FILTERS 479 



Apparatus Required: 



Filtering apparatus: The actual filter candle that is used must 

 be the one it is intended to test and must be previously care- 

 fully sterilised; the arrangement of the apparatus will natur- 

 ally vary with each different form of filter, one or other of those 

 already described (vide pages 42-48). 



Plate-levelling stand. 



Case of sterile plates. 



Case of sterile pipettes, 10 c.c. (in tenths). 



Case of sterile pipettes, i c.c. (in tenths). 



Tubes of nutrient gelatine. 



Flask containing sterile normal saline solution. 



Sterile measuring flask, 1000 c.c. capacity. 



METHOD. 



1. Prepare surface cultivations, on nutrient agar in 

 a culture bottle, of the Bacillus mycoides, and incu- 

 bate at 20 C., for forty-eight hours. 



2. Pipette 5 c.c. sterile normal saline into the culture 

 bottle and emulsify the entire surface growth in it. 



3. Pipette the emulsion into the sterile measuring 

 flask and dilute up to 1000 c.c. by the addition of sterile 

 water. 



4. Pour the emulsion into the filter reservoir and 

 start the filtration. 



5. When the filtration is completed, pour six agar 

 plates each containing i c.c. of the filtrate. 



6. Incubate at 37 C. until, if necessary, the comple- 

 tion of seven days. 



7. If the filtrate is not sterile, subcultivate the organ- 

 ism passed and determine its identity with the test 

 bacterium before rejecting the filter since the filtrate 

 may have been accidentally contaminated. 



8. If the filtrate is sterile, resterilise the candle and 

 repeat the test now substituting a cultivation of B. 

 prodigiosus a bacillus of smaller size. 



9. If the second test is satisfactory, test the candle 

 against a cultivation of a very small coccus, e. g. f 

 Micrococcus melitensis, in a similar manner; in this 

 instance continuing the incubation of cultivations from 

 the filtrate for fourteen days. 



