208 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



necessary dilution to secure isolated colonies is attained by drawing 

 a platinum needle infected with bacteria several times across the sur- 

 face of sterile, slanted gelatin, agar, blood serum, blood agar or^ other 

 solid medium, each time covering an area not previously touched. 

 Eventually a degree of dilution is reached where discrete colonies are 

 discernible. 



The plating method and streak method possess advantages and 

 disadvantages. A considerable proportion of the growth in plates 

 inoculated in the fluid state is beneath the surface, where it 

 is less characteristic than surface colonies. The distribution of 

 organisms, however, is more uniform, and small numbers of bacteria 

 occurring in mixture with larger numbers of undesirable organisms 

 are somewhat less likely to be overlooked. It is possible, moreover, 

 to obtain a quantitative estimation of the numbers of bacteria in 

 mixtures by the plate method. The streak method is advantageous 

 both with respect to the economy of time necessary to inoculate the 

 medium, and in that the colonies are wholly upon the surface of the 

 medium. There is less danger of contamination when u fishing" from 

 streak plates than from the regular method of plating, because there 

 is no chance for submerged colonies to underlie those upon the surface. 



The use of certain kinds of media, as that of Endo, of blood agar, 

 and Loffler's blood serum, requires that surface inoculation shall be 

 made. The possibility of missing or overlooking small numbers of 

 the less hardy types of bacteria is greater with the streak method of 

 isolation. 



3. The Barber Method for the Isolation of a Single Cell. It is occa- 

 sionally necessary, in very refined bacteriological studies, to be abso- 

 lutely certain that the starting point of a pure culture is a single 

 organism. Theoretically, single cells are the progenitors of the colonies 

 observed in media inoculated by the plate or the streak method, and 

 such is usually the case. Undoubtedly it may happen that a chain of 

 streptococci may remain adherent and their descendants appear as a 

 single colony, and it is equally certain that two alien bacteria may 

 occasionally become adherent by intertwining of flagella or adhesion 

 of viscid capsular substance and develop into a mixed colony. The 

 apparatus of Barber, 1 which consists essentially of a delicate capillary 

 pipette mounted in the substage of the microscope, and capable 

 of upward and downward motion in the optical axis of the instrument, 

 is designed to circumvent this possibility. In practice a very thin 



1 Univ. Kansas Science Bull., No. 1, March, 1907. 



