Acid-Fast Bacteria

ACID-FAST BACTERIA, a group of microbes with strictly defined properties. Acid-fast bacteria are difficult to stain: for their staining, it is necessary to use either prolonged action of the staining solution (e.g., 24 hours in the cold), or a combination of the action of the dye with heating (staining while heating to boiling), or finally, the addition to the dye solution of substances acting as a mordant (staining with carbolic dye solutions, etc.). Once stained, acid-fast bacteria are not decolorized by the action of diluted acids (acid-fastness), diluted alkalis (alkali-fastness), and alcohol (alcohol-fastness; not all acid-fast bacteria possess this). Acid-fastness (respectively, alkali- and alcohol-fastness) is not an absolute property; it varies among different representatives of the group and is also not uniform within the same species of microbes. The existence of non-acid-fast varieties of the leprosy bacillus has been proven (Much); there is also reason to think about the existence of non-acid-fast tubercle bacilli. Regarding the question of the causes of acid-fastness, there are two different opinions. The majority of researchers think that the cause of acid-fastness is certain substances included in the composition of the body of acid-fast bacteria. Most often, such substances are considered to be fatty acids (Much) and wax-like substances, which are always present in acid-fast bacteria; according to Aronson, the cause of acid-fastness is true wax; according to Stoltzner—chitin; according to Koganei—cephalin. Other researchers believe that the cause of acid-fastness is protein or, perhaps, its combination with one or another fatty fraction. The chemical theory is opposed by another, which does not link acid-fastness to the presence of certain substances in bacteria and explains it by the entire physicochemical appearance of the microbes. The question of which of these two theories is correct cannot be considered solved. Arguments against a purely chemical understanding of the nature of acid-fastness are provided by studies of recent years, which testify to the fact that the extraction of fatty and fat-like substances from acid-fast microbes is not accompanied by their loss of acid-fastness; it is also necessary to keep in mind that bacilli can lose acid-fastness under the influence of exposures that leave their chemical structure unchanged. Acid-fast bacteria are not decolorized by Gram staining. They are non-motile; they do not liquefy gelatin, they do not coagulate milk. In relation to oxygen, they are aerobes, and in some of them, the need for oxygen is very sharply expressed. Regarding nutrients, they are not very fastidious, but for abundant growth, the presence of glycerin or glucose in the nutrient substrate is necessary. Morphologically, acid-fast bacteria are short or longer rods, solid or with granular inclusions. Acid-fast bacteria include pathogenic microbes and very numerous saprophytes. There are few pathogenic acid-fast bacteria: these are the tubercle bacillus (Koch), the leprosy bacillus (Hansen), the bacillus of cattle enteritis (Johne), and the rat leprosy bacillus (Stefansky). A group transitional to pathogenic microbes consists of acid-fast bacteria of tuberculosis of cold-blooded animals, which include tubercle bacilli of fish and reptiles; Küster isolated an acid-fast microbe from the organism of frogs, Friedmann—from a turtle, etc. Opinions on the nature of tubercle bacilli of cold-blooded animals differ greatly; most researchers think, however, that this is not a matter of true tubercle bacilli, but of acid-fast saprophytes that accidentally entered the organism of cold-blooded animals. Acid-fast saprophytes are widely distributed in nature, occurring in soil, water, dust, and also on the skin and mucous membranes of healthy humans and animals. Among the bacilli of this group, one should note the smegma bacilli (Alvarez), earwax bacilli, bacilli found in the nasal mucus (Karlinski), in sputum during pulmonary gangrene (L. Rabinovitsch), in the saliva and on the skin of healthy people (Laabs, Moeller), in the feces of horses, cows, and humans (Severin), and others. A whole series of acid-fast microbes has been found in milk and butter [Petri bacilli, L. Rabinovitsch bacilli, Korn bacilli (types I and II), Tobler bacilli (types I-V), milk bacilli (Moeller's, etc.)]. Among the bacilli occurring in nature outside the human or animal organism, one should name the Timothy grass bacillus, first found by Moeller and widely distributed on the stems of grasses. Acid-fast bacteria have also been described in distilled water, on the inner surface of wind instruments, etc. The differences between acid-fast bacteria consist in the unequal degree of their acid-fastness (the greatest is in tubercle bacilli), in the different relation to nutrients (the tubercle bacillus requires glycerin for its growth, the Timothy grass bacillus grows better in the presence of glycerin, milk and butter bacilli grow easily on ordinary nutrient media, etc.), in the unequal temperature optimum (37-38° for tubercle bacilli, 25-30° for acid-fast saprophytes), in morphological dissimilarity (the tubercle bacillus is more often a thin granular rod; acid-fast saprophytes are more often solid, thick, clumsy rods). The chemical composition of acid-fast bacteria also presents certain differences; thus, according to Long, the percentage of unsaponifiable matter in acid-fast bacteria drops sharply upon transition from true tubercle bacilli to acid-fast saprophytes. In bacilli typi humani it constitutes 77.1%, in typ. bov.—60.0%, in typ. galli—35.7%, leprae—27.2%, turtle tubercle bacillus—28.3%, frog—33.7%, Timothy grass—9.4%, and smegma—4.6%. Attempts to transform pathogenic acid-fast microbes into saprophytes and vice versa are very numerous, especially in the old literature, but in general, they are not conclusive. Among the newest works of this kind, it is necessary to note the studies of Kolle, Schlossberger, and Pfannenstiel, who managed to transform acid-fast saprophytes into virulent forms close to true tubercle bacilli; these works, like the studies of old authors, have generally not found confirmation. The kinship of acid-fast bacteria among themselves is proven by Much, who uses the complement deviation reaction for this purpose; sera of tuberculous and tuberculosis-suspected people reacted with an emulsion of tubercle bacilli as an antigen in 69.6%, whereas with an emulsion of leprosy bacilli—in 33.3%, with an emulsion of Timothy grass bacilli—in 12%. There is no rational classification of acid-fast bacteria, and the most correct is the opposition of pathogenic acid-fast bacteria (respectively, parasites) to the vast group of acid-fast saprophytes. To denote the latter, the term "paratuberculous bacilli" is often used, with Calmette referring all acid-fast saprophytes to them, whereas other authors (Baldwin, Petrow, Gardner)—only acid-fast saprophytes found in the excretions of humans and animals. The frequently used term "pseudotuberculous bacilli" to denote acid-fast saprophytes only misleads, because this name is also borne by pseudotuberculous bacilli that do not possess acid-fastness but are capable of causing changes in the organism macroscopically similar to tuberculous ones (e.g., Bacillus pseudotuberculosis rodentium). Pathological-anatomical changes associated with the introduction of acid-fast bacilli into tissues proceed most often in the form of so-called infectious granulomas; less often they have the character of exudates or purulent foci. The tendency to granulomatosis is apparently conditioned by the resistance of the acid-fast bacilli themselves, which play, as it were, the role of foreign bodies.