HOW WE GET BACTERIA FOR STUDY 181 



PROBLEM H. WHAT ARE BACTERIA AND WHERE ARE 

 THEY FOUND? 



The simplest plants, called thallophytes (Lat. thallus, young' 

 branch; Gr. phyton, plant), have many forms. They may be 

 single-celled or many-celled. They may or may not have chloro- 

 phyll, but they never possess the organs of root, stem, and leaves 

 found in the higher plants. 



The bacteria are probably the smallest and simplest in structure 

 of all the organisms. They are usually classified as thallophytes. 

 They have cell walls but do not have any chlorophyll, and are 

 therefore not able to manufacture their own carbohydrate food. 



How bacteria were discovered. As early as 1683 Leeuwen- 

 hoek is believed to have seen bacteria with his newly invented 

 microscope. But it was not until 1865 that Louis Pasteur, the 

 famous Frenchman, discovered the relation between bacteria and 

 disease in silkworms. Pasteur had shortly before this proved 

 that bacteria caused fermentation and that when floating germs 

 got into nutrient fluids such fluids would " go bad " and would 

 decay. Pasteur and Robert Koch, one in France, the other in 

 Germany, were the first people to actually apply the idea of pro- 

 tecting animals against disease by inoculating them with injec- 

 tions of a culture of weakened organisms that caused the disease. 

 Pasteur made this application to man in his treatment for the 

 prevention of rabies or hydrophobia. 



Demonstration 1. To prepare and sterilize culture media. To a 

 100 c.c. of hot filtered beef broth add 1^ grams of the seaweed agar- 

 agar. If agar cannot be obtained, use gelatin. Add a little baking 

 soda, if necessary, so that the liquid is faintly alkaline. Boil the mix- 

 ture and filter through several layers of absorbent cotton into a sterilized 

 Erlenmeyer flask. Close the mouth of flask securely with a plug of 

 cotton and boil flask half an hour in a sterilizer. If the agar mixture is 

 not clear, it should be filtered again. 



Pour the hot nutrient agar into Petri dishes which have been 

 sterilized with dry heat for several hours. Keep Petri dishes in a dry 

 place r free from dust until ready to use them. 



How we get bacteria for study. To obtain cultures of bacteria 

 for study, it is first necessary to find some material in which they 

 will grow, then to kill all living matter in this food material by 



