STRUCTURE AND FUNCTION OF BACTERIAL CELLS
I
Introduction: The size and shapes of bacterial cells
• Size of bacteria: a typical bacterial
cell is about 1 micrometer
(1/1,000,000 of a meter) in diameter
• The bacterium Staphylococcus
mixed with red blood cells. Staph
cells are 1 micrometer in diameter;
RBC’s are 10 micrometers in
diameter
• coccus (cocci): spherical
• Staphylococcus aureus
“coccus” means berry or grape;
“staphylococcus” refers to
“grape-like clusters” of cells
• Streptococcus pyogenes
“streptococci” are “cocci in chains”
• Bacillus (bacilli) are
rod-shaped bacteria
about 1 micrometer
in diameter. “bacillus”
means stick or rod
•Bacillus anthracis
in characteristic chains of cells
• Curved rods are called vibrios
• Vibrio cholerae the agent of cholera
• Spiral shaped bacteria are called
spirilla
I. Anatomical regions of the bacterial cell
A. Appendages: proteinaceous structures attached to the cell
surface
B. Cell Envelope: various layers of the cell coat and surface
C. Cytoplasm: structural and molecular components enclosed by
the plasma membrane
II. Appendages
A. Flagella: organelles for swimming movement
1. General properties
• Flagella are long whiplike filaments composed of protein that
originate in the cell membrane.
• Flagella rotate and impart swimming movement on the cells
a. Physical structure
b. Chemical structure – each structural component is
a protein
c. Arrangement on the cell surface
d. Mechanism of motility – flagellum rotates and imparts
movement in a fluid environment
• Filament is rotated by a protein
“motor” in the cell membrane
• Motor is powered by proton motive
force (pmf) on the outside of membrane
2. Ecological significance
a. Survival value
• escape predatory protozoa and whithe blood cells (phagocytes)
• swim into favorable environments; swim out of unfavorable
conditions
• chemotaxis
b. Tactic responses
- swim towards or away from a chemical
• aerotaxis swim toward or away from O2
• phototaxis swim into the best wavelength of light for
photosynthesis
• magetotaxis swim towards the North Pole or South Pole
B. Other means of movement in procaryotes
1. Gliding movement and Gliding Bacteria
2. Axial filaments (endoflagella) and Spirochetes
3. Twitching movement in Pseudomonas
C. Fimbriae and pili
1. General properties
• Pili (also called fimbriae) are short hair-like structures composed
of protein on the cell surface
2. Sex pilus and conjugation
• A special type of pilus called the
sex pilus is used in mating between
bacteria
3. Common pili (fimbriae)
a. For adherence to surfaces
• Shigella dysenteriae uses its
fimbriae to attach to the intestine
and then produces a toxin that
causes diarrhea
• Neissera gonorrhoeae, the cause of the
STD, gonorrhea, uses pili to attach to the
urogenital and cervical epithelium when
it causes disease
b. Protection against phagotrophic engulfment
and/or killing
• Some pathogenic bacteria are protected from attack by phagocytes by
fimbrial –like filaments on their surface
III. Cell envelope
A. Capsules, including slime layers and glycocalyx
1. Composition
• usually polysaccharide, occasionally polypeptide
• Negative stain of Streptococcus
pneumoniae outlining its notorious
polysaccharide capsule. The capsule helps
prevent uptake of the bacterium
by phagocytes in the lung.
2. Types of capsules
a. glycocalyx (microcapsule)
b. true capsule
c. slime layer or biofilm
3. Functions of capsules
a. attachment (adherence) to surfaces
b. protection against phagotrophic engulfment and
killing
c. protection against desiccation
d. nutrient reserves
e. biofilm protection against antimicrobial agents