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Gram-positive bacteria are
those that are stained dark blue or violet by
Gram staining. This is in contrast to
Gram-negative bacteria, which cannot retain the
crystal violet stain, instead taking up the
counterstain (safranin or fuchsine) and
appearing red or pink. Gram-positive organisms
are able to retain the crystal violet stain
because of the high amount of peptidoglycan in
the cell wall (Figure 1). Gram-positive cell
walls typically lack the outer membrane found in
Gram-negative bacteria.
Figure 1.
Gram-positive bacterial cell wall components.
Protection against
Gram-positive bacterial infection:
Antibodies produced by B lymphocytes such as
immunoglobulin M (IgM) are effective against
extracellular bacterial pathogens. Antibodies
can defend against Gram-positive bacteria via
opsonization; that is, antibodies
specific for an antigen embedded in the cell
wall coat the bacteria and neutralize their
action. The antibody that is attached to the
bacteria can also attach to phagocytes such as
macrophages through Fc receptors on the
phagocyte (Figure 2). This promotes phagocytosis
and bacterial elimination. Recall that the tail
portion of the antibody is referred to the FC
(fragment crystallizable) region (Figure 3), and interacts with cell
surface receptors. In addition, complement bound
to bacteria can attach to phagocytes through
complement receptors (CR), which promotes
phagocytosis (Figure 2).
Figure 2. Examples
of antibody-mediated mechanisms against
Gram-positive bacterial infections.
Figure 3. FC
region of an antibody.
The following characteristics
are displayed by Gram-positive bacteria:
The following list of
Gram-positive bacteria have been profiled:
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