60 Microbes and You 



4. Be sterile— in a bacteriological sense. 



5. Provide desired physical properties (clarity, liquid, solid, etc.). 



Since modern microbiology is founded upon our abilits' to 

 isolate pure strains of specific species of organisms through the use 

 of culture media, it seems wise at this point to elaborate on these 

 five prerequisites of a good microbiological medium. 



PREREQUISITE I: PROPER MOISTURE CONTENT 

 When we analyze bacterial cells, we find that their moistiue 

 content approaches 80%, with a range approximatelv 15% either 

 side of this figure. The technic generally used in the determination 

 of water content of cells is to dry them at 100-110° C. in the air, 

 or in a vacuum oven at a lower temperature, and to observe their 

 weight loss. Slimy capsular layers surrounding some organisms 

 tend to raise their relative moisture level. In compounding media 

 we try to provide a high water content, usually 75 to 95%, to ful- 

 fill this growth requirement of microorganisms. 



Bacteria are more closely related to aquatic plants than to 

 terrestrial plants; they thrive best when the organisms are sur- 

 rounded by moisture containing readily available food substances. 

 An abundance of water is just as important for bacterial growth 

 as is the presence of available food, since moisture serves as a 

 vehicle for the food and provides transportation for the egress of 

 waste products built up within the cell during metabolism. Water 

 is the most universal solvent. Its specific heat aids in the ab- 

 sorption of heat liberated during metabolism of the cells, and 

 equally important is the conductivity of water which facilitates 

 the dissipation of heat generated by living organisms. 



The concentration of food in solution directlv affects the speed 

 of flow and the direction of flow of water with respect to the 

 suspended cells. Too high a concentration of food draws water 

 from the organism and tends to shrink the cell, while too low a 

 concentration of nutrients induces the entrance of excessive water 

 through the cell wall with resulting swelling and eventual harm to 

 the organism. 



