The spores of bacteria have an enormous power of resisting agents destructive to their vitality, very much greater than that of the fully-developed bacteria. Thus it happens that a quantity of an antiseptic, which is quite sufficient not only to prevent the spores of bacteria from developing so long as they remain in it, but to destroy fully-formed bacteria, will not destroy the vitality of the spores or hinder them from germinating as soon as they are removed from the influence of the antiseptic and transferred to a proper soil.

Yet the power to destroy the vitality of the spores completely is what is required in an antiseptic, for we wish to destroy the infectious material, and prevent it from causing disease, rather than to administer substances to an animal which will hinder the germs from developing in the blood after their introduction into it; although this may be desirable when infection has already taken place.

It is therefore necessary to test the effect of drugs in destroying the germs completely.

Method Of Experimenting

This is done by adding to a fluid, containing bacteria and their spores, varying quantities of an antiseptic, and allowing the mixture to stand for a longer or shorter time. A drop of this fluid is then introduced by a sterilised platinum wire or glass pipette into some sterilised Cohn's fluid or beef-tea. This is watched, to see whether bacteria will develop in it or not. If they do develop, it is clear that the spores have not been killed by the admixture with the disinfectant in the original fluid; if they do not develop, then the disinfectant has been sufficiently powerful to destroy them.

1 Buchanan Baxter, Practitioner, vol. i. pp. 343, 350.

The plan usually employed is to take a number of test-tubes, plug their orifices with cotton-wool, and destroy any germs that may be attached to them by thoroughly heating them to about 300° F. in a hot chamber, or in the flame of a Bunsen's lamp. They are then allowed to cool, and a small quantity of a liquid (about 5 cc.) in which bacteria readily grow is placed in each. This also must be previously thoroughly boiled, in order to destroy any germs which may be present in it. The liquid recommended by Conn consists of ammonium tartrate one gramme, potassium phosphate and magnesium sulphate of each five grammes, calcium phosphate .05 gramme, distilled water 100 cc. This is filtered and boiled before use. To the tubes the different agents to be tested are added, the solutions of each having been carefully sterilised by boiling, and the pipette used being superheated in each case before it is employed. If the drugs are added in solution, a similar quantity of boiled water must be added to the first tube, which is to serve as a standard. To each of them is then added a single drop of a liquid containing bacteria.

The mouths of the tubes are then stopped with the cotton-wool and placed for a few days in a warm chamber at about 40° C. The standard liquid will then be found to be opalescent or milky. The degree of the opalescence in the other tubes will be less according to the effect of the drug which has been added, in preventing the development of bacteria.

Where it has completely hindered the development, the solution will remain quite clear, and as its strength diminishes, the opalescence will become greater until it is equal to that of the standard.

In performing this experiment it is best to use one definite form of bacterium, instead of a mixture of several unknown kinds. This is referred to again in speaking of the experiments of Dr. Koch, who generally employs the micrococcus prodigiosus as an example of an organism easily acted upon, and the spores of bacillus anthracis, or of a bacillus found in earth, as examples of resistant organisms.

It is found by this mode of experiment that a smaller quantity of poison will prevent the development of bacteria than will destroy them after they are developed.

By experiments on the comparative action of different drugs on bacteria the results contained in the following table have been obtained by N. de la Croix, and these have been to a considerable extent confirmed by Koch.

It will be seen by looking at the table that the exact limit of the power of each drug to destroy bacteria is not determined, but that two concentrations of each antiseptic are given, one of which is sufficient to do it, and the other is insufficient. The disinfecting limit therefore lies between the two experiments. But the limit of disinfection is not an invariable one for each drug, as its power to destroy bacteria is modified not only by the concentration of the solution employed, but by the length of time during which it acts, and by the temperature.

Prevent the development of bacteria taken from meat infusion.

Do not.

Prevent the reproduction of undeveloped bacteria.

Do not.

Kill developed bacteria.

Do not.

Prevent the reproduction of developed bacteria.

Do not.

Prevent development of spores in boiled meat infusion.

Do not.

Prevent reproduction of spores.

Do not.

Prevent development of spores in unboiled meat infusion.

Do not.

Prevent reproduction.

Do not.

Corrosive sublimate . . . .

1:25250

1:50250

1:10250

1:12750

1:5805

1:6500

1:1250

1:5250

1:10250

1:12750

1:6500

1:10250

1:7168

1:8358

1:2525

1:3358

Chlorine . . . . . .

1:30208

1:37649

1:4911

1:6824

1:22768

1:30208

1:431

1:460

1:28881

1:34589

1:1008

1:1027

1:15606

1:23182

1:1061

1:1364

Chlorinated lime ....

1:11135

1:13092

1:488

1:578

1:3720

1:4460

1:170

1:258

1:3148

1:4716

1:109

1:134

1:286

1:519

1:153

1:286

Sulphurous acid ....

1:5448

1:8515

1:135

1:223

1:2009

1:4985

1:190

1:273

1:8515

1:12649

1:325

1:422

1:12649

1:16782

1:135

1:223

Bromine . . . . . . . . .

1:6308

1:7844

1:769

1:1912

1:2550

1:4050

1:336

1:550

1:13931

1:20875

1:493

1:603

1:5597

1:8375

1:875

1:1153

Sulphuric arid ....

1:5734

1:8020

1:205

1:306

1:2020

1:3353

1:116

1:205

1:5734

1:8020

1:306

1:420

1:3353

1:5734

1:72

1:116

Iodine . . . . . . .

1:5020

1:6687

-

1:2010

1:1548

1:2010

1:410

1:510

1:10020

1:20020

1:510

1:724

1:2010

1:2867

5:843

1:919

Aluminium acetate . . . . .

1:4268

1:5435

1:59

1:80

1:427

1:835

1:64

1:92

1:4268

1:4778

1:937

1:1244

1:6310

1:7535

1:478

1:584

Oil of mustard ....

1:3353

1:5734

1:220

1:306

1:591

1:820

1:28

1:40

1:3353

1:5734

1:77?

1:108 ?

1:3353

1:5734

1:40?

1:60?

Benzoic acid . . . . . .

1:2867

1:4020

1:50

1:77

1:410

1:510

1:121

1:210

1:2877

1:4020

1:50

1:77

1:1439

1:2010

1:77

1:121

Borosalicylate of sodium . . .

1:2860

1:3777

1:303

1:394

1:72

1:110

1:30

1:50

1:1343

1:1694

1:35

1:50

1:2860

1:3777

1:35

1:50

Picric acid . . . . . .

1:2005

1:3041

1:706

1:841

1:1001

1:1433

1:150

1:200

1:2005

1:3041

1:200

1:300

1:2005

1:3041

1:100

1:117

Thymol . . . . . . .

1:1340

1:2229

1:109

1:212

1:109

1:212

1:20

1:36

1:1340

1:2229

1:109

1:212

1:1340

1:2229

1:20

1:36

Salicylic acid ....

1:1003

1:1121

1:343

1:454

1:60

1:78

-

1:35

1:3003

1:6004

1:603

1:1003

1:1121

1:1677

1:343

1:450

Permanganate of potassium . . .

1:1001

1:1433

1:100

1:150

1:150

1:200

1:150

1:200

1:2005

1:3041

1:101

1:150

1:300

1:403

1:35

1:50

Carbolic acid ....

1:669

1:1002

1:22

1:42

1:22

1:42

1:266

1:4

1:402

1:502

1:22

1:42

1:502

1:669

-

1:10

Chloroform . . . . . .

1:90

1:112

-

1:08

1:112

1:134

-

1:08

-

-

-

-

1:103

1:134

-

1:122

Borax . . . . . . .

1:62

1:77

-

1:14

1:48

1:69

-

1:12

1:30

1:43

-

1:14

1:107

1:161

-

1:37

Alcohol . . . . . .

1:21

1:35

1:44

1:8

1:44

1:6

-

1:118

1:11

1:21

1:177

1:203

1:21

1:30

-

1:142

Eucalyplol . . . . . .

1:14

1:20

-

1:203

1:116

1:205

-

1:583

1:20

1:29

-

1:14

1:205

1:308

-

1:30

Chlorate of potassium .

-

1:30

-

-

-

-

-

-

-

-

-

-

-

1:13

-

-