404 PLANTS AND MAN 



or COCCI, and the spiral or corkscrew-shaped bacteria or spirilla. 

 The relative abundance of the various types is in the order above 

 noted. Many of these bacteria are able to produce spores, or 

 heavy walled resting structures, in which form they are very 

 resistant to extremes of temperature and drying; some are able 

 to withstand boiling in water for over one-half hour. Fortu- 

 nately, from man's viewpoint, some of the worst disease produc- 

 ing forms — the typhoid, undulant fever, and tuberculosis 

 bacteria — do not form spores, so that the heating of milk to a 

 temperature of about 145° F. for thirty minutes— the process 

 known as pasteurization — renders it safe from contagion by any 

 of these organisms. 



The identification of bacteria and a knowledge of their nature 

 and activities is essential to the understanding and treatment of 

 disease. This can be accomplished only by the isolation of a 

 particular bacterial organism and its growth in a pure culture, 

 which is a growth of only one kind of individual. Of the hundreds 

 of bacterial species, many are practically identical as to shape 

 and size so that it is necessary to resort to tests to determine the 

 function as well as the form of the particular bacterial cell being 

 studied. Shape and size of bacteria is determined by staining 

 with dyes derived from coal tar compounds; in addition to this, 

 certain forms give characteristic staining reactions with special 

 dyes, which permits their separation from other forms of like 

 size and shape but giving different reactions. Bacterial function 

 is further studied by growth of pure strains upon various types 

 of culture substances or media, which result in varying sizes, 

 shapes and colors of colonies produced upon agar, gelatin, potato, 

 and milk media. Optimum temperatures for bacterial growth 

 also give important clues as to their identity; human pathogenic 

 bacteria grow best at a temperature near that of the human body, 

 while soil bacteria have their optimum at a considerably lower 

 point. 



Human Disease 



With large numbers of bacteria present at all times in the air, 

 and also common in water, milk, foods, and particularly on the 

 human hands, one often marvels at the small amount of trouble 



