Mycobacterium tuberculosis (MBT) belongs to the family of Mycobacteriacae, of order Actinomycetalis, and genus Micobacterium.
Etymologically, “mycobacterium” is derived from the Greek words ‘myces’ for fungus and ‘bakterium’ meaning small rod. The name “fungus” is derived because of the tendency of these mi-croorganisms to spread diffusely over the surface of liquid medium in a mold like growth pattern. According to the modern concept of clinical medicine, the term “Mycobacterium tuberculosis” which was discovered by the German scientist Robert Koch in the year 1882, unites the complex of four kinds of mycobacterium: including M. tuberculosis (MBT), M.bovis and its Bacille Calmette-Guerin (BCG) variant, M.africanum and M.microti. There is a high degree of genetic relatedness in this group. Micobacterium tuberculosis (MBT) is the major cause of tuberculosis in man. M.bovis and M.africanum can cause a disease clinically indistinguishable from classical tuberculosis. M.microti is not considered to be pathogenic for human beings but it can cause a disease resembling tuberculosis in rats. BCG is not pathogenic for humans.
The given materials about “Tuberculosis” in this textbook refers only to the disease caused by M. tuberculosis (MBT), tubercle bacilli of Koch (BK).
Mycobacterium tuberculosis (MBT) – Facultative intracellular parasite.
The natural reservoir – human, domestic and wild animals, birds.
MBT are slender, curved rods that are resistant to acids, alkalis, and dehydration. The cell wall contains complex waxes and glycolipids.
MBT can multiply in macrophages, as well as extracellularly, in tissues of different organs.
Multiplication of MBT is comparatively slow, by means of simple cell division. On enriched media, multiplication of MBT doubles and lasts from 18 to 24 hours. Clinical strains of MBT may require 4 to 6 weeks to grow in such a medium.
Genetic structure of MBT.
The sequence and annotation has been published in the international databases. The sequence of MBT is 4,411,529 b.p long. MBT do not have ability to move. The temperature limits of growth are between 29-42 C (optimum between 37-38 C). MBT have resistance to physical and chemical agents. MBT can survive at very low temperatures, as well as at a high temperature of 80° C for a period of 5 minutes. MBT is rather resistant to external environment. In water it can survive for about 150 days. Dried up MBT can cause tuberculosis to Guinea pigs in 1 -1,5 year, lyophilized and frozen are viable for 30 years. The viability of MBT is sharply reduced at an intensive sunlight and at high temperature of the environment. On the contrary, in darkness and dampness, viability of MBT is rather significant. Outside the body, they remain alive for many months, particularly in dark, damp rooms. MBT has acid resistance (acid-fastness) that differs them from many other causative agents of the disease. Acid-fastness becomes apparent at preservation of the coloring even after decoloration by acids, alkali, and alcohol. These properties are resulting from high content of micolic acid and lipid content of their cell walls.
Morphological changes of MBT.
The morphology and the sizes of MBT considerably change, depending on the age of bacteria and, especially, on the condition of the existence and content of cultures.
The cord-factor.
The lipids of the external membrane of MBT determine its virulence and its capacity to form plait-like congestions (cord-factor) in culture. Koch noted about the cord factor in his initial report on the etiological agent of tuberculosis. First of all the cord factor was associated with virulence of M. tuberculosis. Formation of plait-like congestions was subsequently observed to occur among other mycobacterial species of lesser or having no virulence. Cord factor, later identified as a highly unusual biological compound, trehalose 6,6-dimycolate, was observed to cause highly virulence, often lethal consequences when injected into experimental animals. However, the role of this compound in the pathogenesis of tuberculosis is unclear. Resistance of MBT to acids, alkalis and alcohols is related to the lipid fraction of the external layer of MBT ’s membrane.
L-forms.
One of the important features of MBT is its ability to produce L-forms The L-forms are characterized by reduced level of metabolism and weak virulence. Remaining viable L-forms can long time survive in the host and produce anti-tuberculosis immunity. The L-form differs from usual MBT by the expressed functional and morphological alterations. It is discovered, that the transformation of MBT into the L-forms accelerated under long anti bacterial therapy and under other factors, which inhibit the MBT growth, duplication and cell membrane formation.
It is established, that in the sputum of “MBT-negative” patients with destructive form of tuberculosis L-forms could be found, capable, under the appropriate conditions, to be modified in rode-like variant which may cause reactivation of the tubercular process. Hence, elimination of MBT from cavities of such patients yet does not mean their sterilization from MBT.
MBT is tolerant to many antibiotics.
This property is connected first of all with highly hydrophobic cell surface, which serves as a physical barrier for chemical agents and antibiotics. The main reason of resistance is coded in the MBT genome. MBT can develop tolerance (resistance) against the action of antituberculosis drugs. Simultaneously resistance of the MBT to several antituberculosis drugs considerably reduces efficiency of tuberculosis treatment in the last years.
The laboratory researches have shown that the occurrence of MBT resistance is connected with nucleotide replacements (mutations) in genes, encoding various enzymes, which directly influence with drugs. For example, the mutations of rpoB gene, coding beta-subunit of RNA-polymerase (in a fragment of length 81 nucleotide pairs), in 96 % of cases result in resistance of MBT to Rifampicin. Mutations in the gene katG, resulting in replacement of some amino acids in enzymes catalase and peroxidase, and nucleotide replacements in regulatory and complicated coding areas of the loci inhA are associated with resistance of some MTB strains to Isoniazid. The resistance of MTB to Streptomycin is connected with missens mutation in a gene rpsL, coding S12 mitochondrial protein, or with nucleotide replacements in a gene rrs, coding 16S RNA.
As a result, the modern public health services, do not simply deal with the dangerous causative agent of tuberculosis but also with the whole set of strains which has resistance against different drugs. In practice, for good organization of effective tuberculosis treatment, it is important not only to discover MBT, but at the same time to determine its resistance fast within two-three days so as to prescribe an effective chemotherapy.
At the end of 80s of the last century in hands of the researchers the method has appeared, which has helped them significantly to shorten time of such analysis. New diagnostics is based on selective amplification of nucleic acids (DNA or RNA) in vitro with the help of polymerase chain reaction (PCR). This PCR method has the large opportunities and serves as the base of exact DNA-diagnostics, which allows to identify any strain of MBT and to define the reason of the drug resistance.
These data open new opportunities to design antimicrobial drugs, vaccines, and other elements able to change immune reactions against this devastating pathogen.
Above submitted mutations in MBT genome are the only limited examples of formation of its resistance to anti-tuberculosis drugs. On this basis it is possible to make the following conclusion: introduction of new drugs in chemotherapy of tuberculosis leads to new mutations in МBT, resulting to resistance without exception of all used drugs and in this circumstance it is necessary constantly to take into consideration about tactics of tuberculosis treatment.