52 INFECTIOUS AND PARASITIC DISEASES. 



To separate one species of bacteria from 

 Spores. another, bacteriologists find it necessary 

 to note with the greatest accuracy the 

 minutest differences in size, shape, arrangement, 

 motihty or non-motihty, etc., of the individuals of each 

 species; also the appearances of the species when 

 viewed as masses with the naked eye. To do this the 

 bacteria are grown upon various artificially prepared 

 foods, called "culture media," which are valuable in 

 bringing out the peculiarities of the different species. 

 By means of cultures, the various species are also 

 separated one from the other, and the individual 

 characteristics studied. But these distinguishing char- 

 acteristics can have little interest for anyone unless 

 he works with bacteria in the laboratory. However, 

 one biological characteristic is of very great importance 

 to the practical sanitarian. Reference is made to a 

 resting form of certain of the bacteria that is known as 

 a spore. A spore is a glistening oval body into which a 

 bacterium is converted when the environment is un- 

 favorable to its growth. It corresponds to the seeds 

 of a plant. The spore-form of bacteria represents 

 more than a resting stage, however, since by virtue of 

 an investing membrane, they are far more resistant to 

 every destructive agency than the yegetative form. 

 Thus while a temperature of 60° Centigrade (140° F.) 

 kills the vegetative form of almost every bacterium in a 

 few minutes, spores are able to withstand the boiling 



