The Germicidal Value 249 



When it is desired to secure information concerning the 

 progress of the germicidal action of reagents, body-fluids, 

 etc., especially in the unusual and interesting cases in which 

 the material subjected to the test may exert a restraining 

 action for a time only, or bring about destruction of some or 

 many, but not all of the germs, it is imperative to plate the 

 culture and count the colonies. For this purpose the Petri 

 dish can be used with advantage. 



For example, if it be desired to determine whether blood 

 serum be germicidal or not, the following method can be em- 

 ployed. Into about 5 c.c. of the serum contained in a test- 

 tube, two or three loopfuls of any desired bacterium, in liquid 

 culture, are added. The tube is well agitated and imme- 

 diately one loopful is transferred to a tube of melted gelatin, 

 distributed through it, and poured into a Petri dish. After 

 one minute the operation is repeated, in five minutes again, 

 and so on as often as is desired. 



The dishes are stood away until the colonies of bacteria 

 develop and can be counted with a Wolfhugel or other appa- 

 ratus. On the first dish there may be 100 colonies; on the 

 second, 80; on the third, 50; on the fourth, 20; on the fifth, 

 30; on the sixth, 150; on the seventh, 1000, etc.; indicating 

 that the serum exerted a destructive action upon some, but 

 not all, of the bacteria, and that this power disappeared 

 after the lapse of a certain time, allowing the bacteria to 

 develop ad libitum. 



Control experiments are indicated in this kind of work, as 

 the very fact that the bacteria are transferred from one 

 medium to another is commonly sufficient to cause the death 

 of a large number. 



Gaseous Disinfection. If the germicide to be studied be 

 a gas, as in the case of sulphurous acid or formaldehyd, a 

 different method must, of course, be adopted. 



It may be sufficient to place a few test-tube cultures of 

 various bacteria, some with plugs in, some with plugs out, 

 in a closed chamber in which the gas is evolved. The germi- 

 cidal action is shown by the failure of the cultures to grow 

 upon transplantation to fresh culture media. This crude 

 method may be supplemented by an examination of the dust 

 of the room. Pledgets of sterile cotton are rubbed upon the 

 floor, washboard, or any dust-collecting surface present, and 

 subsequently dropped into culture media. Failure of growth 

 under such circumstances is very certain evidence of good 



