13 |Glistening,|Strong | Do. |Weak |In old stab-cultures |yellowish. |curdling. | | |a brown discoloration | | | | |of the culture-bed is | | | | |noticed along the | | | | |length of the stab. | | | | |Differs in the gelatin | | | | |stab-culture from the | | | | |common bacterium _Coli | | | | |commune_likewise in the | | | | |curdling of the milk. | | | | | 38 |Yellow |Milk | Do. |Not noticed | -- |glistening |is not | | | |deposits. |changed. | | | | | | | | 40 |White, | Do. | Do. |Weak. Is only | -- |glistening.| | |noticed in culture-| |Only very | | |bed containing | |slight | | |grape-sugar. | |growth. | | | | | | | | | 42 |Sulphur |No change | Do. |Strong only in | -- |yellow, |of the | |media containing | |glistening.|milk. | |grape-sugar. | | | | | | 43 |No growth | Do. | Do. |None | -- | | | | | 44 |Weak devel-|Curdling | Do. |Very weak. Only in | |opment. The| | |media containing | |culture is | | |grape-sugar. | |sulphur | | | | |yellow. | | | | | | | | | 45 |Weak |Milk is | Do. |Not noticed | -- |yellowish |not | | | |deposit. |changed. | | | --------+-----------+----------+------+-------------------+--------------------- The practical difficulty is to keep such cultures uniform during propagation, and so far this has prevented their introduction in practice. Similar difficulties have influenced the use of pure cultures of yeast in the brewing of English beers, although the use of a single species of yeast is common in the low fermentation breweries on the Continent.