Microorganisms in Relation to Man 575 



is melted and then boil for several minutes. Test for acidity with 

 litmus paper, and add enough baking soda to neutralize and give an 

 alkaline reaction. Filter while hot through absorbent cotton in a 

 glass funnel two thirds full, into which the mixture is slowly poured ; fill 

 sterilized test tubes about one third full, and close with a cotton plug. 

 The tubes with their contents should be steamed for half an hour or 

 more to insure sterilization. This should be repeated two or three 

 times at intervals of a day. 



Exercise : Cultures. The medium thus prepared may be inocu- 

 lated in the tubes or melted and poured into watch glasses or Petri 

 dishes for plate cultures. Plate cultures have several advantages, 

 such as separating the individual bacteria and showing to the naked 

 eye the separate colonies when developed. To inoculate, melt the 

 mixture in several tubes and add to each any of the following materials : 

 a cubic centimeter of faucet water, a cubic centimeter of boiled water 

 in which a small piece of putrefying meat has been shaken, a drop of 

 saliva, a hair from the head, a bit of substance gathered on a tooth- 

 pick from the teeth, or some dust from the floor. Mix the contents 

 of each tube thoroughly and pour into the watch glass ; cover imme- 

 diately. Label each and make a record of the preparations. Fill 

 three more dishes with the mixture, covering them at once. When 

 cool, touch the surface of one with the finger ; expose another to the 

 air of the room when the class is moving about ; expose another when 

 all is quiet. Place all the dishes where the light is dim and the tem- 

 perature is likely to remain about 70 F. (as in the furnace room). 

 Observe at intervals and make a record of growth, number of colonies, 

 color, and general appearance. Put on a glass slide and cover with a 

 thin glass a drop of some stagnant water in which a crumb of bread or 

 cake has been left for several days. Examine it with a high-power 

 microscope. Examine also a drop of some water in which a bit of 

 decaying meat has been shaken. 



Forms of bacteria. The form of the microorganisms which 

 may be observed in these preparations is spherical. This is the 

 simplest type of bacteria, and is exceedingly small. The name 

 given to this group is coccus (cocci, plural), (Fig. 171 a). The 

 cocci may be congregated in groups varying in number, size, 

 and shape. The grouping may be due to the manner of re- 

 production by division, as determined by external conditions 

 of warmth, moisture, and nutritive materials. If the division 

 occurs in one plane, forming a pattern like a string of beads, 



