LECTURE 3 (Microbial genetics) Flashcards
What is Genetics?
The science of heredity which includes the study of genes, how they are replicated, expressed and passed on from one generation to another
What is the Genome?
The genetic information in a cell including its chromosomes and plasmids
What are Chromosomes?
Structures containing DNA that physically carry hereditary information (genes)
What are Genes?
Segments of DNA that code for functional products
What does a nucleotide consist of?
- Nitrogen containing organic base
- Pentose sugar
- Phosphate group
How is Ribose different to Deoxyribose?
Ribose contains one more oxygen atom than deoxyribose
What is the difference between Genotype and Phenotype?
Genotype = The genetic makeup of the organism (the DNA that codes for all the particular characteristics of the organism)
Phenotype = The manifestation of the genotype
Describe the process of DNA Replication
1) Helicases unwind the parental double helix and single-strand binding proteins stabilise the unwound parental DNA
2) The leading strand is synthesised continuously in the 5’-3’ direction by DNA polymerases
3) The lagging strand is synthesised discontinuously. Primase synthesises a short RNA primer, which is extended by DNA polymerase to form an OZAKAZI FRAGMENT
4) After the RNA primer is replaced by DNA by another DNA polymerase, DNA ligase joins the Okazaki fragment to the growing strand
DNA replication allows the flow of genetic information from one generation to the next. This is called _________ ______ __________
Vertical gene transfer
What is Transcription?
The synthesis of a complementary strand of RNA from a DNA template
Explanation: Begins at the promoter and ends at the terminator. The mRNA made carries info from DNA to ribosomes to make proteins
Describe the process of Transcription
1) RNA polymerase binds to the promoter and the DNA unwinds at the beginning of the gene
2) RNA is synthesised by COMPLEMENTARY BASE PAIRING of free nucleotides with the nucleotide bases on the template strand of DNA
3) The site of synthesis moves along DNA and DNA that has been transcribed rewinds
4) Transcription reaches the terminator and RNA and RNA polymerase are released. DNA helix reforms.
Describe the process of Translation
1) Start codon (Methionine) is first recognised by ribosome subunits. mRNA code and tRNA anticodon bind to each other.
2) Amino acids add to growing polypeptide chain until the ribosome reaches a Stop codon
3) No new amino acid is added by the stop codon, instead it facilitates the release of the last amino acid from tRNA -> releasing the polypeptide
4) Polypeptide may now fold into a functional protein
What can changes in DNA/genetic variations result in?
- Biofilm formation
- Pathogenicity
- Antibiotic resistance
Explanation: A cell’s DNA can be changed by mutations and horizontal gene transfer
What is the difference between “Base substitution mutations” and “Frameshift mutations” and what they can cause?
Base substitution reactions = a single DNA base pair is altered
- could change a codon to encode a different amino acid
- small change in protein produced
Frameshift reactions = DNA base pairs are added or removed from the sequence, causing a shift in sequence reading
- can alter gene/proteins so no longer can function
Why is Nitrous acid a mutagen?
Nitrous oxide can convert the base Adenine to a form that pairs with CYTOSINE instead of thymine -> When DNA containing modified adenine replicates -> One daughter DNA molecules will have a base-pair sequence different from that of parent DNA
Why is Nucleoside analog a mutagen?
Structurally similar to normal nitrogenous bases but have altered base-pairing properties -> Nucleoside analog randomly incorporated into cellular DNA in place of normal bases -> during DNA replication, analogs cause mistakes in base pairing -> incorrectly paired bases copied during DNA replication
Why are X-rays, Gamma rays and UV light mutagens?
X-RAYS & GAMMA RAYS: form free radicals, ionised atoms and molecules that alter the structure of other molecules
UV LIGHT: form harmful covalent bonds between pyrimidine bases
What are the different mechanisms of bacterial gene transfer?
- TRANSFORMATION = Lysis of donor cell releases DNA into medium which is taken up by recipient cell and recombination occurs between donor DNA and recipient DNA
- CONJUGATION = Donor DNA is transferred directly to recipient through a connecting tube (contact + transfer promoted by specialised plasmid in donor cell)
- TRANSDUCTION = Bacteriophage infects a cell -> lysis of donor cell and donor DNA is packaged in release bacteriophage -> Donor DNA is transferred when phage particle infects recipient cell
How is Conjugation different from Transformation?
- Conjugation requires direct cell-to-cell contact
- Conjugating cells must be opposite of mating type (donor cells must carry the plasmid, recipient cells do not)
Describe Generalised Transduction and Specialised Transduction
GENERALISED TRANSDUCTION
1) Bacterial host cell (donor cell) is infected by phage and host cell DNA is broken down into smaller pieces. Proteins and phage DNA is also synthesised.
2) Bacterial host DNA is packaged into viral capsids that are released through lysis of bacterial cell
3) Transducing phage with host DNA infest new recipient cell. Recombination occurs.
4) Recombinant new cell has mixture of donor DNA and its own DNA. Genotype is different.
SPECIALISED TRANSDUCTION
1) Prophage DNA is integrated into bacterial DNA and cuts incorrectly and exchanges its DNA into that of bacterial host cell (prophage DNA + host DNA mix)
2) Phage capssids contain bacterial host DNA which is transferred to a new bacterial recipient cell
3) Transducer phage is transferred to a new cell + recombination occurs
4) Recombinant cell DNA has a different genotype, different from donor or recipient cell genotype
What are the properties of Streptococcus?
- Gram-positive
- Non-motile
- Catalase-negative
- Anaerobic cocci that occur in chains/pairs + widely distributed in nature
What are the different Streptococcus?
- STREPTOCOCCUS PYOGENES (GROUP A) = found on skin and oropharynx in small numbers, β-hemolytic group, causes “strep throat” which can lead to rheumatic fever + heart disease
- STREPTOCOCCUS AGALACTIAE (GROUP B) = occurs in vagina, the most frequent cause of sepsis in newborns
- STREPTOCOCCUS PNEUMONIAE = found in oropharynx, a leading causes of pneumonia and meningitis in persons of all ages
- VIRIDANS STREPTOCOCCI = found in oropharynx
- ENTEROCOCCI + ANAEROBIC STREPTOCOCCI = located in colon
What can Streptococcus be classified into?
- α-hemolytic
- β-hemolytic
- γ-hemolytic
Which cell wall carbohydrates (C carbohydrate) are the most common causes of serious disease?
Group A & Group B
Explanation: Groups C, E, F, G, H and K-U streptococci infrequently cause human disease
What are the two important antigens of β-hemolytic streptococci?
- C CARBOHYDRATE/Lancefield antigens = determines the group of β-hemolytic streptococci and is located in the cell wall
- M PROTEIN = the most important virulence factor
[group A streptococci are divided into more than 100 serotypes based on antigenic differences in the M PROTEIN]
Strains of S. progenies that produce certain M protein types are ____________ whereas strains of S.pyogenes that produce other M protein types are _____________
RHEUMATOGENIC & NEPHRITOGENIC
Where is group D carbohydrate found?
In the genus Enterococcus
What are the 5 types of β-hemolytic Streptococci Toxins?
- Erythrogenic toxin = causes the rash of scarlet fever. It is produced only by certain strains of S. pyogenes lysogenised by a bacteriophage carrying the gene for the toxin
- Streptolysin O = pore forming and antigenic
- Streptolysin S = hemolysin that is not inactivated by oxygen
- Pyrogenic exotoxin A = Toxin responsible for most cases of streptococcal toxic shock syndrome
- Exotoxin B = protease that rapidly destroys tissue and is produced in large amounts by strains of S. pyogenes and is what causes necrotising fasciitis
Why are antibodies not formed against S. pyogenes’ capsule?
Antibodies are not formed against the capsule because hyaluronic acid is a normal component of the body and humans are tolerant to it
What are the 4 types of β-hemolytic Streptococci hemolysins?
- C5a peptidase degrades complement
- Hyaluronidase = degrades hyaluronic acid and is known as spreading factor
- Streptokinase (fibrinolysin) = activates plasminogen to form plasmin which dissolves fibrin in clots, thrombi and emboli
- DNase (streptodornase) = degrades DNA in exudates or necrotic tissue
Summarise Group A Streptococcus (GAS) disease
- Primary sources of infection are respiratory droplets or direct contact with the skin
- Impetigo (highly contagious skin infection) results from minor trauma such insect bites
- In streptococcal toxic shock, StrepSAqs producing GAS in superficial lesion spread into the bloodstream
Describe the three mechanisms that Group A Streptococci cause disease
- PYOGENIC INFLAMMATION = induced locally at the site of the organism in tissue (pharyngitis and cellulitis)
- EXOTOXIN PRODUCTION = can cause widespread systemic symptoms in areas of the body where there are no organisms (scarlet fever and toxic shock syndrome)
- IMMUNOLOGIC = occurs when antibody against a component of the organism cross-reacts with normal tissue/forms immune complexes that damage normal tissue (rheumatic fever and acute glomerulonephritis)
Group A Streptococci infections can be divided into which categories?
- Diseases attributable to invasion by S pyogenes (β-hemolytic Group A Streptocci)
- Disease attributable to local infection with S pyogenes and their byproducts
- Invasive Group A Streptococcal infections (Streptococcal toxic shock syndrome and scarlet fever)
- Post streptococcal diseases (Rheumatic fever, Glomerulonephritis)
Which diseases are attributable to invasion by S pyogenes (β-hemolytic Group A Streptococci)?
- Erysipelas
- Cellulitis
- Necrotising fasciitis
What is Erysipelas?
An infection of the upper dermis and superficial lymphatics and often affects the lower extremities and face
SYMPTOMS:
- fever/chills
- generally feeling unwell
- a red, swollen and painful area of skin with a raised edge
- blisters on the affected area
- swollen glands
What is Cellulitis?
An acute, rapidly spreading infection of the skin and subcutaneous tissues
SYMPTOMS:
- pain in the affected area
- redness/inflammation of skin
- swelling
- fever
- leaking of yellow, clear fluid or pus
What is Necrotising fasciitis?
A very rare but serious infection that can destroy skin, fat and tissue covering the muscles within a very short time. Also called “flesh-eating bacteria” and is called “Fournier’s gangrene” when it occurs on the genitals.
SYMPTOMS:
- fatigue/weakness
- fever with chills and sweating
- nausea/dizziness
- vomiting
- infrequent urination
Which diseases are attributable to local infection with S pyogenes and their byproducts?
- Streptococcal sore throat
- Streptococcal pyoderma
- Scarlet fever
What is Streptococcal sore throat?
Also known as “Pharyngitis”, is the most common infection caused by β-hemolytic S pyogenes
SYMPTOMS:
- sudden fever
- sore, red throat with white patches
- headache
- chills
- loss of appetite
- swollen lymph nodes in the neck
- trouble swallowing
What is Streptococcal pyoderma?
Also known as “Streptococcal impetigo” or “impetigo contagiosa”, is a local infection of superficial layers of skin
What is Scarlet fever?
A bacterial infection caused by group A Streptococcus. It is a strep throat with a rash.
DIAGNOSIS:
- swab test
- blood test
- blood test for bacterial identification
[sometimes confused with Rubella, Kawasaki disease, Drug eruption]
SYMPTOMS:
- very red, sore throat
- fever
- red rash with sandpaper feel
- bright red skin in underarm, elbow
- “strawberry” (red + bumpy) and whitish coating on tongue
- headache or body aches
- nausea, vomiting or abdominal pain
- swollen glands
TREATMENT:
- penicillin (oral antibiotic for 10 days)
What are the long-term health problems from Scarlet fever?
- Rheumatic fever (inflammatory disease affecting heart, joints, skin and brain)
- Kidney disease (inflammation of kidneys “post-streptococcal glomerulonephritis”)
- Otitis media (ear infections)
- Skin infections
- Abscesses of the throat
- Pneumonia
- Arthritis
What is Streptococcal Toxic Shock Syndrome?
A toxin-mediated acute life-threatening illness characterised by shock, bacteremia, respiratory failure and multi organ failure
DIAGNOSIS:
- physical examination
- symptoms
- blood and urine test for bacteria identification
- blood test for liver and kidney function
SYMPTOMS:
- vague myalgia (muscle aches + pain)
- chills
- pain at the infected site
- nausea, vomiting and diarrhoea followed by hypotension
TREATMENT:
- IV antibiotic therapy under monitoring
- medication to stabilise blood pressure
- IV fluids to fight dehydration
- gamma globulin injections to suppress inflammation + boost immune system
What is Acute Rheumatic Fever?
An autoimmune disease, post-infection complication of streptococcal infection that occurs 2-3 weeks after group A streptococcal pharyngitis
DIAGNOSIS:
Jones criteria is used - two major criteria or one major and two minor plus a history of strep throat
MAJOR CRITERIA
- arthritis in several joints (polyarthritis)
- heart inflammation (carditis)
- nodules under the skin (subcutaneous nodules/Aschoff bodies)
- rapid, jerky movements (Sydenham’s chorea)
- skin rash (erythema marginatum)
MINOR CRITERIA
- fever
- high ESR (erythrocyte sedimentation rate = sign of inflammation)
- joint pain
- ECG changes
- other lab findings (elevated C-reactive protein, elevated/rising streptococcal antigen signs)
TREATMENT
- penicillin
- erythromycin
- azithromycin
What is Glomerulonephritis?
An immune mediated disease causing inflammation and damage to the filtering part of the kidneys
SYMPTOMS:
- hematuria
- proteinuria
- red blood cell casts in urine
- edema
- hypertension with/without oliguria
- fever
- malaise
PATHOGENESIS
- Group A streptococcal infection of skin/pharynx
- antibody against M protein cross-reacts between cell wall of S. pyogenes and glomeruli of kidney
- activation of complement cascade
DIAGNOSIS
- antibody titer in blood
- urine analysis
- chest radiographs + renal ultrasound
- histologic tests
- renal biopsy
TREATMENT
- antibiotic therapy
- immunosuppressant
What is S. agalactiae Group B Streptococcus?
The Group B Streptococcus colonises the vagina, GI tract and the upper respiratory tract of healthy humans. It causes postpartum infection and is the most common cause of neonatal sepsis.
[also causes pneumonia, endocarditis, arthritis, cellulitis and osteomyelitis in adults]
DIAGNOSIS
- Isolation GBS from blood
- GBS antigen detection in blood
- chest radiography, CT scanning, echocardiography
- CAMP factor +ve
- Hippurase +ve
TREATMENT
- penicillin
- ampicillin
- vancomycin
- clindamycin + cefazolin (pts allergic to penicillin)
What is Pneumococcal disease?
Any infection caused by Streptococcus Pneumoniae/Pneumococcus
that begins abruptly with a shaking chill and high fever
SYMPTOMS:
- shaking chill followed by bloody sputum
- lung consolidation typically lobar
PATHOGENESIS:
colonisation -> crossing of mucosal barrier -> invasion of bloodstream (leads to meningitis sepsis) + local invasion (Ottis media, sinusitis, non-bacteraemic pneumonia -> can lead to bacteraemic pneumonia -> can lead to invasion of bloodstream)
TREATMENT
- pneumococcal vaccine
- antibiotic therapy with broad spectrum antibiotic
- levofloxacin
What are the different Streptococcus diagnostic labaratory tests?
- Specimen (throat swab, blood test, serum antibody determinants -> for culture identification)
- Smears
- Cultures
- Antigen detection tests
- Serologic tests
