he wishes to study. When the needle is 

 then touched to a fresh agar slant, a new 

 colony of the same kind will form be- 

 cause some bacteria have been "trans- 

 planted." In this way it is easy to start 

 pure cultures, that is, cultures containing 

 only one kind of bacterium. 



Identifying bacteria. Some bacteria can 

 be identified merely from the appearance 

 of the colony. Some colonies are smooth, 

 others are rough; some are white, others 

 have a distinct color such as yellow or 

 red. Generally one cannot determine the 

 exact kind of bacteria by the appearance 

 of the colony. Examining bacteria 

 through a microscope helps somewhat 

 in identifying them. But even after mi- 

 croscopes had been improv^ed to mag- 

 nify looo times or more, it was still diffi- 

 cult to see bacteria because they were so 

 transparent and so tiny. Koch and other 

 bacteriologists discovered chemicals that 

 dyed or stained the bacteria but did not 

 stain other cells. This was of great value 

 because staining made it possible to see 

 and identify bacteria in body tissues. 



If you have a school laboratory, it 

 will be possible to prepare a pure culture 

 and to stain bacteria, as described in 

 Exercises 4 and 5. If you have dishes of 

 nutrient agar, you can easily test for the 

 presence of bacteria in air, in water, and 

 on objects outside the body. See Exer- 

 cises 6 and 7. 



Favorable and unfavorable conditions 

 for growth. Bacteria will grow and repro- 

 duce onlv if kept moist and at a proper 

 temperature. Like other organisms that 

 lack chlorophyll, they depend on manu- 

 factured food. By doing Exkkcisk 8 you 

 can determine for yourself to what ex- 

 tent external conditions afl'ect the 



Constant Care Is Needed for Health unit v\ 



Fig. 290 How iiia7iy colonies of bacteria can 

 you find oti this agar plate? How many bacteria 

 were introduced? (Brooklyn botanic garden) 



growth of bacteria. In general, bacteria 

 are not killed by low temperatures but 

 they fail to reproduce unless the temper- 

 ature is favorable. Bacteria have been 

 known to survive at temperatures be- 

 low the freezing point. \'ery high tem- 

 peratures, on the contrary, kill bacteria. 

 The temperature must be extremely high 

 to kill all kinds. Even boiling does not 

 kill all species; some have been known 

 to survive five hours of boiling. This is 

 possible because in some species the cell 

 may secrete a very resistant wall around 

 itself; it is then called a spore. Fortu- 

 nately for us, very few pathogenic bac- 

 teria form spores. Bacteria in the spore 

 form can be killed by heating them in 

 steam at great pressure. This method is 

 used in the home when a pressure 

 cooker is employed in preserving fruits 

 or veoetables. Before pressure steriliza- 

 tion was invented objects were sterilized 

 by hcatiniT them in ordinary steam for 

 an hour three da\ s in succession. Al- 

 though slow, this method was successful 



