinine when used with Cohn's solution. When bacteria are cultivated in beef-tea instead of Cohn's solution, potassium picrate is less powerful.

Sulphocarbolates and strychnine have considerable power, though a good deal less than quinine; berberin and sesculin have hardly any power to destroy bacteria at all. Sodium hyposulphite has very little action; sodium sulphate has a destructive action, but is about ten times less strong than quinine.1