Bacterial Genetic Variation, Gene Transfer and Evolution of Virulence

Question 1

Pathogenic strains have evolved by

Answer 1

the acquisition of new genes that allow them to survive and cause disease in human hosts.

Question 2

Regulation of transcription of bacteria

Answer 2

can increase or decrease in response to environmental conditions. Accomplished by DNA-binding proteins that interact with the promoter regions (the region of the gene in which RNA polymerase must interact prior to initiating transcription) of regulated genes. Many genes are regulated in response to concentration of free iron in the cells surrounding. Virulence genes (e.g. diptheria toxin) are often expressed only under conditions of low-iron-conditions that are encountered in the host.

Question 3

Control of transcription by DNA rearrangement

Answer 3

different type of control involving semistable mechanism known as PHASE variation.

Question 4

Salmonella phase variation

Answer 4

involves a relatively rapid (10^-4 - 10^-5), reversible switching in the synthesis of two alternative flagellar antigens (H1 and H2). The molecular switch that determines which flagellar gene will be transcribed is a small invertible segment of DNA within which lies the promotor of the H2 gene.

Question 5

Neisseria gonorrhoeae phase variation

Answer 5

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.

Question 6

Spontaneous Mutation

Answer 6

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. Typically changes are deleterious or neutral.
In rare instances, a mutation may confer a selective advantage.

Question 7

Sponateous Mutation rates.

Answer 7

As in higher organisms, the rate of spontaneous mutation in bacteria is very low. Spontaneous mutation to antibiotic resistance occurs once in approx 108 – 1010 organisms

Question 8

Examples of spontaneous mutations that are of medical importance include

Answer 8

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.

Question 9

Recombination

Answer 9

Either site-specific or homologous recombination within a particular organism, or genetic exchange and recombination between closely related organisms can contribute to the emergence of strains with new properties.

Question 10

Antigenic variation in

Answer 10

Borrelia recurrentis and Neisseria gonorrhoeae are examples of how recombination between duplicated genes can give rise to new antigenic variants. Example he gave in class is the promoter sequence switching to allow for transcription of H1 antigen for salmonella.

Question 11

acquisition of new DNA segments

Answer 11

increasing evidence that bacterial pathogens have acquired new genes by LATERAL transfer from other bacteria, even from unrelated species and possibly eukaryotic organisms. In most cases the origins of these new genes are obscure. Acquisition of new genes may alter the virulence potential, survival characteristics or antimicrobial resistance of the microorganism.

Question 12

Acquisition of transposable elements (transposons, IS elements, and “complex transposons”)

Answer 12

A transposable element (transposon) is a discrete segment of DNA (5-10 nucelotides long) which is capable of moving itself (or a copy of itself) from one chromosomal location to a new location within the cell. Transposons are not capable of self-replication except as part of another self-replicating molecule (i.e., the bacterial chromosome, virus, or plasmid). These elements typically encode one or more proteins mediates transposition (transposase). Transposition is not dependent upon regions of extensive homology and does not require host recombination machinery. OFTEN CODE ANTIBIOTIC RESISTANCE GENE.

Question 13

Insertion sequences

Answer 13

(IS elements) are transposons that simply encode transposase. Play a role in genome evolution by inactivating genes into which they transpose, or turning on expression of adjacent genes.

Question 14

Complex transposons

Answer 14

Carry additional genes such as those encoding antibiotic resistance, toxins, adhesins and other virulence factors.

Question 15

Bacteriophage conversion

Answer 15

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.

Question 16

Acquisiton of plasmids

Answer 16

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. Plasmids can carry virulence genes and genes conferring antibiotic resistance.

Question 17

Acquisition of “Pathogenicity Islands”

Answer 17

Pathogenicity Islands are generally relatively large segments of DNA present in the chromosome of some, but not all strains of a particular bacterial species. EX) SALMONELLA acquired 2 pathogenicity island (SPI1 which does invasion of interstinal epithelial cells. and SPI2 which codes for survival in macrophages.)

Question 18

Transformation for genetic exchange btwn bacteria

Answer 18

Genetic transfer of naked DNA. Griffith, Avery. 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. SEE PAGE 5 of HANDOUT FOR TRANSFORMATION STEPS!

Question 19

Transduction

Answer 19

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.

Question 20

Growth of “virulent” bacteriophage

Answer 20

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.

Question 21

Temperate phages and lysogenic bacteria.

Answer 21

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.

Question 22

Prophage

Answer 22

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. 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.

Question 23

Mechanism of transduction

Answer 23

Of the several known mechanisms of phage- mediated gene transfer, the simplest is generalized transduction, a process in which any segment of the donor cell genome (chromosome or resident plasmids) may be passed into another cell.

Question 24

Errors in DNA packaging

Answer 24

Occasionally, the phage’s packaging system will insert a “headful”-sized piece of bacterial DNA into a maturing phage capsid in place of a normal phage DNA molecule. These transducing particles contain no viral genetic information, but they are still able to attach to other host cells and inject the bacterial DNA which they contain. The injected DNA may then recombine with homologous segments in the recipient genome to produce a genetic recombinant, or transductant.

Question 25

Bacterial plasmids

Answer 25

including those which confer antibiotic resistance, may also be transferred by generalized transduction. This mechanism of antibiotic resistance transfer is particularly prevalent among the Gram positive bacteria.

1) Self-replicating, extrachromosomal DNA elements
2) Often circular
3) Range in size from couple genes to few % of chromosome
4) Not essential for viability
5) May encode a variety of functions - resistance to antibiotics, virulence factors, metabolic functions, self-transmission

Question 26

Bacteriophage conversion (lysogenic conversion).

Answer 26

Certain temperate bacteriophage encodes gene(s) which may be expressed during the lysogenic state and cause the appearance of a new phenotypic trait. In the lysogenic hostIn 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.

Question 27

Bacterial conjugation

Answer 27

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

Question 28

Mechanism of conjugaton: The plasmid F paradigm.

Answer 28

The F plasmid contains genetic information encoding the following traits:
i. Autonomous replication of the plasmid DNA
ii. Synthesis of sex pili (F pili) which are essential for mediating pair
formation between donor and recipient cells
iii. Conjugative transfer of F DNA to recipient (F- ) cells iv. Ability to integrate into the bacterial chromosome

Question 29

Conjugative transposons

Answer 29

are mobile elements which mediate conjugation between pairs of cells, in which the transferred DNA is the conjugative transposon itself. Once transferred, the element transposes to the chromosome of the recipient cell. Therefore these elements encode both transfer (tra) genes and transposition genes. Conjugative transposons may encode antibiotic resistance, especially resistance to tetracycline (tetM)

Question 30

Pathogenic bacteria evolve

Answer 30

2) slow incremental changes to non-pathogenic progenitors by mutations that modify the function of existing genes
2) quantum changes by the acquisition of entirely NEW genetic material by lateral transfer

Question 31

Even within a single species

Answer 31

there can be tremendous differences between individual bacteria isolates in their virulence and disase-causing potential.

Question 32

Bacterial Genome consists of

Answer 32

1) Chromosome = DNA molecule(s) that encode genes essential for growth under “normal”conditions. (usually a single chromosome, though some like Cholera have 2)
2) Plasmids - usually circular DNAs in the cell too.
3) Bacterial viruses

Question 33

Cholera needs

Answer 33

2 virulance bacterial viruses to become virulance. 1 will not do it.

Question 34

Escherichia coli

Answer 34

Some are important in our gut and do not make us sick and are the most abundant aerobic normal flora of the gut. OTHERs make us SICK. There are as follows:
ETEC-Plasmid-encoded enterotoxin (travellers diahhrea). EPEC-Plasmid-mediated histopathology. EIEC Plasmid-mediated invasion and epithelial cell destruction. EHEC Bacteriophage-encoded toxins, and plasmid-encoded virulence factors (GET FROM FOOD E.coli 0157). EAEC Plasmid-mediated adherence and pathology