The introduction of this method for the isolation and study of bacterial species in pure cultivation certainly constitutes the most im portant stimulus to the development of modern bacteriology. By it results were placed upon a more secure basis than ever before, and a con fidence in the work such as had never existed was awakened in the minds of all students of the subject.
The studies that had been made by Pasteur upon fermentation; upon the souring of wines; upon the maladies of silk worms, and upon certain fatal epizootics of fowls and domestic cattle; together with Koch's fundamental studies upon the infections of wounds and the appropriate methods of analyzing them were nch in suggestion to the pioneers in this new field. Within a comparatively brief period af ter the adoption of the new methods our Imowl edge of the exciting causes of many hitherto obscure diseases was greatly extended; it was shown to be possible to determine the modes of their transmission and the channels through which infection occurred. The conditions most favorable to the successful action of a host of substances employed for the purpose of disin fection were accurately determined. And early in the work observations were made that indi cated the possibility of successful vaccination against disease through the use of attenuated (weakened) living cultures of specific disease produdng bacteria. One of the most important outgrowths of modern bacteriology has resulted from its application to the problems of the sani tary engineer. As a result of these studies we know that sewage, polluted waters and polluted soils tend naturally to revert to a state of purity if their pollution be checked and that this pro gressive purification is due in large part to the activities of the bacteria located within them. It has been found that by the appropriate ad justment of conditions the normal activity of the bacteria may be so greatly accentuated as to constitute them the most important factors in the purification of polluted waters and sewage. The utilization of these facts is conspicuously illustrated in the purification of water by the process of natural sand filtration and in the purification of sewage by irrigation; by the sep tic tank process, etc. In these methods liv ing bactena and other living microscopic organ isms, and they alone, are the instruments through which the results are attained. The sand grains in the filters and the particles of soil in the irrigation fields serve only as objects to which the micro-organisms can attach them selves and multiply. By the normal life proc esses of the bacteria the polluting organic matters in the fluids to be purified are used up and inert matters given off as a result.
In the study of agricultural phenomena from the bacteriological standpoint Imowledge has been equally extended. At one time it was taught that atmospheric *nitrogen—represent ing roughly 80 per cent of the air by volume — was of no direct biological significance. This view has in late years been entirely revised. We have learned that the leguminous plants when assisted, symbiotically, by certain soil bacteria, are enabled to make up their nitrogen deficit in large part from the free nitrogen of the air; a fact that sheds important light upon the signifi cance of plants of this type in the practice of °rotation of crops.° tinder normal conditions instead of impoverishing the soil, the legumens —clover, peas, beans, etc.— with the aid of the bacteria attached to their roots, may actually enrich it. The application of bacteriological methods to the study of dairy processes has re vealed the interesting fact that the delicate flavors to which butters and cheeses owe their commercial value are directly due to the prod ucts of growth of certain species or groups of species of bacteria and more highly organized molds. A number of such species have been
isolated and are kept in pure cultivation — so that by purposely inoculating the fresh cream with them butter of uniform flavor may with comparative ease be produced.
Probably the most important results of ap plied bacteriology are those in connection with preventive medicine. Early in the course of the work it was discovered by Pasteur that certain virulent pathogenic bacteria when kept under particular conditions gradually lost their dis ease-producing power, wholly or in part, with out their other life properties being conspicu ously disturbed. If injected into animals when in this attenuated state the result was a mild, temporary and modified form of infection usu ally followed by recovery. With recovery the animals so treated were immune from the ac tivities of the fully virulent bacteria of the same species; in other words, they had been protected from the fatal injection by vaccination with an attenuated species. The subsequent develop ments growing out of this observation have re sulted in the annual saving of millions of money through the successful vaccination of sheep, horses and bovines against the fatal infection Imown as splenic fever or anthrax, and, though less successful, of other domestic animals against other infections also. In the closer analysis of the means by which infective bac teria cause disease it soon became evident that it is through the elaboration of specific poisons; sometimes easily separated from the bacteria, at others so intimately associated with the bac terial tissues as to make their separation diffi cult or impossible. The question arose as to the effect of the poisons, separated from the living bacteria, upon the animals susceptible to inf ec tion by the bacteria themselves, and it was found that fatal intoxications often accom panied by the same constitutional symptoms and pathological lesions followed the use of the poisons, just as they would follow inoculations by the bacteria by which they were produced In pursuance of this topic it was discovered that if very small, only mildly intoxicating, doses of these specific poisons of bacterial origin were repeatedly injected into susceptible animals, after a while the latter acquired not only a sort of tolerance to them, but a tolerance that was accompanied by the presence in the circu lating blood of an antidote for these poisons — an °antitoxin,'" as it is called. This reaction has been shown to be possible for a number of specific infections, and in the case of diphtheria has met with such practical success as to be deservedly regarded as the triumph of modern medicine.
Bibliography, Abbott, Principles of Bacteriology' (Philadelphia 1909) ; Ball, (Es sentials of Bacteriology) (ib. 1913); Mugge, (Die Mikroorganisme& ; Friedlander, ( Mi kroskopische Technik) (Berlin 1900); Koch, (Zur Untersuchung von pathenogenen Organts men) (1881) ; id., (Untersuchungen fiber die Aetiologie der Wundinfections-Krankheite& ; Leettwenhoek, Antonio von, (Arcana Natura' (1695) ; Loffler, (Vorlesungen *Ober die ge schichtliche Entwickelung der Lehre von den Bakterien>; Mallory and Wright, 'Pathological Technique' (Philadelphia 1913) ; Mason, "Water Supply from the Sanitary Standpoint'; (Metchnikoff, (L'Immunite dans les maladies infectieuses' (1901) • Muir and Ritchie, 'Manual of Bacteriology' New York 1913); Newman, and the Public Health' (Phila delphia 1904) ; Park, 'Bacteriology in Medicine and Surgery' ; Reckets and Dick, 'Infection, Immunity and Serum Therapy' ; Russel, (Out lines of Dairy Bacteriology' (1899); Sternberg, (Bacteriology) (New York 1901) ; Thoma, 'Lehrbuch der pathologischen Anatomic' (trans. by Bruce, London 1896) ; Tyndall, 'Essays on the Floating Matter in the Air' ; Vallery-Radot, 'Life of Pasteur); Zinsser, 'Infection and Re sistance' (New York 1914).