Microbiology & Infectious Diseases Flashcards
What is a virus?
Briefly describe its structure
An infectious, obligate intracellular parasite
Comprised of genetic material (DNA OR RNA) surrounded by a protein coat and/or a membrane
Big difference between Virus and Bacteria?
Virus IS dependent on host cell (bacteria is not)
Bacteria has cell wall, organelles, DNA AND RNA and are alive
Viruses have none of these things and are not alive
When virus is outside the cell, what is it called?
Virion?
How do viruses replicate?
Describe the 6 stages
- Attachment to specific receptor on host cell
- Cell entry - only central viral core (carrying the nucleic acids and some proteins) enters host cell
- Interaction with host cells - uses cell materials (e.g. enzymes, amino acids) for their replication
- Replication may localise in nucleus, cytoplasm or both
- Assembly - occurs in nucleus, cytoplasm or cell membrane
- Release - bursting open of cells OR leaking from cell over period of time
How do viruses cause disease?
Give examples
Damage by :
- direct destruction of host cells e.g. HIV
- modification of host cell structure or function e.g. rotavirus
- over-reactivity of host host as response to infection e.g. hepatitis
- cell proliferation and cell immortalisation e.g. HPV
- evasion of extra AND intracellular host defences
What is a pathogen?
Organism that causes or is capable of causing disease
What is a commensal?
Organisms which colonises the host BUT causes no disease in normal circumstances
What is an opportunist pathogen?
Microbe that only causes disease if host defences are compromised
Define virulence / pathogenicity
Ability to cause disease once established
What is asymptomatic carriage?
When a pathogen is carried harmlessly at a tissue site where it causes no disease
What do gram negative bacteria have that gram positive don’t?
DOUBLE CELL MEMBRANE
2 membranes separated by lipoprotein, periplasmic space and petidoglycan
State and describe the types of bacterial toxins
ENDOTOXIN - part of outer membrane of bacteria
EXOTOXIN - proteins secreted by gram pos and neg bacteria
What does the coagulase test distinguish?
Distinguishes S.aureus from other staph
- will be COAGULASE POSITIVE
Types of gene mutation
Base sub
Deletion
Transfer
Types of gene transfer
Transformation e.g. plasmid
Transfunction e.g. via phage
Conjugation e.g. via sex pilus
Describe how to gram stain
- Apply primary stain (crystal violet) to heat fixed bacteria
- Add IODIDE - binds to crystal violet and fixes it to cell wall
- DECOLOURISE with ethanol or acetone
- COUNTERSTAIN with safranin (pink)
What does gamma haemolysis imply?
NO haemolysis
Why does alpha haemolysis occur?
How does the agar appear?
BC production of hydrogen peroxide oxidising haemoglobin
GREEN
Why does beta haemolysis occur?
Bc lysis of RBCs by haemolysis such as Streptolysin O produced by S. pyogenes
What does an oxidase test test for?
If micro-organisms contains a cytochrome oxidase
implies organism able to use O2 as terminal e- acceptor
Gram pos bacteria stain what colour?
Give an example
PURPLE
Staph. Aureus - coagulase positive
What are the groups of Streptococci classification?
- Haemolysis
- Lancefield typing
- Biochemical properties
What is lancefield grouping?
A method of grouping catalyse neg, coagulase neg bacteria
Based on bacterial carbohydrate cell surface antigens
How do gram-pos bacteria spread?
Aerosols, surface-to-surface, colonisation of prostheses
Virulence factors of Gram Neg bacteria?
- Colonisation factors - adhesins, invasins
- Toxins (effectors) - usually secreted proteins - damage, subversion
Name the 4 major groups of Gram-Neg bacteria
Describe their shape
- Proteobacteria - ALL rod-shaped
- Bacteroids - rodshaped
- Chlamydia - round pleimorphic
- Spirochaetes - spiral/helical
Why can is be difficult to culture pathogens in the lab?
Bc of dependency on host
3 Types of Worms
- Nematodes (roundworms)
- Trematodes (flatworms)
- Cestodes (tapeworms)
What type of nematodes can you get?
Intestinal
Larva migrans
Tissue (filaria)
What type of trematodes can you get?
Blood
Liver
Lung
Intestinal
What type of cestodes can you get?
Non-invasive
Invasive
What is the pre-patent period?
Interval between infection and appearance of eggs in the stool
What is a protozoa?
Single-celled eukaryotic organism with a definitive nucleus
Name medically important mycobacteria
M. TB
M. leprae - leprosy
M. avium - disseminated infection in AIDS
M.kansasii - chronic lung infection
M.marinum - fish tank granuloma
M.ulcerans - buruli ulcer
Rapidly growing mycobacteria
What is a Mycobacteria?
Aerobic, non-spore forming, non-motile bacillus
Slow reproduction
Slow response to treatment
Slow growing ∴ hard to culture
Describe the cell wall of Mycobacteria
Contains high molecular weight lipids
Weakly gram-pos (or colourless)
Survive inside macrophages, even in low pH environments
What is acid fast bacilli used for?
To identify organisms with wax-like, thick cell walls
E.G. MYCOBACTERIA (which are resistance to gram stain)
Name some direct lab methods to diagnose bacterial infections
Gram stain
Acid fast stains
Wet film
KOH (fungi)
India ink
Name some culture lab methods to diagnose bacterial infections
Solid media
Liquid media
Blood culture
How do fungi move?
By growth or through generation of spores - carried thru air/water
Describe fungi
Eukaryotic
Chitinous cell wall
Heterotrophic
What is a dimorphic fungi?
Fungus that exists as both yeast and mould - switches between the 2 when conditions suit
What are yeasts?
Small single celled organisms that divide by budding
What do moulds form?
Multicellular hyphae and spores
What does selective toxicity in treating fungal disease rely on?
Target does not exist in humans
Target is signif different to human analogues
Drug is conc in organism cell (respect to humans)
↑ permeability to compound
Human cells being ‘rescued’ from toxicity by alternative metabolic pathways
Why can fungi be hard to treat?
Relatively few classes of effective antifungals
Why can the total worm burden in a human not increase (unless constant re-exposure)?
Bc adult worms can’t reproduce in the body (without a period of development outside) ∴ if worm can’t get out, it will die
Can produce larvae/eggs but once grown, will die
Define invasiveness
Capacity to penetrate mucosal surfaces to reach normally sterile sites
5 Major groups of protozoa
Flagellates
Amoebae
Sporozoans
Ciliates
Microsporidia
