METHODS OF EESEARCH : PREPARATION OF CULTURE MEDIA 103 



Preparation of Raw Vegetable Substances. — ^Select, as 

 before, sound tubers and roots, etc., wash thoroughly with 

 hand-rubbing or brush-scrubbing. Instead of paring, plunge 

 them one minute into alcohol (to drive air out of the crevices), 

 and then 30 to 40 minutes in 1 : 1000 mercuric chlorid water, 

 then remove, dry with a sterile cloth, or with sterile filter paper, 

 and cut on a sterile surface into suitable slabs with a cold sterile 

 knife; one of large size such as is used in kitchens is suitable. 

 As soon as cut, the slices are removed without touching the 

 cut surface and placed in sterile deep culture dishes in pairs, one 

 of which may be inoculated and the other held as a check. 

 If this work is done in clean still air, with sterile instruments, 

 and if the cut surface is never touched with the hands or with 

 contaminating instruments, there will be little opportunity for 

 intruding organisms to obtain a foothold. Undoubtedly some 

 bacteria on the roots are not killed even by the long soaking 

 in mercuric chloride water but their surface layers are so im- 

 pregnated with the poison that they seldom develop or give 

 any trouble in the dishes. The amount of poison dragged across 

 the cut surface in making the slices is wholly negligible, if the 

 roots are dry when cut. 



Fluid Vegetable Substances. — The juices of vegetables 

 diluted with water are also useful as culture-media. They may 

 be used in 10-cc. portions in test tubes, or filled into suitable 

 fermentation tubes, or used in flasks. These are generally 

 steam-heated a short time on 3 consecutive days before using. 

 The same care should be used in the preliminary surface prepa- 

 ration of vegetables, the juice of which is to be used for culture- 

 media as has been recommended already for the preparation of 

 steam-sterilized potato cylinders, etc. This juice is obtained 

 either by cooking or by crushing or grinding and pressure. It is 

 very easy to sterilize such media if they have not been con- 

 taminated by carelessness in the initial stages of the prepara- 

 tion. These fluids must be filtered before they are tubed, 

 first through two folds of cheesecloth or surgeon's gauze and 

 afterward through filter paper. Substances containing starch 

 filter with difficulty after they have been boiled. Therefore, 

 if juices are to be extracted from starchy vegetables by heat 



