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50 



The feature by which the alteration in vitality of lactic acid bacteria may be most 

 directly observed, is the quantity of acid formed from a given sugar under uniform con- 

 ditions, and even more so, the rate at which souring takes place; this can, in the case 

 of milk cultures at like temperatures, be measured by the rate of coagulation. As the rate 

 of souring (in contrast to the degree of acidity in the cultures after 14 days) is influenced 

 to a high degree by the quantity sown out. We always inoculated the 10 cm^ of milk 

 employed with the same platinum loop, from a previous (just curdled) milk culture. 

 Where greater fluctuations were observed in these respects, the extreme limits are noted 

 in the tables. The highest amount of acid is shown first in the case of sugars whose fermen- 

 tation has declined in course of years, and last in the case of sugars whose fermentation 

 has grown stronger. 



If the lactic acid bacteria are to be of practical use, and keep down the detrimental 

 bacteria, then it is primarily necessary that they shall sour rapidly and strongly, and this 

 is just where laboratory cultures are often liable to fall short. If their acidulating power 

 has once declined, then it is as a rule very difficult to restore it completely, even when 

 they are transferred daily from milk to milk. Surprising results may, however, often be 

 obtained in this respect by using a larger quantity of inoculating material. We have 

 had strains of Streptococcus cremoris, for instance, used for the souring of cream in making 

 butter, which could not be brought to produce more than 4.7°/oo lactic acid in twenty-four 

 hours at 25°, however frequently we might transfer them with the platinum loop to a new 

 10 cm^ of milk. When, on the other hand. We proceeded to a daily inoculation of 10 cm^ 

 culture in 200 cm^ milk, we could then, after the lapse of two or three weeks, obtain in 

 20 hours the 7 "/qq lactic acid required for maintenance of activity in the pasteurised, 

 though not therefore by any means germ-free, dairy cream. Next to unsuitable com- 

 position of the nutritive substrate and preservation at too high tempera- 

 tures, the slight quantity of inoculating material generally used in la- 

 boratories is the main cause of the frequent degeneration in laboratory 

 cultures^). 



That an abundant quantity of inoculating material should give such favourable re- 

 sults is due to the fact that we then arc more sure of transferring some of the strongest indi- 

 viduals in the culture. If a pure culture be spread on agar or gelatin, and a series of new 

 strains isolated therefrom, then the latter will never behave altogether alike towards the 

 difi'erent sugars, and all will as a rule — until they have been repeatedly transferred for some 

 time — form a somewhat smaller quantity of acid than the original culture, as it would 

 be a very fortunate chance to obtain on spreading, some of the most powerful individuals 

 which stamp the culture as a whole. As a matter of fact, the majority of indivi- 

 duals in a bacteria culture are weakened, and have but slight power of 

 resistance; this was very distinctly apparent from our heating experiments. This 

 explains why pure cultures are never at first so powerfully effective in practice as the 

 original culture previously employed, and shows, that it is by no means a matter of indif- 

 ference whether a person becomes infected by a few cells of a disease bacteria or a great 

 number of the same. 



') In the case of stab cultures of comparatively anaerobic lactic acid bacteria, as for instance the 

 thermobactcria, it will be necessary' to inoculate from the bottom of the stab, as the cells situated nearer 

 to the surface will alwaj's be fjreatly weakeued by the o.xygen in the air. 



