Bacteria in health
BACTERIA - Characteristics - GRAM + or Gram - The Bacterial Cell Envelope
Bacterial Characteristics:GRAM POSITIVE
or Gram Negative?
The Bacterial Cell Envelope
The Cell Envelope comprises the cytoplasmic membrane,
cell wall plus an outer membrane, called a capsule (if present):
- Capsule
(Only some species of bacteria) - Extra protective - outer capsule
composed of polysaccharides (complex carbohydrates). Capsules play a number of roles,
but the most important are:
• To keep the bacterium from drying
out
• To protect it from phagocytosis (engulfing)
by larger microorganisms - a major virulence factor (ability to cause disease) in
the major disease-causing bacteria, such as Escherichia
coli and Streptococcus pneumoniae.
Non-encapsulated mutants of these organisms are avirulent ( i.e. don't cause disease).
- Cell Wall
- Each bacterium is enclosed by a rigid cell wall composed of peptidoglycan,
a protein-sugar (polysaccharide) molecule.
• Gives the cell
its shape and surrounds the cytoplasmic membrane - protecting it from the
environment.
•
Helps to anchor appendages like the pili (tiny whiskers
used to exchange genetic material) and flagella
(whip-like
flagellae used to propel bacterium in water). These originate in the cytoplasm
membrane and protrude through the wall to the outside.
• Wall strength
keeps cell from bursting. The primary function of the wall is to protect
the cell from internal pressure caused by much higher concentrations of proteins
and other molecules inside the cell compared to outside;
• Unique to bacteria
is the presence of peptidoglycan. Located immediately outside the cytoplasmic
membrane;
• Cell wall composition
varies widely amongst bacteria and is one of the most important factors in bacterial
species analysis and differentiation. E.g. Cell wall thickness determines
whether bacteria is
gram-positive
or gram-negative (see below)
- Cytoplasmic
membrane. Composed of a phospholipid bilayer, similar to a eukaryotic
cell membrane, and similarly acts as a permeability barrier for most molecules and
serves as a transport zone for molecules into the cell
Gram-positive
(Thick-walled) or gram-negative (Thin-walled)? In 1884, Danish
physician Hans Christian Gram devised a staining and washing technique to differentiate
between bacteria with thick cell walls and those with thin walls. When exposed to
a "gram stain",
gram-positive
bacteria retain the purple color of the stain because the structure of their cell
walls traps the dye. In
gram-negative bacteria,
the cell wall is thin and releases the dye readily when washed with an alcohol or
acetone solution.
ACID-FAST bacteria.
Won't hold gram stain, so are difficult to define as
gram-positive
or
gram-negative
gram-positive
Gram-negative
Staphylococcus
spp
Streptococcus
spp
Clostridium
spp
Listeria
spp
Bacillu S
spp
Corynebacterium
Acinetobacter
spp
Neisseria
Haem ophilus
spp
Bordetella
Helicobacter spp (curved rod/helix)
Campylobacter
Pseudomonas
spp
Legionella
spp
Bacteroides
Enterobacteriaceae
family, such as:
Escheririchia coli
Salmonella spp
Proteus
Klebsiella
spp
Shigella
Citrobacter
Serratia
Morganella
Yersinia
Enterobacter
Comparison of BACTERIAL CELL WALLS in
Gram-positive and
Gram-Negative bacteria
Gram Negative
Cell Wall (in detail)
Peptoglycan layer in the cell wall of
Gram-negative bacteria is much
thinner than the
gram-positive
bacteria - comprised of only 20% peptidoglycan
Unique to Gram-negative
bacteria is an outer membranelayer - high in lipid content, it is formedby
lipopolysaccharide (LPS) layer, lipoproteins, proteins, aphospholipid layer and
porins:
• Periplasmic
space - separates cell plasma membrane from the peptidoglycan layer
(contains proteins which destroy potentially dangerous foreign matter present in
this space).
• Phospholipid
layer - located in inner layer of outer membrane attached by
lipoproteins to the exterior peptidoglycan layer;
Many G- bacteria are pathogenic
associated with its endotoxin layer
The Black Plague wiped out a third of the
population of Europe
- It was caused by the tiny G-
rod, Yersinia pestis
• PS layer
(also known as endotoxin) - this outer layer of the outer membrane
is comprised of lipopolysaccharides; their lipid portion is embedded in the outer
membrane and is called Lipid A, a toxic substance responsible for most pathogenicity
of G- bacteria. Also contributing to
toxicity are polysaccharides, called O polysaccharides, extending outward from the
bacterial surface.
The highly charged lipopolysaccharides give the
G- cell wall an overall negative charge
The center segment, called the core polysaccharide, contains
sugars which are highly phosphorylated; it's these phosphate groups which contribute
the negative charge.
• Porins -as
a phospholipid bilayer, the lipid part of the outer membrane is impermeable to charged
molecules. Channels, called porins, span the outer membrane and allow passive transport
of solutes, manyions, sugars and amino acids across the outer membrane into and
out of the bacterial cytoplasm