ee —— 
METHODS OF ISOLATION. II 
quite constantly, but merely as followers of something else. When possible, 
therefore, diseased plants should be examined for the suspected pathogen, in large 
nuinbers, in different years, and from widely separated localities. Of course, if fungi 
are also present they must likewise be examined as to constant occurrence and 
pathogenic properties. 
Under (6) all of the standard nutrient media should be tried, and that repeatedly, 
until the student is entirely familiar with the appearance and behavior of the 
organism. It is usually best to isolate the organism for experiment from selected 
portions of the tissue by means of Esmarch roll-cu!tures or by the use of poured 
plates (Petri-dish cultures), generally the latter. 
Isolations may also be made by inserting a sterile platinum needle or loop into 
the diseased tissue, obtaining therefrom a little fluid, and drawing this over the 
Fig. 6.* 
surface of slant agar, gelatin, or potato a number of times. This is an old method 
introduced by Koch in 1881. If ten or twelve tubes are used, the final streaks will 
often consist only of scattering colonies, from one or more of which the subcultures 
may be made. ‘The plate method has the great advantage of showing just how 
many kinds of bacteria are present in the tissues (provided they will all grow in the 
medium used and under the conditions of the experiment), and just how numerous 
they are. In case of viscid organisms, or those forming compact zooglcece in the 
*Fic. 6—Cross-section of root of plant No. 53 (turnip) parasitized by Bacterium campestre, 
showing an early stage in the formation of a bacterial cavity. The original section was made from 
material fixed in alcohol, infiltrated with paraffin, stained with carbol-fuchsin, and washed in a mix- 
ture of alcohol and water. Drawn from a photomicrograph. X 500. 
