198 Report of the Department of Bacteriology of the 



extremely numerous. Quantitative work, therefore, fell more and 

 more into disfavor until in 1902 Remy 1 stated that mere deter- 

 minations of the number of bacteria were of no use. Remy realized 

 the importance of knowing not only the numbers of bacteria, but 

 also the kinds present and their functions. He considered a complete 

 study of this sort, however, too colossal a task to be undertaken. 

 As a practical substitute for a complete qualitative study, Remy 

 suggested a method of obtaining a qualitative knowledge of soil 

 bacteria without counting them or separating the different kinds 

 from each other. This was accomplished by measuring the chemical 

 changes which the total flora of any soil was capable of producing 

 when inoculated into special liquid media. Bacteriological methods, 

 today, have improved to such an extent that it may soon be 

 possible to make a more complete study of soil bacteria, including 

 determinations of the number of each kind of bacteria present 

 as well as the total number, and also of the functions of the various 

 bacteria. Quantitative as well as qualitative methods, however, 

 must be perfected before a complete study of this sort becomes 

 possible. 



The usual method employed in quantitative work has been to 

 count the colonies developing upon a plate of nutrient gelatin or 

 agar inoculated with a small amount of soil infusion of definitely 

 known dilution. It is known that the composition of the nutrient 

 gelatin or agar has considerable influence upon the results, but the 

 best possible composition has not yet been determined. The 

 present investigation is a study of the relative merits of various 

 culture media for this purpose. The results are to be considered 

 as merely preliminary; but they are published as an aid to others 

 who are striving after satisfactory media for soil bacteriological 

 work. 



USES AND LIMITATIONS OF THE PLATE METHOD. 



The weaknesses of the plate method of counting bacteria are too 

 well known to need much discussion. It is an indirect method, for 

 by its use the bacteria are counted only by means of the colonies 

 they produce on the plates. In interpreting the colony count as 

 though it were a count of the bacteria themselves, it has to be assumed 

 that every bacterium mixed with the culture medium develops into 

 a macroscopic colony, an assumption that is not justified unless the 

 composition of the medium, the temperature of incubation, and the 

 length of time allowed to elapse before counting are such as to 

 permit the growth of every kind of organism present, and unless 

 the bacteria are so well separated from each other that no colony 

 represents more than a single individual. These conditions have 

 never been fully met, and probably never can be. 



1 Remy, Th. Bodenbakteriologische Studien. Ccnibl. Bakt., Abt. II, 8:657-662, 

 699-705," 728-735, 761-769. 1902. 



