Aug. 19,1918 Reaction on Nitrogen-Assimilating Bacteria 
3i9 
method of determining reaction. More recent study has shown that 
bacterial processes are influenced by the hydrogen-ion concentration of 
the medium rather than by the total acidity. Therefore it was planned 
in this study to measure true acidity, concentration of hydrogen ions, 
as well as total acidity of the culture media, 
INFLUENCE OF ACIDITY AND ALKALINITY ON THE GROWTH OF 
BACTERIA 
The data reported deal with the effect of varying reactions on the mul¬ 
tiplication of bacteria from some of the more important legumes, as well 
as on the multiplication of two different strains of Azotobacter. The 
term “strain” as used in this paper is applied to the same species of an 
organism isolated from different sources; for instance, the writers studied 
eight strains of the alfalfa organism, all of which were separated in pure 
culture from plants grown in widely separated sections and from soils of 
different reaction. 
IDENTIFICATION OF LEGUME BACTERIA 
To prove that the organisms employed were pure and true to name, 
all cultures were replated at least twice before their general character¬ 
istics were studied. Table I shows some of the cultural characteristics 
of these microorganisms. On standard nutrient-agar slopes growth is 
moderate, at first colorless, later a faint brown. In standard gelatin 
stab, growth is slow, chiefly at the top of the medium, brownish in 
color, and no liquefaction is noted for one or two weeks; however, 
in older cultures, three months, the gelatin is slowly liquefied. No gas 
is produced from dextrose, lactose, or saccharose broths, although the 
media become cloudy, and frequently a white membrane is formed 
which covers the surface. The organisms grow slowly in nitrate broth 
without gas production. In neutral litmus milk no change is noted 
during the first week; but at the end of the second or third week this 
medium becomes alkaline, and the dye is partly reduced. After two 
weeks bromcresol-purple milk inoculated with the legume bacteria 
becomes alkaline. The difference between the legume bacteria of dif¬ 
ferent plants and different strains is perhaps best demonstrated by the 
rate of growth on mannitol-agar stroke cultures. On this medium the 
legume bacteria may be divided according to their amount of growth 
into three groups: scanty, moderate, and abundant growers. The 
organisms from different sources show decided variations in their growth 
in standard nutrient broth. The presence or absence of a membrane 
however, seems to bear no relation to the growth on mannitol-agar 
slopes. Except in the media already described, the legume bacteria 
exhibit close agreement in their cultural characteristics. 
