_Bacteria in Cheese-making._ The cases where it has been possible to trace bacterial disease to the consumption of butter and cheese have been rare. Notwithstanding this fact, it must not be supposed that therefore cheese contains few or no bacteria. On the contrary, for the making of cheese bacteria are not only favourable, but actually essential, for in its manufacture the casein of the milk has to be separated from the other products by the use of _rennet_, and is then collected in large masses and pressed, forming the fresh cheese. In the course of time this undergoes ripening, which develops the peculiar flavours characteristic of cheese, and upon which its whole value depends. We have said that the casein is separated by the addition of rennet, which has the power of coagulating the casein. But this precipitation may also be accomplished by allowing acid to develop in the milk until the casein is precipitated, as in some sour-milk or cottage cheeses. The former method is of course the usual one in practice. It has been suggested that the bacteria contained in the rennet exert a considerable influence on the cheese, but this, although rennet contains bacteria, is hardly established. It is not here, however, that bacteria really play their rôle. After this physical separation, when the cheese is pressed and set aside, is the period for the commencement of the ripening process. That bacteria perform the major part of this ripening process, and are essential to it, is proved by the fact that when they are either removed or opposed the curing changes immediately cease. If the milk be first sterilised, or if antiseptics, like thymol, be added, the results are negative. It is not yet known whether this peptonising process is due to the influence of a single organism or not. The probability, however, is that it is to be ascribed to the action of that group of bacteria known as the _lactic-acid_ organisms. Nor is it yet known whether the peptonisation of the casein and the production of the flavour are the results of one or more species. Freudenreich believes them to be due to two different forms. However that may be, we meet with at least four common groups of bacteria more or less constantly present in cheese-ripening, either in the early or late stages. First, there are the _lactic-acid bacteria_, by far the largest group, and the one common feature of which is the production by fermentation of lactic acid; secondly, there are the _casein-digesting bacteria_, present in relatively small numbers; thirdly, the _gas-producing bacteria_, which give to cheese its honeycombed appearance; lastly, an indifferent or _miscellaneous_ group of extraneous bacteria, which were in the milk at the outset of cheese-making, or are intruders from the air or rennet. All these four groups may bring about a variety of changes, beneficial and otherwise, in the cheese-making.