20 BACTERIOLOGY. 



reaction or chemical composition of the soil, may profoundly 

 affect the form and size of bacteria. 



It is evident, therefore, that a given species of bacteria 

 does not possess a constant form or size. Under certain 

 conditions of environment it may be a large, thick bacillus, 

 while at other times it may appear almost like a coccus. 

 By the typical form of a given species is meant that form 

 which is met with when the best conditions of temperature, 

 soil, oxygen supply, etc., are provided. The slightest 

 variation from these optimum conditions will, as indicated 

 above, cause a deviation from the typical form. 



Variations in form and size may be considered as 

 arising from natural causes, the environment; and from 

 artificial causes due to methods of manipulation. 



When a perfectly pure specimen is examined under the 

 microscope more or less marked differences in the size of 

 the various cells will be observed. Such differences must 

 be expected among living, actively growing forms. The 

 small, young cells will always be present beside the large, 

 old cells. Again, most of the bacterial cells in a given 

 specimen may be single, but, now and then, some will be 

 found forming threads, or filaments, many times longer 

 than the single cell. These thread-like forms are not con- 

 taminations due to the presence of a different species, as 

 might at first thought be supposed. 



Another variation may be expected when the adult 

 cell develops a spore or seed. Very characteristic forms 

 result from the presence of a spore within the bacterial 

 cell. These will be given special attention in a subsequent 

 chapter. 



The composition of the medium on which the bacteria 

 grow will exert a marked influence upon their form charac- 

 teristics. The s'ame organism planted on solid media, such 

 as coagulated blood serum, agar and potato, will show dif- 

 ferences in form. The addition of small amounts of glycerin- 



