268 PHYSICAL INFLUENCES 



too, show considerable resistance, while some bacteria, e.g.,B.carotarum 

 and Ps. radicicola, are readily killed. 



The resistance of microorganisms is influenced greatly by the me- 

 dium on which they are placed for drying. Hansen found that yeast 

 cells dried on cotton were still alive after two to three years, while if 

 dried on platinum wire some died in five days and others lived as long as 

 100 days. Compressed beer-yeast mixed and dried with powdered char- 

 coal kept as long as ten years; Ps. radicicola dried on a cover-glass 

 or filter paper died within twenty-four hours ; on seeds, this same organism 

 was still alive after fourteen days and in the dried nodules of legumes a 

 few cells were able to reproduce after more than two years. Soil con- 

 taining an average number of 17,000,000 bacteria per gram was dried for 

 two years; the total number of organisms averaged then 3,250,000, 20 

 per cent of the bacteria, therefore, could resist desiccation. Dried cul- 

 tures of microorganisms are commonly sold for several purposes, as 

 dairy-starters and the so-called "magic yeast" and "yeast foam" used for 

 bread-making. Such cultures are dried on milk, sugar, starch, flour or 

 similar porous and absorbing material. Starters are usually guaranteed 

 only for a certain 'length of time, from one to twelve months. The 

 advantage of the dry culture is its better keeping qualities. Liquid 

 cultures produce substances harmful to themselves, and die rapidly 

 after a short time, while the dry cultures show little change. 



The resistance of pathogenic bacteria to desiccation is of consider- 

 able importance in the spreading of contagious diseases. Many patho- 

 genic bacteria die after desiccation of a few hours to a few days, and 

 spreading of such diseases by dust is highly improbable. Protozoa of 

 soil decrease in number by drying, but all are not killed. 



