82 



General Bacteriology. 



EXERCISE LXVII. RELATION OF BACTERIA IN THE AIR TO DUST PARTICLES. 



a. Pour a tube of gelatin into a Petri dish and solidify. 



b. Remove the lid and shake a dust-brush or cloth over it. 



c. 18-24 hours later, examine under low power of microscope to determine the rela- 

 tion of the developing colonies to the dust particles. 



EXERCISE LXVIII. ESTIMATION OF NUMBER OF BACTERIA IN SOIL. 



a. With a sterile knife collect a sample of soil in a sterile test-tube 

 or Petri dish. Samples at various depths can be secured by means of an 

 earth borer. (Fig. 15). 



b. Weigh out 1 gram and dilute 1000 times with sterile water. 



c. Make three gelatin plate cultures using 1 cc., cc. and rV cc. of 

 this suspension. Incubate. 



d. Count the colonies as they develop and estimate the number of 

 bacteria per gram of soil. 



e. Many of the bacteria of the soil are anaerobic and can only be 

 grown in the absence of free oxygen. See Part II. Chap. VII. for methods of 

 cultivation. 



REFERENCES. A. 556; H. 394; L. & K. 389; McF. 174; N. 444; S. 

 567. 



Fig. 15. Fraenkel's 

 Soil Borer. 



EXERCISE LXIX. WATER ANALYSIS. 



QUANTITATIVE ANALYSIS. 



a. Collect a sample of water in a sterile test-tube or bottle. Fig. 

 16 shows a form of apparatus used in taking samples of water at vari- 

 ous depths. 



b. Make two gelatin plates using ^ cc. and T V cc. of the water. 



c. Count the colonies as they appear, and estimate the number 

 per cc. 



d. Make agar plates and compare results with those obtained 

 above. 



e. Analyze a surface water (lake or river) , a deep well and a 

 spring water. 



QUALITATIVE ANALYSIS. 



a. Detection of putrefactive organisms. Examine gelatin plates, 

 made above and (1) determine number of liquefying organisms per cc. 

 (2) search for the presence of proteus forms. (B. vulgans.) 



b. Detection of Faecal Bacteria. 



1) Inoculate a fermentation-tube containing glucose bouillon (1%) with 1 cc. 

 of water. 



2) Make litmus lactose agar plate using 1 cc. water. 



3) Incubate both at 38 C. 



4) Compare growth obtained with that of B. coli. 



REFERENCES. A. 526; H. 373; L. & K. 396; McF. 169; M. & R. 79; N. 422; 

 P. 245; S- 553. For the determination of the various species present see Frankland's 

 Micro-organisms of Water; Fuller: Report Am. Public Health Assoc., 1899, 580. 



---c. 



FIG. 18. Russell's 

 Water Sampler. 



