Studies on Physiology of Some Plant Pathogenic Bacteria 19 
agreed in showing the influence of initial hydrogen ion concentration upon 
final hydrogen ion concentration. Their studies show for each acid a “criti¬ 
cal pH,” which is the mean of the most acid value of pH which permits growth 
and the least acid value which inhibits growth. In the case of Welches’ bacil¬ 
lus, the “critical pH” with hydrochloric acid was 4.78; with lactic, 4.68; with 
formic, 4.91; with malic, 4.66; with acetic, 4.88; with butyric, 4.99; and with 
succinic, 4.88. Bacillus sporogenes was less tolerant to these acids as shown 
by the “critical pH” 4.87 with hydrochloric; 4.87 with lactic; 4.91 with formic; 
5.08 with acetic; and 4.96 with butyric. 
Wyeth (9) in experimentation with B. coli extended the observations of Wolf 
and Harris (8) and concluded in agreement with them, that the initial hydro¬ 
gen ion concentration of the medium and the nature of the acid used to produce 
this initial reaction are both factors which modify the final reaction by a cul¬ 
ture. In addition, he found that in 2 per cent peptone adjusted with hydro¬ 
chloric acid a “critical pH” for B. coli of 4.27 was obtained, but when phtha- 
late was substituted for peptone, as a buffer material, this value became pH 
4.55; with lactic acid the corresponding “critical pH” values are 4.47 and 4.73 
and with acetic, 4.68 and 4.83 in peptone and phthalate respectively. 
Foster (10) has recently noted another interesting condition which modifies 
the range of tolerance. In 1 per cent glucose broth the limits of growth of 
Streptococcus hemolyticus were pH 6.35 and pH 8.50. If, however, 5 per cent 
horse serum was added to this broth, these limits were extended to pH 5.90 
and pH 9.25. 
Manifestly, then, there is no definite hydrogen ion concentration at which 
growth ceases or which can be tolerated by bacteria which is not modified by 
one or more of the following factors: 
1. Kind of organism 
2. Kind of acid used in adjustment or the medium 
3. Initial reaction of the medium 
4. Kind of buffer material 
5. Concentration of buffer material 
6. Kind of fermentable carbohydrate 
7. Concentration of fermentable carbohydrate 
8. Presence of substances like serum which stimulate growth and fermenta¬ 
tion probably through their content of food accessory substances. 
9. Physical state of medium, i. e. whether solid or liquid. 
A number of other factors, some of which are more or less well known, such 
as strain differences, age of the parent culture, “salt” effects, temperature, auto¬ 
toxins, etc., have been noted to exert an influence. 
In summary, therefore, studies on tolerance to or inhibition by H or OH ions 
are of little or no value unless accompanied by definitely specified conditions 
which must include a statement of the exact composition of the experimental 
media. 
References : 
(1) Friedentlial, H. Absolut und relativ Disinfektionskraft von Elementen 
und chemischen Verbindungen. Biocliem. Zeitschr. 94: 47-68, 1919. 
(2) Paul, T. und Kronig, B. Ueber das Verlialten der Bakterien zu chemi¬ 
schen Reagentien Ztsclir. f. physik. Chem. 21: 414, 1896. 
(3) Paul, T., Birstein G., und Reuss, A. Beitrage zur Kinetik der Giftwir- 
kung und golosten Stoffen. Bioc-hem. Ztschr., 29: 202, 1910. 
(4) . Einfluss der Neutralsalze 
und Temperatur auf die Disinfektionsgeschwindigkeit von Sauren. 
Biochem. Ztschr. 29: 249, 1910. 
(5) Wright, J. H. Importance of uniform culture media in the bacteriologi¬ 
cal examination of disinfectants. Jour. Bact. 2: 315-346, 1917. 
(6) Michaelis, L. und Macora, F. Die Saure productivitat des Bacterium 
coli. Ztschr. f. Immunitatsforch, u. exper. Tlier., 14: 170-173, 1912. 
(7) Clark, W. M. the final hydrogen ion concentration of cultures of 
Bacillus coli. Jour. Biol. Chem. 22: 87-98, 1915. 
(8) Wolf, C. G. L. and Harris, J. E. G. (Contributions to the biochemistry 
of pathogenic anaerobes, III. The effect of acids on the growth of 
Bacillus Welcliii (B. perfringens) and Bacillus sporogenes (Metchni- 
koff). Biochem. Jour. 11: 213-245, 1917. 
