Immunology - Pathogens Flashcards
What are pathogens?
disease causing micro-organisms
What are the 4 groups of pathogens?
- Bacteria
- Viruses
- Fungi
- Parasites
What are bacteria?
Bacteria are prokaryotic micro-organisms
What are the 5 shapes of bacteria?
- Cocci
- Bacilli
- Spirilla
- Vibrios
- Spirochaete
Describe bacilli shape
Also known as rods, these are long and thin.
Describe cocci shape
These spherical micro-organisms are found grouped together, as staphylococci (clusters), streptococci (lines) or diplococci (paired).
Describe spirilla shape
Spiral-shaped bacteria, although these are less common.
Describe vibrios shape. What is an example?
Flagellated (tailed) organisms, a notable example of which is Vibrio cholerae, the causative organism of cholera.
Describe spirochaete shape. What is an example?
These are tightly coiled. An example of is Treponema pallidum, the causative organism of syphilis.
Which bacteria causes syphilis?
Treponema pallidum
What does gram-staining depend on?
Depends on the thickness of peptidoglycan present in the cell wall:
- Gram-positive bacteria have a thick layer of peptidoglycan
- Gram-negative have a thin layer.
Can Mycobacterium tuberculosis be stained?
No - gram-indeterminate.
Describe the results of gram-positive staining
Gram-positive organisms have a thick cell wall of peptidoglycan and so retain the crystal violet stain when washed with acetone/methanol. When safranin is added, it is retained but obscured by the crystal violet.
Therefore these cells stain purple.
Describe the results of gram-negative staining
Gram-negative organisms have an outer lipopolysaccharide layer. When acetone is added these lipids dissolve, exposing the relatively thin peptidoglycan membrane. Crystal violent/iodine complexes are able to exit which decolourises the cell.
Therefore when the red counterstain is added, gram-negative bacteria stain red
The final way of classifying bacteria is into aerobic and anaerobic, depending on their ability to survive with or without oxygen.
Anaerobic vs aerobic bacteria?
Aerobic bacteria can survive in the presence of oxygen, and obligate aerobes absolutely require oxygen to survive.
Anaerobic bacteria can survive without oxygen, and obligate anaerobes can only survive in an environment without oxygen.
What classification (gram stain and shape) is:
Staphylococcus aureus
Coagulase-negative staphylococcus
Beta-haemolytic streptococci
Streptococcus pneumoniae
Enterococcus faecalis
Gram-positive cocci
What classification (gram stain and shape) is:
Listeria monocytogenes
Bacillis anthracis
Bacillus cereus
Gram-positive bacilli
What classification (gram stain and shape) is:
Neisseria meningitidis
Neisseria gonorrhoeae
Moraxella catarrhalis
Gram-negative cocci
What classification (gram stain and shape) is:
Escherichia coli
Klebsiella pneumoniae
Salmonella typhi
Pseudomonas aeruginosa
Haemophilus influenzae
Gram-negative bacilli
Classification of Staphylococcus aureus?
Gram-positive cocci (clusters)
Classification of Neisseria meningitidis and Neisseria gonorrhoeae?
Gram-negative cocci
Classification of Listeria monocytogenes?
Gram-positive bacilli
Classification of E.coli?
Gram-negative bacilli
Classification of Haemophilus influenzae?
Gram-negative bacilli
What do viruses consist of?
A nucleic acid core, either DNA or RNA, which is either single or double stranded.
The virus is covered by a protein coat known as the “capsid”.
Some viruses also have an outer envelope.
What are the two types of RNA found in a virus?
The RNA can be either ‘positive sense’ or ‘negative sense’ depending on the polarity of the nucleic acid
Difference between positive and negative sense RNA?
Positive sense (5′-3′) RNA is directly translatable into viral proteins, while negative sense (3′ to 5′) is not.
Transcription must occur first with negative sense strands to create mRNA
What does the cell capsid, or outer envelope if present, have attached to it?
Glycoproteins
What do these glycoproteins on the outside of a virus bind to?
These bind to appropriate receptors on certain host cells
For example, glycoprotein 120 on the HIV virus binds to CD4 receptors on host T cells. This allows the virus to replicate and establish a viral infection.
Are the herpes viruses enveloped or not?
Enveloped
Describe structure (DNA or RNA, enveloped or not) of Hepatitis B
DNA, enveloped
Describe structure (DNA or RNA, enveloped or not) of varicella zoster
DNA, enveloped (one of the herpes viruses)
Describe structure (DNA or RNA, enveloped or not) of smallpox
DNA, enveloped
Describe structure (DNA or RNA, enveloped or not) of Human Papillomavirus
DNA, non-enveloped
Describe structure (DNA or RNA, enveloped or not) of Adenovirus
DNA, non-enveloped
Describe structure (DNA or RNA, enveloped or not) of HIV
RNA, enveloped
Describe structure (DNA or RNA, enveloped or not) of Influenza
RNA, enveloped
Describe structure (DNA or RNA, enveloped or not) of Hep C
RNA, enveloped
Describe structure (DNA or RNA, enveloped or not) of SARS-COV-2 (COVID)
RNA, enveloped
Describe structure (DNA or RNA, enveloped or not) of Norovirus
RNA, non-enveloped
Describe structure (DNA or RNA, enveloped or not) of Hep A
RNA, non-enveloped
Process of viral replication:
- The virus is adsorbed onto the host cell membrane.
- Through the process of pinocytosis, the virus enters the cell in a vacuole.
- Uncoating occurs, where the outer protein coat is stripped to expose the genomic material.
- If an RNA virus, mRNA is generated directly. If a DNA virus or a negative sense RNA virus, transcription occurs to create mRNA.
- Viral mRNA hijacks host machinery to generate viral proteins. Viral nucleic acid is generated to facilitate further replication.
- The virion is assembled, which is an immature, inactive version of the virus. This contains the newly synthesised viral proteins and viral genomic material.
- The virion exits to infected another host cell, and the cycle repeats.
What can fungi be subdivided into?
- Yeasts, which are single-celled
2. Moulds, which are multicellular.
What are Candida albicans and Pneumocystis jirovecii examples of?
Yeasts
What does Candida albicans cause?
Thrush infections
What does Pneumocystis jirovecii cause?
respiratory infections in susceptible individuals.
What can parasites be subdivided into?
- Protozoa (single-celled)
2. Helminths (worms)
What are the 6 stages of viral replication?
- Attachment
- Entry
- Uncoating
- Replication
- Assembly
- Release
Describe the attachment stage of viral replication
The virus becomes attached to the cell by specific cellular receptors which can be glycoproteins, phospholipids or glycolipids.
Describe the entry stage of viral replication
Following attachment the virus can enter the cell, most commonly via receptor mediated endocytosis. This is the same process by which many hormones enter the cell.
Describe the uncoating stage of viral replication
Once inside the host cell, the viral capsid must be uncoated to release the viral nucleic acid. Uncoating may be achieved by host or viral enzymes that will degrade the capsid.
Describe the replication stage of viral replication
Once uncoated, viruses (DNA or RNA) replicate by switching the host machinery from cellular protein synthesis to viral synthesis and viral proteins are produced.
Describe the assembly stage of viral replication
Newly synthesised viral proteins are post-transcriptionally modified and packaged into virions that can be released from the infected host cell to infect other cells.
Describe the release stage of viral replication
Virions are released from the cell either by lysis or budding. In lysis, the infected cell dies and the virions are released. In budding, the virion takes some of the host cell’s membrane with it as it leaves – this normally does not kill the infected cell
Which cell does HIV target?
Which glycoprotein does it use to do this?
CD4+ T helper cells
Using a protein called gp120 on its viral envelope to attach to the CD4 molecule of the T cell.
Which enzyme is used for HIV to combine itself into the host cell’s genome?
Viral integrase
What is viral integrase?
Retroviral integrase is an enzyme produced by a retrovirus that integrates—forms covalent links between—its DNA into that of the host cell it infects.
