BACTERIA 221 



( 190 C.) for periods varying from six hours to forty-two 

 days without killing it. The retardation of bacterial growth 

 ri low temperature is of importance from the public-health 

 standpoint, since it makes possible the shipping and temporary 

 preserving of perishable foods in cold storage. . 



Heat in sufficient amount kills all bacteria whether in the 

 i?pore or vegetative state. Steam heat is more effective than 

 dry; a few minutes of steam heat at 120 C. will kill spores 

 that would take 180 C. of dry heat to destroy. 



Light. Contrary to the effect produced upon green plants, 

 light has an unfavorable action upon bacteria. Bright sunlight 

 serves to kill the vegetative cell and weakens the spores ; 

 diffuse light retards growth;- in the absence of all light they 

 grow best. This destructive action is intensified by moisture 

 and fresh air. 



Oxygen. Pasteur was the first to demonstrate that some 

 bacteria live without free oxygen. He divided all bacteria into 

 chree classes : aerobic, those species that can grow only in the 

 presence of air ; anaerobic, those that can grow only in the 

 absence of air ; and facultative, those that can grow either 

 with or without air. Bacteria that grow in the inner tissues 

 of the body of a plant or animal are examples of anaerobic 

 species ; they do not grow without oxygen, but get a supply 

 by breaking down organic substances that contain it. The 

 majority of bacteria are aerobic, as evidenced by the many 

 cases of decay which begin on the surface and work toward 

 the center. 



Work of bacteria. Like other fungi, bacteria are parasitic 

 (attacking living plants and animals), saprophytic (feeding 

 upon dead or waste animal or plant matters), and symbiotic 

 (living in plants to the mutual benefit of bacterium and 

 plant). Because some species can attack living" tissue and 

 produce disease, all bacteria have come to suggest disease 

 to the popular mind. This reputation is as unjust to the 



