PURE CULTURE METHODS 11] 
Gelatin Colonies. See the directions given for agar colonies. If 
the culture is a liquefier, observations should be made at frequent 
intervals before the colonies have spoiled the plate. 
Gelatin Stab. A straight needle should be used and the tube 
stabbed to the bottom. It should be incubated at 20° C. and frequent 
observations made to determine the amount o! liquefaction. The 
amount of liquefaction may be determined by means of a millimeter 
rule. The Descriptive Chart adopted at the 1907 meeting of the 
Society of American Bacteriologists recommended that gelatin tubes 
should be held for six weeks to determine liquefaction. This is a test 
upon which some work must be carried out in order to secure better 
methods for determining gelatin liquefaction in tthe shortest time. 
There is little trouble or error involved in reporting positive results 
but the time is an important element with negative results. Some 
bacteria will liquefy gelatin only after several months. This indicates 
that another method, by which to more sharply distinguish between 
the liquefiers and the non-liquefiers, is needed. 
The Committee has proposed to make the method of Rothberg 
(1917) provisional until it may be tried out. Rothberg gives the 
organism a preliminary cultivation in a 1 per cent gelatin solution at 
25° or 37°; then inoculate the surface of gelatin in a test tube and 
incubate for 15 days at 20° C. 
Potato. The inoculation of potato slants should be made in the 
same way as for agar slants. After the observations called for on the 
Descriptive Chart have been made, the potato may be tested for 
diastasic action according to the directions given under “ Potato 
Starch Jelly.” 
Potato Starch Jelly. Iodine should be used to determine the presence 
of diastasic action. Pour the contents of the culture tube into a beaker 
and dilute with distilled water. If the dilution is sufficient and if the 
organism possesses a diastasc, the various colors of the starch decom- 
position products with iodine will be obtained. Some of these are as 
follows: 
Starch... ccc ce cee eee blue color with iodine 
Soluble starch...............4. .blue color with iodine 
Erythordextrin...............-- red color with iodine 
Achroodextrin.... 0 ........2.05. .no color with iodine 
Maltose.........0.22 cee eee no color with iodine 
Dextrose. .....-ccee cece eee eeece -no color with iodine 
