GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND INHERITANCE OF TRAITS

19.3. INHERITANCE OF TRAITS AND GENETIC RECOMBINATION

19.3.6. Plasmids

Many Bacteria (if not all) can harbor extrachromosomal DNA elements. These small, circular, double-stranded DNA molecules, compared to the bacterial chromosome, are called Plasmids. When cultivated under normal conditions, bacteria can survive without plasmids: Cells cured of plasmids through UV irradiation, mitomycin C, or acridine dye Treatment grow well on standard nutrient media. Plasmids are recognized by the specific traits they confer upon the host Cell.

Fertility factors (F-factors). Some plasmids enable a cell to conjugate with other cells. We encountered such plasmids when studying conjugation. These plasmids can integrate into the bacterial chromosome, mobilize its Genetic information, and facilitate its transfer to another cell.

Resistance factors (R-factors). Bacteria resistant to certain Antibiotics were first discovered in the 1950s in Japan. These were strains of the dysentery pathogen Shigella isolated from patients treated with antibiotics. A notable characteristic was that these bacteria exhibited multiple resistance, which could be transferred to other bacteria. As research revealed, resistance factors contain genes that make The Cell resistant to sulfonamides, streptomycin, kanamycin, tetracycline, etc. For instance, the R68.45 plasmid encodes resistance to three antibiotics: ampicillin (300 mcg/ml), kanamycin (100 mcg/ml), and tetracycline (40 mcg/ml). Some R-factors confer resistance to as many as eight antibiotics simultaneously, while others confer resistance to heavy toxic metals. An R-plasmid carries two groups of genes: 1) genes responsible for plasmid transfer via conjugation (tra genes), which form the so-called "resistance transfer factor" (RTF); 2) genes that directly determine resistance (constituting a small fraction of the plasmid).

The resistance transfer factor (RTF factor) combines all the genes responsible for the cell-to-cell transfer of the R-factor via conjugation. Thus, the R-factor, much like the F-factor, is infectious. Certain R-factors exhibit a wide host range—they can be transferred among several bacterial genera, significantly promoting their dissemination.

The Mechanism of Antibiotic Resistance conferred by R-factors may differ from that of chromosomal inheritance. If streptomycin resistance depends on a chromosomal Gene, it is associated with an alteration in the 30S ribosomal subunit, meaning the bacterium lacks the target for the antibiotic. In contrast, R-factor-mediated resistance is based on the inactivation of the antibiotic through adenylylation driven by Enzymes. Enzymatic chemical modification of antibiotics is frequently the cause of plasmid-encoded resistance. In the presence of R-factors, genetic recombination may occur, resulting in a novel gene combination capable of conferring additional antibiotic resistance. Therefore, the existence of R-factors serves as further evidence against the indiscriminate use of antibiotics, as R-factors can spread through bacterial populations just like infectious agents.

Bacteriocins. Many bacteria synthesize Proteins capable of killing or inhibiting the growth of closely related strains and species. These proteins—bacteriocins—are encoded by specific plasmids known as bacteriocinogenic plasmids. Bacteriocins have been isolated from Escherichia coli (colicins), Pseudomonas aeruginosa (pyocins), and Bacillus megaterium (megacins).

Other traits encoded by plasmids. Genes for enzymes required to degrade unusual substrates (such as naphthalene, camphor, salicylic acid, octane, etc.) can be harbored on plasmids. Nitrogen Fixation, nodule formation, indole-3-acetic acid synthesis, and Hydrogenase production represent just a partial list of properties encoded by plasmids.

Incompatibility. Many bacteria contain plasmids of various sizes. The coexistence of different plasmids within a single cell indicates that these plasmids are compatible. However, two closely related plasmids cannot coexist in the same cell—they are incompatible. All plasmids are classified into incompatibility groups. Plasmids belonging to the same group are mutually incompatible.