Bacteria in Soil 



239 



measured quantity of the secured soil is thoroughly mixed 

 with liquefied sterile gelatin and poured into a Petri dish 

 or solidified upon the walls of an Esmarch tube. The 

 colonies are counted with the aid of a lens. Fliigge found 

 in virgin earth about 100,000 colonies in a cubic centimeter. 



Samples of earth, like samples of water, should be exam- 

 ined as soon as possible after being secured, for, as Giinther 

 points out, the number of bacteria changes because of the 

 unusual dryness, warmth, exposure to oxygen, etc. 



The most important bacteria of the 

 soil are those of tetanus and malignant 

 edema, in addition to which, however, 

 there are a great variety of organisms 

 pathogenic for rabbits, guinea-pigs, and 

 mice. 



In the "Bacteriological Examination 

 of the Soil of Philadelphia," Ravenel * 

 came to the conclusion that 



1. Made soils, as commonly found, are 

 rich in organic matter and excessively 

 damp through poor drainage. 



2. They furnish conditions more suited 

 to the multiplication of bacteria than do 

 virgin soils, unless the latter are contami- 

 nated by sewage or offal. 



3. Made soils contain large numbers 

 of bacteria per gram of many different 

 species, the deeper layers being as rich 

 in the number and variety of organisms 

 as the upper ones. After some years 

 the number in the deeper layers probably 

 becomes proportionally less. Made soils 

 are more likely than others to contain 

 pathogenic bacteria. 



In seventy-one cultures that were iso- 

 lated and carefully studied by Ravenel, 

 there were two cocci, one sarcina, and 

 five cladothrices ; all the others were 

 bacilli. 



Fig. 71. Tip of 

 Frankel' s instru- 

 ment for obtaining 

 earth from various 

 depths for bacterio- 

 logic study. B 

 shows the instru- 

 ment with its cav- 

 ity closed, as it 

 appears during bor- 

 ing; A, open, as it 

 appears when twis- 

 ted in the other di- 

 rection to collect 

 the earth. 



* "Memoirs of the National Academy of Sciences," First Memoir, 

 1896. 



