90 



A MANUAL OF BACTEEIOLOGY 



Is there a sharply defined line between infected and noninfected 

 tissues or do they gradually merge into each other ? 



2. Cut slices about 1 cm. thick from a sound carrot, using a 

 flamed knife. Place them on a piece of moist filter paper in a 

 Petri dish. Inoculate by scratching them with a needle beariag 

 the organism from a pure culture. Scratch other pieces with the 



same needle after sterilization 

 in the flame. Keep the Petri 

 dish in your desk and make 

 examination after three to 

 five days. 



3. By reference to the lit- 

 erature above cited acquaint 

 yourself with the characters of 

 pectinase, the cytolytic enzyme 

 which this organism produces. 

 The following method, de- 

 scribed by Jones, is a con- 

 venient one for demonstrating 

 this- enzyme. 



Pour a tube of melted agar 

 into a sterile Petri dish to a 

 depth of 3 mm. Allow this 

 to become hard, then inocu- 

 late an area of 2 or 3 mm. in 

 the center of the dish, using 

 the platinum loop, with care 

 that the layer of agar is not 

 punctured. Keep the dish in the incubator until you have a 

 surface colony about 1 cm. in diameter. 



With a flamed knife cut a slice from a fresh turnip root 

 and place it at once in a sterile Petri dish. Carefully remove 

 the agar layer bearing the bacterial colony and transfer with 

 sterile instruments to the surface of the turnip slice. Cover at 

 once to prevent contamination. Prepare a control by covering 

 another turnip shoe with a layer of agar bearing no bacteria 

 (Fig. 36). 



d 



^- 



Fig. 36. Method for showing diffusibility 

 of enzymes through agar. (After Jones) 



A, surface view ; JB, vertical section along 

 dotted line e-e ; a-a, sterile slice of living 

 turnip root ; 6-6, layer of nutrient agar bear- 

 ing the bacterial colony c-c ; d-d, region of 

 most active enzyme action 



