bacteria have cell walls made up of
polysaccharides that give them strength
and rigidity this is important since
bacteria often experience variations in
osmotic pressure due to the different
solutions they encounter and it is their
cell walls which prevent them from
shrinking or swelling as a reminder as
Moses is the process by which solvent
molecules pass through a semipermeable
membrane from a less concentrated
solution to a more concentrated one
equalizing the concentration on either
side of the membrane nearly all
bacterial cell walls have a peptide
polysaccharide layer called
peptidoglycan also known as moraine
peptidoglycan is a polymer made up of
sugars and amino acids which forms a
kind of mesh
bacteria can be classified based on
their reaction to the Gram stain which
identifies them as gram positive or gram
negative based on the chemical and
physical properties of their cell walls
gram positive bacteria have a thick cell
wall which consists of up to around 30
layers of peptidoglycan this cell wall
surrounds a mono derm which is a single
plasma membrane gram-negative bacteria
have a much thinner cell wall consisting
of a single layer of peptidoglycan
this layer of peptidoglycan is
sandwiched between two lipid bilayer
membranes called dye terms
we can differentiate between
gram-positive and gram-negative bacteria
by dyeing them with crystal violet and
then washing them with a decolorizing
solution then a counter stain is added
for example safranin or fruit scene gram
positive bacteria will retain the
crystal violet dye and remain purple
while the gram negative bacteria will be
stained pink note that gram positive
bacteria also pick up the pink color of
the counter stain however this is not
visible when they are dyed with the
darker purple color of the crystal
violet stain
the reason for these staining
differences is due to differences in
cell wall structure which is the chief
difference between gram-positive and
gram-negative bacteria the Gram stain
detects peptidoglycan
and since gram positive bacteria have a
thick multi-layered peptidoglycan layer
they retain the crystal violet dye
gram-negative bacteria do not retain the
dye for two reasons they have an outer
membrane getting in the way of the
crystal violet and they lack
peptidoglycan to retain the stain
although both gram positive and gram
negative bacteria can be pathogenic
gram-negative bacteria are more
resistant to antibodies because of their
impenetrable cell wall unfortunately
these bacteria also develop resistance
more quickly
not all bacteria can be reliably
classified through gram staining
acid-fast bacteria and gram variable
bacteria for example do not respond to
gram staining acid-fast bacteria are
bacteria whose cell walls retain stains
particularly well although they aren't
closely related to gram positive
bacteria they can't appear purple after
the Gram stain test Gram variable
bacteria show a mix of pink and purple
cells when stained if you liked this
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