322 PHARMACEUTICAL BACTERIOLOGY 



The chief purpose in disinfection is to check and prevent the spread of 

 communicable diseases, by destroying the primary causes thereof, namely, 

 the pathogenic bacteria or other disease producing organisms. The agents 

 or substances which have disinfecting powers or properties are legion. 

 We can only refer to a few of the more important ones, those which are 

 commonly employed, giving the methods of their use and explaining their 

 action. 



Disinfectants differ greatly as to germ destroying powers and attempts 

 have been made from time to time to standardize them or, in other words, 

 to determine their comparative germicidal efficiency, but thus far no satis- 

 factory or generally acceptable method has been devised. All methods 

 appear, to have some objectionable features. The technic and principles 

 involved in the standardization of antiseptics include the following: 



1. Selecting some antiseptic as the unit of comparison, as a solution of 

 phenol. 



2. As test objects, definite quantities of bacterial cultures are used; as 

 bouillon cultures of the t3T3hoid bacillus, colon bacillus, hay bacillus, etc. 

 Some experimenters first air dry the bacteria before exposing them to the 

 disinfectants to be tested. There are a number of methods known as the 

 "silk-thread method," the "garnet method," "the glass-rod method," 

 "the platinum-loop method," "the spoon method," and others. 



3. Exposing the bacteria from a standard culture, for definite periods 

 (uniform for the series of tests) of time, to varying strengths of the disinfec- 

 tants to be tested. 



4. Plating out (in Petri dishes) the exposed bacteria in order that the 

 death point may be ascertained. 



The results are expressed numerically by dividing the strength of the 

 disinfectant tested which will kill- a given organism in a given time by the 

 strength of the phenol solution which under the same conditions will kill 

 the same organism in the same time. To illustrate, we will suppose that a 

 1-40 solution of formaldehyde will kill the typhoid bacillus in ten minutes 

 at 37° C. and that a i-iio solution of phenol will kill the same organism in 

 the same length of time and at the same temperature, then we get as the 

 phenol coefficient of formaldehyde, 0.36 (*%io =0.36), which means that 

 formalin is only about one-third as active as phenol as far as the destruc- 

 tion of the typhoid bacillus is concerned. The phenol coefficient* is 

 also known as the Rideal-Walker (R-W) coefficient, named after the Eng- 



' For a full dhcussion of the methods for determining the phenol coefficient and 

 the albumen coagulation coefficient, the student is referred to the following books: 

 Schneider. Bacteriological Methods in Food and Drug Laboratories. P. Blakiston's 

 Son and Co., 1915; or, Tanner. Bacteriology and Mycology of Foods, John Wiley 

 and Sons, iqiq. 



