186 BACTERIOLOGICAL CHEMISTRY 



streptococci and pneumococci in dilutions varying from 

 1 in 40,000 to 1 in 3000. It is seen that they have 

 properties similar to the polypeptide antibiotics gramicidin 

 and tyrocidine, the resemblance being especially close to 

 the latter. The effects are eliminated by the presence 

 of phosphatides, with which all the substances combine. 

 It is possible that they may be active by interference 

 with the phosphatide metabolism of the organisms. 



Antibiosis appears to be a very widespread 

 phenomenon, existing among micro-organisms of all 

 sorts. It is probably the chief mechanism by which the 

 majority of species manage to survive in natural sur- 

 roundings, particularly in the soil where they must be 

 subjected to intense competition. Obviously the anti- 

 biotic substances produced by micro-organisms influence 

 profoundly the ecology of their surroundings. 



Antibiotic substances have found several applications 

 in the control of plant and animal diseases, the out- 

 standing example in human treatment being the use of 

 penicillin. It is possible that the closed plaster-cast 

 method of treating wounds may depend for its efficacy 

 on the antagonistic action of saprophytic organisms 

 towards any pathogens which might be present originally. 



Another use of antibiotics has been in the preparation 

 of selective media for the isolation of bacteria from mixed 

 cultures. For instance, the inclusion of penicillin in 

 Bordet-Gengou medium suppresses the growth of most 

 organisms occurring in the throat and enables Hcemophihis 

 "pertussis to be isolated with greater ease from cough 

 plates or swabs. 



For further reading : — 



A. Waksman, " Antagonistic Relations of Micro-organisms." Bart. 



Reviews, 5, (1941), 231. 

 A. AVaksnian, " Miorof)ial Antagonisms and Antil»iotic .Substances.'" 



The Commonwealth Fund. New York, 1945. 



