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Oral cavity bacteria: reservoir of determinants of antibiotic resistance

The upper respiratory tract, including the nose, oral cavity, nasopharynx and oropharynx, are colonized by a wide range of gram-positive and gram-negative flora, aerobes lacking the cell wall, as well as micro- anaerobic organisms. The composition of the microflora of the oral cavity is dynamic and varies according to the age, the hormonal context, the diet and the general state of health of the individual. In addition, a large number of various microorganisms are constantly sucked from the outside into the respiratory tract and enter the gastrointestinal tract. The exact species composition of the microflora in the oral cavity varies considerably from person to person, as well as from the same individual at different times. In total, up to 300 different types of microorganisms are isolated from periodontal pockets, and up to 100 species can be isolated from a site.

Such a variety of microorganisms represents optimal opportunities for the transmission of resistance determinants, the reservoir of which is the normal human microflora. The possibility of exchanging genetic information between bacteria in the genitourinary tract and the oral cavity has been demonstrated; and in laboratory conditions - between species of taxonomically distant microorganisms. The prophylactic use of antibiotics in cases where they are not indicated (dental procedures, periodontal diseases, oral abscesses) makes a significant contribution to this process. The most commonly prescribed antibiotics for these indications are beta-lactams, tetracyclines and metronidazole. Macrolides, clindamycin and fluoroquinolones are less used, aminoglycosides are generally not used.

The first beta-lactamase in oral bacteria was described on a plasmid of Haemophilus influenzae in the early 1970s. It was found to be identical to TEM-1, first described in E.coli. The TEM-1 enzyme has also been described in Haemophilus parainfluenzae, Haemophilus paraphrohaemolyticus and other Haemophilus species. This enzyme is generally found on high molecular weight conjugate plasmids specific to the genus Haemophilus, which also carry the determinants of resistance to chloramphenicol, aminoglycosides and tetracycline.

Around the same time, TEM-1 beta-lactamase appeared on plasmids in strains of Neisseria gonorrhoeae; these plasmids could be transmitted to other strains. These plasmids are closely linked to the low molecular weight plasmids H.ducreyi and H.parainfluenzae. It is assumed that H.parainfluenzae may be the most likely source of plasmids encoding the beta-lactamase genes. Periodically, reports indicate the discovery of similar plasmids in Neisseria meningitidis; however, no strain isolated in vivo has been subjected to independent analysis. At the same time, the possibility of conjugative transfer of a plasmid coding for beta-lactamases from N..gonorrhoeae to N.meningitidis has been demonstrated in the laboratory..

Reports indicate the discovery of TEM beta-lactamases in many non-pathogenic Neisseria species (Table 1), whose genes are generally located on low molecular weight plasmids genetically closer to plasmid RSF1010 E. coli than gonococcal plasmids. Plasmids linked to RSF1010 can also code for the genes for resistance to sulfanilamide and streptomycin. Larger plasmids encoding genes for resistance to tetracyclines, aminoglycosides and TEM beta-lactamase genes have been described in N.sicca. Multiple resistance strains of Moraxella catarrhalis have been identified at the Centers for Disease Control as non-pathogenic neisseria.

ROB genes of beta-lactamases in H. influenzae have been found on low molecular weight plasmids, which are almost identical to those of animal pathogens from animal microorganisms: Actinobacillusspp. and Pasteurella spp.

Recently, beta-lactamases have been found in mandatory gram-negative anaerobes: Bacteroides forsythus, Fusobacterium nucleatum, Porphyromonas asaccharolytica, Prevotella spp., Veillonella spp. Only an insignificant part of them has been characterized (Table 1) and the localization of the genes (plasmid or chromosome) has not been determined.

Resistance to penicillin in microorganisms that transform easily in vivo (Haemophilus, Neisseria, Streptococcus) may be associated with the replacement of certain genes coding for penicillin binding proteins (PSB), corresponding regions of the genome of resistant microorganisms. This resistance mechanism is less common than the enzyme, associated with the production of beta-lactamases. In N.meningitidis, the regions of the genome that determine resistance are close to commensal genes such as N.flavescens and N.cinerea. One of the PSA genes (penA) was found to be unusually heterogeneous: in 78 strains studied, 30 different mosaic genes were described. The mosaic PSB of S. pneumoniae contains sites obtained from S.mitis and other streptococci.

Another non-enzymatic resistance mechanism found in methicillin-resistant S.aureus is the presence of the dude A gene, a genetic determinant that codes for the additional PSB (called PSB2a) with low affinity for beta-lactams. The gene is located on a 30-40 kb DNA fragment and codes for resistance to all beta-lactams. When screening for the presence of the mec A gene in 15 different types of staphylococci, hybridization was detected in 150 S.sciuri strains. As all S.sciuri strains are not resistant to penicillins, a gene homologous to dude A gene probably fulfills a certain physiological function in this type of staphylococcus which is not not linked to beta-lactamam resistance.

18 determinants have been described which code for resistance to tetracycline through two main mechanisms: active elimination of the antibiotic from the microbial cell and protection of the ribosome. The distribution of the various Tet determinants varies considerably, which is partly due to the ease of transmission of specific determinants between strains and species. The TetB gene, which codes for the active excretion of antibiotics in Gram-negative microorganisms, is the most common and is found in a number of oral bacteria (Table 2). Periodontal disease is caused by both A.actinomycetemcomitans and T.denticola. The TetB determinant is detected on the Actinobacillus and Haemophilus conjugate plasmids. Plasmids carrying the tet (B) determinants of A.actinomycetemcomitans can be transferred to H. influenzae. The determinants of TetB from a small number of strains studied Moraxella and Treponema could not be mobilized.

Recently, we have been able to detect genes characteristic of gram-positive microorganisms coding for the active elimination of tetracyclines in certain gram-negative bacteria of the oral cavity (Table 2). The strain Haemophilus aphrophilus isolated from a patient with periodontal disease in 1990 had the tet (K) gene. Tet (L) or tet (Q) genes have been found in some V.parvula strains, however, most strains had the determinant tet (M). In streptococci isolated from the oral cavity, the tet (M), tet (Q), tet (K), tet (L) genes were found both in isolated form and in combination (Table 3). Recently, other genes protecting the ribosome have been found in enterococci. The determinant TetS was found in S.milleri; furthermore, tetracycline-resistant streptococci which do not possess any of the known tet genes have been isolated. The determinant Tet (M), which codes for a protein associated with ribosomes, is widespread among gram-positive and gram-negative bacteria (Table 2, Table 3).

Tetracycline resistant streptococci not shown in the table have Tet K, L, M, O or other unknown determinants. Some types of urogenital peptostreptococci have Tet K, L, M, O. It is possible that these four determinants are present in peptostreptococci living in the oral cavity.

The determinant tet (Q), first detected in Bacteroides of the large intestine, is generally isolated from Gram-negative anaerobes linked to bacteria (for example, in Pretella) (Table 2). Tet (Q) genes have been found in several strains of V.parvula, however, tet (M) genes were characteristic of most strains. The Mitsuokella and Capnocytophaga strains generally contain tet (Q) genes.

Resistance to metronidazole is found in a number of oral bacteria, but its genetic mechanism is unknown. In Bacteroides spp. from the colon, the nimA, nimB, nimC, nimD genes are described and sequenced. They are located either on the chromosome or on various plasmids. The nim genes probably code for 5-nitroimidazole reductase, which reduces 5-nitroimidazole to 5-amino derivatives.

Enzymes that acetylate, phosphorylate or adenylate aminoglycosides have been found in pneumococci, streptococci, staphylococci and, more recently, in saprophytic pastries and hemophiliacs. A strain C.ochraceus has been found to be resistant to aminoglycosides, chloramphenicol and tetracycline.

The first erythromycin-resistant S.pneumoniae strains possessed an ErmB class rRNA, which modifies the only adenine residue in 23S RNA and ensures resistance to macrolides, linkcosamides and streptogramin B We found RNA methylase in the strains A. actinomycetemcomitans and Campylobacter rectus. The genes encoding the methylase RNA in the two strains were located on conjugated plasmids; they could have been transferred to Enterococcus faecalis and from A.actinomycetemcomitans to H. influenzae. Many other microorganisms in the oral cavity are known to be resistant to erythromycin or clindamycin.

Oral bacteria are an important reservoir of determinants of antibiotic resistance. The danger of their appearance reflects the excessive or unjustified use of antibiotics, which creates prerequisites for the transfer of the determinants of resistance to more pathogenic species.