Overview of Bacteriology (page 2)
(This chapter has 6 pages)
© Kenneth Todar, PhD
STRUCTURE AND FUNCTION OF PROCARYOTIC CELLS
Procaryotic cells have three architectural regions (Figure 4): appendages
(proteins attached to the cell surface) in the form of flagella
and pili; a cell envelope consisting of a capsule,
wall and plasma membrane; and a cytoplasmic region
contains the cell genome (DNA) and ribosomes and various
sorts of inclusions.
Figure 4. Schematic drawing
of a typical bacterium.
Flagella are filamentous protein structures attached to the
surface that provide swimming movement for most motile procaryotic
The flagellar filament is rotated by a motor apparatus in the plasma
allowing the cell to swim in fluid environments. Bacterial flagella are
powered by proton motive force (chemiosmotic potential) established on
the bacterial membrane, rather than ATP hydrolysis which powers
flagella and cilia. Procaryotes are known to exhibit a variety of types
behavior, i.e., the ability to move (swim) in response to
stimuli. For example, during chemotaxis a bacterium can sense
quality and quantity of certain chemicals in their environment and swim
towards them (if they are useful nutrients) or away from them (if they
are harmful substances).
Figure 5. Vibrio cholerae
a single polar flagellum for swimming movement. Electron Micrograph of
cholerae by Leodotia Pope, Department of Microbiology, University
Texas at Austin.
Fimbriae and Pili are interchangeable terms used to
short, hair-like structures on the surfaces of procaryotic cells.
are shorter and stiffer than flagella, and slightly smaller in
Like flagella, they are composed of protein. A specialized type of
the F or sex pilus, in some way stabilizes the transfer of DNA
bacteria, but the function of the smaller, more numerous common pili
is quite different. Common pili (often called fimbriae)
are usually involved in adherence (attachment) of procaryotes to
in nature. In medical situations, they are major determinants of
virulence because they allow pathogens to attach to (colonize) tissues
and to resist attack by phagocytic white blood cells.
Figure 6. Fimbriae of Neisseria
gonorrhoeae allow the bacterium to adhere to tissues. Electron
by David M. Phillips,
Unlimited, with permission.
The Cell Envelope
Most procaryotes have a rigid cell wall. The cell wall is an
essential structure that protects the delicate cell protoplast from
lysis. The cell wall of Bacteria consists of a polymer of disaccharides
cross-linked by short chains of amino acids (peptides). This molecule
a type of peptidoglycan, which is called murein. In the
bacteria (those that retain the purple crystal violet dye when
to the Gram-staining procedure) the cell wall is a thick layer of
In the Gram-negative bacteria
(cells which do not retain the crystal violet dye)
the cell wall is relatively thin and is composed of a thin layer of
surrounded by a membranous structure called the outer membrane.
is a substance unique in nature to bacterial cell walls. Also, the
membrane of Gram-negative bacteria invariably contains a unique
lipopolysaccharide (LPS or endotoxin), which is
to animals. The cell walls of Archaea may be composed of protein,
or peptidgolycan-like molecules, but never do they contain murein. This
feature distinguishes the Bacteria from the Archaea.
Although procaryotes lack any intracellular organelles for
or photosynthesis, many species possess the physiologic ability to
these processes, usually as a function of their plasma membrane.
example, the electron transport system that couples aerobic respiration
and ATP synthesis is found in the plasma membrane. The photosynthetic
that harvest light energy for conversion into chemical energy are
in the membrane. Hence, the plasma membrane is the site of oxidative
phosphorylation and photophosphorylation in procaryotes, analogous to
the functions of mitochondria
and chloroplasts in eucaryotic cells. The procaryotic plasma membrane
also a permeability barrier, and it contains a variety of different
that selectively mediate the passage of substances into and out of the
The membranes of Bacteria
are structurally similar to the cell membranes
of eucaryotes, except that bacterial membranes consist of saturated or
monounsaturated fatty acids (rarely polyunsaturated fatty acids) and do
not normally contain sterols. The membranes of Archaea form
bilayers functionally equivalent to bacterial membranes, but archaeal
are saturated, branched, repeating isoprenoid subunits that attach to
via an ether linkage, as opposed to the ester linkage found in
of eucaryotic and bacterial membrane lipids. The structure of archaeal
membranes is thought to be an adaptation to their existence in extreme
Most bacteria contain some sort of a polysaccharide layer outside of
the cell wall or outer membrane. In a general sense, this layer is
a capsule or glycocalyx. bacteria and archaea may also
have an additional proteinaceous coat called an S-layer. Capsules,
slime layers, glycocalyx and s-layer
are known to mediate attachment of bacterial cells to particular
Capsules also protect bacteria from engulfment by predatory protozoa or
white blood cells (phagocytes) and from attack by antimicrobial agents
of plant or animal origin. Capsules in certain soil bacteria protect
from perennial effects of drying or desiccation.
Importance of Surface Components
All of the various surface components of a procaryotic cell
important in its ecology since they mediate the contact of the cell
its environment. The only "sense" that a procaryote has results from
immediate contact with its environment. It must use its surface
to assess the environment and respond in a way that supports its own
and survival in that environment. The surface properties of a
are determined by the exact molecular composition of its plasma
membrane cell wall, including LPS, and the function of surface
as flagella, fimbriae and capsules. Some important ways that
use their surface components are (1) as permeability barriers that
selective passage of nutrients and exclusion of harmful substances; (2)
as "adhesins" used to attach or adhere to specific surfaces or tissues;
(3) for protection against engulfment by phagocytic white blood cells
predatory protozoa: (4) as enzymes to mediate specific reactions on the
cell surface important
in the survival of the procaryote; (5) as "sensing proteins" that can
to temperature, osmolarity, salinity, light, oxygen, nutrients, etc.
in a signal to the genome of the cell that will cause a biological
to a changing environment.
Figure 7. The complex surface
of Streptococcus pyogenes. Electron micrograph of Streptococcus
pyogenes by Maria Fazio and Vincent A. Fischetti, Ph.D. with
Laboratory of Bacterial Pathogenesis and Immunology, Rockefeller
The cytoplasmic constituents of bacteria invariably include the
chromosome and ribosomes. The chromosome is typically one
circular molecule of DNA, more or less free in the cytoplasm,
although intermittently associated with membranes.
sometimes possess smaller extrachromosomal pieces of DNA called plasmids.
The total DNA content of a cell is referred to as the cell genome.
During cell growth and division, the procaryotic chromosome is
in the usual semi-conservative fashion before for distribution to
cells. However, the eucaryotic processes of meiosis and mitosis are
in procaryotes. Replication and segregation of procaryotic DNA is
by the plasma membrane.
The distinct granular appearance of procaryotic cytoplasm is due to
the presence and distribution of ribosomes The ribosomes of
are smaller than cytoplasmic ribosomes of eucaryotes. Procaryotic
are 70S in size, being composed of 30S and 50S subunits. The 80S
of eucaryotes are made up of 40S and 60S subunits. Ribosomes are
in the process of translation (protein synthesis), but some details of
their activities differ in eucaryotes, Bacteria and Archaea. Protein
using bacterial 70S ribosomes occurs in eucaryotic mitochondria and
and this is taken as a major line of evidence that these organelles are
descended from bacteria.
Often contained in the cytoplasm of procaryotic cells is one or
of some type of inclusion granule. Inclusions are distinct
that may occupy a substantial part of the cytoplasm. Inclusion granules
are usually reserve materials of some sort. For example, carbon and
reserves may be stored as glycogen (a polymer of glucose) or as
acid (a type of fat) granules. Polyphosphate inclusions are reserves of
PO4 and possibly energy; elemental sulfur (sulfur globules)
are stored by some phototrophic and some lithotrophic procaryotes as
of energy or electrons. Some inclusion bodies are actually membranous
or intrusions into the cytoplasm which contain photosynthetic pigments
or specialized enzyme complexes.
Figure 8. Bacterial colonies
growing in a petri dish containing nutrients.
Hans Knoll Institute, Jena,