58
BACTERIOLOGY
[general
ployed in pure cultures in American dairies, and in Denmark certain butters are said to keep fresh much longer owing to the use of pure cultures and the treatment employed to suppress the forms which cause rancidity. Quite distinct is the search for the germs which cause undesirable changes, or “ diseases ”; and great strides have been made in discovering the bacteria concerned in rendering milk “ ropy,” butter “ oily ” and “ rancid,” &c. Cheese in its numerous forms contains myriads of bacteria, and some of these are now known to be concerned in the various processes of ripening and other changes affecting the product, and although little is known as to the exact part played by any species, practical applications of the discoveries of the decade 1890-1900 have been made, e.g., Edam cheese. The Japanese have cheeses resulting from the bacterial fermentation of boiled Soja beans. That bacterial fermentations are accompanied by the evolution of heat is an old experience; but the discovery that the “spontaneous” combustion of sterilized Thermo cotton-waste does not occur simply if moist and freely exposed to oxygen, but results when the bacteria. washings of fresh waste are added, has led to clearer proof that the heating of hay-stacks, hops, tobacco, and other vegetable products is due to the vital activity of bacteria and fungi, and is physiologically a consequence of respiratory processes like those in malting. It seems fairly established that when the preliminary heating process of fermentation is drawing to a close, the cotton, hay, &c., having been converted into a highly porous friable and combustible mass, may then ignite in certain circumstances by the occlusion of oxygen, just as ignition is induced by finely divided metals. A remarkable point in this connexion has always been the necessary conclusion that the living bacteria concerned must be exposed to temperatures of at least 70° C. in the hot heaps. Apart from the resolution of doubts as to the power of spores to withstand such temperatures for long periods, the discoveries of Miquel, Globig, and others have shown that there are numerous bacteria which will grow and divide at such temperatures, e.g., B. thermophilus, from sewage, which is quite active at 70 C., and B. Ludivigi and B. ilidzensis, &c., from hot springs, etc. The bodies of sea fish, e.g., mackerel and other animals, have long been known to exhibit phosphorescence. This phenomenon is due to the activity of a whole r Fig. 8.—A similar preparation to Fig. 7, except that two slit-like openings of series of marine bacteria of various genera, the es°*£t ° ' equal length allowed the light to pass, and that the light was that of the examination and cultivation of which have been bacteria. electric arc passed through a quartz prism and casting a powerful spectrum on the plate. The upper slit was covered with glass, the lower with successfully carried out by Cohn, Beyerinck, quartz. The bacteria were killed over the clear areas shown. The lefthand boundary of the clear area corresponds to the line F (green end of the Fischer, and others. The cause of the phosphorescence is blue), and the beginning of the ultra-violet was at the extreme right of the a mystery. The suggestion that it is due to the upper (short) area. The lower area of bactericidal action extends much still farther to the right, because the quartz allows more ultra-rays to pass than oxidation of a body excreted by the bacteria seems andoes glass. The red-yellow-green to the left of F were without effect. swered by the failure to filter off or extract any such body. (Original.) Beyerinck’s view that it occurs at the moment peptones are worked up into the protoplasm cannot be regarded milk examined in Paris, and one in every nineteen in Washington, contained tubercle bacilli. Hence the de- as proved, and the same must be said of the suggestion sirability of sterilizing milk used for domestic purposes that the phosphorescence is due to the oxidation of phosbecomes imperative. Ho milk is free from bacteria, because phoretted hydrogen. The conditions of phosphorescence the external orifices of the milk-ducts always contain them, are, the presence of free oxygen, and, generally, a relatively but the forms present in the normal fluid are principally low temperature, together with a medium containing those which induce such changes as the souring or sodium chloride, and peptones, but little or no carbo“ turning ” so frequently observed in standing milk (these hydrates. Considerable differences occur in these latter were examined by Lord Lister as long ago as 1873-77, respects, however, and interesting results were obtained though several other species are now known), and those by Beyerinck with mixtures of species possessing different which bring about the various changes and fermentations powers of enzyme action as regards carbohydrates. in butter and cheese made from it. The presence of foreign Thus, a form termed Photobacterium phosphorescens by germs, which may gain the upper hand and totally destroy Beyerinck will absorb maltose, and will become luminous the flavours of butter and cheese, has led to the search for if that sugar is present, whereas P. Pjlugeri is indifferent those particular forms to which the approved properties to maltose. If then we prepare densely inseminated are due. A definite bacillus to which the peculiarly fine plates of these two bacteria in gelatine food-medium to flavour of certain butters is due, is said to be largely em- which starch is added as the only carbohydrate, the bac-
bacterial pigments, solution in water does not occur, though alcohol extracts the colour readily. Finally, there are a few forms which yield their colour to neither alcohol nor water, e.g., the yellow Micrococcus cereus-jiavus and the B. berolinensis. Much work is still necessary before we can estimate the importance of these pigments. Their spectra are only imperfectly known in a few cases, and the bearing of the absorption on the life-history is still a mystery. In many cases the colour-production is dependent on certain definite conditions—temperature, presence of oxygen, nature of the food-medium, &c. Ewart’s important discovery that some of these lipochrome pigments occlude oxygen, while others do not, may have bearings on the facultative anaerobism of these organisms. A branch of bacteriology which offers numerous problems of importance is that which deals with the organisms so common in milk, butter, and cheese. Milk is a me( bacteria lium not only admirably suited to the growth of bacteria, but, as a matter of fact, always contaminated with these organisms in the ordinary course of supply. Lafar has stated that 20 per cent, of the cows in Germany suffer from tuberculosis, which also affected 17‘7 per cent, of the cattle slaughtered in Copenhagen between 1891 and 1893, and that one in every thirteen samples of
