l6o PHARMACEUTICAL BACTERIOLOGY. 



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 adopted. 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 i per cent, 

 solution of phenol. 



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

 bouillon cultures of the typhoid 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 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 pest bacillus in ten minutes at 37 C. and 

 that a i-no 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 co- 

 efficient of formaldehyde, 0.36 (40/110=0.36), which means that formalinis 

 only about one-third as active as phenol as far as the destruction of the pest 

 bacillus is concerned. The phenol coefficient is also known as the Rideal- 

 Walker (R-W) coefficient, named after the English investigators who 

 worked out the details of the method. In time no doubt an international 

 standard method for testing disinfectants will be adopted. This would be 

 of inestimable value for comparative purposes. 



i. Physical and Mechanical Disinfectants. 



The following is an outline of the physical and mechanical means of 

 disinfecting. 



A. Cleanliness. That is, bacterial cleanliness, or absence of bacteria, 



