Lecture 4

Source:  Lecture 4    Tag:  gram positive flagella

Lecture 4: Prokaryotic Cell Biology - Structures external to the cell membrane
The cells of bacteria can be distinguished from those of all other forms of cellular life (fungi,
protozoans, plants and animals) by their prokaryotic organization
- In contrast to the situation with eukaryotic cells, the DNA of prokaryotic cells is not enclosed in
a separate nuclear compartment
= Also, the DNA of eucaryotic cells is always associated with specific chromosomal proteins,
the histone.  Histone-like proteins are found in some prokaryotic cells, but are not common
= The DNA of prokaryotic cells does not become condensed into the "chromosomes" with
which we're familiar in eucaryotic cells; nevertheless, we refer to the DNA molecules of
prokaryotic cells as chromosomes.
- Prokaryotic cells also lack other membrane-bound organelles - mitochondria, endoplasmic
reticulum, chloroplasts - found in eucaryotic cells
- It's important to note that, with all of these differences, prokaryotic cells carry out all of the
functions of cells; for example, while they lack mitochondria for respiration, they may carry
respiratory enzymes in the cytoplasmic membrane
- Although prokaryotic bacterial cells may seem relatively "simple" when compared to those of
eucaryotes, they do exhibit considerable structural complexity (Tortora et al., Figure 4.5)
Bacterial cells are usually very small, less than 2 µm (2 1/1000ths of a millimeter) in diameter.
- The shape (cocci, bacilli, or spirilla) and arrangement of bacterial cells can be very useful for
identification in the laboratory (Tortora et al., Figures 4.1, 4.2 and 4.3)
- It's important to note that environmental conditions can influence the shape and arrangement of
bacterial cells
The cytoplasmic membrane of a typical prokaryotic cell is surrounded by a cell wall; a variety of
structures may be found external to the cell wall.
- The glycocalyx is a polymeric material secreted from the cell
= Chemically, the glycocalyx may be made up of polysaccharide, protein, or both
= If the glycocalyx is tightly organized and firmly attached to individual cells, it is referred to as
a capsule
= If the glycocalyx seems loosely organized, with cells dispersed in it, it is called a slime layer
= Possession of a glycocalyx may be important to the "lifestyle" of a bacterium
· Pathogenic (disease-causing) bacteria frequently possess capsules, which are thought to
interfere with phagocytosis by white blood cells
· A glycocalyx may also also enhance a bacterium's ability to adhere to surfaces; in nature,
most bacteria live in biofilms composed of extracellular polysaccharide
- Bacterial flagella are long protein filaments used for locomotion
= Bacterial flagella are much simpler than the flagella of eucaryotic cells, and are anchored into
the cytoplasmic membrane through a series of rings (Tortora et al., Figure 4.7)
= Flagellar arrangement is useful for identification purposes (Tortora et al., Figure 4.6)
· Monotrichous - A single polar flagellum
· Amphitrichous - One or more polar flagellum at both poles
· Lophotrichous - Multiple flagella at one pole
· Peritrichous - Flagella distributed over the entire cell
= The movement of a prokaryotic flagellum results from rotation of the basal body (see Tortora
et al., Figure 4.7) in a manner analogous to an electric motor
= The movements of flagellated bacteria alternate between "runs", in which the flagella are all
rotating in the same direction and the cell appears to be swimming, and "tumbles" caused by
abrupt reversal of flagellar rotation (Tortora et al., Figure 4.8)
= By varying the frequency of runs and tumbles, bacteria are able to exhibit taxis in response to
positive and negative stimuli= Spirochaetes possess flagella-like axial filaments that are used for locomotion (Tortora et al.,
Figure 4.9)
- Pili and fimbriae are protein filaments, thinner than flagella, found on the surface of some
bacteria (Tortora et al., Figure 4.10)
= Like the filaments of flagella, pili and fimbriae are composed of helically arranged molecules
of a single protein (pilins in pili, flagellins in flagella)
= The general function of pili and fimbriae is to aid in attachment to surfaces, including tissue
The shape of a bacterial cell is determined by the cell wall, which serves to protect the cell from
osmotic lysis
- The major polymer of most bacterial cell walls is peptidoglycan, which forms a continuous
network around the cell
= One part of peptidoglycan consists of polysaccharide chains composed of repeating units of
two monosaccharides, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
(Tortora et al., Figure 4.11)
= Attached to each molecule of NAM is a chain of four amino acids, the "tetrapeptide side
chain"; the amino acids include D-isomers, making them different from protein polypeptides
= The tetrapeptide side chains are covalently linked, sometimes through additional amino acids,
forming a strong three-dimensional network (Tortora et al., Figure 4.12a)
The cell walls of Gram-positive and Gram-negative bacteria are very different from one another
(Tortora et al., Figure 4.12)
- Gram-positive bacteria are surrounded by a relatively thick layer of peptidoglycan (Tortora et al.,
Figure 4.12b)
= Negatively-charged teichoic acids are covalently bound to the peptidoglycan
= The teichoic acids probably assist in binding positively-charged molecules and surfaces to the
- Gram-negative cells possess a much thinner layer of peptidoglycan, surrounded by an outer
membrane (Tortora et al., Figure 4.12c)
= Like the cytoplasmic membrane, the outer membrane is based on a phospholipid bilayer, and
contains macromolecules unique to these bacteria
· Lipoproteins consist of a lipid portion embedded in the "inner face" of the outer
membrane and a protein portion covalently bound to the peptidoglycan; lipoproteins
anchor the outer membrane to the peptidoglycan layer
· Lipopolysaccharides (LPS) consist of a lipid portion embedded in the "outer face" of the
outer membrane and a complex polysaccharide extending away from the membrane; like
teichoic acids, the polysaccharides of LPS are negatively charged
· The outer membrane is quite porous, due to the presence of protein channels composed of
porins and specific channel proteins
= In the clinical setting, LPS is known as endotoxin; when released from dying cells, it causes
profound metabolic responses in host animals, including fever and shock.
- Certain natural and artificial defenses are targeted to peptidoglycan cell walls
= Lysozyme, an enzyme found in many secretions (including tears), catalyzes hydrolysis of the
covalent bond between NAG and NAM
= The antibiotic penicillin acts by interfering with formation of the peptide cross-links; thus,
bacteria growing in the presence of penicillin construct weakened cell walls and may lyse
- Structural and functional differences between Gram-positive and Gram-negative cell walls can be
exploited for identification and treatment of bacterial infections (see Tortora et al., Table 4.1)