Bacterial gene variation, gene transfer, and evolution. Flashcards
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Mechanisms of genetic variation
-Spontaneous Mutations
-Recombination
Antigenic variation
Genetic exchange between related organism
-Acquisition of new DNA segments- e.g. lateral
transfer from other bacteria (higher organisms??)
Transposable elements
(IS elements and complex transposons)
Bacteriophage conversion
Acquisition of plasmids
Acquisition of “pathogenicity islands”
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Spontaneous Mutation
Note:
Pseudomonas aeruginosa is a frequent and virulent pulmonary pathogen in patients with cystic fibrosis. If colonization is not prevented, P aeruginosa becomes permanently established and nearly always mutates into a mucoid strain. The alginate-containing matrix of the mucoid strain is thought to allow the formation of protected microcolonies and provide increased resistance to opsonization, phagocytosis, and destruction by antibiotics. As a result, conversion to the mucoid phenotype is associated with a significant increase in morbidity and mortality.
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Recombination
In Salmonella typhimurium, where it was first discovered, phase variation involves a relatively rapid (10-4 - 10-5), reversible switching in the synthesis of two alternative flagellar antigens (H1 and H2). As shown on the following page, the molecular switch that determines which flagellar gene will be transcribed is a small invertible segment of DNA within which lies the promoter of the H2 gene.
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Recombination
In Neisseria gonorrhoeae, phase variation involves the successive alternation between several antigenic forms of pili expressed on the cell surface. Each strain of gonococcus possesses an expressed copy of the pilin structural gene, plus multiple, silent, non-expressed copies of variant pilin genes. Recombinational exchange between the expressed and a non- expressed copy of the pilin genes results in a new pilin gene at the expression site and production of a new, antigenically distinct pili on the cell surface.
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Acquisition of new gene segments
What is a transposable element?
Acquisition of new DNA segments- e.g. lateral
transfer from other bacteria (higher organisms??)
-Transposable elements–>Transposable element (transposon) is a discrete segment of DNA contained within a bacterial or phage chromosome, or within a plasmid that has the property of being enzymatically moved from one DNA location to another.
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Acquisition of new gene segments
Bacteriophage conversion
Acquisition of new DNA segments- e.g. lateral
transfer from other bacteria (higher organisms??)
Bacteriophage conversion. Certain virulence genes (including those encoding diphtheria toxin, cholera toxin, streptococcal pyrogenic toxins, botulism toxins and certain LPS antigens) are carried on bacteriophage and are not a “normal” component of the respective bacterial genome. Therefore, the respective virulence factor is only carried and expressed by bacterial strains that have become lysogenized and the bacteriophage genome is stably maintained by the bacterium.
- Discuss how spontaneous mutation and selection can interact to determine the genetic composition of bacterial populations.
What is Spontaneous Mutation? These are single base changes, deletions and insertions occur spontaneously within a population.
–>Under appropriate selective pressure (e.g., a patient receiving streptomycin), the preferential growth of a pre-existing mutant within a population is selected.
Mutation rates. As in higher organisms, the rate of spontaneous mutation in bacteria is very low, typically in the range from 10-6 to 10-10 per cell- generation, depending on the phenotypic trait being studied.
Examples of mutations that are of medical importance include i) increased resistance to antimicrobials in Pseudomonas and Mycobacterium tuberculosis, and ii) Streptococcus pyogenes strains with an increased likelihood of causing invasive disease due to a single amino acid change in pyogenic exotoxin B.
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence
Acquisition of plasmids
Bacterial plasmids are autonomously replicating*, usually circular, extrachromosomal DNA’s ranging in size from a couple genes to a few percent the size of the bacterial chromosome. Often they can be transferred from one bacterium to another by conjugation or transduction (see below). Plasmids can carry virulence genes and genes conferring antibiotic resistance.
- Describe the mechanisms that generate genetic diversity within a bacterial species and how these contribute to the evolution of virulence.
Acquisition of pathogenic islands
-Pathogenicity Islands are generally relatively large segments of DNA present in the chromosome of some, but not all strains of a particular bacterial species.
PIs encode genes that contribute to the virulence of these isolates.
Bacterial isolates that lack the PI may be avirulent, or have a somewhat different disease-causing potential. It is not certain where PIs originated, or how they were first transferred to the ancestral cell. Acquisition from a heterologous organism is often implicated. In some cases, PIs contain site-specific recombination sequences similar to those of bacterial viruses, suggesting that they may have been transmitted by viruses.
- Distinguish between transformation, transduction and conjugation as mechanisms of gene transfer. Identify the salient features of each mechanism.
Transformation
What is the active component in transformation?
Transformation was the first example of bacterial gene transfer to be discovered. Very early studies by F. Griffith (1928) and later by Avery (1944) showed that crude extracts, and ultimately pure DNA, taken from virulent, encapsulated strains of the pneumococcus (S forms) could convert avirulent, nonencapsulated strains (R form) to the virulent phenotype.
Transformation has been observed in several different species of Gram-positive and Gram-negative bacteria.
The active component in transformation is naked DNA, probably DNA released from lysing cells. Transforming DNA can be either chromosome fragments or plasmids from the donor cell.
Many transformable species become competent for DNA uptake at only certain points in the growth cycle, and competence requires synthesis of specialized proteins to mediate the uptake. Some species of bacteria that are not normally transformed (e.g., E. coli) can be induced to become competent for transformation by treatment with **calcium chloride and **low temperature.
In nature, transformation probably occurs most frequently between members of the same species, even though competent cells readily take up heterologous DNA fragments, which may contribute to acquisition of new genes and genetic potential of a given species.
- Distinguish between transformation, transduction and conjugation as mechanisms of gene transfer. Identify the salient features of each mechanism.
Transduction
Transduction is gene transfer mediated by a bacteriophage. In transduction, bacterial viruses (bacteriophages) transfer segments of DNA (a couple genes up to a couple hundred genes) from one cell to another.
Prevalent in gram positive bacteria.
- Distinguish between transformation, transduction and conjugation as mechanisms of gene transfer. Identify the salient features of each mechanism.
Transduction
- Distinguish between transformation, transduction and conjugation as mechanisms of gene transfer. Identify the salient features of each mechanism.
Conjugation
Is a form of genetic transfer that is dependent upon physical contact between the donor and recipient cells, and is usually mediated by certain types of bacterial plasmids.
- Distinguish between transformation, transduction and conjugation as mechanisms of gene transfer. Identify the salient features of each mechanism.
Conjugation
The paradigm is conjugation mediated by the plasmid “F” (for “fertility”)
Plasmids “Extrachromosomal DNA elements”
• Conjugative Plasmids: self-transmissible, mediate their own transfer between cells.
• Non-Conjugative Plasmids
Mobilizable: can be passively transferred
during conjugation
Non-mobilizable
Non-conjugative plasmids may still be transferred by transformation or transduction
Characteristics of some bacterial plasmids
- Self-replicating, extrachromosomal DNA elements
- Often circular
- Range in size from couple genes to few % of chromosome
- Not essential for viability
- May encode a variety of functions, including:
–Resistance to antibiotics, heavy metals
–Virulence factors; e.g., toxins, adherence pili, capsule
–Metabolic functions e.g., sugar utilization, degradative pathways (tol)
– Self-transmission
- Discuss the properties of bacterial viruses. Distinguish between the lytic and lysogenic state.
What is the latent period of a virulent bacteriophage?
Which paghes do not kill their susceptible host cells?
Growth of “virulent” bacteriophage. Virus adsorbs to the bacterial cell surface and injects its nucleic acid into the cell. The viral genome is replicated, and the viral genes are transcribed and translated. This is termed the “latent period”, during which viral components are being synthesized, but no assembled, infectious virus particles are present within the infected cell. Once the components are synthesized, progeny virus are assembled, and are subsequently released (usually) upon lysis of the infected cell.
Temperate phages and lysogenic bacteria.
Certain phage species, the “temperate” bacteriophages, do not invariably kill their susceptible host cells. Infection by temperate phages may instead elicit either a lytic response, leading to phage multiplication and host cell lysis, or a lysogenic response, in which the host cell remains viable and the infecting phage DNA is maintained by the host cell in a noninfectious state known as prophage.
a. The prophage often consists of phage DNA which is linearly inserted into the host cell genome where it becomes passively replicated as part of the bacterial chromosome. As shown in the following diagram, a prophage can be induced enter the lytic state, resulting in viral replication, production of progeny virus and lysis of the infected cell. The lysogenic state is maintained by a prophage-encoded repressor protein that blocks expression of the phage genes necessary for viral DNA replication and lytic development. Under certain conditions, usually stress conditions, the prophage is induced to initiate lytic development with the production and release of new viral particles.
- Describe how errors in bacteriophage development can lead to phage-mediated gene transfer.
- Define lysogenic conversion. Distinguish between lysogenic conversion and generalized transduction
Bacteriophage conversion (lysogenic conversion).
Certain temperate bacteriophage encodes gene(s) which may be expressed during the lysogenic state and cause the appearance of a new phenotypic trait (e.g. toxin production in C. diphtheriae) in the lysogenic host. This process is bacteriophage conversion (lysogenic conversion). In lysogenic conversion the genes controlling the new phenotypic trait are found only as a component of the phage genome; that is, the converting genes are not found alone as normal constituents of the bacterial genome. Diphtheria toxin, scarlet fever toxin, cholera toxin and certain types of botulism toxin are all bacteriophage-encoded toxins and produced by strains of bacteria that have become lysogenized by the respective bacteriophage.
