Bacterial Genetics Flashcards
(35 cards)
1
Q
T/F Bacteria are adaptable.
A
True
2
Q
What does the bacterial genome consist of?
A
An organisms complete set of DNA:
- chromosome- essential and nonessential proteins
- plasmids
- bacteriophage- prophage
- IS elements, transposons, PAI’s (GEI’s)
- integrons
3
Q
Bacterial chromosome? shape?
A
- one, circular, dsDNA (majority), some have more than one, some are linear
- no histones or introns
- encodes for essential and non essential genes, better fit to survive in environment
- not housed in a membrane bound nucleus, but nucleiod, a region of the cell where the chromosome resides
4
Q
Are bacteria haploid or diploid?
A
haploid -one copy of each gene, mutations are expressed from mother to daughter
5
Q
Bacterial gene expression? essential vs nonessential?
A
- genes required for basic survival are constitutively expressed (DNA, cell wall, RNA, translation)
- expression of non essential genes can be positively (inducers) and negatively (repressors) regulated, usually at the level of transcription, some genes are present on DNA as operons
- operons
- polycistronic mRNA
6
Q
What are operons?
A
sequence of DNA that contains multiple genes to produce multiple proteins for a single purpose (not in eukaryotic cells), multiple genes coordinately expressed
-multiple genes with single promoter
7
Q
Polycistronic mRNA?
A
mRNA that has information for more than one gene or protein product
-there are sites where ribosome can bind to get independent proteins
8
Q
What do bacteria adapt to?
A
they quickly adapt to environmental changes for survival by modulating gene expression
- availability of sugars
- iron concentration
- presence of an antibiotic
9
Q
what is an example of regulation for survival in bacteria?
A
the Lac operon
- when glucose is available the lac operon encodes genes that allows for metabolism of lactose
- using regulators and repressors, allosteric changes
- positive and negative regulation
- adapts to environmental changes to survive
10
Q
What is gene regulation by quorum sensing? low vs high density
A
- organism can change its phenotype based on density of bacteria
- low density- signal molecules for harmless type
- high density- signals bind receptors that bind upstream of genes to make organism more pathogenic by making biofilm of special toxins
11
Q
The expression of many virulence genes are under regulation at the level of ______.
A
transcription
12
Q
What are the mechanisms of mutations in bacteria?
A
- base substitution or point mutation (transition or transversion), G->A, G->C
- deletion or addition of nucleotides
- insertional mutation- disruption of a gene by a transposon (jumping gene)- makes organism more pathogenic
- recombination
13
Q
Results of mutations?
A
- missense mutation- changes in amino acids sequence of protein product
- frameshift mutation- shift in reading frame of gene
- inversion mutation- segment of DNA is removed and reinserted in reverse direction
- inactivation of gene
14
Q
Clinical significance of mutations?
A
- changes in amino acids of bacterial structures can result in resistance to antibiotics
- non resistant bacteria multiply, some develop mutation, non resistant bacteria die from antibiotic, drug resistant mutations thrive
- base mutations, frameshift and gene arrangements can cause antigenic and phase variation of major surface components
- antigenic variation
- phase variation
15
Q
antigenic variation?
A
- changes in composition or structure of a molecule
- pilli on organism is changed because amino acids look different, antibodies used need to be different
- population- shift back and forth
16
Q
phase variation?
A
- turning on or off of the expression of a molecule
- capsule is expressed, then it changes to be not expressed
- some without variation can bind those that can in order to be protected from antibodies
17
Q
What structures can undergo phase and antigenic variation?
A
- capsule
- flagella
- pilli
- lipid A cannot because it is part of cell wall, O antigen can undergo variation
18
Q
When do these genetic changes normally take place? what are the consequences?
A
DNA synthesis
- polymerase makes error
- changes expression
- organism looks different on surface
- immune system struggles to recognize
- antibiotics may not work
19
Q
What are mobile genetic elements (MGEs)?
A
- elements that can move in and out of the genome
- organisms that can share
- elements that are responsible for bacteria becoming more resistant to antibiotics
- plasmids
- bacteriophage
- insertion sequences
20
Q
What are plasmids?
A
- small circular supercoiled DNA, extra chromosomal DNA
- 1 to 5 copies/cell, large plasmids, 10 to 20 copies/cell small plasmids
- replicate independently of the chromosome, inherited by daughter cells
- conjugative plasmids (large) encode transfer enzymes, and sex pili for transfer to other cells
- can carry genes that confer phenotypic advantages to bacteria
- can allow organisms to share DNA, confers a survival advantage
- can piggyback
21
Q
What is the clinical significance of plasmids?
A
- plasmids may carry genes that provide resistance to antibiotics (R-plasmids) or that make proteins that act as toxins or other pathogenic factors
- R plasmids may carry resistance to multiple antibiotics
- use of antibiotics has applied a strong selective pressure for bacteria that can resist them
22
Q
Bacteriophage? structure? genome?
A
viruses that infect bacteria:
-specific, used to subtype bacteria (phage typing)
structure:
- protein capsid that houses genome
- may have base and tail
- no membrane envelope
Genome:
- DNA or RNA
- single or double strand
- linear or circular
23
Q
Life cycles of bacteriophage?
A
Lytic (virulent):
- after binding -> rapid replication
- 20 mins from infection to lysis, leads to cell death and production of new (progeny) phage, takes over machinery, can treat diseases
Lysogenic (temperate):
- infection leads to cell lysis, or integration of phage DNA into chromosome or the formation of a plasmid (prophage-piece of DNA)
- inject genome into chromosome and repress expression of their genes and stably replicate their genome prophage can encode for toxins to make organism more pathogenic
24
Q
lysogenic conversion of bacteriophage?
A
- prophage conversion may increase virulence of bacteria
- diptheria toxin is encoded on a prophage, only those with prophage can cause disease
25
What are insertion sequences?
IS- the smallest and most simple transposable elements
- jumping genes- go from one place to another
- they have machinery to excise themselves, replicate genome, and move to another place (genes for transposition)
26
What are transposons?
- more complex insertion sequences, transposable elements that contain genes for transposition in addition to genes that encode for antibiotic resistance and other virulence properties
- encode for toxins, fimbriae, increase virulence
27
What are genomic or pathogenicity islands?
- large segments of DNA that contain mobile genetic sequences
- they can encode for genes that enhance the virulence of a pathogen
- really large transposons
- in S. aureus there are 12 PAIs that encode for a variety of super antigen toxins
28
What is horizontal gene transfer?
- lateral gene transfer (LGT)
- tranformation
- conjugation
- transduction
29
Transformation?
donor -\> cell lysis: release DNA fragments -\> DNA enters recipient cell and integrates into DNA
- uptake of naked DNA by bacteria in state of competency
- under right conditions, the DNA can be incorporated into genome of recipient and result in change in phenotype
- naturally competent DNA have the ability to take up exogenous DNA across membrane without manipulation (strep, neisseria, gonorrhoeae)
30
Conjugation?
- intimate association with two individuals, cell contact
- male (F+) and female (F-) cells come together
- conjugative plasmid makes copy of itself and shares it with F-
- after reaction, there are two F+ cells
- conjugative transfer of plasmids with resistance genes is an important cause of spread of antibiotic resistance
- Gram - use pili and Gram + use other systems
- some transposons are conjugative
31
Transduction?
- transfer of genetic info from one bacteria to another by bacteriophage
- prophage- DNA incorporated into chromosome
- lysogenic conversion
- donor organism goes through lytic phase with temperate phage
- takes up DNA from donor and its own DNA to make progeny
- infects another organism, DNA from donor will recombine with genome of recipient
- transduction and conjugative play important role in antibiotic resistance
32
Clinical significance of transfer mechanisms?
1. evolution of new pathogens
- non pathogenic strain of E coli aquires foreign DNA to produce E coli strains that cause diarrhea or UTI
2. spread of antibiotic resistance
- VRE and MRSA share info to make them resistant to vancomycin (MVRSA)
33
Mechanisms of antibiotic resistance?
1. decreased permeability
- mutation of porin
2. alteration of target
- new PBP that does not bind penicillin
3. inactivation of antibiotic
- secretion of beta lactamases
34
Beta lactams examples? method of actions? bacteria method of resistance?
-penicillin, methicillin, oxacillin, nafcillin, cephalosporins
MOA:
-bind transpeptidases (PBP), preventing peptidoglycan cross linking which leads to cell death
MOR:
- cleavage of beta lactam ring by beta lactamases
- modification of PBP (ex. MRSA)
- spontaneous mutations of porins in Gram -
35
Vancomycin method of action? bacteria method of resistance to it?
MOA:
-interacts with D-alanine, D-alanine termini, prevents cross linkage of peptidoglycan layers
MOR:
- change of binding site to D-alanine, D-lac
- thickening of cell wall that traps vancomycin
