206 VETERINARY BACTERIOLOGY 



handled, would " rot." Ayers and Johnson have shown recently 

 the falsity of this assumption. There are practically always 

 present in any given sample of raw milk one or more strains of 

 the lactic-acid organism, which will resist the temperature of the 

 pasteurizer, and will bring about normal souring in the milk 

 pasteurized. 



The production of lactic acid in milk is of considerable im- 

 portance in preventing the growth of undesirable bacteria. Milk 

 entirely freed from these forms, as by heating to the boiling-point, 

 does not sour, but undergoes a putrefactive fermentation, brought 

 about by organisms ordinarily held in abeyance by the develop- 

 ment of the lactic acid. 



Pathogenesis. Heinemann, by a series of animal inoculations, 

 has shown it possible to exalt the virulence of typical Str. factious 

 so that it would kill rabbits as quickly as virulent Str. pyogenes. 



The close relationship of the two forms can scarcely be ques- 

 tioned. Before the work of Heinemann it had been concluded 

 by many workers that the presence of streptococci in milk was 

 necessarily an indication of udder infection, and examination for 

 streptococci was made a part of the routine work of certain 

 board of health laboratories. Inasmuch as it is now known that 

 the Str. lacticus is normally present in practically all milk, it is 

 evident that such examinations are of little use. A streptococcic 

 milk standard is not practicable. 



Utilization. Different strains of Streptococcus lacticus are 

 used in pure cultures as starters in the dairy. The flavor of butter 

 is largely dependent upon the development of a peculiar flavor 

 and aroma, largely through the agency of the lactic-acid bacteria 

 present. It is customary, therefore, to pasteurize cream to destroy 

 undesirable organisms and to add to it the starter, which is allowed 

 to increase and produce the desired aroma and flavor. The 

 Str. lacticus is likewise of importance in the manufacture of cheese; 

 the change of the lactose present to lactic acid is an essential 

 preliminary in most cases to the ripening process. 



