WHAT ARE BACTERIA? 7 



rods. The contents of one of the rods collects itself in a 

 spherical or oral body in the center of the rod. This later 

 breaks out of the rod, the rest of the individual then dying and 

 disappearing. The oval body itself is a spore, and is capable, 

 when placed under proper conditions, of developing into a 

 new rod. Inasmuch as only a single spore arises from a 

 single bacterium, it is not a multiplication. Its purpose is not 

 so much to increase the number of individuals as to enable 

 the bacteria to endure adverse con- 

 ditions without being killed. The 

 ordinary bacteria are likely to be 

 killed by being dried and will 

 readily succumb to moderate heat, 

 a temperature of 160 J being suf- 

 ficient to kill almost any of them. 

 But these spores are covered with 

 a hard case which enables them to 

 resist the conditions which the 

 active, growing and multiplying 

 forms cannot resist. They may be 

 dried for months, and even years, FIG - 5 SPORE PRODUCTION 

 and still retain their vitality. They 



may be heated very much hotter than the active forms without 

 injury; indeed, some of these spores may be boiled in boiling 

 water for many minutes, for an hour even, or longer, without 

 having their vitality destroyed, since, if the material is subse- 

 quently cooled, the spores are capable of generating and growing 

 into new bacteria. As the result of this it will follow that, while 

 it is very easy to kill ordinary bacteria by heat, it is far more 

 difficult to destroy spores. Some species of bacteria produce 

 such spores while others do not, and hence some are much 

 more easily killed by heat than others. Milk, for example, 

 contains many kinds of bacteria. By the simple boiling or. 



1 Temperatures used in this book always refer to the Fahrenheit scale. 



