Theme I: Infectious agents & diseases Flashcards
What is the pathogenesis of prion diseases
- A normal protein undergoes a change into an abnormal mis-folded prion form
- The prion form binds to normal proteins in the host’s brain & convert these into misfolded proteins.
- Infectious as spread cell to cell
- These accumulate & aggregate to form amyloid fibrils (stacked Beta sheets).
- Plaques form & attack brain tissue causing neurodegenerative diseases.
Why viruses are called obligate intracellular parasites
- They cannot reproduce independently
- require chemical & structural components of host cells.
- they have no ribosomes, metabolism, or genes that encode enzymes for DNA replication.
- Use the host’s metabolic machinery
What is a nucleocapsid. what is the capsid’s function. and what is the envelope of viruses.
- Capsid + nucleic acid
- Capsid= a protein coat of repeating subunits that surrounds the nucleic acid.
- Capsid protects the NA, and helps it enter into host cells.
-Some virions have a phospholipid membrane envelope surrounding it. The envelop is derived from infected host cell membrane and glycoproteins.
Difference between virus & virion
-Virus= acellular (no organelles, ribosomes etc.) infectious agent that only replicates inside living organisms.
- Virion is the extracellular state of a virus. It is the infectious viral particle outside the cell
- becomes a virus when it enters a cell.
Protein synthesis of: DNA viruses, positive sense ssRNA, and negative sense ssRNA virus.
(ss = single stranded)
- DNA = transcription then translation
- Positive sense RNA = can be directly used as mRNA for translation
- Negative sense RNA = require synthesis of complementary RNA to the viral RNA.
Lytic Replication of a virus
=Reproduction of a virus using a host cell.
- Attaches to the cell, enters it, synthesises its viral nucleic acids & proteins, and assembles new viruses.
- The host cell is destroyed and the new viruses are released via budding where it acquires a portion of the host’s virus-encoded glycoproteins.
Lysogenic replication of a virus
- Involves the integration of the viral genome into the host cell’s DNA. Host daughter cells are produced which carry the viral DNA
- host DNA not destroyed
What is recombinant DNA technology
- Genetic engineering that manipulates the genome of an organism to introduce new genes of desired characteristics.
- DNA is constructed & inserted into cells via a vector/plasmid
- Bacteria that take up this plasmid can be identified & selected and then cultured & cloned.
What is a plasmid
- Small, circular DNA, replicates independently from chromosomal DNA
- Carries extra features eg. antibiotic resistance
- Can be transferred between cells.
What are the 3 types of horizontal gene transfer
- Transformation: When a cell dies, its DNA is released and taken up by recipient cell
- Transduction: Bacterial phage (virus) transfers DNA from 1 cell to another
- Conjugation: Donor bacteria injects DNA into another via a pilus.
Goals of genetic manipulation
- To understand the function of a gene, to take out unessential genes, to change individual amino acids in proteins, to clone proteins.
- Helps to develop vaccines, to generate improved probiotic bacteria, and produce recombinant proteins such as insulin
- Can be used to produce blood proteins, interferon, IL-2, lysozyme etc.
What is a vector and what do they contain
- Vectors act as vehicles to carry recombinant DNA into host cells.
- Usually plasmids, also bacteriophage, cosmids
1- Origin of replication
2-restriction sites - for insertion of new DNA (poly linker) that can be used in cloning.
3-Gene(s) to allow selection of bacteria that takes up the plasmid. Eg. antibiotic resistance gene. Bacteria that take this up will be able to survive and grow on an agar plate with ampicillin.
How recombinant DNA technology could prevent tooth decay. What else could recombinant DNA be used to produce
- Create a genetically altered strain of streptococcus mutans (SMaRT) to contain a gene that would make it not produce lactic acid.
- SMaRT bacteria would grow & colonise in the mouth and outcompete bacteria causing tooth decay.
What are biofilms, what the matrix is made of, (macro and micro molecules) and what it means by heterogeneity.
- Community of microbes, enclosed in a sticky matrix of extracellular polymers, found on interfaces.
- Infection is confined to a particular site.
- Matrix made up of polysaccharides, proteins & nucleic acids. Nutrients, metals, signalling molecules may also become trapped.
-Heterogeneity means they are diverse 3D structures containing a mixture of species. Distribution of nutrients/ waste products is non-uniform, so have pockets of extremely high/low oxygen & growth.
Why are biofilms so resistant. 6 major points
- Bacteria is slow growing so cell wall growth is slow - antibiotics rely on fast growth.
- They have persister cells which are inactive/ dormant so difficult to kill. Form small colonies that cling to surfaces and cause delayed infection when they eventually become active.
- Bacteria are protected against the host’s immune system by the matrix. Immune cells cannot penetrate the matrix so resistant to phagocytosis
- Efflux pumps pump out toxic material (antibiotics)
- Electrostatic negatively charged matrix traps cationic antimicrobials so cannot penetrate
- Adhere to surfaces so difficult to remove
- Antibiotics poorly penetrate, for example positive tobramycin repelled by negative bacteria DNA
What is the human microbiome, metagenome, transcriptome, proteome, metabelome, mycobiome
- Microbiome is the total collection and genetic material of microbes in our body (bacteria, viruses, fungi)
- Metagenome= total DNA of all cells
- Transcriptome= total RNA
- Proteome= total set of proteins encoded by a genome
- Metabelome = total metabolites
- Mycobiome= fungi community
What is the ‘Core’ genome and what affects it, and the ‘Pan’ genome of bacteria
- Core genome is made up of the essential genes, shared by all strains of the species.
- Affected by host environment, health, diet, immune system etc.
-Pan genome is the acquired extra features that are not essential and only present in some strains. May encode key virulence factors which give them advantages and aid in causing disease (eg. protective capsule protecting it from the host’s immune system)
Chromosomal & pathogenicity islands of bacteria
- Chromosomal islands are regions of chromosome of foreign origin, so have been acquired by horizontal transfer
- Pathogenicity islands are chromosomal islands that contain genes & properties for virulence/ harmfulness
What did the human microbiome project study?
- It discovered the roles of microorganisms in the different parts of our body
- It identified the species of bacteria at different body sites that are present in everyone, to correlate these with the diseases they cause.
Why antibacterial mouthwash not always good. Give an example of how mouth bacteria can be helpful.
- Kills good and bad bacteria.
- bacteria in our mouth can reduce nitrate on our tongue to nitrite in saliva. Nitrite is absorbed & enters our circulation & becomes nitric oxide. This is responsible for dilating blood vessels & lowering blood pressure.
- So if these bacteria are killed there is less nitric oxide so vessels are more constricted causing higher blood pressure.
What gene is present and unique to all bacteria. What gene is found in fungi
- Bacteria= 16S rRNA gene
- Fungi= 18S rRNA
What is the miasma theory and how did Koch disprove this. What did he prove
- Miasma- rancid smelling air was causing disease
- Koch disproved the idea
- He discovered that a specific microorganism (Bacillius anthracis) was present in the blood of all animals with Anthrax disease, and did experiments to show that the bacteria was causing the disease.
- He proved the germ theory - microorganisms cause disease.
What were Koch’s 4 postulates for determining cause of disease (in 1884), but the limitations later discovered in modern times
1-The microorganism must be present in all cases of disease but not in healthy individuals (BUT people can be asymptomatic carriers)
2-Organism must be isolated from a diseased host & grown in pure culture (BUT some viruses & prions are difficult/ impossible to culture. And some infections are polymicrobial)
3-Isolated organism must cause disease when introduced in a suitable animal (BUT animal models may not be available/ not representative of human disease)
4-The organism must be re-isolated from said infected animal
What are the current postulates for disease aetiology
1-DNA sequence of a pathogen should be present in most cases of disease, preferably found in organs known to be diseased.
2-Few/ no copies should be found in the host or tissues with no pathology
3-As the disease resolves, it should decrease & be undetectable
4-Sequence copy number should correlate with severity of the disease
5-Proof of filterability (eg. viruses filtered out as smaller than bacteria)
6-Detection of immune response to the virus
7-Virus can be isolated from diseased host, and cause a healthy person to become diseased when injected into them
8-Sequences should be reproducible
What is legionella bacteria. What is their link to dental clinics
- Grow best in warm water and they survive in another organism called an amoeba which protects them from harsh environments.
- Deaths in dental clinics have been linked with contamination of dental waterlines with legionella. So all dental practices in the UK must have a legionella risk assessment.
What is a pathogen and an opportunistic pathogen
- A pathogen is an organism that can cause disease in an otherwise healthy individual
- Opportunistic pathogen is an organism that would not usually cause disease, but can do if the host has a defect in its defences/ is compromised (eg. injury, immunosuppression)
Different between infection and disease
- Infection is when a non-native organism grows in a host, with or without causing damage.
- Disease arises when there is damage to the host.
Virulence meaning, and examples of virulence factors of bacteria (what are endotoxins and exotoxins)
-Virulence= Severity of disease a bacteria can cause.
- The virulence factors= properties of a bacteria that contribute to its virulence - help it survive and multiply in the host.
- Toxins manipulate host cell functions causing fatal disease: Endotoxins (LPS released by gram-negative bacteria when they die, & cause inflammation & septic shock) and Exotoxins (secreted by gram negative & positive).
- Tissue-degrading enzymes (eg. collagenase, haemolysin)
- Adhesins, invasins etc.
Difference between endogenous and exogenous infection
- Endogenous: caused by infectious agents already present in the body, previously dormant.
- Exogenous: agents acquired from the environment. Most common
Mechanisms of how bacteria causes disease. Stages of infection
1-Adherance to host tissue (pilus interact with host receptors/ glycoproteins)
2-Enter the cell- invasion through epithelium
3-Infection= Growth and reproduction of the bacteria and its virulence factors/ toxins in the host cell. They do this to obtain nutrients and because its an appropriate environment for them.
4-Bacteria spread, either in local or distant sites
5-Damage to the host - cell destruction
Give examples of defence mechanisms for disease
- Innate: Skin providing a physical barrier to stop penetration of bacteria, stomach acidity to inhibit growth, commensal bacteria for outcompeting bad bacteria, flushing of urinary tract to prevent infection, mucous & cilia in trachea to remove microbes, secretions (lysozyme dissolves cell walls, bile salts)
- Adaptive: Phagocytosis, inflammation to carry immune cells to infected site, fever, antibodies to bind to pathogen which will then be engulfed by macrophages.
Why infection (such as c difficile) spread easily in hospitals
- Outbreaks common for bacteria that produce spores which are highly resistant and stay on many surfaces which people touch then ingest
- Difficult to clean curtains and bedding etc. properly so bacteria stay on these surfaces
- Hospitals are usually hot environments which bacteria thrive in.
- Lots of patients are ill and compromised so easier for them to become ill with infection
- Hospitals have thousands of patients and visitors every day so easily transmitted from person to person. Lots of people touching lots of things.
