Lactic acid bacteria

“ Bacillus lacticus (Bacterium acidi lactici, Zopf) is always found in milk which has stood for some time, and in sour foods (cabbage, cucumbers, etc.) ; it turns the milk sour by producing lactic acid fermentation in the sugar contained in the milk; the lactic acid formed, eventually causes the coagulation of the casein. It resembles the Vinegar-bacteria, occurring as small cylindrical cells, rarely in short rows; not self-motile.—Several other Bacteria appear to act in the same way, some occurring in the mouth of human beings; some of these Bacteria give to butter its taste and flavour. ”

“ It includes not only the true lactic acid types in which no gas is formed, but those species capable of producing gases and various kinds of acids. These organisms are the distinctively milk bacteria, although they do not predominate when the milk is first drawn. Their adaptation to this medium is normally shown, however, by this extremely rapid growth, in which they soon gain the ascendency over all other species present. It is to this lactic acid class that the favorable flavor-producing organisms belong which are concerned in butter-making. They are also indispensable in cheese-making. ”

“ This organism is a powerful lactic acid ferment, forming large amounts of lactic acid from milk sugar while forming very little alcohol. The organism grows well in milk and on some media containing an abundance of soluble carbohydrates, as, for instance, in malt extracts. We may take the behavior of B. bulgaricus in the digestive tract as being typical of efficient lactic acid bacilli in general. There are two questions which we must put to ourselves regarding the therapeutic effects of such bacteria. ”

“ While, however, the attention of the dairyman has been chiefly directed to the propagation of acid-producing organisms and the use of pure cultures of lactic acid bacteria in their relation to butter and cheese making, a new sphere in micro-biology has been disclosed by the study of the effects caused by the combined growth of two or more different classes of organisms in milk and the consequent production of lactic, alcoholic, and gaseous fermentations. ”

“ Professor Russell is of opinion that the lactic-acid organisms are to be credited with greater ripening powers than the casein-digesting organisms, but it must not be forgotten that these two great families of bacteria are still more or less on trial, and it is not yet possible finally to dispose of either of them. ”

“ It appears that there is always at first a marked increase in the number of micro-organisms, which is soon followed by a more gradual decline. While the casein-digesting and gas-producing classes suffer a general and more or less rapid decline, the lactic-acid bacteria develop to an enormous extent, from which fact it would appear that cheese offers ideal conditions for the development of the latter. ”

“ This organism has the power of transforming milk sugar and other saccharoses into lactic acid, with evolution of carbonic acid gas. It is rarely found in the saliva or mucilage of the teeth. In these are two micrococci, both of which cause the production of lactic acid, but which manifest differences in their development under cultivation. There are also two pigment forming bacteria, Micrococcus prodigiosus, which produces intensely red spots, and the yellow micrococcus of osteomyelitis. ”

“ Milk is not so frequently the cause of dissemination as the other factors, but where milk supplies become contaminated, epidemics of considerable magnitude are wont to occur. The danger from milk is also aggravated by the fact that the typhoid bacillus is capable of withstanding considerable amounts of acid, and consequently finds, even in raw milk containing the normal lactic acid bacteria, conditions favorable for its growth. In a considerable percentage of cases, the disease is not sufficiently severe to cause the patient to take to his bed. ”

“ Lactic Acid as a Bactericide.—Then came Metchnikoff's announcement that his studies showed sour milk to be an extremely valuable food material, but much more than that, an important auxiliary for the lessening of microbic life in the intestines. He seemed to be able to demonstrate that a great many bacteria, whose products, absorbed from the intestines, hastened that process of deterioration in the tissues that we call old age, were inhibited when sour milk or lactic acid bacteria were present. The general health of the person who took sour milk was, as a consequence, much better. ”

“ 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. ”

“ In several cases the writer has found a not inconsiderable number of liquefying bacteria mixed with the selected organism. Molds not infrequently are found in cultures put up in the dry form. Doubtless the effect of these accidental contaminations is considerably less in the case of a starter composed of a distinctively lactic acid-producing organism than with a form which is less capable of thriving vigorously in milk, and it should be said that these impurities can frequently be eliminated by continued propagation. ”

“ They are also powerful germicides and scavengers, destroying or neutralizing the products of other bacteria which in the absence of these bacteria and the 22lactic acid produced by them would play havoc with the food and produce putrefaction or disease. Everybody who handles milk knows that pure sour milk or buttermilk in which lactic acid bacteria abound keeps well for a long time, free from other fermentations which have no chance to develop in their presence. ”

“ In all pasteurized milk as it flows from the machine, there will remain some living bacteria. The spores will not be destroyed by any pasteurizing process, and under commercial conditions, vegetating bacteria are also present. If the milk is not quickly chilled after heating, these forms will grow, and their development is particularly hastened by the destruction of the lactic bacteria, the acid of which would otherwise hold them in check. ”

“ Where careless conditions obtain under which dust and manure particles find their way into milk, it becomes more abundantly seeded with gas-generating bacteria, and consequently, the type of fermentation is undesirable. If, however, the milk is drawn into clean utensils and care is taken to exclude dirt, the pure lactic acid types are able to control the character of the changes produced, and a clean, pleasant tasting liquid results. It will be seen that things are well arranged by nature ”

“ Whether the organisms producing this or that series of changes prevail or not depends upon the initial seeding, and the conditions under which the milk is kept. Ordinarily, the lactic acid organisms grow so luxuriantly in the milk that they overpower all competitors and so determine the nature of the fermentation; but occasionally the milk becomes infected with other types of bacteria in relatively large numbers and the conditions may be especially suitable to the development of these forms, thereby modifying the course of the normal changes that occur. ”

“ The danger with soured milk is, that in the process of culture we develop the best condition for the increase of these when they preponderate, or when, through the use of bad cultures, the lactic-acid-producing bacteria are absent, or present only in small numbers. ”

“ It does not liquefy gelatine. It grows best at blood-heat; but much above that it fails to produce its fermentation, and it ceases to grow under 10° C. It inverts milk-sugar and changes it to dextrose, from which it then produces lactic acid. Sugars do, however, differ considerably in the degrees to which they respond to the influence of the lactic ferment, and some which are readily changed by the alcoholic ferment are untouched by the Bacillus acidi lactici. It will be necessary to refer again to this micro-organism when we come to speak of milk and other dairy products. ”

“ Not only do organisms of this group produce far more acid than the normal lactic bacteria; they are also more resistant to acid, and are able to develop in milk to which 0.5 per cent. hydrochloric acid has been added. A comparatively high percentage of alcohol seems to encourage growth, and this was obtained in milk containing 4 per cent. alcohol. This would no doubt tend to explain the phenomenon observed by Kuntze that milk is not so rapidly fermented by organisms of this group as when cultures of diplococci and yeasts are added. ”

“ The methods of isolating these bacteria, and the compositions of the media employed, would demand a treatise on bacteriology; but, for general purposes, a good liquid medium for the cultivation of puer bacteria is a gelatin peptone broth, made by digesting 10 grm. gelatin with 6 1/2 grm. 80 per cent. lactic acid in 100 c.c. water under pressure for three hours, neutralizing with ammonia, adding 1 grm. potassium phosphate, making up to 1000 c.c., and filtering. A sterile infusion of fresh dung may be used, but it is troublesome to prepare and not easy to get uniform in strength or composition. ”

“ Such poisonous microbes thrive and multiply very well in the alkaline contents of the intestines, but the growth of such 185dangerous bacteria can be greatly hindered by the introduction therein of acid substances, especially lactic acid. ”

“ Obviously, we can not wait for the manufacture of any poison, but must make it an impossibility, if we can, without rivaling any of the toxic effects by our remedies. Acquaintance is occasionally made with the ptomaine before the guilty micrococcus is known; in such cases even more care must be exercised.
Following the investigations of Lister and Hueppe, the ordinary fermentation of milk is traced to the growth of a microorganism known as Bacterium lactis, which converts the milk-sugar into lactic acid. ”

“ It is attended by the one danger which is not always duly appreciated, that unless its use is restricted to herds on ground that is already infected, it endangers the infection of new districts. The spores are not absolutely sterilized at 85° C. Arloing and Cornevin and later, Nocard and Roux have shown that the addition of lactic acid to the liquid which has been weakened for inoculation, restores it to its former virulence, making it a most deadly agent. ”

“ This receives support from the fact that soured milk beverages prepared by the use of ordinary lactic bacteria, distinct from those of the Bulgaricus 124 type, often exert a beneficial influence upon human beings even although the organisms responsible for the fermentation are incapable of growth at blood temperature.
It may be mentioned in conclusion that cultures prepared by the use of organisms of the type Streptococcus lacticus combined with Bac. bulgaricus possess a more agreeable flavour and aroma than those prepared from a pure culture of Bulgaricus alone. ”

“ Apart from those organisms associated with acute infective diseases, disturbances of function and physical lesions may be the result of abnormal bacterial activity in the canal; and these disturbances may be both local and general. Many of the bacteria commonly present produce putrefactive changes in the contents of the tract by their metabolic processes. They render the medium they grow in alkaline, produce different gases and elaborate more or less virulent toxins. Other species set up an acid fermentation, seldom accompanied by gas or toxin formation. ”

“ Desirable acid-forming bacteria. If milk is produced under clean conditions, it is not likely to have a disagreeable odor or taste at any time, even when it is sour; rather the taste is agreeable like that of good butter milk. The curd is perfectly homogeneous, showing no holes or rents, due to the development of gas, and there is but little tendency for the whey to be expressed from the curd. This type of fermentation is largely produced by the group of bacteria to which has been given the name, Bacillus lactis acidi. ”

“ The organism is aërobic, and liquefies gelatine. It stains readily with the ordinary aniline dyes. It does not produce spores, though certain refractile bodies inside the protoplasm of the bacillus in old cultures have been regarded as such. The virulence of the bacillus is readily attenuated, and both the virulence and morphology appear to show in different localities and under different conditions of artificial cultivation a large variety of what are termed involution forms. Unless the organism is constantly being sub-cultured, it will die. Acid, even the .2 per cent. ”

“ If a difference of action in certain reagents, in ammonia, for example, were sufficient to determine the limits of the animal and vegetable kingdoms, might we not argue that there must be a very great and natural difference between moulds and bacteria, inasmuch as the presence of a small quantity of acid in the nutritive medium facilitates the growth and propagation of the former, whilst it is able to prevent the life of bacteria and vibrios? ”

“ It has been clearly proved that the microbes which produce lactic acid can, and do, control the growth of other microbes within the body, and that the lactic microbe is so much at home in the human body that it is to be found there several weeks after it has been swallowed. ”

“ A comparative survey of the anatomy of the vertebrates shows that as a general rule the longer the intestine the shorter the life. He does not advocate its extirpation by surgery or its disinfection by chemicals, but he would crowd out its wild and poisonous flora by harmless cultivated species. Among the friendly microbes he regards the lactic acid bacilli as most useful for this purpose. These act upon milk or fruit sugar, converting it into lactic acid, which is destructive to most other microbes, including some of the most dangerous. ”