Laboratory Outline for General Bacteriology 



tube it is called facultative anaerobic. Observations on growth 

 and tests for alkalinity or acidity are made usually at 2, 7, and 14 

 days. 



STARCH AGAR 



Starch agar is like the ordinary agar with .2 ( /< of soluble starch 

 added. It may be poured hot into plates. When cool it is inocu- 

 lated by making one streak across the center. Do not let the loop 

 cut down into the agar. Invert to incubate. 



After the culture has grown the required length of time, usually 

 7 days, it is covered with a solution of iodine. If the starch is 

 unchanged by the bacteria the entire plate will turn blue. When 

 diastase is present a clear zone will appear along the line of inocu- 

 lation. The width of the clear zone shows the intensity of the 

 diastatic action. This is measured in millimeters from the edge 

 of the growth to the edge of the blue. Records should be made 

 at once as the blue reaction disappears within a few minutes. 



MILK AND LITMUS MILK 



Fresh skimmed milk is tubed and sterilized. For the litmus 

 milk enough litmus solution is added to give it a distinctly blue 

 color. 



In testing reaction of milk sterilize loop and take a loopful 

 out onto litmus paper. If it is necessary to take a second loopful 

 sterilize loop before taking it to avoid contaminating the culture. 

 Test an uninoculated tube at the same time for comparison. Fresh 

 milk has what is called an amphoteric reaction, that is, it turns 

 red litmus slightly blue and blue litmus slightly red. If bacteria 

 have produced a change of reaction litmus paper will be turned 

 redder or bluer than the control. Milk and litmus milk tubes from 

 the same culture should show the same reaction. 



Coagulation is when milk becomes thick like sour milk. Pep- 

 tonizatiori is shown by a clearing or whey like appearance due to 

 destruction of the casein. It generally begins at the surface and 

 works downward. The milk may or may not be curdled. Sometimes 

 when a hard curd is formed whey is pressed out. This is called 

 "extrusion of whey" and can usually be distinguished from pep- 

 tonization by the clearness of the whey. Reduction refers to the 

 destruction of litmus color. When complete the litmus milk looks 

 like the plain milk. Reduction usually begins at the bottom of the 

 tube and works upward. Sometimes the color becomes pale through- 

 out the entire tube. A change of color to red or blue indicates 

 change of reaction not reduction. 



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