STERILIZATION ANTISEPSIS FOOD PRESERVATION 8 1 



the same bacteria in beef broth or in feces or in urine, on the other. 

 For these reasons it is not always possible to draw conclusions 

 from the results of laboratory experiments as to the value of a 

 germicidal agent for practical disinfecting purposes. 



Method. To 15 c.c. of sterile water in a 60 c.c. Erlenmeyer 

 flask add 2 c.c. of a virulent culture of the test-organism. Then 

 add a solution of the substance under investigation in the pro- 

 portion necessary to give the dilution wished. Mix thoroughly, 

 and allow this " action-flask " to stand as long as it is desired to 

 have the germicide in contact with the test-organism (action- 

 period). Transfer 0.5 c.c. from the action-flask to a flask con- 

 taining 200 c.c. of a solution of some chemical capable of decom- 

 posing the substance being tested with the formation of inert or 

 insoluble compounds. In this " inhibition-flask " the strength 

 of the solution should be such that molecular proportions of the 

 chemical are present in sufficient quantity to combine with all 

 the germicide carried over. The inhibition-flask is shaken for 

 30 seconds, and i c.c. transferred from it to 100 c.c. of sterile 

 water in another, the " dilution-flask." After two minutes, 

 three agar tubes are inoculated with i c.c. each from the dilution- 

 flask, plated, and growth watched for. 



Control-experiments should be performed to determine that 

 the dilution of the test-culture is not too great when carried through 

 the three flasks. It likewise should be determined that the in- 

 hibiting chemical has no effect on the bacteria. 



What the inhibiting chemical shall be must be determined 

 for each individual case. For salts of the heavy metals ammo- 

 nium sulphide answers well; for mercury salts, stannous chloride 

 may be used; for formaldehyde, ammonium hydrate; for carbolic 

 acid, sodium sulphate. 



The testing of gaseous disinfectants, such as sulphur dioxide 

 and formaldehyde, must be conducted under conditions as nearly 

 parallel to actual practice as possible. The test-organisms may 

 be exposed on threads or cover-glasses, and acted upon by a known 

 volume strength of disinfectant 'for a known length of time. 

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