METHODS OF IDENTIFYING BACTERIA 395 



may divide every twenty minutes. A single cell may produce mil- 

 lions of cells within twenty-four hours. As bacteria divide and 

 cling together, the mass of similar cells is 

 known as a, colony. 



If conditions are unfavorable, many bac- 

 teria will give off moisture, thus lessening 

 their size, and surround themselves with a 

 thick wall. In this form, the cell remains 

 dormant until conditions are again favor- 

 able for development. This is known as a 



spore. All bacteria cannot form spores. are found among bacteria. 



Sometimes the spore forms 



Spore-formation in bacteria is not a type at one end of the ceil and 



. sometimes in the center. 



of reproduction because no new cells are 



developed. It is a method by which the cell exists during un- 

 favorable conditions. Spores may be dried without injury. Some 

 spores may be heated to a high temperature, and the organism 

 will still remain uninjured. 



Methods of identifying bacteria. Bacteria are so small that 

 other methods of identification must sometimes be used besides 

 their shape, size, and flagella. Some are recognized by their abil- 

 ity to hold an acid stain, or to give particular color reactions 

 with certain stains. Others are recognized by the changes they 

 bring about in or on various substances on which they are grown. 

 These substances are called media. The type of colony formed 

 is also a means of identification. Some produce colonies with 

 smooth, scalloped, or fringed edges. In some, the colonies are 

 opaque; in others, they glisten. Different colonies of bacteria 

 are characterized by different pigments. Some of the pigments 

 are gray, yellow, pink, or brown. Some colonies develop on the 

 surface of the media, others beneath the surface. The place and 

 character of the spore formed are other means of identification. 

 In some, the spore forms at one end of the cell ; in others, in the 

 middle. A tetanus bacillus forms a spore at one end of the cell. 



