THE GROWTH OF BACTERIA 



107 



air contained many bacteria ; if the dish contains few colonies, 

 the air contained few bacteria. Bacteria are everywhere in 

 the air, but they are less numerous out-of-doors than indoors, 

 less abundant in light places than in dark places, less abundant 

 in well ventilated rooms than in poorly ventilated rooms. 



We can show the presence of bacteria in water, milk, and 

 dust by removing covers from three separate dishes of agar and 

 quickly putting a few drops of water in one, a few drops of milk 

 in another, and a few flecks of dust in a third. If these vessels 

 are set aside in a warm, dark place, colonies of bacteria develop 

 in each. Agar merely touched with the fingers or with a pencil 

 point develops colonies, showing the presence of bacteria on the 

 body and on all the articles we use. Water fresh from the faucet 

 does not produce so many colonies as water which has been 

 standing in the room ; fresh milk does not produce so many as 

 sour milk; clean fingers give less than dirty fingers; and 

 dust from open sunshiny places less than old dust from dark 

 damp corners. 



Bacteria surround us on all sides, but we can lessen the num- 

 ber in our homes by ventilation and sunshine, in our bodies 

 by personal cleanliness, in our food by protection against air 

 and dust. To protect ourselves and our food against bacteria 

 we must know something of their growth and reproduction. 



The growth of bacteria. When we examine a bacteria colony 

 under the microscope we see that it is composed of an infinite 

 number of individual bacteria. Each bacterium is a simple 

 one-celled organism. It has no green coloring matter, and can- 

 not make its own food, but ^ 



and wherever it can get it. FIG. 45. Division of a bacterium into two 



The bacterium absorbs the daughter cells. 



stolen food materials and increases to a full-grown cell. Then 



tt divides crossways into two halves (Fig. 45), and forms two 



