METHODS OF BACTERIAL ANALYSES OF AIR^ 



By G. L. A. RuEHLE, 



Assistant Bacteriologist, New York Agricultural Experiment Station, Geneva, N . Y. 



INTRODUCTION 



This study of methods of making bacterial analyses of the air was 

 undertaken at the New York Agricultural Experiment Station in con- 

 nection with the problem of determining the relation of the bacterial 

 content of the stable air to the amount of bacterial contamination of 

 milk before it leaves the cow stable. The latter problem forms part of 

 a larger one which has been the subject of investigation at this Station 

 for a number of years and which is now being completed in cooperation 

 with the Illinois Agricultural Experiment Station. Briefly stated, this 

 larger problem is a study of the relative importance of various barn 

 operations in the production of sanitary milk. The work already re- 

 ported can be found in bulletins of the New York Agricultural Experi- 

 ment Station (8-ii).^ 



In the present investigation two general methods of bacterial air 

 analyses have been tested: One, methods of determining the number of 

 bacteria in a given volume of air ; and the other, methods of determining 

 the amount of bacterial precipitation on a known area in a definite 

 period of time. The greater part of the work, however, is devoted to a 

 study of the former technique and involves the determination of the 

 filtering efficiency of two aeroscopes frequently used by investigators 

 and a modification of one of them. 



HISTORICAL REVIEW 



A brief review of the efforts of bacteriologists to devise satisfactory 

 methods for the bacteriological analysis of the air will show the impor- 

 tant stages in the development of the present technique. The methods 

 which have been devised for quantitative purposes generally involve 

 one of the following principles: 



(a) Bubbling the air through a liquid, which acts as a filter. The 

 liquid may be either nutritive or nonnutritive. In either case the liquid 



' Credit is due to Dr. H. A. Hardins, formerly bacteriologist at the New York Agricultural Experiment 

 Station, for having suggested the original plan of the work and for valuable aid in carrying out the first 

 part of it. Dr. Robert S. Breed bore a similar relation to the later part of the work, and assisted mate- 

 rially by his criticism of the manuscript. Dr. John Weinzirl, member of the Committee on Standard 

 Methods for the Examination of Air, has also given several helpful suggestions during the course of the work. 

 Mr. W. L. Kulp, Student Assistant in Bacteriology, made the series of analyses recorded in Tables III 

 and IX. The writer wishes to express his gratitude for the aid thus given. 



2 Reference is made by number to "Literature cited," p. 366-368. 



Journal of Agricultural Research, Vol. IV, No. 4 



Dept. of Agriculture, Washington, D. C. July 15, 1915 



N. Y. (Geneva)— 3 

 (343) 



