DIFFERENT METHODS OF ATTACK 



man, rat, frog, etc., will reduce this to give lactic acid, 

 CHg . CHOH . COOH. A culture of Aerobacter aerogenes grown 

 at pH 8 will attack it by hydrolysis to give acetic and 

 formic acids: 



CHg . CO . COOH -f H2O = CH3 . COOH + H . COOH. 



The same organism grown in the same medium but adjusted 

 to ^H 6 will utilise a third method of attack by decarboxyla- 

 tion : 



2CH3.CO.COOH= CH3.CO.CHOH.CH3 + 2CO2, 



while another organism, Propionihacterium, will reduce 

 pyruvic acid to propionic acid CH3. CHg. COOH. The fact 

 that a given culture of a bacterium will attack a certain 

 substance in a certain way means nothing more than that a 

 culture of that identical organism grown and tested under 

 identical conditions will attack that substance in that way; 

 vary the growth conditions, the experimental test conditions, 

 the strain, species, genus, or family of organism and we cannot 

 say, without further experiment, anything about the reaction 

 that will occur. At first sight it would seem as though we 

 have here a biological problem which is uncontrollable from a 

 chemical point of view by reason of its possible variations. 

 But the situation is not as hopeless as it may first seem; 

 the nature of the attack on a given substance by a bacterium 

 depends upon 



1. the bacterium, 



2. the conditions under which it grows, 



3. the conditions under which it is tested. 



By taking typical organisms and studying their chemical 

 reactions under various growth and experimental conditions, 

 we have already acquired a considerable amount of knowledge 

 concerning the factors governing the variations under (2) and 

 (3). Once we have covered the ground with one organism, 

 we can repeat with others closely and distantly related, and, 

 fortunately, we often find that there are certain fundamental 

 principles underlying the variation of activity with environment. 



