36. Bacterial Genetics Flashcards
(42 cards)
key genetic differences making bacteria unique?
haploid
no nucleus
no introns
30S (INCL 16S rRNA) + 50S ribosomes
the presence of 16S rRNA sequences are used for what?
to identify and quantify individual bacterial species from a complex mixture of human cells and microbial cells (ie from stool sample/microbiome)
conserved regions vs hypervariable regions of 16s rRNA?
conserved: used to design universal PCR primers for amplification of the 16s gene across all bacterial species
hypervariable: used for species specific identification w/in 16S rRNA genome
which can replicate independently from the chromosome: plasmids or transposons?
plasmids - have own replication genes but are sometimes integrated into chromosome
transposons cannot replicate independently (must be inserted into another replicating element like the chromosome, plasmid, or bacteriophage)
the bacteriophage and pathogenicity island of bacteria are on the chromosome or plasmid?
chromosome
what enzyme lets transposons jump b/w plasmids and genes?
transposase
transposons (Tn) are simple elements that contain genes that allow them to transpose…along with what other elements?
one or more genes encoding a phenotype - mostly antibiotic resistance like B-lactamase
besides conferring a new phenotype like abx resistance, insertion of a Tn (transposon) can do what?
inactivate genes or can mediate other rearrangements (duplication, deletion, inversion)
Tns are flanked on either side by what?
inverted repeats ( what causes the hairpin loop often seen on EM)
what are insertion sequences (IS)?
essentially transposons without an obvious phenotype like abx resistance - can mediate rearrangements or inactivate target genes. Have transposase (tnp) genes plus inverted DNA sequences at ends
IS elements can form part of more complex transposons called what?
composite transposons (can lock phenotypic gene like abx resistance or toxin in the middle)
what are bacteriophages?
viruses that infect bacteria, not mammalian cells - there are 2 lifestyles for bacteriophages:
- lytic phages can lyse and kill cells (virulent phage)
- lysogenic phage (temperate phage or prophage) do not immediately lyse cells but can insert their DNA into the chromosome and replicate along with the cell. Many toxins can be encoded on phage genome and so if the phage DNA is incorporated into the chromosome, the bacteria can express the toxin
lytic phages attach to the outside of the cell and inject their genome into the bacterial cell. New copies of the phage genome and hte phage proteins are produced inside the bacterial cell and assemble into complete phage particles. The cell is then lysed and the phage can now infect other cells….why is this clinically relevant?
therapies to kill abx-resistant bacteria
life cycle of a temperate bacteriophage?
phage DNA is integrated into the bacterial chromosome and the lytic cycle is suppressed. Bacterial cell can grow and express the genes encoded on the phage (sometimes toxins) and the viral DNA is replicated along with the chromosomal DNA. At some point, a signal (eg UV light, heat) can induce the lysogenic phage to become lytic and the lytic cycle results
what are pathogenicity islands?
large blocks of DNA containing multiple virulence genes (located on the region of house-keeping genes present in non-pathogens (house-keeping genes encode normal growth and metab of non-pathogenic and pathogenic bacteria)
often have different DNA composition from the rest of the genome, suggesting that the entire block was via horizontal gene transfer
eg. LEE pathogenicity island of E.coli O157:H7 can lead to HUS
what are the 3 major ways in which bacteria can exchange genetic info?
transformation, conjugation, and transduction
transformation?
the process whereby naked DNA from one cell is taken up by another cell through the cell wall without ant participation of other bacteria or virus….
the Griffith experiment: pathogenic strep pneumoniae (pneumococcus) have a smooth polysacch. capsule on their surface, which acts as an antiphagocytic factor
- life smooth pneumococci injected into mice causes death and smooth pneumococci are recovered from dead mice
- head killed smooth pneumococci don’t kill mice
- rough pneumococci do not kill mice
- live rough (do not kill mice on their own) and killed smooth (do not kill mice on their own) are injected together into mice, death results and smooth, live cells are recovered
conjugation?
occurs via cell-to-cell contact (sex)
mediated by a pilus encoded on a plasmid
explanation of discovery and how it works:
model conjugation system was orig described in E.coli and is encoded in a plasmid called “F” (fertility factor). Strain with the F plasmid = F+ and can serve as a donor. A strain w/o the plasmid =F- and can serve as the recipient.
ssDNA from the plasmid enters into the recipient cell where the complementary strand is synth to form a complete ds circle.
complementary strand in the donor cells is also synth so that both cells have complete copies of the plasmid
the F plasmid is a special kind of plasmid called what? (ie how does it integrate into the chromosome)
an episome that can integrate into the chromosome (via IS elements on teh plasmid that recombine w/identical IS elements on the chromosome)
when the F plasmid is integrated into the chromosome, it is called what?
an HFR (high frequency recombination) because it can transfer adjacent chromosomal genes into a recipient strain, not just plasmid sequences
transduction?
gene transfer involving phage, which can pick up genes from the host cell and incorporate them into the progeny phage.
the phage w/foreign DNA can attach to a recipient cell and inject the phage DNA along with the foreign DNA into the recipient cell.
why do bacteria have to change the expression of their own genes?
to rapidly adapt to new environments
bacteria have multiple genes physically linked that are co-regulated in what?
an operon
multiple operons can be linked together in a regulatory network (not necessarily physically linked) in a _____.
regulon
