Acidity

ACIDITY of solutions, according to modern concepts of physical chemistry, is a complex concept characterized by three quantities: 1) active reaction (see), 2) titration acidity (see Alkalimetry), 3) their buffer properties (see). Solutions having the same active reaction may differ in titration acidity and their buffer properties. Only all the above-mentioned quantities, taken together, make it possible to fully characterize the A. of a solution. On the A. of gastric juice-see Stomach, as well as Hydrogen ions. ACID-FAST BACTERIA, a group of microbes with strictly defined properties. A. b. with difficulty accept dye: for their staining it is necessary either prolonged action of the dye solution (e.g. 24 hours in the cold), or combined action of the dye with heating (staining when heated to boiling), or finally the addition to the dye solution of substances acting as a mordant (staining with carbolic solution of dyes, etc.). Once stained, A. b. do not decolorize when acted upon by dilute acids (acid-fastness), dilute alkalis (alkali resistance) and alcohol (alcohol resistance; not all A. b. possess this). Acid-fastness (resp. alkali- and alcohol resistance) is not an absolute property; it varies among different representatives of the group and is also not the same within the same species of microbes. It has been proven (Kedrovskii) the existence of non-acid-fast varieties of the leprosy bacillus; there is also reason to think about the existence of non-acid-fast tubercle bacilli.-As to the causes of acid-fastness, there are two different opinions. Most researchers think that the cause of acid-fastness is certain substances that make up the body of A. b. Most often such substances are fatty acids (Much) and waxy substances, always present in A. b.; according to Aronson, the cause of acid-fastness is true wax; according to Stoltzner-chitin; according to Koganei-kephalin. Other researchers believe that the cause of acid-fastness is protein or perhaps its combination with one or another fat fraction. A chemical theory is opposed by another, not connecting acid-fastness with the presence in bacteria of certain substances and explaining it by the entire physicochemical appearance of the microbes. The question of which of these two theories is correct cannot be considered settled. Research of recent years speaks against a purely chemical understanding of the nature of acid-fastness, showing that the extraction of fatty and fat-like substances from acid-fast microbes is not accompanied by loss of their acid-fastness; one must also keep in mind that bacilli may lose acid-fastness under the influence of influences that leave their chemical structure unchanged. A. b. do not decolorize when stained by Gram's method. They are immobile; they do not liquefy gelatin, do not curdle milk. With respect to oxygen they are aerobes, and in some of them the need for oxygen is expressed very sharply. In relation to nutrients they are not very demanding, but for abundant growth their presence in the nutrient substrate of glycerin or glucose is necessary. Morphologically A. b. represent short or longer rods, solid or with granular inclusions. Acid-fast bacteria include pathogenic microbes and very numerous saprophytes. Pathogenic A. b. are few: these are the tubercle bacillus (Koch), the leprosy bacillus (Hansen), the bacillus of enteritis of cattle (Johne) and the leprosy bacillus of rats (Stephany). A transitional group to pathogenic microbes is formed by A. b. of tuberculosis of cold-blooded animals, which include tubercle bacilli of fish and reptiles; Kroster (Köster) isolated an acid-fast microbe from the body of frogs, Friedman (Friedmann)-from a turtle, etc. Opinions on the nature of tubercle bacilli of cold-blooded animals differ greatly; however, most researchers think that it is not a matter of true tubercle bacilli, but of acid-fast saprophytes that have accidentally entered the body of cold-blooded animals. Acid-fast saprophytes are widely distributed in nature, occurring in soil, water, dust, as well as on the skin and mucous membranes of healthy people and animals. Of the bacilli of this group, it is necessary to note the bacilli of smegma (Alvarez), earwax, bacilli found in nasal mucus (Karlinski), in sputum in pulmonary gangrene (L. Rabinovitsch), in saliva and on the skin of healthy people (Laabs, Moeller), in the excrements of horses, cows and humans (Severin), and others. A whole series of A. microbes has been found in milk and butter [bacilli of Petri, bacilli L. Rabinovitsch, bacilli Korn (types I and II), bacilli Tobler (types I-V), milk bacilli (Moeller and others)]. Of the bacilli found in nature outside the human or animal organism, it is necessary to name the bacillus of Timofeev's grass, first found by Möller and widely distributed on the stems of grasses. A. b. are also described in distilled water, on the inner surface of wind instruments, etc. Differences between A. b. consist in the varying degree of their acid-fastness (greatest-in tubercle bacilli), in different relations to nutrients (tubercle bacillus requires glycerin for its growth, bacillus of Timofeev's grass grows better in the presence of glycerin, milk and oil bacilli easily grow on ordinary nutrient media, etc.), in different temperature optimum (37-38° for tubercle bacilli, 25-30° for acid-fast saprophytes), in morphological dissimilarity (tubercle bacillus-more often a thin granular rod; acid-fast saprophytes-more often solid, thick, clumsy rods). The chemical composition of A. b. also shows certain differences; thus, according to Long (Long), the percentage of non-saponifiable substance in A. b. sharply decreases when passing from true tubercle bacilli to acid-fast saprophytes. In bacilli typ. humani it amounts to 77.1%, in typ. bov.-60.0%, in typ. galli-35.7%, leprae-27.2%, tubercle bacillus of turtle-28.3%, frog-33.7%, Timofeev's grass-9.4% and smegma-4.6%. Attempts to turn pathogenic A. microbes into saprophytes and vice versa are very numerous, especially in the old literature, but in general they are not conclusive. Of the latest works of this kind, it is necessary to note the research of Kolle, Schlossberger and Pfanenstiel, who succeeded in turning A. saprophytes into virulent forms close to true tubercle bacilli; these works, like the research of old authors, in general have not found confirmation. The relationship of A. b. to each other is proven by Much, using for this purpose the complement deviation reaction; sera of tuberculous and suspicious of tbc people with emulsion of tubercle bacilli as antigen reacted in 69.6%, while with emulsion of leprosy bacilli-in 33.3%, with emulsion of bacilli of Timofeev's grass-in 12%.-There is no rational classification of A. b., and the most correct is the opposition of pathogenic A. b. (resp. parasites) to the extensive group of acid-fast saprophytes. For the designation of the latter, the term 'paratuberculous bacilli' is often used, and Calmet refers to them all A. saprophytes, while other authors (Baldwin, Petrow, Gardner)-only A. saprophytes found in secretions of humans and animals. The often used term 'pseudotuberculous bacilli' for the designation of A. saprophytes only leads into error, since this name is also borne by pseudotuberculous bacilli, not possessing acid-fastness, but capable of causing in the organism changes macroscopically similar to tuberculous (e.g. Bact. pseudotuberculosis rodentium). Path.-anat. changes associated with the introduction into tissues of A. bacilli most often proceed under the guise of so-called infectious granulomas, more rarely they have the character of exudates or purulent foci. The tendency to granulomatosis is probably conditioned by the stability of the A. bacilli themselves, playing the role of foreign bodies.