58 Microbes and You 



upon the permeability of its membrane. The passage of fluids 

 across a membrane is termed osmosis. 



We all know individuals who are particular about their diets; 

 they refuse to eat this or that because they just don't like it. 

 Microbes also exhibit this type of rejection, in some cases to a 

 marked degree. While differences in the permeability of the cell 

 walls partially explains this selectivity, we find some foods that can 

 be absorbed only to be rejected by a cell that doesn't thrive on 

 them. Some organisms have had their own way for such a long 

 time that thev are no longer able to metabolize certain foods. Since 

 a number of organisms are unable to swim around in their quest 

 for food, the nutrients must be provided nearbv at the right time, 

 and in an acceptable concentration, otherwise the cells may starve 

 to death. Such extreme dependence upon a narrowh^ defined diet 

 sometimes works to the disadvantage of pathogenic bacteria. 



When more than a bare minimum of food is available the organ- 

 ism gets larger, just as man puts on weight, but the microbe goes 

 us one step better. When it reaches a predetermined size, it splits 

 in two by a process called binary fission ( equal division ) . Under 

 optimum conditions the dividing process occurs once in about 

 twenty to thirty minutes, with notable exceptions including Mi/co- 

 bacterium tuberculosis whose generation time is much greater. 

 But this splitting cannot go on indefinitely, because natural checks 

 and balances come into play to keep other forms of life from being 

 forced from the face of the earth bv growth of overwhelminsj 

 numbers of microbes. 



Some persons keep cows, others keep pigs or sheep, and the 

 breed of scientists we call a microbiologist, tends microbes. In each 

 case the nutritional needs of the living things being cared for must 

 receive careful attention by the keepers. While minute amounts of 

 food are required by a single bacterial cell, the masses of organ- 

 isms capable of being generated in only a few hours put increasing 

 demands upon the food supply. In general, we are not interested 

 in prolonging the life span of microbes the way we are striving 

 to increase man's life expectancy. However, in the process of 



