30 EXPERIMENTAL DAIRY BACTERIOLOGY 



colonies, the number of liquefying and nonliquefying colonies on 

 the gelatin plates; and express the results as the number of each 

 contained in a cubic centimeter of the milk. 



Isolation of pure cultures. A pure culture is one which 

 contains but a single kind of organism. Under natural con- 

 ditions the bacterial flora of any substance is made up of a 

 number of kinds. If it is desired to study the different kinds 

 in detail, a separation must be made and each obtained in 

 " pure culture." Each colony on the culture plate represents 

 the progeny of a single cell or of a number of adherent cells 

 of one kind; hence the cultures obtained from a single col- 

 ony are termed "pure cultures." If from the isolated colony 

 further cultures are made in proper media, the morphological 

 and physiological study of the organism can be carried out. 



FIG. 7. PLATINUM NEEDLES 



A, a straight needle; B, an ordinary loop; C, a standard loop (the bend indi- 

 cates the depth to which the loop is dipped in the liquid each time so as to 

 remove equal amounts) ; D, Ravenel's needle 



Platinum needles. The bacteria are transferred from one 

 culture vessel to another, or are handled by means of platinum 

 needles. These are made of No. 25 or 27 gauge platinum wire 

 for ordinary work. For special purposes lighter or heavier 

 wire is used. A piece 50 mm. long is fused into a glass rod 

 or tube about 180 mm. long and 2.0-2.5 mm. diameter. The 

 danger of the glass cracking at the point of insertion of the 

 wire when the needle is heated can be lessened by fusing on 

 the end of the ordinary glass rod a bead of readily fusible 

 glass, into which the wire is inserted. Jena glass, No. 397, 

 III, is excellent for this purpose. 



