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Bacterial Genetics. Medical Microbiology. You have known …. Science of genetics describes and analyze heredity of physiologic functions that form the properties of organism. These properties are determined by the total of all the genetic information named genome . - PowerPoint PPT Presentation
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You have known…You have known…Science of Science of geneticsgenetics describes and analyze describes and analyze heredityheredity of of physiologic functions that form the properties of organism. physiologic functions that form the properties of organism.
These properties are determined by the total of all the These properties are determined by the total of all the genetic information named genetic information named genomegenome..
Basic unit of genetics isBasic unit of genetics is genegene, a segment of DNA that , a segment of DNA that carries in its nucleotide sequence information for a specific carries in its nucleotide sequence information for a specific biochemical or physiologic property.biochemical or physiologic property.
A geneA gene is relatively stable but occasionally may undergo a is relatively stable but occasionally may undergo a nucleotide change, such a change is called as nucleotide change, such a change is called as mutationmutation. .
Mutations may occur Mutations may occur spontaneouslyspontaneously or can be or can be inducedinduced by a by a number of physical or chemical agents. number of physical or chemical agents.
Bacteria may have changes including:Bacteria may have changes including:Morphological and/or structural changesMorphological and/or structural changes
Variations of cultural characteristics and Variations of cultural characteristics and
biochemical reactionsbiochemical reactions
Changes in virulenceChanges in virulence
Variation in antigenicityVariation in antigenicity
Changes in drug resistanceChanges in drug resistance
These variations can be divided into two types:
▲ Phenotypic variation:Phenotypic variation: non-heritable▲ Genotypic variation:Genotypic variation: heritable (mutation)
Bacterial Bacterial GenomeGenome
DNA/Genome:
Genetic materials relative to bacterial mutation.
A. chromosome
B. out of chromosome: a) plasmid
b) phage
c) transposable genetic element
1. Chromosomal DNA1. Chromosomal DNA
Bacterial chromosome consists of a
single, circle of double-stranded DNA.
▲ 2 mm long in average
▲ Usually < 5 Mb (1 Mb = 1024 Kb)
Transfer property
▲ Conjugative plasmid : mediate conjugation through sex
pilus
▲ Non-conjugative plasmid: can not mediate conjugation
because of no gene for encoding sex pilus
Phenotypic effect
▲ Fertility plasmid (F factor): has a sex pilus-encoding gene
▲ Resistance plasmid (R factors): contains genes that
encoding enzymes to destroy antibiotics
2. Plasmid2. Plasmid
Structure of R Factor
RTF (Resistance Transfer Factor)
▲ Conjugative plasmid
▲ Transfer genes
Tn 9
Tn
21
Tn 10
Tn 8RTF
R determinant
R determinant▲ Resistance genes
▲ Transposons (Tn, 转座子 )
3. Phage3. Phage
Phages are obligate
intracellular parasites that
specially propagate in bacteria
(bacterial virus).
▲Nucleic acid: DNA (dsDNA or
ssDNA) or RNA (dsRNA or ss
RNA).
▲ Protein: function in infection
and protect nucleic acid.
▲ Head: DNA or RNA+protein coat (capsid)
▲ Tail: is composed of a hollow core surrounded by a sheath with base plate and at the end.
Tail
Tail Fibers
Base Plate
Head
Contractile Sheath
CapsidDNA
Structure of T4 phage
Interaction between phages and bacteria
Phages are wide spread in nature but usually has a
specificity of host bacteria.
Infection with a virulent phage results in phage replication
with the production of new phage particles and their
subsequent release causes death of host bacteria.
adsorption penetration biosynthesis maturation /
release
Infection with a temperate phage does not necessarily
lead to bacterial death, but phage’s nucleic acid integrates
into bacterial chromosome without new phase production.
adsorption penetration integration
I. Adsorption
Recognition of host bacterial surface receptors by tail fibers
Phage’s nucleic acid is injected through hollow core into bacterial cytoplasm.
II. Penetration
Virulent phage
Temperate Phage (Prophage (Prophage formation) formation)
I. adsorptionII. peneration
III. integrate of phage DNA into host genome
IV. prophage replicates along with chromosome of lysogenic bacterium
4. Transposable Genetic 4. Transposable Genetic ElementsElements
Types of Transposable Genetic Elements ▲ Insertion sequences (IS) ▲ Transposons (Tn)
Definition: segments of DNA that have the capacity to
move from one bacterial DNA molecule (bacterial chromosome or plasmid) to another or from one location to another in the same one DNA molecule
(jumping gene / movable gene)
Insertion sequence (IS)
▲ Definition: a type of transposable Genetic Elements
only has a transposase-encoding gene alone.
▲ Structure: a small DNA has reverse repeated
sequences at the two ends that are involved in
transposition. In the middle there is a transposase
coding gene.
▲ Function: introduction of an insertion sequence into
a bacterial gene will result in inactivation of the
gene.TransposaseABCDEFG GFEDCBA
Transposon (Tn)
▲ Definition: a type of transposable Genetic Elements has both
insertion sequences (IS) and other genes.
▲ Structure: the extra genes are located between the terminal
repeated sequences.
▲ Function: Since transposons can jump from one DNA molecule
to another and frequently carry antibiotic-resistant genes,
the transposons mediate drug resistance in bacteria.
IS ISResistance Gene(s)
IS ISResistance Gene(s)
Types of Mutation Self Mutations: Spontaneously occur with low frequency.
Gene transfer and recombination: high frequency.
One bacterium (recipient) uptake exogenous DNA
segment from another bacterium (donor) or phage (Gene transfer) and then the DNA segment is incorporated into DNA of itself (recombination). Major mechanisms of bacterial gene transfer
▲ Transformation ▲ Transduction
▲ Conjugation ▲ Lysogenic conversion
Transformation
Definition: a bacterial recipient uptake naked DNA segment offered by bacterial donor in environment and then the DNA segment recombinate with the recipient’s chromosomal DNA.
Significance for transformation
Transformation occurs in nature and it can lead to increased virulence ( e.g. S. pneumoniae) and drug resistance.
Transduction
Definition: a bacterial donor’s chromosomal DNA
segment transferred to a bacterial recipient by way
of a phage, and then the DNA segment recombinate
with the recipient’s chromosomal DNA . ▲ Generalized Transduction: incorrect assemblage (any
genes).
▲ Specialized transduction: Sometimes, during
activation of prophage, the excised phage’s DNA
contains host DNA segments at its ends.
Lysogenic conversion
Definition: a bacterial recipient is infected
with phage from a bacterial donor, and the
phage’s genes recombinate with recipient’s
chromosomal DNA.
As an example, Corynebacterium diphtheriae
will produces diphtheria toxin after it is
infected by the β- phage, because the gene
encoding the toxin is carried by the phage.
Conjugation
Definition: Gene transfer from
a donor to a recipient by direct
contact between two bacterial
cells through sex pili.
▲ Donor: a bacterium with F factor
can produce sex pili.
▲ Recipient: a F factor-absent
bacterium.
Donor
Recipient
F+ factor-depecdent Conjugation
The F+ bacterium transfers extra chain of F+ factor and then the completed F+ factors in the two bacteria is synthesized by rolling circle replication.
F+ F+F+ F+
F+ F- F+ F-F+ F-
High-frequency recombination (Hfr)F factor is integrated into bacterial chromosome.
Bacterial DNA is transferred with a high frequency.
F factor is difficulty to be transferred.
F-Hfr Hfr F-
Hfr
Hfr F-
F-
F factor in bacterial DNA is excised but it carries
bacterial chromosomal DNA at its two sides (F’).
F’ factor-depecdent Conjugation
F’Hfr F’ F’F’ F-
SummarySummary
The most important contents in this lecture are
displayed as the followings:1) The Agents (plasmid, phage, bacterial chromosomal DNA)
associated with bacterial mutation.
2) Concepts of Transformation, Transduction, Conjugation
and Lysogenic conversion.
3) The Significance of bacterial mutation (e.g. bacterial
virulence, drug resistance and antigenicity).
Bacterial PathogenicityTerms :Terms :
Pathogen: disease-causing microbe.
Opportunistic pathogen: A microbe does not
cause diseases in normal conditions, but can
cause diseases in some certain conditions.
Pathogenicity: ability of a microbe to cause
diseases.
Virulence: quantitative ability of a microbe to
cause disease (invasiveness and toxigenicity).
LD50: the number of pathogen required to cause
death in half of the exposed hosts.
Source of infectionSource of infection
Exogenous infection
Infectious agents come from environment or
hosts (patient, diseased animal or carrier).
▲ carrier: individuals (human and animal) infected with microbes but no clinical signs or symptoms.
Endogenous infection
Normal flora act as infections agents under
some certain conditions (opportunistic
infection).
Normal flora
Microbial populations inhabit on skin
and mucosa of healthy normal
persons. ▲ Gastrointestinal tract
▲ Urogenital tract
▲ Skin and Conjunctiva (结膜 )
Physiological role of normal flora
▲ Antagonism: a) biolfilm; b) antimicrobial
agents.
▲ Trophism: a) digestion of foods; b)
production of vitamins (K and B).
▲Immunoenhancement: promotes
development of mucosal immune system.
▲ Anti-tumorigenesis and anti-apolexis( 衰老 ):
e.g. eliminate nitrite and anti-oxidation
(SOD).
Opportunistic infectionsOpportunistic infections
▲ Translocation of normal
flora.
▲ Suppression of normal flora.
▲ Low immunity of human
body.
The conditions required by The conditions required by normal flora to cause normal flora to cause diseases.diseases.
Hospital acquired
infections
(Nosocomial
infections)New infectionsinfections after 48 hours of hospital admission.
Pathogenic process of bacteria
Generally, infection process caused by a
bacterial pathogen involves the four steps
as the following: ▲Adhesion (chemotaxis) Adhesion (chemotaxis)
▲▲ Survival / propagation Survival / propagation
▲▲ Penetration and sPenetration and spreadpread
▲▲ Tissue injuryTissue injury
Virulence: adhesins, invasive enzymes and
toxins.
adhesinadhesin
EPITHELIUMEPITHELIUM
receptorreceptor
BACTERIUMBACTERIUM
1. AdhesionA process of recognition and combination of microbial adhesins with receptors on the surface of host cells▲ lipoteichoic acid (G+ bacteria)
▲ ordinary pilus and outer membrane proteins (G- bacteria)
2. Survival / propagation2. Survival / propagation
•Anti-phagocytosis: e.g. capsules.
•propagation: probably due to
stress.
3. Penetration and Spread3. Penetration and Spread
▲ Collagenase: hydrolyze collagens.
▲ Hyaluronidas: hydrolyze hyaluronic acid.
▲ Coagulase: agglutinate fibrinogen.
▲ Streptokinase/fibrinolysin: the former activates fibrinogenase
to thrombin, and the latter directly hydrolyze fibrin.
▲ Cytolysins: 1) hemolysin (to lyse erythrocyte or tissue cells )
2) Leukocisin (to kill leukocyte or tissue cells )
4.1. Exotoxin
▲ Cytotoxin: e.g. diphtheria toxin
▲ Nuerotoxin: e.g. tetanospasmin
▲ Enterotoxin: e.g. cholera enterotoxin
diphtheria toxin
ToxicToxicBindingBinding
AA
Cell surfaceCell surface
BB
A-B type exotoxinsA-B type exotoxins
▲ Subunit A is toxic, while subunit B is the ligand of cell’s receptor to mediate A subunit into host cells.
4.2. Endotoxin (LPS)4.2. Endotoxin (LPS)
LPS
▲Fever: a typical pyrogen.
▲ Leukocytoreaction: leukopenia/leukocytosis.
▲ DIC: disseminated intravascular coagulation.
▲ Shock: dilatation of small blood vessels
▲ Inflammation: IL-1β , TNF-αand IL-6.
Major difference between endotoxin and Major difference between endotoxin and exotoxinexotoxin
Property Endotoxin Exotoxin
Chemical nature Lipopolysaccharide( MW = 800-1, 000 kDa )
Protein or peptide( MW = 50-1, 000 kDa )
Relationship to bacteria Part of outer membrane Extracellular
Denatured by boiling No Usually
Antigenicity Yes Yes
Form toxoid No Yes
Toxicity Relatively low( > 100 µg )
Relatively high( approximate 1 µg )
Pyrogenicity Yes Occasionally
Specificity to host cells Low degree High degree
Enzymatic activity No Usually
4.3. Immunopathological reaction
Human immune responses to bacteria may cause tissue injury by:
1. Numerous cytokines and complement activation.
2. Continuously generated bacterial antigens will
subsequently causet humoral antibodies and cell
mediated immunity, which resulting in chronic
immunopathological injury.
3. Some of bacterial antigens (e.g. M protein of
Streptococcus) react with host tissue antigens to cause
autoimmunity.
Number and route▲ Invaded mumber of bacteria:
Different pathogens need different bacterial number to cause diseases.
▲ Suitable invading route of bacteria:
Most of bacteria require suitablec invading routes to cause diseases (e.g. Clostridum tetani causes disease through wounds and Mycobacterium tuberculosis has multiple invading routes).
Types of infection
According to infectious state:According to infectious state:
▲ Inapparent or subclinical infection: The
infection with no manifesting clinical
signs and symptoms.
▲ Latent infection: The infection is inactive
but maintain potential to cause diseases.
▲ Apparent infection: clinical signs and
symptoms.According to infectious site:According to infectious site:
▲ Local infection; ▲ Systemic infection
Systemic infection
▲ Bacteremia: Bacteria enter bloodstream without propagation in bloodstream.
▲ Toxemia: Exotoxin or endotoxin rather than bacteria enters bloodstream.
▲ Septicemia: Bacteria enter bloodstream with propagation and release virulent factors.
▲ Pyemia:Pyogenic bacteria enter bloodstream with propagation and release virulent factors, and then spread through bloodstream into the target organs to form pyogenic foci.
SummarySummaryThe most important contents in this lecture are displayed as the
followings:
1) Concepts of virulence, normal flora, hospital acquired
infection, latent infection, toxemia, septicemia, endotoxemia
and pyemia.
2) The physiologic role of normal flora.
3) The conditions for generation of opportunistic infection.
4) The difference between exotoxin and endotoxin.
5) The pathogenic effects of endotoxin.
Innate ImmunityPhagocytosis: mononuclear-
macrophages, neutrophils.
Complements: lyse bacteria
Others: lysozyme, antibacterial peptide.
Acquired Immunity
Antibody: antibacterial antibody and
antitoxin (IgM, IgG and SIgA).
T lymphocyte: cytotoxicity (CD4Th1, CTL).
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