302 STATE BOARD OF AGRICULTURE. 



BACTERIAL ASSOCIATIONS IN THE SOURING OF MILK 



BY CHARLES E. MARSHALL AND BELL FARRAND. 



Special Bulletin No. 42. 



INTRODUCTION. 



Previous contributions, "A Preliminary Note on the Associative Action 

 of Bacteria in the Souring of Milk/' Special Bulletin 23; "Additional Work 

 upon the Associative Action of Bacteria in the Souring of Milk," Special 

 Bulletin 29, and ''Extended Studies of the Associative Action of Bacteria 

 in the Souring of Milk," Special Bulletin 33, all covering the same central 

 idea, and leading to the same end, will be simply confirmed, and the con- 

 ception much extended in this paper, dealing -with a more intimate and ex- 

 tensive study of milk existing under ordinary dairy conditions. 



This study involves approximately the entire mic^-obial content of each 

 sample. Hitherto, our investigations have been confined to selected micro- 

 organisms. In this work, the micro-organisms isolated from a definite 

 sample of milk are employed for purposes of association "with the lactic 

 micro-organism also isolated from the same sample. 



A sample of milk produced under ordinary conditions is selected, the 

 micro-organisms isolated by plating, pure cultures of each species estab- 

 lished and these associated in cultures with the lactic micro-organism, also 

 isolated along with the others from the same sample of milk. The dominant 

 lactic micro-organism of each sample of milk, therefore, becomes the cen- 

 tral biological agent with which each micro-organism isolated from the 

 sample is associated. 



Following in line with previous articles, results are based upon appear- 

 ances of cultures, development of acid, and the number of micro-organisms, 

 as compared with checks. The general plan of this work may be illustrated 

 by the following scheme. Let the samples of milk be represented by I, II, 

 III and IV; then allow the various kinds of micro-organisms of a given 

 sample of milk, as (I), to be represented by 



la lb Ic Id le If Ig Ih 



Let Ih stand for the dominant lactic micro-organism; then Ih ^vill be asso- 

 ciated separately in turn with la, lb, Ic, Id, le, If, Ig, and the combination 

 in each instance being represented as la + Ih, lb + Ih, Ic + Ih, Id + Ih, 

 le + Ih, If + Ih, Ig -I- Ih. Whenever a combination is subjected to ob- 

 servation, pure cultures of the individual species are run parallel with the 

 combined culture, as: 



Culture Culture Culture 



la la + Ih Ih 



By this means it becomes possible to note any deviations occurring in 



