Bacterial Genetics Flashcards
T/F Bacteria are adaptable.
True
What does the bacterial genome consist of?
An organisms complete set of DNA:
- chromosome- essential and nonessential proteins
- plasmids
- bacteriophage- prophage
- IS elements, transposons, PAI’s (GEI’s)
- integrons
Bacterial chromosome? shape?
- 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
Are bacteria haploid or diploid?
haploid -one copy of each gene, mutations are expressed from mother to daughter
Bacterial gene expression? essential vs nonessential?
- 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
What are operons?
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
Polycistronic mRNA?
mRNA that has information for more than one gene or protein product
-there are sites where ribosome can bind to get independent proteins
What do bacteria adapt to?
they quickly adapt to environmental changes for survival by modulating gene expression
- availability of sugars
- iron concentration
- presence of an antibiotic
what is an example of regulation for survival in bacteria?
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
What is gene regulation by quorum sensing? low vs high density
- 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
The expression of many virulence genes are under regulation at the level of ______.
transcription
What are the mechanisms of mutations in bacteria?
- 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
Results of mutations?
- 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
Clinical significance of mutations?
- 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
antigenic variation?
- 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
phase variation?
- 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
What structures can undergo phase and antigenic variation?
- capsule
- flagella
- pilli
- lipid A cannot because it is part of cell wall, O antigen can undergo variation
When do these genetic changes normally take place? what are the consequences?
DNA synthesis
- polymerase makes error
- changes expression
- organism looks different on surface
- immune system struggles to recognize
- antibiotics may not work
What are mobile genetic elements (MGEs)?
- 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
What are plasmids?
- 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
What is the clinical significance of plasmids?
- 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
Bacteriophage? structure? genome?
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
Life cycles of bacteriophage?
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
lysogenic conversion of bacteriophage?
- prophage conversion may increase virulence of bacteria
- diptheria toxin is encoded on a prophage, only those with prophage can cause disease
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)
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
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
What is horizontal gene transfer?
- lateral gene transfer (LGT)
- tranformation
- conjugation
- transduction
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)
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
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
Clinical significance of transfer mechanisms?
- evolution of new pathogens
- non pathogenic strain of E coli aquires foreign DNA to produce E coli strains that cause diarrhea or UTI - spread of antibiotic resistance
- VRE and MRSA share info to make them resistant to vancomycin (MVRSA)
Mechanisms of antibiotic resistance?
- decreased permeability
- mutation of porin - alteration of target
- new PBP that does not bind penicillin - inactivation of antibiotic
- secretion of beta lactamases
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 -
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
