138 Microbes and You 



growth conditions are not favorable, or they divide into two small 

 cells if the environment permits. Each time that a cell divides it 

 carries over some of the parent cell, and this leads Frobisher to 

 ask the question, "Are bacteria immortal?" 



There appears to be an interesting correlation, with some ex- 

 ceptions, between the size of a living thing and its generation time 

 and life expectancy. Going up the scale with respect to size we 

 can tabulate the speed of multiplication, or the gestation period, 

 of a few living things as follows: 



Bacteria 20 minutes up to several hours 



Mice 20-21 days 



Albino rats 21-25 days 



Rabbits 30-34 days 



Guinea pigs 68-71 days 



Humans 270-295 days 



Horses 330-380 days 



Elephants Indian 607-641 days 



African 641 days. 



Bacteria have svich a relatively short life span that they are not 

 confronted with retirement problems. They pack a great deal of 

 living into a short time, but in spite of this fact Pasteur once made 

 the statement, "Gentlemen, it will be the microbes who have the 

 last word." 



When we speak of growth curves of bacteria it should be borne 

 in mind that they refer to pure cultures living without all of the 

 competition with which they are faced in nature where many 

 species may be growing in a given environment under highly com- 

 petitive conditions. The curve to be discussed in this chapter 

 represents laboratory conditions and cultures of single species of 

 bacteria. 



If we plot against time the number of living cells of a pure 

 culture growing in broth under optimum conditions, a line char- 

 acteristic of a skewed frequency curve is obtained. The curve ex- 

 presses the general law of growth by Pearl. Whether we are deal- 

 ing with the population of organisms in a limited environment, 

 with people in a country, or with fruit flies in a bottle, the same 



