Microbiology of Influenza
Introduction
The influenza virus is the only member
of the orthomyoxviruses family.
 There are three types, A, B and C.

Influenza Virus

The influenza virus
contains ssRNA in
its core
 This is surrounded
by a matrix protein
membrane
 A lipid bilayer
envelopes the virus
 The outer layer is
studded with
prominent
glycoprotein spikes
Courtesy of
http://micro.magnet.fsu.edu/cells/viruses/infl
uenzavirus.html
Glycoprotein Spikes
Glycoprotein spikes help the virus
attach to a cell.
 There are two types of glycoprotein
spikes.

– Hemagglutin (HA)
– Neuraminidase (NA)

The number of HA and NA on the
surface determines the name of the
specific virus, ex. H5N1, avian flu.
Glycoprotein Spikes




Hemagglutin
Endocytosis is triggered
when it binds to pulmonary
epithelial cells.
Image courtesy of
http://upload.wikimedia.org/
wikipedia/en/a/a1/Hemaggl
utinin_molecule.png
Neuraminidase
Hydrolyzes mucous in the
lungs so virus can attach to
the cell surface.
Image courtesy of
http://en.wikipedia.org/wiki/Ima
ge:Neuraminidase_Ribbon_Di
agram.jpg
Influenza A, B & C
Influenza A
Influenza B
Eight –ssRNA  Eight –ssRNA
molecules
molecules
 Infects only
 Infects humans
humans
and animals
 Can cause
such as pigs
serious
and birds
infections
 Can cause
serious
infections

Influenza C
Seven –ssRNA
molecules
 Infects only
humans
 Can cause only
minor infections
usually in a
family or
classroom

Antigenic Variations
The influenza virus mutates easily and
can change from year to year.
 This is why you need a flu shot every
year to prevent the flu.
 Influenza can mutate in two different
ways.

– Antigenic drift
– Antigenic shift
Antigenic Variations
Antigenic Drift
Minor changes in HA and 
NA
 Occurs in influenza A and 
B

 Antibodies are not
effective against new
mutation
 Causes a minor epidemic 
about every two years

Antigenic Shift
Major changes in HA and
NA
Occurs only in influenza A
Two different influenza
viruses enter cell and
combine causing a new
influenza strain
Causes major epidemic
about every 10 years
Influenza Microbiology
The stomach flu is usually caused by
other viruses or bacteria, not influenza.
 No matter what flu shots come out,
some people still get the flu.
 The virus is constantly changing and
mutating making it hard for the body to
combat what would seem like a simple
RNA
