BACTEKIA '2-2\ 



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

 (lays without killing it. The retardation of haeterial growtli 

 in low temperature is of importance from tiu* })ul)lic-healt]i 

 standpcunt, since it makes possible the s!iip[)ing and temporary 

 preserving of perishable foods in cold storage. 



Heat in sutfieient amount kills all bacteria whether in the 

 spore or vegetative state. Steam heat is more etTective than 

 dry; a few minutes of steam heat at 1:2(1° ('. will kill spores 

 that would take 1S0° (". of dry heat to destroy. 



Light. Contrary to the effect jjioduced U])on green [)lants, 

 light has an unfavorable action upon bacteria. iJright sunlight 

 serves to kill the vegetative cell and weakens the spores ; 

 diffuse liii'ht retards g^rowth ; in the absence of all light thev 

 grow best. This destructive action is intensilied by moisture 

 and fresh air. 



Oxygen. Pasteur was the tirst to demonstrate that some 

 bacteria live without free oxygen. I le divided all bacteria into 

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

 l)resence of air: anaerobi(% those that can grow only in the 

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

 with or without air. liacteria that grow in the iiuier tissues 

 of the bodv 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 decav whicli begin on llie surface and woi k towar<l 

 the center. 



Work of bacteria, hike other fungi, bacteria are parasitic 

 (attacking living plants and animals), saprophytic (feeding 

 upon dead or waste ainmal or [)huit matters), and synd)iotic 

 (livhig in plants to the mutual benefit of bacterium and 

 plant). Because some species can attack li\ ing tissue and 

 produce disease, all bacteria have come to suggest disease 

 to the popular mind. This rei>utatioii is as unjust to the 



