CHAPTER XI 

 ANTIBIOTICS 



THE term antibiotics is used of those substances 

 produced by micro-organisms which have an an- 

 . tagonistic effect, usually specific, on other organisms. 

 Antibiosis results, therefore, from the growth of an 

 organism evolving an antibiotic, in presence of another, 

 susceptible, organism, in contrast to symbiosis which 

 occurs when two micro-organisms grow together with 

 mutual benefit (see pp. 102, 109). 



The antagonistic effect of some micro-organisms on 

 others has been known for many years. The anthrax 

 bacillus, for example, was shown by Pasteur to be in- 

 liibited by aerial contaminants ; lactic acid bacilli will 

 overgrow CI. hutyricum in the butyl alcohol/acetone 

 fermentation, because the large amount of lactic acid 

 which they form produces conditions under which CI. 

 butyricum cannot survive (see p. 315). Substances like 

 lactic acid which act in a non-specific way by altering 

 the physical condition of the environment are not, as a 

 rule, called antibiotics ; the expression antibiotic is 

 reserved for substances which act specifically on a few 

 species of organisms and which are usually active in 

 very small amounts. That is they have an action which 

 is almost the reverse of that of growth factors, probably 

 by interfering with enzyme systems involved in meta- 

 bolism. Antagonism also results from other causes such 

 as the " swamping " of a slow growing organism by a 

 fast growing one which competes successfully for the 

 available nutrients, or by the production by one organism 

 of conditions of oxidation-reduction potential unfavour- 



