OXYGEN. Some species cannot live without a supply of free oxygen (_Aerobic_), _e.g._ the Vinegar-bacteria, the Hay-bacilli, the Anthrax-bacilli, the Cholera-_Microspira_. Other species again thrive vigorously without supply of free oxygen, and are even checked in their development by the admission of air (_Anaerobic_), _e.g._ the butyric acid Bacterium (_Clostridium butyricium_ = _Bacillus amylobacter_). A distinction may be drawn between obligate and facultative aerobics and obligate and facultative anaerobics. Several Bacteria, producing fermentation, may grow without the aid of oxygen when they are living in a solution in which they can produce fermentation; but, if this is not the case, they can only grow when a supply of oxygen is available. A great number of the pathogenic Bacteria belong to the facultative anaerobics. A luminous Bacterium (_Bacillus phosphorescens_) which in the presence of a supply of oxygen gives a bluish-white light, has been found in sea-water. Phosphorescent Bacteria have frequently been observed upon decaying sea-fish, as well as on the flesh of other animals; by transferring the Bacteria from cod fish to beef, etc., the latter may be made luminous. _Organic carbon compounds_ are indispensable for all Bacteria, (except, as it appears, for the nitrifying organisms), as they can only obtain the necessary supplies of _carbon_ from this source. The supplies of _nitrogen_, which also they cannot do without, can be obtained equally as well from organic compounds as from inorganic salts, such as saltpetre or ammonia-compounds. The various “ash-constituents” are also essential for their nourishment. While Moulds and Yeast-Fungi grow best in an acid substratum, the _Bacteria_, on the other hand, generally thrive _best_ in a _neutral_ or slightly _alkaline_ one. In _sterilization_, _disinfection_, and _antisepsis_, means are employed by which the Bacteria are killed, or checked in their development, for instance, by heat (ignition, cooking, hot vapours, hot air, etc.), or poisons (acids, corrosive sublimate). The process of preserving articles of food, in which they are boiled and then hermetically sealed, aims at destroying the Bacteria, or the spores of those which already may be present in them, and excluding all others. As the Bacteria are unable to assimilate carbon from the carbonic acid of the air, but must obtain it from the carbon-compounds already in existence in the organic world, they are either _saprophytes_ or _parasites_. Some are exclusively either the one or the other, _obligate_ saprophytes or parasites. But there are transitional forms among them, some of which are at ordinary times saprophytes, but may, when occasion offers, complete their development wholly or partly as parasites--_facultative parasites_; others are generally parasitic, but may also pass certain stages of development as saprophytes--_facultative saprophytes_.