Human Microflora

HUMAN MICROFLORA. The cavities of the human body remain free of microbes only in the first hours after birth; then, being in constant communication with the outside world, these cavities are colonized by microbes, which inhabit the body throughout the rest of a person's life, constituting their microflora. The latter is divided into two groups of microbes: some are its permanent inhabitants, others are accidental and temporary. As is known, in combinations of living beings, symbiosis and parasitism are distinguished; in the first case, both cohabitants derive benefits from their joint existence, in the second, one of the cohabitants is a parasite of the other, using the juices and tissues of the latter. In the vast majority of cases, microbes of the human body are parasites; the presence of only a small number of species (a group of lactic acid bacteria) can be recognized as beneficial to humans. Most of the microbes inhabiting the human body seem harmless to the host organism, but this harmlessness is conditional: given certain circumstances, harmless saprophytes become pathogenic parasites. At the present time, the question of the relationship between the human body and the microbes inhabiting it is being resolved from the perspective of the doctrine of so-called microbes de sortie. This term, which is difficult to translate into Russian (closest is 'exit' microbes), is applied to microbes that normally live in an animal's body and are capable of manifesting (ready to exit) a pathogenic effect when favorable conditions for this are created in the organism. These microbes primarily include banal bacteria living in the organism either as saprophytes or as non-virulent forms of pathogenic microbes. Such are Bac. proteus, Bact. coli commune, various spirochetes, staphylococcus, Friedländer's bacillus, Bac. perfringens, and many others. Having settled in the human body immediately after birth, these microbes sometimes remain in it until death. Another category of microbes de sortie is strictly pathogenic microbes, which, having penetrated the organism, do not cause its disease, but remain in it in an inactive state; such are the influenza bacillus, diphtheria bacillus, catarrhal micrococcus, actinomycetes, tetanus bacillus, etc. Not manifesting a pathogenic effect under 'normal' conditions, microbes de sortie, in the presence of certain causes, are activated and cause disease. The causes of their transition to an active state are internal or external; the former is the property of variability inherent in all microbes, the causes of which in most cases still escape us; external causes are moments that weaken the macroorganism (malnutrition, trauma, sharp fluctuations in external temperature and humidity, diseases, etc.). The place of residence of the microbes normally inhabiting the human body is, in essence, all its cavities communicating with the outside world. In general terms, it can be pointed out that the flora of the respiratory and digestive tracts is not identical: in the former, pneumococcus, catarrhal micrococcus, staphylococcus, and streptococcus predominate; in the latter—Bacillus coli, Bac. proteus, Bac. perfringens, spirochetes, etc. The mechanism of self-defense of the macroorganism and 'self-cleansing' of the body differs depending on which body cavity we are dealing with. Along with the mechanical method of self-cleansing (removal of microbes by the flow of mucus, with feces, with lacrimal fluid, etc., by the action of the cilia of the ciliated epithelium), the reactions of the environment, the lack of nutrient material, the action of bactericidal substances (lysozyme), etc., are of great importance in this regard. 1. Microflora of the oral cavity. The conditions for the existence and development of microbes in the oral cavity are highly favorable, and thanks to this, the mouth represents a body cavity with the richest and most diverse flora. These conditions are as follows: the presence of nutrient material, constant humidity, suitable environmental reaction, favorable temperature, and sufficient influx of O₂. The nutrient substrate consists of mucus, saliva, desquamated epithelium, dental pulp, and especially food residues getting stuck in the spaces between the teeth. Saliva provides the microbes with water and salts, and food residues and waste products of the dental pulp provide proteins and carbohydrates; often being in a state of irritation and mild inflammation, the dental pulp secretes serum, which is an excellent nutrient material. The reaction of the environment in the oral cavity is neutral or slightly alkaline, highly favorable for most bacteria; the temperature on average equals 37°; under the tongue, it has a constant character and corresponds to body temperature; between the jaws and cheeks, in the vestibule of the mouth, it is lower and not constant; here, thanks to this, there are conditions for the existence of bacteria whose temperature optimum is below 37°. The influx of O₂ in the oral cavity is very large, and thanks to this, even strict aerobes find favorable conditions for existence here. On the other hand, in the cavity of the teeth, there are also conditions ensuring the development of anaerobes. According to some authors (van der Reis and others), saliva is devoid of bactericidal properties; however, according to the latest research, lysozyme (see) is present in saliva, and therefore it is impossible to deny with absolute certainty the possibility of its bacteria-killing action. The microbes of the mouth are naturally divided into two categories: some constitute its usual flora (permanent microbes of the mouth) and are found in every person; others get there accidentally depending on the type of food and other external conditions (accidental inhabitants of the mouth). The permanent microbes include the group Leptothrix buccalis (see), bacteria, vibrios, spirochetes, cocci, and some animal protozoan microbes. Among the bacilli, one must name Bacillus maximus buccalis—a very large, thick, non-motile, Gram-positive rod; some forms related to it form streptobacilli, and sometimes a fusiform rod (Bac. fusiformis, Bac. Vincenti) is encountered—a non-motile rod with pointed ends and a thickened middle part, which often does not take up ordinary staining, which gives the microbe the appearance of a diplobacillus (two triangles facing each other with their bases). The size of the rod is not identical: there are forms of 3-4 μm and along with them rods of 10-30 μm. It does not stain by Gram; it does not form spores; it stains well by Giemsa. The growth of the fusiform rod on sugar broth and agar with the addition of ascitic fluid or blood serum was obtained for the first time by Levkovich (1902) under conditions of anaerobiosis. The microbe is sensitive to the action of high temperature and is not very stable. In symbiosis with a spirochete, it has etiological significance in ulcerative processes (Vincent's angina, stomatitis). Among the cocci, the following are encountered: Jodococcus magnus, Jodococcus parvus, Micrococcus rosaceus, Streptococcus brevis, Streptococcus longus, Streptococcus salivarius, capsular diplococcus, Staphylococcus salivarius. Iodococci form small chains enclosed in one common sheath; these cocci contain starch and, as a result, stain blue with Lugol's solution. The capsular diplococcus is similar to pneumococcus but is not virulent to animals. Streptococcus salivarius has the appearance of a diplococcus or forms very short chains. This microbe is the causative agent of lactic acid fermentation and takes part in the carious process of teeth. Among the spirochetes in the oral cavity, 3 species are encountered (Mühlens, Hartmann): Sp. dentium, Sp. denticola, Sp. buccalis, and Sp. media. The first is thin and forms regular, even, and deep curves; the second forms large, uneven curves. Spirochetes do not stain by Gram and are very motile. Spirochaeta dentium was obtained by Mühlens in 1906 in pure culture. Noguchi grew spirochetes on sheep serum and then on serum agar; he obtained cultures of Sp. macrodentium and Sp. microdentium in the process; the pure culture of Mühlens, in Noguchi's opinion, represents a mixed culture of these two spirochetes. Repaci grew a spirochete in anaerobic conditions, intermediate between Sp. buccalis and dentium. Hoffmann added one more species, which he named Sp. trimerodonta. A classification of oral spirochetes does not yet exist; at the present time, the most rational is the morphological division of Hoffmann into the following species: 1. Sp. buccalis (Cohn): a) crassa, b) tenuis, c) inaequalis, 2. Sp. media oris (Hoffmann and Prowazek), 3. Spirochaetae: a) dentium orthodonta, b) scoliodonta (Hoffmann), and c) trimerodonta (Hoffmann). Of these, Sp. buccalis tenuis and Sp. dent. orthodonta are forms encountered in Vincent's angina in symbiosis with fusiform rods. Here, one can also point to animal parasites, although they do not belong to the microflora: Entamoeba buccalis, Entamoeba maxillaris, Protozoon Ellermann, Protozoon Baumgartner. Williams, Sholly, Rosenberg, and Mann, who examined 1,678 schoolchildren aged 5 to 16, found amoebae 966 times; Goadby and Wellings believe that amoebae exist in the oral cavity only in the presence of food residues; these residues, as well as pathogenic microbes, are eliminated by amoebae. According to Fischer, there is no connection between the presence of Entamoeba buccalis and the unhygienic state of the oral cavity; their presence depends on the time of year and weather conditions.

The majority of bacteria inhabiting the oral cavity are devoid of pathogenic properties and are saprophytes; this does not, however, exclude the fact that some of them possess pathogenic action. The role of oral bacteria is most significant in the process of dental caries (see Caries). Besides permanent inhabitants, various pathogenic microbes often enter the oral cavity as accidental guests; thanks to this, the oral cavity can in certain cases serve as a portal of entry for infections; of greatest importance in this regard are the lymph follicles, as well as carious teeth. Among the pathogenic microbes that accidentally enter the oral cavity, one must name the influenza bacillus, pneumococcus, and the tubercle bacillus. Often they remain in the oral cavity for a long time without manifesting pathogenic action. 2. Microflora of the stomach. The acidic reaction of the stomach contents, depending on the presence of free HCl in it, creates conditions unfavorable for the existence and reproduction of bacteria. Thanks to this, there are significantly fewer bacteria in the stomach than in the oral cavity and especially in the large intestine. Among the bacteria encountered in gastric contents, bacilli predominate; among them, one must name Bac. mesentericus, Bac. subtilis, Bact. coli commune, and Bac. lactis aerogenes. Among other forms, sarcinae and enterococci are almost constant inhabitants of the stomach. Several species of sarcina are encountered, most often Sarcina ventriculi (slightly yellow in color) and Sarcina flava (yellow). Yeast cells, mold fungi, etc., are also found. The stomach contains the most bacteria immediately after the intake of food. With a high content of HCl in the gastric contents (hyperaciditas), the stomach can be completely free of microbes; with reduced acidity, Bact. coli commune, enterococci, and yeast are determined in it with the greatest constancy. 3. Microflora of the small and large intestines - see Intestine, intestinal flora. 4. Microflora of the normal conjunctival sac. The conjunctival sac communicates with the external air, and therefore the conditions for the entry of microbes into it are quite favorable. Despite this, the flora of the conjunctival sac is not particularly rich, and in any case, there are fewer microbes in it than on the corresponding edges of the eyelid skin. On the other hand, this flora is distinguished by a certain uniformity; this testifies to the fact that many microbes, undoubtedly entering the conjunctival sac from the surrounding world, find unfavorable conditions for their existence in it. It is necessary to assume the presence in the conjunctival sac of certain adaptations protecting it from contamination by microbes from the outside. These adaptations are as follows: mechanical removal of bacteria through the nasolacrimal duct into the nasal cavity by the flow of lacrimal fluid; the temperature of the conjunctival sac, which is not very suitable for the development of the majority of bacteria; the absence in it of the nutrient material necessary for this development. To the number of these protective factors, one must also add the direct bactericidal action of tears, manifested by them in relation to a whole series of saprophytes and depending on the presence of lysozyme (see) in tears in significant quantity. Among the microbes most often encountered in the conjunctival sac are Bact. xerosis and white staphylococcus. The xerosis bacillus (Corynebacterium xerosis) is morphologically very similar to the diphtheria bacillus, but is monomorphic; it contains polar granules (Babes-Ernst) that are clearly visible upon staining, and is Gram-positive. Of the sugars, it decomposes glucose and sucrose, but does not decompose galactose; it is aerobic; it is not virulent for guinea pigs. The xerosis bacillus received its name because it was first found in the eye during xerosis conjunctivae (Kuschbert and Neisser; 1884), but it has no etiological significance for this process. It is encountered very often in the normal conjunctival sac; for instance, Rymovich discovered it 94 times in 100 cases, and Heinersdorff 83 times. Pillat found the xerosis bacillus on the eyelids in 96.9%, on the conjunctiva of the eyelids in 81.9%, and on the conjunctiva of the eyeball in 81.2%; the corresponding figures for staphylococcus were 71.9%, 65.6%, and 62%. These newest data generally coincide with the figures of Rymovich and Heinersdorff. Staphyl. albus is also a frequent inhabitant of the normal conjunctiva; in his 100 cases, Rymovich discovered it 79 times, and Heinersdorff 85 times. Besides Bact. xerosis and staphylococci, Staph. pyogen. aur., pneumococcus, sarcinae, and yeast and mold fungi are found on the conjunctiva. Golden staphylococcus and pneumococcus are discovered in 6-8% of cases, but Pillat, using the Lindner method ("Epithelabstrich" - examination of an epithelial smear), found pneumococci on the conjunctiva of the eyeball in 37.5%, whereas on the conjunctiva of the eyelids they were discovered in 1%, and on the skin of the eyelids they were absent altogether. Such inhabitants of the conjunctival sac as Staphylococcus albus, aureus, and pneumococci can sometimes be the cause of diseases of the eyelids, conjunctiva, and cornea. 5. Microflora of the nasal cavity. In the nasal cavity, microbes are encountered in small quantities; the nose is, as it were, a filter that cleans the air penetrating into it of bacteria. Thus, in the experiments of Thomson and Hewlett, air that contained 29 fungal spores and 9 bacteria in 1 cm³ proved to be sterile after passing through the nose; according to Wright, the air we inhale loses 3/4-4/5 of the microbes contained in it in the nose. Among the adaptations that rid the nose of microbes are the hairs located in the nostrils, the ciliated epithelium covering its mucosa, and the constant flow of mucus, which moves from the inside to the outside and carries with it the microbes located on the mucosa. The role of leukocytes, which engulf bacteria, is comparatively not great. The bactericidal capacity of nasal mucus has very substantial significance, although literary data on this question are generally contradictory; thus, according to Klemperer, Park, and Wright, nasal mucus is not capable of killing bacteria. On the other hand, Malato Calvino insists on the bacteria-killing capacity of the mucus. Schousboe, whose experiments are distinguished by great thoroughness, found that some bacterial species are killed by nasal mucus, the growth of others is slowed, while on a third group this mucus exerts no action at all. These data represent the key to solving the question of the action of nasal mucus on microbes; they should be compared with the data cited above (Microflora of the normal conjunctival sac) concerning lysozyme; nasal mucus is rich in it, but lysozyme acts differently on different species of microbes. The poverty of the nasal cavity in microbes also depends on the fact that nasal mucus is a poor nutrient substrate for microbes; exceptions are leprosy bacilli (Gerber, Kuster), and diphtheria and pseudo-diphtheria bacilli, for which nasal mucus represents a favorable nutrient medium. Turning to the question of the frequency of finding microbes in the nasal cavity, it must be noted that a whole series of authors obtained negative or close to negative results in such an examination, whereas other researchers had positive results. In general, the microflora of the nose is not constant; among the various microbes encountered in the nose, one or another pathogenic microorganism is not rare. 6. Microflora of the respiratory tract and lungs. In the opinion of the majority of authors, the lungs are sterile (Klipstein, Muller, Bartel); other authors believe that bacteria penetrate into the lungs with inhaled air, but then they quickly perish in the tracheobronchial glands (Arnold) or in the pulmonary tissue (Paul). According to the experiments of G. E. Platonov on rabbits, there were no bacteria in the lungs and bronchi; in the lower section of the trachea, isolated microbes were discovered, whereas in the culture of mucus from the larynx there were quite a few of them. Analogous results were obtained by other researchers as well. As for the species of bacteria encountered in the respiratory tract, it is most often a matter of staphylococci, catarrhal micrococcus, streptococcus, etc. It is necessary to note the presence of various species of micrococci, and they are encountered most often in the nasopharynx. Such are Microc. catarrhalis, Diploc. pharyng. cinereus, Dipl. siccus, Dipl. flavus, Microc. crassus (see Micrococci). 7. Microflora of the vagina - see Vagina. 8. Microflora of the urethra. In the anterior part of the urethra, both in men and in women, microbes are always contained; in the deep parts, they are usually absent. Among the microbes in the urethra, cocci are encountered most often: streptococcus (Streptococcus giganteus urethr. of Lustgarten and Mannaberg), Gram-positive diplococcus, diplostreptococcus (Finkelstein) which is Gram-positive and often possesses pathogenicity for both humans and certain animals (white mouse), and staphylococcus. Among bacillary forms, Pfeiffer's streptobacillus (a Gram-positive saprophyte), pseudo-diphtheria bacilli, and also acid-fast microbes (Bac. smegmae, Bac. Marpmann'a) were often found. As accidental inhabitants, Bac. perfringens and others have been described. 9. Microflora of the skin.

The most diverse types of microbes, which land on the skin from the air, water, and also partly from human secretions, find unfavorable conditions for existence in it and consequently multiply poorly; they are most numerous where humidity is high (skin folds, inner surface of the thighs, axillary cavity, etc.). Nevertheless, thanks to the constant influx of bacteria from the outside world, the skin always contains many microbes, but this flora is variable, just as the environment surrounding the skin is inconstant. The most frequent inhabitants of the skin are staphylococci, streptococci, pseudodiphtheria bacilli, diphtheroids; mold and yeast fungi are also encountered. Among the microbes found exclusively on the skin, one should mention the seborrhea microbacillus (described by Unna and Sabouraud) and Unna's bottle bacillus (Flaschenbacillus Unna). The seborrhea microbacillus is a bacillus (0.5–1.5 x 0.3 µ), which stains by Gram; it causes an inflammatory process near the hair roots. The bottle bacillus is an oval cell with a small head at one of the ends; it is polymorphic (size from 3 to 15 µ); it is found together with a large coccus in various skin diseases. The deep layers of the skin are usually free of microbes; among the species found there, staphylococcus (Staph. albus, aureus, citreus, etc.) should be noted. In recent Russian literature, the question of the microflora of the skin, in particular the flora of the hand, is linked to industrial factors. According to data from V. I. Rozhansky and M. E. Khesina-Lurie, each type of production is characterized by its own "bacterial formula" with a different quantity of bacteria and different types. Thus, in workers at a distillery, the hay bacillus predominates, followed by air cocci and staphylococcus. In workers at metallurgical plants, staphylococcus occupies first place, followed by the hay bacillus, etc. 10. Microflora of the blood, bile, and urine. Practically, these fluids are sterile, but in known cases, microbes are discovered in them. In the blood, microbes have been found during the digestion of food (Nocard). In clinically non-tuberculous individuals, the blood may sometimes contain tubercle bacilli; from the blood, they penetrate into the bile, the fluid with which these microbes (in faeces) are excreted from the organism. Streptococci from a local focus (carious tooth, etc.) often penetrate into the blood in small quantities without causing bacteremia. A normal inhabitant of the intestine, Bac. perfringens, often gets from there into the blood during various infectious diseases. Microbes usually penetrate into the urine from the blood; they can also enter the bile from the intestine (enterococcus, Bacillus coli). In cases of bacillus carriage, typhoid and paratyphoid bacilli can remain in the bile for a long time.

V. Lyubarsky.