SYMBIOSIS 43 



substances goes on they become gradually liquefied. The pepton- 

 izing and liquefying property of certain pathogenic bacteria is very 

 important from a diagnostic standpoint, and we therefore divide 

 pathogenic microorganisms into two groups, peptonizing or liquefying 

 and non-liquefying bacteria. 



Other fermentative products which are formed by certain enzymes 

 of pathogenic bacteria (colon bacillus) in the splitting of glucose are 

 hydrogen and carbon dioxide; also occasionally alcohol and pro- 

 pionic acid. The anthrax bacillus when grown in the presence of 

 sugar (glucose) forms lactic, formic, acetic, and sometimes succinic 

 acid. The bacillus of malignant edema, under anaerobic condi- 

 tions, decomposes glucose into ethyl alcohol, formic, butyric, and 

 lactic acid. The Bacillus lactis aerogenes which occurs in the intes- 

 tinal tract of man and the domestic animals forms lactic acid as its 

 main product from sugar. Some pathogenic bacteria bring about 

 putrefactive changes in milk and other albuminous materials. When 

 milk is in the intestinal tract, however, these changes do not take 

 place due to the presence of growing colon and lactic aerogenes 

 bacilli. 



Change of Reaction in Culture Soil. Bacteria in general and patho- 

 genic bacteria in particular require, as has been pointed out above, 

 a certain delicate reaction of the medium in which they grow. 



As a rule, the medium must be either neutral or slightly alkaline; 

 more rarely slightly acid. Excesses in these slight degrees of acidity 

 or alkalinity are fatal. Man) pathogenic bacteria growing in culture 

 media change their reaction considerably, and this change, partic- 

 ularly in the presence of certain substances, may be so great that 

 the bacterium becomes weak or even dies. This is particularly true 

 when sugar or glycerin are present. Many pathogenic bacteria form 

 acids from these substances which may increase the acidity of the 

 medium to such a degree as to destroy the bacterial life. Some 

 bacteria, for instance the diphtheria bacillus, first increase the acidity 

 of the medium and then reverse their action, neutralizing the culture 

 soil, and finally make it alkaline to a certain degree. When the reac- 

 tion of a culture medium is changed, other changes may occur depend- 

 ing upon the change of reaction. For instance, alkaline milk may 

 be made acid by a growth of colon bacilli and the casein precipitated 

 in the course of several days. This change is an important character- 

 istic of some bacteria growing in milk, and is valuable as one of the 

 means of identification and diagnosis. 



Symbiosis. It is found that some microorganisms when growing 

 together in an artificial culture medium assist each other materially 

 in their growth, or one may assist the other receiving no benefit itself, 

 but at the same time experiencing no damage or hindrance in its 

 growth. When two bacteria assist each other in this manner we speak 

 of a symbiosis. The following groups of bacilli are properly classed 

 under this head: The anthrax bacillus and the streptococcus; the 



