484 BACTERIOLOGICAL ANALYSES 



exclusion of organisms likely to contaminate the prep- 

 aration, and if possible one which affords character- 

 istic cultural appearances. 



Bile Salt Broth (page 180) combines these desiderata; 

 it permits only the growth of intestinal bacteria, whilst 

 the formation of an acid reaction and the production 

 of gas in subcultures prepared from the germ-germicide 

 mixture is fairly complete evidence of the presence of 

 living B. coli. 



The amount of medium present in each test-tube is a 

 matter of importance, since the medium not only pro- 

 vides pabulum for the test germ, but also acts as a 

 diluent to the germicide, to reduce its strength below 

 its inhibition coefficient. For routine work each sub- 

 culture tube contains 10 c.c. of medium, but it is 

 obvious that if germicide-x possesses an inhibition 

 coefficient of o.i per cent, the addition of o.i c.c. of 

 a 10 per cent, solution to 10 c.c. of medium would effectu- 

 ally prevent the subsequent growth of the test germ 

 after a contact period insufficient to destroy its vitality. 

 Hence the preliminary tests may in some instances 

 indicate the necessity for the presence of 12 c.c., 15 c.c. 

 or more of the fluid medium in the culture tubes. 



/. Incubation Temperature, 37 C. 

 g. Observation Period of the Subcultivations, Seven 

 Days. 



In order to determine whether or no the test germs 

 have been destroyed, observations must always be con- 

 tinued when growth appears to be absent up to the 

 end of seven days before recording ' ' no growth. ' ' 



h. Identification of the Organisms Developing in the 

 Subcultivations after Contact in the Germ + Germicide 

 Solution. 



This is based on the naked eye characters of the 

 growth in the bile salt broth, supplemented where 



