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Micro - Intro to Viruses (i) (intro (contain only 1 type of nucleic acid…
Micro - Intro to Viruses (i)
intro
different viruses cause a vast range of disease
Tx now possible -> antivirals
important role in cancer (e.g. EBV + lymphoma) + immunosuppression (HIV)
infectious
contain only 1 type of nucleic acid
can be circular or linear, ds or ss
technically not living entities
viral genome encodes for viral proteins + enzymes necessary for rep
strict intracellular pathogens
totally dependent on (an appropriate) host cell for replication - hijak cellular processes by invasion
must kill host cell after rep to release new mature virus particles
only visible with EM
Classification
phenotypic characteristics
capsid morphology
cubic/icosahedral
20 faces
each is an equilateral triangle
e.g. adenovirus
helical
protein binds like a cylinder
e.g. coronavirus
complex
non-symmetrical
e.g. poxvirus
bacteriophages (viruses that infect bacteria) have a non-symmetrical capsid head + tail
nucleic acid
DNA viruses
parvovirus
papovaviris
adenovirus
herpes family
HSV1+2 (cold sores)
VZV (chickenpox + shingles)
EBV (infectious mononucleosis aka glandular fever)
poxvirus
Hepadna family (e.g. Hep B)
RNA viruses
picornavirus family
polio
hep a
reovirus
togavirus
coronavirus (the common cold)
orthmyxovirus family
e.g. influenza (HxNx = antigenic variation - haemoglutinin + neuraminidase)
rhabdovirus
paramyxovirus
retrovirus family
HIV
mechanism of rep
hosts they infect + diseases they cause
Components
nucleocapsid
nucleic acid
capsid
proteinaceous coat made up of capsomers
protective shield around genome
role: introduces viral genome into host cell
protein interacts with host cell Rs
contains antigens that stimulate immune system
humoral immunity often insufficient to neutralise infection
different serological characteristics
resistant to drying, heat, detergents, acids, proteases
hence can survive in GIT + environment
spread easily via fomites (vehicles)
all viruses have this
envelope
surrounds capsid
not always present (no envelope - naked capsid)
made up of phospholipid bilayer + glycoprotein
derived from host cell membranes during budding
contains viral proteins that bind to host cell surface Rs
surface polypeptides, e.g. haemaglutinins of influenza
distinctive serological characteristic (diagnostic)
sensitive to drying, heat, detergents, acid
must stay wet during transmission (in large droplets/secretions)
can't survive in GIT
doesn't need to kill cells in order to spread
requires both humoral + cellular response
matrix between envelope + capsid
some carry a small no. of enzymes (e.g. neuraminidase of influenza - cleave sialic acid groups from glycoproteins and are required for rep)
Viral acquisition
vectors (animal/insect -> rabies, dengue)
inhalation (influenza, rhinovirus)
faecal-oral (rotavirus, norovirus-winter vomiting bug, hep A+E)
blood borne (HIV, HepB+C)
sexual (HIV, Hep B+C)
congenital (rubella, CMV, Hep B+C, HIV)
tropism
= a virus's affinity for a specific body tissue
determined by...
cell Rs
cell TFs (must recognise viral promotors + enhancer sequences)
physical barriers
local environment (temp, pH, oxygen tension/PO2, enzymes-e.g. digestive enzymes in bile may inactivate some viruses, secretions)