Basic Challenge 1 Commensal Microbes – With a little
help from our (little) friends
1. What is the structure of gram positive and gram negative? And what is the
inside structure of bacteria? And what is the bacterial classification?
Two most common shapes of bacteria = cocci (coccus) and rods
There are several common features of bacterial cell structure. Bacterial cells are
surrounded by several layers, which are collectively called the cell envelope. The most
common cell envelope layers are the plasma membrane, cell wall, and capsule or slime
layer. The innermost layer of the cell envelope is the plasma membrane, which
surrounds the cytoplasm. Most bacteria have a chemically complex cell wall, which
covers the plasma membrane. Many bacteria surround the cell wall with a capsule or
slime layer.
Because most bacteria do not contain internal, membrane-bound organelles, their
interior appears morphologically simple. The genetic material is localized in a discrete
region called the nucleoid and is not separated from the surrounding cytoplasm by a
membrane. Bacteria also have ribosomes which are scattered through the cytoplasms,
and larger masses called inclusions.
Filamentous structures termed pili (s. pilus) may protrude from the surface, facilitating
gene transfer or attachment to surfaces. Finally, many bacteria use flagella for
locomotion.
,The cell wall is the layer that lies just outside the plasma membrane. It is one of the
most important structures for several reasons: It maintains cell shape and protects the
cell from osmotic lysis, and it can protect the cell from toxic substances.
Christian Gram developed the Gram stain
in 1884, and based on this most bacteria
can be placed into 2 groups based on their
response to the gram-staining procedure.
Gram-positive bacteria stained purple,
whereas Gram-negative bacteria were
pink or red.
The cell walls of typical gram-positive
bacteria consist of a single, 20-80 nm thick
layer of peptidoglycan (murein) lying
outside the plasma membrane. In contrast,
the cell walls of typical Gram-negative
bacteria have two distinct layers: a 2- to 7-
nm-thick peptidoglycan layer covered by a
12- to 14-nm-thick outer membrane.
One important feature in typical Gram-
negative bacteria is a space between the
plasma membrane and the outer
membrane. It also is sometimes observed
between the plasma membrane and cell wall in typical Gram-positive bacteria. This
space is called the periplasmic space. The substance that occupies the periplasmic
space is the periplasm.
Gram-positive bacteria = streptococcus
Most cultured bacteria that stain
Gram positive belong to only two
phyla, Firmicutes and
Actinobacteriota. Most of these
bacteria have thick cell walls
composed of peptidoglycan and
large amounts of other polymers
such as teichoic acids.
,Gram-negative bacteria = E.coli
The peptidoglycan layer of the gram negative bacteria is very thin (2 to 7 nm,
depending on the bacterium) and sits within the periplasmic space. The periplasmic
space is much larger than that of a typical Gram-positive cell, ranging from about 30 to
70 nm wide.
The outer membrane lies outside the thin peptidoglycan layer. It is linked to the cell by
Braun’s lipoprotein, the most abundant protein in the outer membrane, present at over a
million copies per cell. This small lipoprotein is covalently joined to both the outer
membrane and the peptide chain of peptidoglycan.
In contrast to the symmetry of the plasma membrane, the most distinctive structural
feature of the outer membrane is the difference between the inner leaflet (facing the
periplasm) and the outer leaflet (facing the environment). The inner leaflet is composed
of phospholipids as found in the plasma membrane, whereas the external layer of the
Gram-negative cell is comprised of lipopolysaccharides (LPS). These large, complex
molecules contain both lipid and carbohydrate, and consist of three parts: (1) lipid A, (2)
the core polysaccharide, and (3) the O side chain
Gram positive cell wall vs. gram negative cell wall
Gram positive cell wall Gram negative cell wall
Layers
Single layered Double layered
Composition
Consisting of several layers of Consisting of a single layer of
peptidoglycan peptidoglycan
, Thickness
15 to 80 nanometers 10 nanometers
Periplasmic space
Small Large
Outer membrane
Absent Present
Teichoic Acid
Present Absent
Porins
Absent Present
Lipid content
Very low High
Lipopolysachharides
Absent present
Gram staining procedure
Retains the crystal violet dye and appears It does not retain the crystal violet dye
purple in color and appears pink in color
Classification of bacteria
Taxonomy = the science of classifying living things
• A taxonomic scheme is used to arrange organisms into groups called taxa based
on mutual similarity.
• Domain (bacteria, archaea, or eukarya), phylum, class, order, family, genus, and
species
2. What is the difference between commensal microbiome, opportunistic, and
true pathogens?
help from our (little) friends
1. What is the structure of gram positive and gram negative? And what is the
inside structure of bacteria? And what is the bacterial classification?
Two most common shapes of bacteria = cocci (coccus) and rods
There are several common features of bacterial cell structure. Bacterial cells are
surrounded by several layers, which are collectively called the cell envelope. The most
common cell envelope layers are the plasma membrane, cell wall, and capsule or slime
layer. The innermost layer of the cell envelope is the plasma membrane, which
surrounds the cytoplasm. Most bacteria have a chemically complex cell wall, which
covers the plasma membrane. Many bacteria surround the cell wall with a capsule or
slime layer.
Because most bacteria do not contain internal, membrane-bound organelles, their
interior appears morphologically simple. The genetic material is localized in a discrete
region called the nucleoid and is not separated from the surrounding cytoplasm by a
membrane. Bacteria also have ribosomes which are scattered through the cytoplasms,
and larger masses called inclusions.
Filamentous structures termed pili (s. pilus) may protrude from the surface, facilitating
gene transfer or attachment to surfaces. Finally, many bacteria use flagella for
locomotion.
,The cell wall is the layer that lies just outside the plasma membrane. It is one of the
most important structures for several reasons: It maintains cell shape and protects the
cell from osmotic lysis, and it can protect the cell from toxic substances.
Christian Gram developed the Gram stain
in 1884, and based on this most bacteria
can be placed into 2 groups based on their
response to the gram-staining procedure.
Gram-positive bacteria stained purple,
whereas Gram-negative bacteria were
pink or red.
The cell walls of typical gram-positive
bacteria consist of a single, 20-80 nm thick
layer of peptidoglycan (murein) lying
outside the plasma membrane. In contrast,
the cell walls of typical Gram-negative
bacteria have two distinct layers: a 2- to 7-
nm-thick peptidoglycan layer covered by a
12- to 14-nm-thick outer membrane.
One important feature in typical Gram-
negative bacteria is a space between the
plasma membrane and the outer
membrane. It also is sometimes observed
between the plasma membrane and cell wall in typical Gram-positive bacteria. This
space is called the periplasmic space. The substance that occupies the periplasmic
space is the periplasm.
Gram-positive bacteria = streptococcus
Most cultured bacteria that stain
Gram positive belong to only two
phyla, Firmicutes and
Actinobacteriota. Most of these
bacteria have thick cell walls
composed of peptidoglycan and
large amounts of other polymers
such as teichoic acids.
,Gram-negative bacteria = E.coli
The peptidoglycan layer of the gram negative bacteria is very thin (2 to 7 nm,
depending on the bacterium) and sits within the periplasmic space. The periplasmic
space is much larger than that of a typical Gram-positive cell, ranging from about 30 to
70 nm wide.
The outer membrane lies outside the thin peptidoglycan layer. It is linked to the cell by
Braun’s lipoprotein, the most abundant protein in the outer membrane, present at over a
million copies per cell. This small lipoprotein is covalently joined to both the outer
membrane and the peptide chain of peptidoglycan.
In contrast to the symmetry of the plasma membrane, the most distinctive structural
feature of the outer membrane is the difference between the inner leaflet (facing the
periplasm) and the outer leaflet (facing the environment). The inner leaflet is composed
of phospholipids as found in the plasma membrane, whereas the external layer of the
Gram-negative cell is comprised of lipopolysaccharides (LPS). These large, complex
molecules contain both lipid and carbohydrate, and consist of three parts: (1) lipid A, (2)
the core polysaccharide, and (3) the O side chain
Gram positive cell wall vs. gram negative cell wall
Gram positive cell wall Gram negative cell wall
Layers
Single layered Double layered
Composition
Consisting of several layers of Consisting of a single layer of
peptidoglycan peptidoglycan
, Thickness
15 to 80 nanometers 10 nanometers
Periplasmic space
Small Large
Outer membrane
Absent Present
Teichoic Acid
Present Absent
Porins
Absent Present
Lipid content
Very low High
Lipopolysachharides
Absent present
Gram staining procedure
Retains the crystal violet dye and appears It does not retain the crystal violet dye
purple in color and appears pink in color
Classification of bacteria
Taxonomy = the science of classifying living things
• A taxonomic scheme is used to arrange organisms into groups called taxa based
on mutual similarity.
• Domain (bacteria, archaea, or eukarya), phylum, class, order, family, genus, and
species
2. What is the difference between commensal microbiome, opportunistic, and
true pathogens?
