118 CLASSIFICATION AND DESCRIPTION OF BACTERIA 
In careful research work the exact shade of the indicator should be compared 
with that obtained in standard “ buffer ” solutions, and the results recorded 
in terms of Py. In laboratories where these solutions cannot be obtained, it is 
better to record the results as simply + or — according to the reaction of the 
culture to litmus, than to use the titration method. Under such conditions it 
is possible, however, to obtain a rough idea of the hydrogen-ion concentration 
by the use of Clark and Lubs’ series of indicators without making accurate 
determinations of Py. Four different degrees of acidity can be easily distin- 
guished by this simple method in sugar broth with initial reaction of neutrality. 
The indicator reactions for these different degrees of acidity are listed in table 
II together with the approximate range of Px to which each corresponds. In 
the absence of accurate determinations, these degrees of acidity may be recorded 
by the indefinite terms, ‘ weak,” ‘‘ moderate,” “ strong,” and “ very strong,” 
or by the symbols +, ++, +++, and ++++. 
If this procedure is used in order to secure data, it is possible that 
a more satisfactory method may be developed whereby the actual 
acidities of the culture tubes may be defined. 
Pathogenicity to Plants. Apply a broth culture of the organism 
to that part of the plant for which it is supposed to be specific. 
Loss of Virulence on Culture Media. The loss of any characteristic 
may be determined by comparing it before and after prolonged culti- 
vation on artificial media. Attempts should also be made to bring it 
back after it has been demonstrated to have been lost. 
Ferments. The presence of enzymes may be determined by the 
decompositions which the organisms accomplish. In searching for the 
presence of any special enzyme, a substrate containing the special 
compound should be used. Frankel’s medium or any other such 
medium may be used as a base. 
Toleration to Acids or Alkalis. This characteristic may be deter- 
mined by adding different amounts of normal acid to alkali to plain 
broth before inoculating with the organism. Quite often a large 
amount of acid or alkali may precipitate the proteins which are in 
the media. This precipitation must not be confused with growth. 
Thermal Death-point. The Committee on Identification of Specics 
of the Society of American Bacteriologists has recommended the expo- 
sure of the organism in nutrient broth for ten minutes. The medium 
should be prepared according to standard methods and the time very 
carefully limited to ten minutes. 
