Chapter 1 



Microbial Antagonisms and Production 



of Antibiotics 



Living organisms are not found in nature 

 in separated pigeonholes but are in constant 

 association with one another. During these 

 contacts various types of reactions can be 

 noted. Even though these interactions are 

 often difficult to classify, the following types 

 can be recognized: (1) symbiosis, (2) com- 

 petition, (3) predatoriness, (4) parasitism, 

 and (5) antagonism. In order to simplify the 

 following discussion, these interactions are 

 considered as if they occurn^d only between 

 two individual organisms or groups of or- 

 ganisms. 



Symbiosis 



The word .sijmbiosis is used to designate 

 the harmonious relationship between two 

 organisms which is beneficial to both part- 

 ners. In the world of microorganisms there 

 are many examples of symbiosis. 



The lichens represent a symbiotic associ- 

 ation between an alga and a fungus so har- 

 monious that it forms physiological and 

 morphological types which are different frf)m 

 either fungi or algae. Either one or the 

 other may be the dominant partner, that 

 which encircles the weaker partner and is 

 mainly responsible for the shape of the 

 lichen. In most cases the fungus is the dom- 

 inant partner. The fungal mycelium pene- 

 trates the substratum on which the lichen 

 grows, l)e it rock, l)ark, wood, or soil, and 

 secretes acids and enzymes that dissolve and 

 break down the substratum, often with a 



resulting beneficial nutritional effect. The 

 entrapped alga furnishes the products of its 

 photosynthesis. 



Mycorrhizas are structures formed by 

 interaction of the mycelium of certain fungi 

 and the roots of certain higher plants. The 

 nature of the interrelationship between the 

 roots and the fungi is still the subject of 

 numerous studies, but it seems obvious that 

 the association is beneficial to the plants. 

 The fungus probably helps the plant by 

 absorbing water and nutrients from the soil 

 and benefits from the association by receiv- 

 ing food from the plant. In some instances 

 the association is so successful that the plant 

 produces no chlorophyll and depends on the 

 fungus as a universal pro\-ider. This is true 

 of the Ericaceae, Monotropa (Indian pipe), 

 and the orchid Corallorrhiza. In nature, all 

 orchids depend on fungi for their very life 

 even though most of them ha\'e chlorophyll. 

 In the laboratory, orchid seeds can l)e ger- 

 minated free of mycorrhizal fungi if pro- 

 ^'ided with sucrose which has been auto- 

 claved at an acid pll. It is a fair assumption, 

 then, that fungi fui'uish orchid seeds with a 

 mixture of sugars and sugar degradation 

 products. 



Symbiosis between animals and micro- 

 organisms is represented by certain scale 

 insects and a fungus belonging to the genus 

 Septohasidium. The insect feeds by sucking 

 on a plant, the fungus sending hyphae in- 

 side the insect and growing luxuriant h^ on 



