VL5: Regulation of virulence factors Flashcards
Define pathogenicity, virulence, virulence factors
Pathogenicity: Capacity of a becaterium to cause disease
Virulence: Measure of the pathogenicity of a microorganism (infect, spread, cause tissue damage or disease symptoms)
Virulence factors: aid microbes to invade survive or spread within the host
3 requirements for bacterial virulence with examples
Attachment: Capsules, Pili, Adhesions
Nutrition: Iron acquisition
Survival from host defece: Antigenic variability, phase variation
Name 5 of 10 bacterial virulence mechanisms
Adherence Invasion Byproducts of growth (gas, acid) Toxins (Degradative enzymes, cytotoxic proteins) Endotoxin Superantigen Induction of excess inflammation Evasion of phagocytic and immune clearance Capsule Resistance to antibiotics
5 levels at which gene expression can be regulated
DNA replication Transcription Posttranscriptional processing Translation Posttranslational processing
How can gene expression be regulated by varying the gene copy number?
- plasmid copy number
- tandem gene amplifications: (high frequency, unstable= rapidly reversible)
more gene copies = more gene expression (higher probability of new mutations)
2 ways of gene amplification: Non-reciprocal recomination, rolling circle mechanism
Gene amplification is the most common innitial response when selective problem can be solved by increased gene expression,
allows secondary mutatios to occur,
if mutations lead to more resistance,
copy number of gene can be reduced again
gene amplification can be difficult to observe, since it will not necessarily leave any evolutionary trace in the genome
5 steps in antibiotic resistance development with gene amplification
Initial selection
Gene amplification, growth and clonal expansion
Point mutation in gene (or other target gene)
relaxed selection for amplification
segregation
What is site specific recombination? 2 types?
type of genetic recombination between segments with some sequence homology
- Phase variation: expression of a given phenotype is switched ON or OFF
- Antigenic variation: expression of alternative forms of an antigen on the cell surface
How does phase variation in Salmonella flagella work?
gene hin is flanked by inverted repeats, so it can point either in one or the other direction position 1: expresses fljB (-> FljB flagellin) and fljA (-> FljA represses fliC) position 2 (inverted): fljB is inactive no FljB flagellin, repressor is not made -> fliC is expressed -> FliC flagellin
2 more examples of site-specific inversion:
Antigenic variation of Surface Layer Proteins (SLP) of Campylobacter fetus
Phase variation of type 1 pili (fimA) in E.coli
What are the advantages of site specific recombination?
mixed phase population:
- environmental signals may regulate gene expression in one population (“on” cells)
- the immune system may actively eliminate one population
- environment (e.g. selection for adherence) may enrich one population
- generates phenotype diversity
- important to escape host defences
- occurs at high frequency and phenotypic changes are usually reversible
How does Neisseria gonorrhoeae infect cells?
- attaches to cell surface with pili (PilE)
- becomes more closely attached (Opa)
- phagocytosis, lives in phagosome, varies its surface proteins to evade immune response
How does N.gonorrhoeae regulate gene expression by strand slippage during replication?
genes involved:
1 PilE gene (codes for active PilE)
19 pilS genes (silent, source of genetic variation)
11 opa genes (each subject to strand-slippage)
1-2 pilC genes (required fo pilin biogenesis)
Opa(city) refers to opaque colour of colonies as a result of these proteins on cell surface
all 11 opa genes are trancribed, have variable number of coding repeats (CTCTT) polymerase slipps and
creates proteins in different lengths, frameshift mutation will not be translated
What mechanisms uses N.gonorrhoeae to vary pili?
- antigenic variation (occurs when silent pilin casettes (pilS) recombine with active pilE gene
a) intergenomic (DNA from neighboring Neisseria cells)
b) intragenomic (recA dependent recombination of pilE and pilS silent copies) - Phase variation
pilated (P+) and non-pilated (P-) cells, depends on expression of pilC, can be turned on and off by framshift mutations
What is epigenetic regulation?
- no change in DNA nucleotide sequence
- changes in methylation of DNA sequences in the regulatory regions of phase varying gene or operon
- E.g. pap fimbria operon in E.coli
What is Dam methylation?
methylation of GATC sites (to distinguish between old and new strand) by Dam DNA adenine methylase
How does E.coli regulate pap fimbrial genes?
pap fimbrial adhesin =virulence factor (for adhesion)
controlled via phase variation:
A nucleoproteincomplex binds to unmethylated GATC sites in the operon, turns it off, if GATC is methylated first, pap operon gens can be expressed
Subject to both positive and negative feedback control
Take home message:
Pap fimbriae production is turned ON and OFF stochastically as bacteria grow and DNA is replicated
How are operons structured in bacteria?
several genes are regulated by the same promotor,
lead to 1 mRNA with several ribosome binding sites,
lead to several proteins
Describe how the Fur repressor regulates mRNA transcription
Fur protein: transcriptional repressor of sideophores (+ receptors)
Fur with Fe2+ as a co-receptor binds to operon sequence “iron box” -> repression,
without Fe2+ no binding, no repression
How does H-NS regulate promotr accessibility?
small chromatin associated protein found in enterobacteria, binds to curved DNA
repression in non-intestinal environment
expression in response to intestinal stimuli (such as elevated temperature) or positively acting transcription factors
What is a 2 component system?
one prot senses signal and phosphorylates second prot to produce activated transducer that activated transcription
