Plasmids and Episomes
Plasmids and Episomes
- Genetic elements apart of nucleoid, which possess capacity of independent replication with high incidence of transmission, are termed plasmids and episomes.
- It is considered that episomes are able to integrate with nucleoid, whereas plasmids not. Plasmids were discovered in 1958 by F. Jacob and E. Wollman.
- These elements play a definite role in the evolution of bacteria. In large part due to the broad use of antibacterial agents, the natural environment of bacterial habitation harshly changes.
- To withstand these unfavorable conditions, many representatives of pathogenic and non-pathogenic microflora acquire drug resistance.
- Despite the high variability of resistance mechanisms, they rapidly spread among the bacteria owing to the permanent exchange of genetic transmissible elements, such as plasmids.
- Most of bacteria contain one or more of different plasmids. Plasmids vary in size from a few genes to several hundred. The properties, encoded by the plasmids, endow the bacterial cell with many useful adaptive properties.
- Plasmids can possess infectivity, being transmissible from one cell to another.
- The group of plasmids and episomes includes the fertility factor, the resistance transfer factor that controls the multiple bacterial resistance to antibiotics and other drugs, the factor of bacteriocinogenesis, the hemolytic and enterotoxigenic factors, and many others.
- The number of copies of a plasmid in the single bacterial cell can be various, depending on the plasmid nature. For instance, tetracycline resistance plasmid is a low copy number plasmid being present in only 1-2 copies per microbial cell. Other plasmids can exist in the cell in a quantity of more than 100 copies (high-copy number plasmids).
- Narrow host range plasmids can propagate only in a certain microbial species (e.g., F factor). But some other plasmids, termed as wide host range plasmids, can propagate in a great variety of different species of bacteria.
- Some plasmids and episomes (e.g. R or F factors) contain the information necessary for their transfer from one bacterium to another by conjugation. They are named conjugative plasmids.
- Conjugative plasmids carry so-called tra-operon, which encodes structural elements (sex-pili, different proteins), responsible for conjugation. These plasmids are self-transmissible. Others are non-conjugative plasmids that are non-transmissible.
Fertility factor (F factor)
This factor governs bacterial conjugation, and its action is described in details in the corresponding paragraph, devoted to conjugation (see below).
- Resistance plasmids, or R factors confer resistance to the great variety of antibiotics and metals (e.g., copper, arsenic or mercury).
- They are usually composed of two elements: a resistance transfer factor (RTF) that encodes the transfer of the plasmid by conjugation, and resistance genes (R genes), which control the resistance properties.
- The RTF part is based on tra-operon that governs the transfer of the plasmid into the recipient bacteria. Therefore, these plasmids are conjugative. R genes are responsible for the resistance to commonly used antibacterial drugs (sulfonamides, beta-lactams, aminoglycosides, tetracyclines), as well as to a number of different metals.
- They can encode antibiotic-degrading enzymes, capable of destroying the antibiotics: beta-lactamases for beta-lactam antibiotic hydrolysis, acetyltransferase for chloramphenicol inactivation, etc.
- R factors can be rapidly transferred to neighbouring sensitive bacteria of various species or genera (lateral or horizontal gene transfer), thus spreading the resistance among microbial cells.
- Many R factors can be transferred and reproduced in closely related bacteria (e.g. Enterobacteriacea family representatives – Escherichia, Shigella, Salmonella, Yersinia, Klebsiella, Serratia, or Proteus).
- Some R factors can also be transmitted to less related genera such as Pseudomonas or Vibrio. These R factors pertain to wide host range plasmids.
- Hence, R factor can develop interspecies transmissibility. It is obvious that easy transfer of R factors in some ecological niche spread the drug resistance to all of the susceptible inhabitants of the certain biotope. Thus many different microorganisms in a hospital environment become resistant to a wide number of antimicrobial agents.
The factors of bacteriocinogenesis
- These genetic elements encode the proteins with specific inhibitory activity towards various species of bacteria.
- The factors of bacteriocinogenesis are responsible for synthesis of different inhibitory proteins: colicins in E. coli (this genetic element is called Col-factor); staphylocines in staphylococci; vibriocines in Vibrio cholerae; pesticins in Yersinia pestis, the causative agent of plague; corynecins in Corynebacterium diphlheriae, etc.
- They are transferred by conjugation from bacteriocinogenic to non-bacteriocinogenic strain. Bacteriocin synthesis ensures selective ecological advantages for the bacterial cells.
- Bacteriocins produce severe disorders in affected bacterial cells, destroying target cell DNA or impairing their cell wall. For instance, bacteriocinogenic microflora of gut can suppress susceptible enteropathogenic bacteria.
- Therefore, the bacteriocinogenic property of normal gut microflora is the important factor that supports intestinal colonization resistance and blocks the ability of pathogenic bacteria to attach and colonize the intestinal wall.
Other types of plasmids
Many other types of plasmids can be produced by bacteria. They may encode certain virulence factors (like toxins, capsule, etc.)
Ent-plasmids as well as some temperate bacteriophages contain tox-genes, responsible for gram-negative bacteria enterotoxin production. K88 plasmid controls bacterial capsule synthesis. Hly plasmid is shown to encode hemolysins, produced by enterobacteria strains and streptococci.