EXPERIMENT STATION BULLETINS. 265 



A POPULAR REVIEW OF SPECIAL BULLETIN NO. 33 ON "THE ASSO- 

 CIATIVE ACTION OF BACTERIA IN THE SOURING OF MILK." 



CHABLES E. MAKSHALL. 



[Bulletin No. 229.] 



For the last two years the attention of the writer has been directed to the 

 determination of the favorable influence which certain microorganisms, usually 

 producing no acid, frequently digesting milk, and many times curding milk, 

 exert upon the growth of lactic bacteria (those germs commonly souring milk) 

 accordingly hastening this process. It is understood that these lactic bacteria 

 in milk cause the changing of the sugar of milk into an acid, called lactic acid, 

 and this in turn acts upon the casein of the milk or the cheese content of the 

 milk, producing a lopper. The bacteria which exert a favoring influence through 

 a partial digestion of the milk, find their way into the milk through the filth 

 of the stable, as, hairs of the cow, dirt from the cow, dirt from the floor of 

 the stable, dirt from the feed, dirt from the milking utensils, in fact, they 

 reach the milk through any avenue which is likely to be contaminated in any 

 degree with dirt of one nature or another. It follows, therefore, that there is 

 present in the milk a mass of bacteria consisting of many kinds, — those which 

 are capable of souring milk directly; those which are capable of growing in milk 

 and digesting it, leaving only a watery fluid behind; those which are capable 

 of curding the casein or cheese product under sweet or sour conditions; and 

 many other kinds which are able to produce various alterations. Our asso- 

 ciative studies in this instance are confined to the lactic bacteria and a few of 

 those which usually digest milk without an appreciable formation of any acid. 



It has been suspected for some time that the lactic type of bacteria does not 

 always account for at least occasional peculiarities noticed in connection with 

 the souring of milk. With the knowledge that this souring is produced by a 

 definite lactic micro-organism in most instances, its isolation and its cultivation 

 in sterile milk ought to yield the same results as one would succeed in getting 

 In the souring of milk under dairy environments. This, however, is not always 

 true. Through the discovery of these influencing micro-organisms upon the lactic 

 germs, it is possible in part to explain many of the variations which occur in 

 the production of sour milk by lactic bacteria. 



While the associative studies in connection with the souring of milk are more 

 or less in an inceptive stage, they are along the same line as several other fer- 

 mentations which have already been familiar for many years. The nature of 

 vinegar fermentation, with which we are all somewhat acquainted, is something 

 that will appeal to every layman not fully in touch with the technique of micro- 

 biology. It is a matter of common knowledge that in the production of vinegar, 

 alcohol is first manufactured by a yeast plant from the sugar of the fruit. Fol- 

 lowing the production of alcohol, bacteria develop which convert the alcohol pro- 

 duced by the yeast into an acid, called acetic acid, which yields the vinegar of 

 commerce. This association is required to yield the proper fermentative results. 

 The yeasts are practically necessary for the cultivation of the bacteria and the 

 bacteria are essential in the production of acetic acid or vinegar. In several 

 fermentations, associative life exists, as is met in kephir and koumiss, ginger 

 beer, and others. Again, in disease mixed infection frequently occurs. This 

 mixed infection many times hastens the development of the disease and perhaps 

 there are cases where it acts as a hindrance. Diphtheria in its pure form is 

 caused by a rod-shaped micro-organism and pursues a definite course, but when 

 mixed with a certain spherical micro-organism, an aggravated form of the disease 

 may follow. In the soil are instances of associative growth: for instance. It 

 has been demonstrated that there is a distinct germ which is capable of liberating 

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