150 BACTERIOLOGY 



from the substances to be tested. If properly prepared the plate 

 will exhibit as many colonies, approximately, as there were indi- 

 viduals in the known amount of the material used. This furnishes 

 a working basis for counting the number of specimens in a given 

 quantity of any substance. By exposing a sterilized Petri dish con- 

 taining agar media to the air for a few moments and then cover- 

 ing, in a day or so many colonies will appear, indicating the kinds 

 and number of species which occur in the atmosphere. By plating, 

 these can be isolated, studied, and classified. 



While in normal condition the human body is comparatively 

 free from bacteria, yet the alimentary and respiratory tracts always 

 contain large numbers, and, under abnormal conditions, the tissues, 

 blood, lymph, and urine often become infested. The products of 

 the mucous membrane (especially the hydrochloric acid) and the 

 serum of the blood, which are germicidal, as well as the phagocytes, 

 are generally effectual checks to the invading hosts. 



The outer layers of the skin furnish the abode of many species, 

 and it is claimed to be impossible to dislodge all the germs that 

 occur under the nails. 



There are certain agencies which influence the growth and dis- 

 tribution of bacteria. Electricity arrests growth. Sunlight will 

 kill Bacillus tuberculosis and other species, a few hours of sunlight 

 being sufficient to kill the vegetative cells of Bacillus anthracis. 

 Acids and oils are usually antiseptic. Some species require oxygen, 

 while others will not grow in its presence. Many species can with- 

 stand severe cold> but quickly succumb to a high temperature. The 

 temperature at which a species dies is called its thermal death 

 point. The thermal death point of a considerable number of spe- 

 cies is about 60 degrees Centigrade. There is a certain temperature 

 most favorable to the growth of each species. This for many spe- 

 cies is about 35 degrees Centigrade. 



Laboratory exercise No. 5Q.^-Bacteria in air, water, soil, etc. Expose 

 to the air the agar-agar in a sterilized Petri dish for a few moments. 

 Examine in a day or so and observe and count the colonies upon the 

 surface of the medium. Note that they differ in form and color. In- 

 oculate five test-tubes of agur-ag-ar, one with each of the following sub- 

 stances: Soil, ordinary drinking* water, milk, urine, and saliva. These 

 materials should be collected in sterilized containers. Label each tube, 

 and in a few days examine for any growths which may have appeared. 



