MICRO-ORGANISMS 21 



Micro-organisms can mutate under natural conditions. Staphylo- 

 coccus aureus Rosenbach has mutated until it resists all the common 

 commercially produced antibiotics. Wheat rust develops new races 

 that attack wheat varieties developed by plant breeders to resist rust. 

 The same type of natural mutation and selection undoubtedly occurs 

 in cultures of micro-organisms grown under different conditions from 

 those encountered in nature and must be constantly guarded against. 

 Too, there is always the possibility that cultures may be contaminated 

 with other micro-organisms. 



SOURCES OF MICRO-ORGANISMS 



Despite the variety of natural sources and the ubiquity of micro- 

 organisms, they are abundant only in habitats favorable to their 

 growth. Such habitats or substrata would be the ones to investigate in 

 a search for new types. Viruses and rickettsiae, for example, are 

 strictly parasitic and can be grown only in tissue culture. Since 

 rickettsiae are parasitic only on animals, they would be sought for 

 isolation from some animal showing symptoms of a rickettsial disease. 

 Viruses, depending on the type desired, would be sought and isolated 

 from living plants or animals. If a bacteriophage were desired, it 

 would be sought in old lysogenic bacterial cultures or substrata such 

 as sewage, feces, or polluted streams. 



Pathogenic bacteria, fungi, and protozoa would, like the viruses and 

 rickettsiae, be sought in their respective infected hosts for the most 

 part, or in cases of intestinal parasitism, commensalism, or symbiosis, 

 by isolation from excreta. The isolation of many parasitic strains is 

 made easier by their lack of host specificity and by their generally 

 ready growth in artificial culture. Among the fungi, notable excep- 

 tions would be the powdery mildews (Ascomycetes) and the rusts 

 (Basidiomycetes), which have never been successfully grown in 

 artificial culture. Organisms that may not grow in pure culture or 

 are difficult to obtain often can be maintained in two-member culture 

 with bacteria ; for example, certain protozoa, algae, Myxomycetes, and 

 Acrasiales. 



Most algae and protozoa are free living and can be found where 

 there is sufficient moisture. Stagnant pond water is a good source 

 of fresh- water protozoa and algae, but slow-moving or fast-moving 

 streams, hot springs, ocean water, soil, or dung may be better sources 

 for particular types. Many algae are submerged aquatics, but also a 

 large number contribute, with protozoa, to make up fresh- water and 

 marine plankton. 



Because of the tremendous competition for survival, many micro- 

 organisms survive in nature only because of certain advantageous 

 enzyme systems. Thus, Morchella occurs exclusively in the Temperate 

 regions of the world and is associated with only particular kinds of 

 trees. Furthermore, its fruiting is restricted to but a week or so in 

 the spring of each year. Since reliably identified isolates can only 

 be obtained from the fruiting structures, their availability is likewise 

 restricted. 



Perhaps the best natural sources of saprobic bacteria, actinomycetes, 

 and fungi are soil, dung and other excrement, sewage, decomposing 



