METHODS FOR MICROSCOPIC STUDY OF BACTERIA 239 
lating needle may be pressed much harder against the surface, and a 
more uniform, and hixurant growth is obtained. This "spiral streak" 
method has pr()^'ed to be far superior to the older methods of inoculation 
and inasmuch as the resulting growth is wholly upon the surface, the 
removal of colonies is more readily accomplished than in the method 
of pouring plates. Anaerobic bacteria are readily obtained in pure 
culture. 
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 some- 
FiG. 26. — An agar plate showing colonies obtained by the plate method. Many of the 
colonies are below the surface of the medium. 
what 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 ad\antageous both witli 
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 "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 Euflo, of blood-agar, 
and Loffler's blood serum, requires that surface inocidation shall be 
made. The possibility of missing or overlooking small numbers of 
the less hardy types of bacteria is somewhat greater with the streak 
method of isolation, but this difficulty is largely obviated when the 
spiral streak method is employed. 
