BACTERIOLOGICAL TECHNIC. 87 



Animal pathogen, epizoon . . 

 Plant pathogen, epiphyte. . . 



I Soil.. 

 Milk. . 



Fresh water 



Salt water 



Sewage 



Iron bacterium 



Sulphur bacterium .... 



A. Counting Plate Colonies. If the colonies in a Petri dish culture are 

 few, not exceeding fifty to one hundred, they may readily be counted in full. 

 If the colonies are quite numerous, the counting may be made easier by 

 marking off (by means of a grease pencil or chalk) the bottom of the plate 

 into two right angled cross-lines (quarter sectors) and these again into 

 equal parts (1/8 sectors). Or one of the recommended special counting 

 plates may be used. Either the square or circular plate will answer the 

 purpose (see figures). When colonies are very numerous (200 and more) 

 in a plate culture and quite uniformly distributed, it is not necessary to 

 count them all. Count the colonies in a number of squares or sector areas 

 (square centimeters) and multiply the average of twenty counts by the num- 

 ber of squares representing the entire surface area of the culture plate. As 

 a rule the counting should be complete, however. 



From the plate counts it is possible, by. simple mathematics, to deter- 

 mine the number of microbes in the dilution cultures of water, milk, tinc- 

 tures, fluidextracts, etc., as has already been explained. 



Studying Plate Colonies. The plate colonies should be studied macro- 

 scopically and also with the aid of a pocket lens and under the low power of 

 the compound microscope. Place the dish on the stage of the microscope 

 and focus upon the colonies carefully by means of the coarse adjustment. 

 Note color, outline and other characteristics of the colonies, etc., as already 

 set forth under tube cultures and in the official methods of the Society of 

 Bacteriologists. 



B. Making Tube-cultures (Subcultures). Inoculate test-tubes (con- 

 taining gelatin, agar or other media) with such colonies as it is desired to 

 study further. This is done as follows: Hold the test-tube to be inoculated 

 in left hand. Take up the platinum needle (straight or loop) in the right 

 hand and pass the entire needle and glass rod (excepting the end held) 

 through the flame of a Bunsen burner several times; heat the needle to a 

 glowing red for a few seconds and then allow it to cool a few seconds. Lift 

 the cover of the Petri dish high enough to pass the needle under, touch end 

 of the platinum needle (straight or loop) on colony desired; let the dish cover 



