520 BACTERIOLOGY. 



4. Immediate and thorough contact with the disinfectant 

 of all the organisms present. This can be done only 

 with bacterial suspensions in which each organism is 

 entirely free and separate from others. To obtain such a 

 suspension, it is necessary to filter through glass-wool 

 or absorbent cotton and then to agitate the liquid thorough- 

 ly, at a temperature of about 37, until microscopical exam- 

 ination shows no aggregations of bacteria. Silk threads 

 which have been soaked in bacterial suspensions and then 

 dried are open to the objection that, on treatment with the 

 disinfectant, the organisms are unequally exposed and some 

 may even be protected by their position. The same objec- 

 tion, to a less degree, applies to cover-glasses on which a 

 thin film of the suspension has been deposited. 



5. The number of bacteria in a given experiment. It 

 can be shown readily that the greater the number of 

 bacteria present the more slowly will disinfection take 

 place. In order, therefore, that the results may be com- 

 parable, approximately the same number of organisms 

 should be present in each experiment. This is readily as- 

 certained by diluting a small portion of the bacterial sus- 

 pension with 1000 parts of sterilized water and then 

 making a gelatin plate with one drop of this dilution. 



A more rapid procedure is to dilute 2 or 3 drops of the 

 suspension to 100 c.c. Freshly distilled water containing 

 1 or 2 per cent, of formaldehyde should be used for this 

 purpose. The suspension is transferred to a Thoma-Zeiss 

 counter and the number of bacteria on the ruled square 

 counted. Each of the small squares represents WTJIT cu. mm. 

 and, since there are 400 of these, the total square corre- 

 sponds to iV cu. mm. Hence, the number of bacteria found 

 under this square multiplied by 10,000 will give the number 

 present in 1 c.c. of the diluted suspension. It is well to 

 allow 20 or 30 minutes for the bacteria to settle before 

 counting. 



