Culture (microbiology)

“ It is scarcely too much to say that a fuller knowledge of the part which soil plays in the culture and propagation of bacteria may suffice to revolutionise the practice of preventive medicine. Truly, our knowledge at the moment is rather a heterogeneous collection of isolated facts and theories, some of which, at all events, require ample confirmation ”

“ The bacteria are so extremely small that it is impossible to study individual germs separately without the aid of first-class microscopes. For this reason, but little advance was made in the knowledge of these lower forms of plant life, until the introduction of culture methods, whereby a single organism could be cultivated and the progeny of this cell increased to such an extent in a short course of time, that they would be visible to the unaided eye. ”

“ This repressive action of one species on another is a well-known bacteriological fact, but it must be remembered that such an explanation is only applicable in those cases where the culture organism is better able to develop than those forms that already exist in the cream. ”

“ It was for this reason that I proposed to use a mixed culture of bacteria, especially bacteria from the sweating process (see p. 105) , which secrete a mild form of proteolytic ferment, capable of dissolving the more easily soluble portion of the skin fibres (or certain constituents) , but not capable of attacking the hyaline layer.
(continuation of table to right)
The practical difficulty is to keep such cultures uniform during propagation, and so far this has prevented their introduction in practice. ”

“ To the busy investigator who cannot afford either the time or space in which to maintain a large bacterial family, it is of immense convenience to be able to obtain at a moment's notice a trustworthy culture, say, of typhoid or tuberculosis, or specimens of obscurer origin from air or water for purposes of investigation. These bacterial cultures are all guaranteed pure, free from contamination or admixture with other and alien micro-organisms, and are strictly what they are represented to be. ”

“ The ability to grow in other nutrient substances than the natural one has greatly facilitated the study of these minute forms of life. By the use of suitable culture media and proper precautions, pure cultures of a particular disease-producing bacterium may be obtained with which further experiments may be conducted. ”

“ That is to say, matter containing the microbe, taken from the infected source, must be cultivated in artificial media outside the animal body, under conditions excluding the possibility of the introduction of other microbes, until pure cultures of these microbes are obtained, and these microbes must be transplanted from generation to generation, until it is certain that no trace of non-living matter derived from the original animal body remains in the culture. ”

“ FIG. 178.
For the study of bacteria they are propagated artificially in a test tube—i. e., a substance called a “culture” is prepared from some organic material which, like the substances of the human body, is favorable to their propagation. Such culture media are found in beef blood, gelatine, beef extracts, meat broth, milk, etc. An ordinary test-tube is supplied with some of the culture medium, and is then sterilized over the fire to destroy all interfering germs. ”

“ To cultivate organisms, thus isolated, apart from other associated or contaminating bacteria—i. e., in pure culture.
For the successful achievement of these objects it is necessary to provide nutriment in a form suited to the needs of the particular bacterium or bacteria under observation, and in a general way it may be said that the nutrient materials should approximate as closely as possible, in composition and character, to the natural pabulum of the organism. ”

“ I have discovered that for an efficient bate a pure culture or single species of bacteria is insufficient, and that no single organism hitherto isolated will give results equal to dung. I have also discovered that the production of enzymes of the character above referred to, depends more upon the composition of the nutrient medium in which the bacteria are cultivated, than upon the kind of bacteria, although these latter must be capable of producing the required enzymes, the production of which appears to be due to a symbiotic growth of bacteria at present little understood. ”

“ We may admire the penetration and the perseverance with which scientists have succeeded, with the aid of the best modern microscopes and methods of preparation and coloring, in making so close a study of the organism of the bacteria, determining their physiological properties, and explaining their great importance for organic life by careful experiments and methods of culture. The bionomic or economic position of the bacteria in nature's household has thus secured for these tiny organisms the greatest scientific and practical interest. ”

“ In this method the bacteria are distributed in a gelatine or agar medium liquefied by heat, and the medium is then poured out on sterile glass plates or in shallow glass dishes, and allowed to solidify. Each bacterium capable of growth gives rise to a colony visible to the naked eye, and if the colonies are sufficiently apart, an inoculation can be made from any one to a tube of culture-medium and a pure culture obtained. ”

“ None of these systems, which are chiefly due to the medical bacteriologists, has maintained its position, owing to the difficulty of applying the characters and to the fact that such properties are physiological and liable to great fluctuations in culture, because a given organism may vary greatly in such respects according to its degree of vitality at the time, its age, the mode of nutrition and the influence of external factors on its growth. ”

“ It is demonstrated that microorganisms, capable of development in the culture media usually employed by the bacteriologists, are only found in the blood and tissues of yellow fever cadavers in exceptional cases, when cultures are made very soon after death. ”

“ By means of artificial nutrient media we may further learn the characters of the organism in "pure culture,"7 its favourable temperature, its power or otherwise of liquefaction, the curdling milk, or of gas production, its behaviour towards oxygen, its power of producing indol, pigment, and chemical bodies, as well as its thermal death point and resistance to light and disinfectants. ”

“ Another important method consists in inoculating an animal with some fluid containing the various bacteria. A pathogenic bacterium present may invade the body, and may be obtained in pure culture from the internal organs. This method applies especially to pathogenic bacteria whose growth on culture media is slow, e.g. the tubercle bacillus. ”

“ Up to that time life exists in the vase exposed to air and heat, If we examine the virulence of the culture at the end of two days, four days, six days, eight days, etc., it will be found that long before the death of the culture the microbe has lost all virulence, although still cultivable. Before this period it is found that the culture presents a series of attenuated virulences. Everything is similar to what happens in respect to the microbe in chicken-cholera. ”

“ Culture media. For culture media, many different substances are employed. In fact, bacteria will grow on almost any organic substance whether it is solid or fluid, provided the other essential conditions of growth are furnished. The food substances that are used for culture purposes are divided into two classes; solids and liquids. [Pg 14]
Solid media may be either permanently solid like potatoes, or they may retain their solid properties only at certain temperatures like gelatin or agar. ”

“ In the inoculated animal the same micro-organism must again be found. The chain of evidence will be still more complete if, from artificial culture, a chemical substance is obtained capable of producing the disease quite independently of the living organism. ”

“ Or media in which certain organisms thrive and others not, so that the former soon “crowd out” the latter (unsterilized milk for lactic acid bacteria, inorganic media in soil bacteriology) may be used. A combination of the general methods (b) and (c) is much used in the separation of the organisms of the “intestinal group” in human practice. (d) The inoculation of a susceptible animal with a mixture suspected to contain a given pathogenic bacterium frequently results in the development of the latter in pure culture in the body of an animal, from which it may be readily recovered. ”

“ The microscope reveals much, and the art of bacterial culture now explains how it is true that the superabundant creature, which has persistently followed civilised man into every quarter of the globe, has ever had a share in conveying contagion beyond that of any other household pest. ”

“ This is well proved by the influence of various culture media on the growth of microorganisms. The presence of chemical compounds of well-known nature, even in very small quantities, has been shown by a great many observers to influence much the mode of growth of bacteria. It has even been proved that bouillon obtained from the muscular tissue of various animals, notwithstanding the absence of any very definite active chemical compound causing marked difference, gave cultivation media more or less suitable for the growth of certain organisms. ”

“ The various improvements in methods of research, and especially the introduction of solid culture media and Koch's 'plate method' for isolating bacteria from mixed 'cultures, have placed bacteriology upon a scientific basis. ... It was a distinguished French physician, Davaine, who first demonstrated the etiological relation of a microörganism of this class to a specific infectious disease. ”

“ A bacteriologist makes a “culture” of a drop of blood, multiplying many times the bacteria in it, to determine whether serious disease germs are prevalent. If the influence of a person could be observed in a large way, would that be conclusive as to the person’s character, just as the result of the culture proves the condition of the blood? May there not be an obscure element in the teacher’s character that is having a deleterious 175effect? Or is it only the outstanding features of his conduct that affect the pupils? ”

“ Thus in the analysis of a bacterial culture we can select the one which seems on various considerations to be the most likely cause of an etiologically undetermined disease, but for actual demonstration we must employ synthetic methods ”

“ Their life processes and ecological interactions can't be easily observed directly in the soil with a microscope. Usually, scientists study microorganisms by finding an artificial medium on which they grow well and observe the activities of a large colony or pure culture—a very restricted view. There probably are more species of microorganisms than all other living things combined, yet we often can't identify one species from another similar one by their appearance. We can generally classify bacteria by shape: round ones, rod-shaped ones, spiral ones, etc. ”

“ I have shown128 that simple cultures of bacteria in certain nutrient media, in the absence of amines and other bodies, have an imperfect bating action on skin. This was the case, whether the cultures were pure or mixed, but the mixed cultures have a better action than the pure cultures of any of the organisms I have hitherto used. Bacteriological processes in nature are usually carried on by a mixture of species of bacteria ”

“ I must, however, first direct attention to a well-recognised fact, that bacteriology only touches animal pathology at a few points, and that so far from bacteria being synonymous with disease germs, the majority of these remarkable organisms appear to be beneficent rather than inimical to man. ”

“ The way that the organism grows in different kinds of cultures, the by-products produced in different media, and effect on the animal body when injected into the same are also used as data in distinguishing one species from another.
Conditions favoring bacterial growth. The bacteria, in common with all other living organisms are affected by external conditions, either favorably or unfavorably. Certain conditions must prevail before development can occur. ”

“ Such cultures are called “deep cultures.” To obtain growth of an anaerobic organism on the surface of a medium, in using the plate method, and also for cultures in fluids, the air is displaced by an indifferent gas, usually hydrogen.
In testing the effects of bacteria by inoculation the smaller rodents, rabbits, guinea-pigs, and mice, are usually employed. One great drawback in certain cases is that such animals are not susceptible to a given Inoculation.bacterium, or that the disease is different in character from that in the human subject. ”

“ Experimental medicine has added constantly to the list of diseases caused by bacterial organisms, and the general public has been educated to an adequate conception of the importance of the germ as the chief agency in the transmission of disease, with corresponding advantage to the efficiency of personal and public hygiene. At the same time knowledge of the benign bacteria and the enormous role they play in the industries and the arts has become much more widely diffused. Bacteriology is being studied in colleges as one of the cultural sciences ”

“ The great host of species which are found in water, milk, air, in our mouths or clinging to our skin, and which are almost omnipresent in Nature, are capable of growing well enough in ordinary lifeless organic foods, but just as soon as they succeed in finding entrance into living human tissue their growth is checked at once by these antiseptic agents which are poured upon them. Such bacteria are therefore not pathogenic germs, and not sources of trouble to human health. ”