III. Tolerance to Acids of Certain Bacterial Plant Pathogenes* 
INTRODUCTION 
Cultural studies of plant pathogenic bacteria are generally believed to be in¬ 
complete unless they include a determination of the lethal or limiting hydro¬ 
gen and hydroxyl ion concentration. Recent investigations on forms patho* 
genic to man have demonstrated, by the employment of refined methods, the 
importance and value of hydrogen ion determinations. Similar studies by plant 
pathologists, employing methods of precision in measuring acidity and alkalin¬ 
ity, in place of the meaningless titrimetric methods, would no doubt, show 
that H-ion concentration is of significance in the culture of plant pathogenic 
forms. Certainly in such an important problem as the possible correlation be¬ 
tween tolerance to acid and immunity or resistance no progress has been made 
with methods in vogue and no convincing body of evidence has thus far been 
adduced. Neither will such data be forthcoming until a better understanding 
is had of some of the fundamental factors underlying the inhibitory or anti¬ 
septic action of acids. 
Further, the influence of the hydrogen ion on inhibition of growth and on the 
processes of metabolism in bacteria is generally believed both to be specific 
and an easily explicable matter. It become obvious, however, that such is not 
the case, and that much additional experimentation is necessary before an 
adequate explanation can be offered for the phenomena which may be noted 
if one cultivates several species of bacteria on media which differ only in the 
kind of acids employed in making the pH adjustment. 
In the present investigation, attention has been directed to the nature of the 
acid and to the nature of the medium as factors in the problem of the influence 
of pH. There has arisen, outside the field of plant pathogenic bacteriology, a 
considerable body of literature dealing with inhibition and antiseptic action. 
Some of these investigations, as will be discussed later, point out the interest¬ 
ing fact of the failure of acids of the same pH concentration to inhibit cell 
multiplication, the cause of which has not been satisfactorily explained. This 
observation has been confirmed in the present study with plant pathogenic 
forms and in addition, differences in growth in liquid and solid media are 
noted which further indicate the complexity of the problem of inhibition. 
METHOD 
The organisms employed were Bacillus carotovorus, Bacterium campestre, 
Bacterium angulatum, Bacterium tabacum, Bacterium glycineum and Bac¬ 
terium sojae. They were cultivated in bouillon, consisting of 1 per cent Ar¬ 
mour’s peptone, 0.3 per cent Liebig’s beef extract, and 0.5 per cent NaCl and in 
nutrient agar prepared by adding 2 per cent agar to the bouillon. Two per 
cent of agar and 1 per cent of peptone were arbitrarily employed after some 
preliminary experimentation upon the effect of variation of the agar con¬ 
tent and of the peptone between 1 and 3 per cent. As is well known, both of 
these substances, except in the case of pure agar, modify the “buffer” action, 
and consequently the amount of acid necessary to secure the desired pH, but 
within the range of 1 to 3 per cent, they appear to be without effect on inhibi¬ 
tion of growth. 
♦Reprinted from Phytopath. 11:244-250. 1921. 
