This document discusses phase variation in bacteria. It begins by defining phase variation as a reversible switch in bacterial cells between expressing and not expressing proteins, which allows bacterial populations to adapt to changing environments without random mutations. It then describes 10 common phase variable phenotypes and molecular mechanisms of phase variation, including short sequence repeats, homologous recombination, and site-specific recombination. The document emphasizes that phase variation allows bacterial evasion of host immune responses by varying surface structures like capsules and proteins.
A bacteriophage (informally, phage) is a virus that infects and replicates within a bacterium. The term is derived from "bacteria" and the Greek (phagein), "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes. Phages replicate within the bacterium following the injection of their genome into its cytoplasm. Bacteriophages are among the most common and diverse entities in the biosphere.
Phages are widely distributed in locations populated by bacterial hosts, such as soil or the intestines of animals. One of the densest natural sources for phages and other viruses is sea water, where up to 9×108 virions per milliliter have been found in microbial mats at the surface,] and up to 70% of marine bacteria may be infected by phages. They have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Central Europe, as well as in France. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria (see phage therapy). Nevertheless, phages of Inoviridae have been shown to complicate biofilms involved in pneumonia and cystic fibrosis, shelter the bacteria from drugs meant to eradicate disease and promote persistent infection
A bacteriophage (informally, phage) is a virus that infects and replicates within a bacterium. The term is derived from "bacteria" and the Greek (phagein), "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes. Phages replicate within the bacterium following the injection of their genome into its cytoplasm. Bacteriophages are among the most common and diverse entities in the biosphere.
Phages are widely distributed in locations populated by bacterial hosts, such as soil or the intestines of animals. One of the densest natural sources for phages and other viruses is sea water, where up to 9×108 virions per milliliter have been found in microbial mats at the surface,] and up to 70% of marine bacteria may be infected by phages. They have been used for over 90 years as an alternative to antibiotics in the former Soviet Union and Central Europe, as well as in France. They are seen as a possible therapy against multi-drug-resistant strains of many bacteria (see phage therapy). Nevertheless, phages of Inoviridae have been shown to complicate biofilms involved in pneumonia and cystic fibrosis, shelter the bacteria from drugs meant to eradicate disease and promote persistent infection
A picornavirus is a virus belonging to the family Picornaviridae, a family of viruses in the order Picornavirales. Vertebrates, including humans, serve as natural hosts. Picornaviruses are nonenveloped viruses that represent a large family of small, cytoplasmic, plus-strand RNA viruses with a 30-nm icosahedral capsid.
This presentation contains 45 slides on general virology comprises of topics on viral classification, transmission, pathogenesis, viral cytopathic effect, stages of viral infections, antiviral drugs and viral vaccines. It also have a slide noting an outline of laboratory diagnosis of viral infection. This power point presentation was designed for medical students, nurses and academicians teaching virology and microbiology in medical universities, schools or colleges.
An oncovirus is a virus that can cause cancer. This term originated from studies of acutely transforming retroviruses in the 1950–60s, often called oncornaviruses to denote their RNA virus origin. It now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor virus" or "cancer virus".
A picornavirus is a virus belonging to the family Picornaviridae, a family of viruses in the order Picornavirales. Vertebrates, including humans, serve as natural hosts. Picornaviruses are nonenveloped viruses that represent a large family of small, cytoplasmic, plus-strand RNA viruses with a 30-nm icosahedral capsid.
This presentation contains 45 slides on general virology comprises of topics on viral classification, transmission, pathogenesis, viral cytopathic effect, stages of viral infections, antiviral drugs and viral vaccines. It also have a slide noting an outline of laboratory diagnosis of viral infection. This power point presentation was designed for medical students, nurses and academicians teaching virology and microbiology in medical universities, schools or colleges.
An oncovirus is a virus that can cause cancer. This term originated from studies of acutely transforming retroviruses in the 1950–60s, often called oncornaviruses to denote their RNA virus origin. It now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor virus" or "cancer virus".
Porphyromonas gingivalis belongs to the phylum Bacteroidetes and is a nonmotile, Gram-negative, rod-shaped, anaerobic, pathogenic bacterium. It forms black colonies on blood agar.
It is found in the oral cavity, where it is implicated in certain forms of periodontal disease, as well as in the upper gastrointestinal tract, the respiratory tract, and the colon. It has also been isolated from women with bacterial vaginosis. Collagen degradation observed in chronic periodontal disease results in part from the collagenase enzymes of this species. It has been shown in an in vitro study that P. gingivalis can invade human gingival fibroblasts and can survive in them in the presence of considerable concentrations of antibiotics.P. gingivalis also invades gingival epithelial cells in high numbers, in which cases both bacteria and epithelial cells survive for extended periods of time. High levels of specific antibodies can be detected in patients harboring P. gingivalis. Dr Harshavardhan Patwal , explains the various enzymes enzyme peptidyl-arginine deiminase, which is involved in citrullination.[4] Patients with rheumatoid arthritis have an increased incidence of periodontal disease, and antibodies against the bacterium are significantly more common in these patients.
P. gingivalis is divided into K-serotypes based upon capsular antigenicity of the various types.
Pathogen-Driven Proteomic Changes in Haemolymph of NPV-Infected Silkworm Bomb...Mr. Suresh R. Jambagi
It will explain the structural proteins of baculoviruse, hemolymph proteins of silkworm, Host-pathogen interaction between BmNPV and silkworm and proteomic countermeasures to BmNPV infection by silkworm has been explained with few case studies
Report Back from SGO 2024: What’s the Latest in Cervical Cancer?bkling
Are you curious about what’s new in cervical cancer research or unsure what the findings mean? Join Dr. Emily Ko, a gynecologic oncologist at Penn Medicine, to learn about the latest updates from the Society of Gynecologic Oncology (SGO) 2024 Annual Meeting on Women’s Cancer. Dr. Ko will discuss what the research presented at the conference means for you and answer your questions about the new developments.
Prix Galien International 2024 Forum ProgramLevi Shapiro
June 20, 2024, Prix Galien International and Jerusalem Ethics Forum in ROME. Detailed agenda including panels:
- ADVANCES IN CARDIOLOGY: A NEW PARADIGM IS COMING
- WOMEN’S HEALTH: FERTILITY PRESERVATION
- WHAT’S NEW IN THE TREATMENT OF INFECTIOUS,
ONCOLOGICAL AND INFLAMMATORY SKIN DISEASES?
- ARTIFICIAL INTELLIGENCE AND ETHICS
- GENE THERAPY
- BEYOND BORDERS: GLOBAL INITIATIVES FOR DEMOCRATIZING LIFE SCIENCE TECHNOLOGIES AND PROMOTING ACCESS TO HEALTHCARE
- ETHICAL CHALLENGES IN LIFE SCIENCES
- Prix Galien International Awards Ceremony
New Drug Discovery and Development .....NEHA GUPTA
The "New Drug Discovery and Development" process involves the identification, design, testing, and manufacturing of novel pharmaceutical compounds with the aim of introducing new and improved treatments for various medical conditions. This comprehensive endeavor encompasses various stages, including target identification, preclinical studies, clinical trials, regulatory approval, and post-market surveillance. It involves multidisciplinary collaboration among scientists, researchers, clinicians, regulatory experts, and pharmaceutical companies to bring innovative therapies to market and address unmet medical needs.
- Video recording of this lecture in English language: https://youtu.be/lK81BzxMqdo
- Video recording of this lecture in Arabic language: https://youtu.be/Ve4P0COk9OI
- Link to download the book free: https://nephrotube.blogspot.com/p/nephrotube-nephrology-books.html
- Link to NephroTube website: www.NephroTube.com
- Link to NephroTube social media accounts: https://nephrotube.blogspot.com/p/join-nephrotube-on-social-media.html
Lung Cancer: Artificial Intelligence, Synergetics, Complex System Analysis, S...Oleg Kshivets
RESULTS: Overall life span (LS) was 2252.1±1742.5 days and cumulative 5-year survival (5YS) reached 73.2%, 10 years – 64.8%, 20 years – 42.5%. 513 LCP lived more than 5 years (LS=3124.6±1525.6 days), 148 LCP – more than 10 years (LS=5054.4±1504.1 days).199 LCP died because of LC (LS=562.7±374.5 days). 5YS of LCP after bi/lobectomies was significantly superior in comparison with LCP after pneumonectomies (78.1% vs.63.7%, P=0.00001 by log-rank test). AT significantly improved 5YS (66.3% vs. 34.8%) (P=0.00000 by log-rank test) only for LCP with N1-2. Cox modeling displayed that 5YS of LCP significantly depended on: phase transition (PT) early-invasive LC in terms of synergetics, PT N0—N12, cell ratio factors (ratio between cancer cells- CC and blood cells subpopulations), G1-3, histology, glucose, AT, blood cell circuit, prothrombin index, heparin tolerance, recalcification time (P=0.000-0.038). Neural networks, genetic algorithm selection and bootstrap simulation revealed relationships between 5YS and PT early-invasive LC (rank=1), PT N0—N12 (rank=2), thrombocytes/CC (3), erythrocytes/CC (4), eosinophils/CC (5), healthy cells/CC (6), lymphocytes/CC (7), segmented neutrophils/CC (8), stick neutrophils/CC (9), monocytes/CC (10); leucocytes/CC (11). Correct prediction of 5YS was 100% by neural networks computing (area under ROC curve=1.0; error=0.0).
CONCLUSIONS: 5YS of LCP after radical procedures significantly depended on: 1) PT early-invasive cancer; 2) PT N0--N12; 3) cell ratio factors; 4) blood cell circuit; 5) biochemical factors; 6) hemostasis system; 7) AT; 8) LC characteristics; 9) LC cell dynamics; 10) surgery type: lobectomy/pneumonectomy; 11) anthropometric data. Optimal diagnosis and treatment strategies for LC are: 1) screening and early detection of LC; 2) availability of experienced thoracic surgeons because of complexity of radical procedures; 3) aggressive en block surgery and adequate lymph node dissection for completeness; 4) precise prediction; 5) adjuvant chemoimmunoradiotherapy for LCP with unfavorable prognosis.
ARTIFICIAL INTELLIGENCE IN HEALTHCARE.pdfAnujkumaranit
Artificial intelligence (AI) refers to the simulation of human intelligence processes by machines, especially computer systems. It encompasses tasks such as learning, reasoning, problem-solving, perception, and language understanding. AI technologies are revolutionizing various fields, from healthcare to finance, by enabling machines to perform tasks that typically require human intelligence.
The prostate is an exocrine gland of the male mammalian reproductive system
It is a walnut-sized gland that forms part of the male reproductive system and is located in front of the rectum and just below the urinary bladder
Function is to store and secrete a clear, slightly alkaline fluid that constitutes 10-30% of the volume of the seminal fluid that along with the spermatozoa, constitutes semen
A healthy human prostate measures (4cm-vertical, by 3cm-horizontal, 2cm ant-post ).
It surrounds the urethra just below the urinary bladder. It has anterior, median, posterior and two lateral lobes
It’s work is regulated by androgens which are responsible for male sex characteristics
Generalised disease of the prostate due to hormonal derangement which leads to non malignant enlargement of the gland (increase in the number of epithelial cells and stromal tissue)to cause compression of the urethra leading to symptoms (LUTS
Knee anatomy and clinical tests 2024.pdfvimalpl1234
This includes all relevant anatomy and clinical tests compiled from standard textbooks, Campbell,netter etc..It is comprehensive and best suited for orthopaedicians and orthopaedic residents.
Couples presenting to the infertility clinic- Do they really have infertility...Sujoy Dasgupta
Dr Sujoy Dasgupta presented the study on "Couples presenting to the infertility clinic- Do they really have infertility? – The unexplored stories of non-consummation" in the 13th Congress of the Asia Pacific Initiative on Reproduction (ASPIRE 2024) at Manila on 24 May, 2024.
Acute scrotum is a general term referring to an emergency condition affecting the contents or the wall of the scrotum.
There are a number of conditions that present acutely, predominantly with pain and/or swelling
A careful and detailed history and examination, and in some cases, investigations allow differentiation between these diagnoses. A prompt diagnosis is essential as the patient may require urgent surgical intervention
Testicular torsion refers to twisting of the spermatic cord, causing ischaemia of the testicle.
Testicular torsion results from inadequate fixation of the testis to the tunica vaginalis producing ischemia from reduced arterial inflow and venous outflow obstruction.
The prevalence of testicular torsion in adult patients hospitalized with acute scrotal pain is approximately 25 to 50 percent
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Phase and antigenic variations in bacteria
1. Dr. Dinesh Kr Jain, MD.,
Assistantprofessor,
Department of Microbiology,
SMS Medical college, Jaipur
2. CONTENTS
Introduction and definition
Phase variable phenotypes and moieties.
Biological significance of phase variation
Molecular mechanism-
1.Genetic regulation
2.Epigenetic regulation
3.Cell’s regulatory mechanism
3. PHASE VARIATION
Method of dealing with rapidly varying environments without
requiring random mutation.
It involves the variation of protein expression .
It’s a reversible switch between an “all-or-none”(on/off)
expressing phase, resulting in variation in the level of
expression of one or more proteins between individual cells of
a clonal population.
Majority of daughter cells will retain the expression phase of
the parent but a minority will have switched expression phase.
The switch can be influenced by external factors, the switching
frequency can be modulated.
4. ANTIGENIC VARIATION
Mechanism by which bacteria alters its surface protein in
order to avoid host immune response.
Bacteria will change the variety of cell surface
molecules(proteins and carbohydrates) that can be detected
by specific antisera.
A single strain may express several antigenic variants of a
cellular component.
The genetic information for producing a family of antigenic
variants is available in the cell, but only one variant is
expressed at a given time.
5. Phase variation v/s classical
mutation
In phase variation
genetic or epigenetic mechanism.
reversible between generations.
frequency of this reversion exceeds that of a random mutation.
As high as a change in 1 cell/10 generation to
1cell/103 generation.
6. Phase variable phenotypes and
moieties
1.Colony Morphology and Opacity
2.Capsule
3.Fimbriae and Pili
4.Flagella
5.Other Surface-Exposed Proteins
6.LPS and LOS Modification
7.DNA Restriction-Modification Systems(R/M)
8.DNA Restriction-Modification Systems
9.Regulatory Proteins
10.Metabolism associated genes
7. 1.Colony Morphology and Opacity
Phenotypic variation in bacteria most commonly observed as colony
variation & is associated with the virulence of the strain.
Phase variation of a variety of surface-exposed proteins, capsule,
cell wall - determines the colony morphology.
Dry v/s moist, Ruffled v/s smooth, Opaque v/s translucent.
1.H.influenzae b- 3colony variants( opaque, intermediate, translucent)
Due to variation in capsule production and cell envelope
protein(encoded by oapA) ;determines the property of colonization in
nasopharynx and serum resistance.
2.Streptococcus pneumoniae- opaque more virulent (produce more
capsular protein and less teichoic acid) as compared to translucent.
8. 1.Colony Morphology and Opacity
3.S.gordonii- colony morphology as well as haemolysin production
phase vary.
4.H.pylori-variation in enzyme phospholipase A increases the virulence
due to release of ureases.
5.P.aerugenosa-colony variation determines the property of
aggregation and motility.
Color variation in colonies grown on specific media can be
caused by phase variation of proteins that interact with a dye.
Staphylococcus epidermidis
9. 2.Capsule
The capsule can influence interactions with the host cells and host
environment, including invasion, adhesion, and serum sensitivity,
and is a well-recognized virulence factor.
Phase variation of capsule synthesis has been found to occur in both
gram-positive and gram-negative bacterial species, including
Campylobacter jejuni, Citrobacter freundii, S. pneumoniae, and
specific serogroups of Neisseria meningitidis.
In Bacteroides fragilis, eight different capsule polysaccharides can be
produced per cell. The expression of each is under the control of
on/off phase variation.
In H. influenzae type b cells, the level of expression of the capsule
can be modulated, and an irreversible switch to a nonexpressing
phenotype can occur.
10. 3.Fimbriae
Fimbriae mediated attachment to the host cell is through interaction
with receptors. These interactions occur either by structural subunit
or by fimbrial adhesin with certain chemical groups of host protein
or lipids.
Attachment to inorganic solid surfaces by nonspecific intercations-
biofilm formation.
Phase variation of fimbriae is regulated mostly by mechanisms that
affect transcription originating at the major promoter of the operon,
resulting in variable (on/off) expression of genes in the fimbrial
operon.
The S.enterica serotype Typhimurium genome encodes at least 11
fimbrial operons - pef, lpf, fim phase vary.
fimbriae in Proteus mirabilis is encoded by pap operon.
11. 4.Pili
Type IV pili function as adhesins and are involved in interaction with
eukaryotic cells, and thus these variations are important for
pathogenesis .
S. enterica serotype Typhi- phase-variable expression of the PilV
affects the pilus-associated property of cellular auto-aggregation .
N. gonorrhoeae can produce over a million different, antigenically
distinct pilin subunits for its type IV pili, in addition pilus associated
protein PilC also phase vary.
12. 5.Flagella
Flagella mediate bacterial motility; adhesion and virulence are
enhanced by flagellar expression and motility.
Flagellin is recognized by innate immune system.
It’s antigenic property forms a significant part of serological
classification scheme.
S. enterica serotype Typhimurium -biphasic antigenic variation
between H1(FliC protein) and H2(FljB protein).
Campylobacter coli-FlhA(flagellin),
Helicobacter pylori-fliP(flagellar basalbody),
Bordetella pertussis-BvgAS(regulatory system).
13. 6.Other Surface-Exposed Proteins
Proteins that are integrated in the cell wall in gram-positive
organisms or in the outer membrane in gram-negative organisms
can have a variety of functions these proteins include.
- Transporters,
- Porins,
- Receptors,
- Colonizing factors,
- Enzymes.
Antigenic or phase variation can occur.
14. 6.Other Surface-Exposed Proteins
S. pyogenes, expression of the cell wall-associated surface proteins
C5a peptidase, M protein, type IIa IgG Fc receptor phase vary, as
well as expression of the capsule and pyrogenic exotoxin.
N. gonorrhoeae and N. meningitidis-family of outer membrane
opacity proteins (opa) that facilitate adhesion.
Mycoplasma gallisepticum- phase variation , whereas the
homologous proteins in M. synoviae undergo antigenic variation . M.
hyorhini , M.hominis, M.penetrans undergoes both phase variation
and antigenic variation.
E. coli, H. influenzae- phase var. Of colonizing factors,
Borrelia spirochetes- multiphasic antigenic variation,
Campylobacter fetus-Surface layer proteins(SLP) absence
complement activity increased.
15. 7.LPS and LOS Modification: Variation in
Expression of Surface Epitopes
LPS consists - lipid A moiety(endotoxin)
- a core of polysaccharide
-O antigen-LPS variability among species and serotypes
occurs.
Modifications impact antigenicity, serum sensitivity and adhesion.
Phase-variation between encapsulated (sialyltransferase ) and
noncapsulated (glycosyltransferase) H. Influenzae.
Helicobacter pylori – phase variation of 3 fucosyl transferase
genes(futA,B,C) occurs, due to which variable carbohydrate modifications
occur in LPS that resemble structures of the Lewis group of antigens of
human blood groups.(host mimicry)
Among Neisseria species ChoP expression shows phase-variable and can be
expressed either in LPS(commensal) or typeIVpili(pathogenic).
Legionella pneumophila, S. enterica serotype Typhimurium -Antigenic or
phase variation of LPS.
16. 7.LPS and LOS Modification: Variation in
Expression of Surface Epitopes
In some species, the core lacks the multiple O-linked saccharide
units and is often therefore referred to as lipooligosaccharide (LOS)
Certain combinations of LOS modification may facilitate colonization
or survival in different host environments.
Ganglioside mimicry(GM1,GM2) of the LOS by Campylobacter jejuni
is an important factor in the development of Guillain-Barre´ and
Miller-Fisher syndromes. Expression of the enzymes involved in the
modification can phase vary.
17. 8.DNA Restriction-Modification
Systems
Most phase variable genes are predicted to be involved in the
biosynthesis of surface structures. Notable exceptions are genes
encoding R/M enzymes. Phase-variable expression of R/M enzymes
has been found in a variety of bacterial pathogens, including-
S. Pneumoniae(modification system), M. Pulmonis(HsdS proteins),
H. Pylori(type III R/M system), P. Haemolytica(type III modification
system), and H. Influenzae(mod gene).
Role of R/M enzyme variation is important during host pathogen
interaction, but the actual significance is currently unclear.
Phenotypic switching of putative R/M enzymes in H. pylori is
induced on contact with human gastric cells. In M. pulmonis a high
level of R/M enzyme (HsdS) variation is induced in vivo during
infection of the rat trachea.
18. 9.Regulatory Proteins
DNA binding proteins that function as activators or repressors can
be categorized as-
1.Global regulators- virulence-associated regulatory protein, (Mga),
BvgS protein in S. pyogenes, two-component BvgAS regulatory
system in Bordetella pertussis.
2.Operon specific or Local regulators-E. coli, expression of the local
regulator PapB(also affects type 1 fimbrial expression).
The expression of multiple regulatory proteins is now known to
phase vary and includes representatives of both groups.
19. 10.Metabolism associated genes
Phase variation of metabolism-associated proteins was identified in
the human pathogen Streptococcus pneumoniae.
A comparison of protein expression patterns between two colony
variants showed that at least three proteins were differentially
expressed, pyruvate oxidase (SpxB), a putative elongation factor,
and a proteinase maturation protein.
SpxB phase variation is related to the hydrogen peroxide that is
produced in conversion of the pyruvate to acetyl phosphate.
The level is sufficiently high to be lethal to other species and may
provide a SpxB isolate with a competitive advantage in a mixed-
species environment -bacterial virulence strategy.
20. Molecular Mechanism
Genetic Regulation
a. Short sequence repeat and slipped strand mispairing- SSM
b. Homologous recombination
c. Site specific recombination
Epigenetic Regulation
a. PAP phase variation
b. Ag43 phase variation
Cell’s Regulatory Network
a. cross regulation
b. environmental regulation
21. SHORT SEQUENCES REPEAT AND SLIPPED-STRAND
MISPAIRING
• SSM is thought to be the most common mechanism of ON/OFF switching
of contingency genes.
• SSM is a process that produces mispairing of short repeat sequences
between the mother and daughter strand during DNA synthesis i.e., DNA
replication, repair and recombination.
• During DNA synthesis template and nascent strand transiently separates
from each other and then reanneal.
• In reannealing step, nascent strand or the template strand can be slipped
either in forward direction or in backward direction and can result in an
increase or decrease in the number of short repeat sequences (1 to 7
nucleotides).
• If the location of these repeats is such that either
transcription or
translation
of a gene is affected it can lead to phase-variable expression of a protein.
22. SHORT SEQUENCES REPEAT AND SLIPPED-STRAND
MISPAIRING
Transcriptional regulation:
1. Promoter region
2. Other region
1.Regulation at the level of transcription occurs when the repeats are
located in the promoter region between the -10 and -35 sites for RNA
polymerase binding .
• e.g. H. influenzae has two divergently oriented promoters and
fimbriae genes hifA and hifB. The overlapping promoter regions
have repeats of the dinucleotide TA in the -10 and -35 sequences.
Through SSM the TA repeat region can undergo addition or
subtraction which results in the reversible ON phase or OFF phase of
transcription of the hifA and hifB.
23. SHORT SEQUENCES REPEAT AND SLIPPED-STRAND
MISPAIRING
Transcriptional regulation:
2. Transcription can also be affected by changes in repeat sequences
located outside of the promoter, may be by affecting the binding of a
regulatory protein or by affecting posttranscriptional event.
• e.g. Phase variation of individual fimbrial genes in B. pertussis is
proposed to occur as a result of a change in a poly(C) tract that
alters the distance between the binding sites of an activator and
RNA polymerase
24. SHORT SEQUENCES REPEAT AND SLIPPED-STRAND
MISPAIRING
Translational regulation:
• Translation of a protein can be affected by SSM if the unit repeats
are located within its coding sequence .
• Open reading frame is disrupted if SSM results in a change in
nucleotide number that is not a multiple of three. In this case, a
non-functional, usually truncated protein is synthesized.
• e.g. phase variation of the expression of the mod gene of H.
influenzae, it contains over 30 repeats of the tetranucleotide (5 -
AGTC) in its coding sequence . By addition of one tetranucleotide
repeat within the coding sequence the reading frame is altered, and,
in addition, a premature stop codon is formed.
• To summarize, SSM can cause a change in the number of unit
repeats consisting of 1 to 7 nt and can affect transcription initiation,
a posttranscriptional initiation event, or translation.
26. Molecular Mechanism
Genetic Regulation
a. Short sequence repeat and slipped strand mispairing - SSM
b. Homologous recombination
c. Site specific recombination
Epigenetic Regulation
a. PAP phase variation
b. Ag43 phase variation
Cell’s Regulatory Network
a. cross regulation
b. environmental regulation
27. HOMOLOGOUS RECOMBINATION/GENE CONVERSION
• Gene conversion is the process by which one DNA sequence
replaces a homologous sequence such that the sequences become
identical after conversion event.
• It results in a unidirectional exchange of DNA. It involves
recombination between one of a repertoire of silent alleles of the
gene and the gene located at the expression site.
• e.g. 1.Type IV pilin antigenic variation in N. gonorrhoeae. There are
several copies of gene coding for this pili but only one is expressed
at any given time. This is referred to as pilE gene. The silent versions
of this gene is pilS. pilS can use homologous recombination to
combine with parts of pilE gene and thus create a different
phenotype.
29. HOMOLOGOUS RECOMBINATION
2. Antigenic variation of the variable major lipoprotein (Vmp) in
Borrelia hermsii and of the VlsE surface proteins in B.
burgdorferi
3. antigenic variation of the SLPs in Campylobacter fetus.
4. variation in the level of capsule production in H. influenzae
type b. and Streptococcus pneumonia serotypes .
30. Molecular Mechanism
Genetic Regulation
a. Short sequence repeat and slipped strand mispairing- SSM
b. Homologous recombination
c. Site specific recombination
Epigenetic Regulation
a. PAP phase variation
b. Ag43 phase variation
Cell’s Regulatory Network
a. cross regulation
b. environmental regulation
31. SITE SPECIFIC RECOMBINATIONS
• Site-specific recombination are
non-homologous
usually short (no more than 30 bp.) and
occur at a single target site within the recombining
sequence.
• requires specific enzymes that act at cognate DNA
sequences known as site-specific recombinase.
• Conservative site-specific recombination (CSSR) can
lead to inversion,
translocation
32. SITE SPECIFIC RECOMBINATIONS-Inversion
i) Inversion of a DNA element by CSSR :
• In DNA inversions ,a segment of DNA is cut, inverted and then
rejoined by site specific recombinases.
• The inverted DNA segment may contain either
a promoter that direct expression of fixed structural genes or
structural genes controlled by fixed promoter.
• e.g. Type 1 fimbrial phase variation: Type 1 fimbriae, encoded
by the fim operon, are the most common fimbrial adhesins in
E. coli isolates.
34. SITE SPECIFIC RECOMBINATIONS-Inversion
Other CSSR-dependent types of phase variation.
• antigenic and phase variation of the PilV protein of the type IVB
pilus in S. enterica serotype Typhi - Inversion is mediated by the
Rci recombinase
• flagellar H1/H2 antigenic variation in S. enterica serotype
Typhimurium: Hin-recombinase.
35. SITE SPECIFIC RECOMBINATIONS-Transposition
2) Insertion and excision of genetic elements from the
chromosome – TRANSPOSITION
• Transposition mechanism leads to either insertion or excision of
transposable element and is restricted to few insertion sequence (IS)
elements.
• mediated by recombinase enzymes known as transposases.
• Classic transposition does not target a specific DNA sequence. In
contrast, transposition mediated by the putative transposase MooV
lead to phase variation requires short sequence identity between the
insertion element and the target sequence.
36. SITE SPECIFIC RECOMBINATIONS-Transposition
• e.g. eps locus in marine bacterium P. atlantica : P.
atlantica contains an eps locus that encodes extracellular
polysaccharide.
• Phase variation of eps expression affects biofilm formation.
• Two recombinases encoded by MooV and Piv mediate the
precise excision and insertion of the insertion element IS492 in
eps locus respectively.
Presence of IS492 in eps locus→ON phase
Excision of IS492→→→→→OFF phase
37. SITE SPECIFIC RECOMBINATIONS-Transposition
• Phase variation of ica expression in Staphylococcus
epidermidis: correlates with the insertion and precise
excision of an insertion element. Expression of the ica
operon results in formation of a polysaccharide adhesin
that facilitates cell-cell interactions and biofilm
formation.
• Phase variation of capsule production in Citrobacter
freundii and in Neisseria meningitidis is also regulated
by insertion and excision of IS-like elements
• Legionella pneumophila, phase variation in expression of
LPS is associated with reversible excision and insertion of
a 30-kb plasmid into the chromosome.
38. Molecular Mechanism
Genetic Regulation
a. Short sequence repeat and slipped strand mispairing- SSM
b. Homologous recombination
c. Site specific recombination
Epigenetic Regulation
a. PAP phase variation
b. Ag43 phase variation
Cell’s Regulatory Network
a. cross regulation
b. environmental regulation
39. EPIGENETIC REGULATION
• Epigenetic regulation of phase variation occurs in the absence of a
change in DNA sequence, therefore maintaining the integrity of
genome.
• It involves differentially methylated sequences in the regulatory
regions of the phase-varying gene or operon.
• the expressed state is heritable but reversible
• The change incurred by methylation alters the binding of
transcription factors. The outcome is the regulation of transcription
resulting in switches in gene expression.
• Methylation-dependent phase variation has been identified in E.
coli and S. enterica serotype Typhimurium as Pap phase
variation and Ag43 phase variation respectively.
40. EPIGENETIC REGULATION
Pap (pyelonephritis associated pili) phase variation in
uropathogenic E.coli: Expression of the pap operon is
dependent on
deoxyadenosine methyltransferase (Dam),
two sequences (GATCdist and GATCprox) in its regulatory region,
leucine-responsive regulatory protein (Lrp), act as both
repressor and an activator at pBA
• pBA is the main promoter for the pap operon.
42. EPIGENETIC REGULATION
Ag43 phase variation: Antigen 43 (Ag43) is an outer membrane
protein in E. coli encoded by the Agn43 gene
• Ag43 causes auto-aggregation and enhances biofilm formation
• Phase variation of Ag43 is mediated by two proteins Dam and the
oxidative stress regulator OxyR.
• The agn regulatory region contains three GATC sequences that are located
within a binding site for OxyR
• When OxyR is bound to the regulatory region of Agn43 overlaps with
the promoter it inhibits transcription OFF phase
• Dam dependent methylation of the GATC sequences inhibits OxyR
binding allowing transcription of Ag43 ON phase
43. Molecular Mechanism
Genetic Regulation
a. Short sequence repeat and slipped strand mispairing- SSM
b. Homologous recombination
c. Site specific recombination
Epigenetic Regulation
a. PAP phase variation
b. Ag43 phase variation
Cell’s Regulatory Network
a. Cross regulation
b. Environmental regulation
44. CELL’ S REGULATORY NETWORK-Cross Regulation
• Cross regulation results in coordinated expression of two cell
surface structures.
• In P. mirabilis, coordinated expression occurs between phase-
varying MR/P fimbriae and non-phase-varying flagella. When
the mrp operon is in the “on” phase and fimbriae expressed,
transcription of the flagellar operon is repressed.
• In E. coli, expression of the phase-varying outer membrane
protein Ag43 and that of fimbriae also appears to be
coordinated. Fimbrial expression results in a repression of
Ag43 on the cell surface.
45. CELL’ S REGULATORY NETWORK-Environmental Regulation
• Environmental regulation of gene expression allows the
bacterium to be optimally suited to its growth
environment
• Iron starvation, for example, increases the frequency of
antigenic and phase variation of N. gonorrhoeae pili
• Stimuli such as temperature, pH, carbon source, and
amino acid concentration serve as signal and affect the
expression of phase-variable fimbriae in E. coli and S.
enterica serotype Typhimurium
46. References
1. Van der Woude DM. Phase and Antigenic Variation in Bacteria. 2004.
American Society for Microbiology 17.3:581–611
2. Ahmad S., Ahmad M. et al. An overview on phase variation,
mechanisms and roles in bacterial adaptation. 2017. J Pak Med Assoc .
67.2:285-291
3. Seifert HS., Magdalene SO. Genetic Mechanisms of Bacterial Antigenic
Variation. 1988, American Society for Microbiology . 52.3:327-336