Medical Microbiology
Lecture 3/
Third class/ Dentistry college
Components External to the Cell Wall
Bacteria have a variety of structures outside the cell wall that can function in
protection, attachment to objects, and cell movement.
Capsules, Slime Layers, and S-Layers
Capsules and slime layers usually are composed of polysaccharides, but they may be
constructed of other materials. For example, Bacillus anthracis has a capsule of poly- Dglutamic acid.
Capsule: Some bacteria have a layer of material lying outside the cell wall. the layer is
well organized and not easily washed off.
Function:
 They help bacteria resist phagocytosis by host phagocytic cells. Streptococcus
pneumoniae provides a classic example. When it lacks a capsule, it is destroyed
easily and does not cause disease.
 Capsules contain a great deal of water and can protect bacteria against
desiccation.
 They exclude bacterial viruses and most hydrophobic toxic materials such as
detergents.
Slime layer: is a zone of diffuse, unorganized material that is removed easily. The Slayer has a pattern something like floor tiles and is composed of protein or glycoprotein .
Function:
 Gliding bacteria often produce slime, which presumably aids in their motility
 Many Gram-positive, Gram-negative bacteria and Archaea have a regularly
structured layer called an S-layer on their surface, where they may be the only
wall structure outside the plasma membrane.
 It may protect the cell against ion and pH fluctuations, osmotic stress, enzymes.
 The S-layer also helps maintain the shape and envelope rigidity of at least some
bacterial cells.
 It can promote cell adhesion to surfaces.
 Finally, the layer seems to protect some pathogens against complement attack and
phagocytosis, thus contributing to their virulence.
Pili and Fimbriae
Many gram-negative bacteria have short, fine, hair like appendages that are thinner
than flagella and not involved in motility.
Fimbriae: They seem to be slender tubes composed of helically arranged protein
subunits. At least some types of fimbriae attach bacteria to solid surfaces host tissues.
Sex pili (s.,pilus): are similar appendages, about 1 to 10 per cell, that differ from
fimbriae in the following ways. Pili often are larger than fimbriae. They are genetically
determined by sex factors or conjugative plasmids and are required for bacterial mating.
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Medical Microbiology
Lecture 3/
Third class/ Dentistry college
Flagella and Motility
Most motile bacteria move by use of flagella (s., flagellum), threadlike locomotors
appendages extending outward from the plasma membrane and cell wall. They are
slender, rigid structures. However, Flagellation patterns are very useful in identifying
bacteria. According to the flagellum position and number bacteria are classify into:
1-Monotrichous bacteria (trichous means hair) have one flagellum; if it is located at an
end, it is said to be a polar flagellum .
2-Amphitrichous bacteria (amphi means “on both sides”) have a single flagellum at each
pole.
3-lophotrichous bacteria (lopho means tuft) have a cluster of flagella at one or both ends
4-peritrichous (peri means “around”) Flagella are spread fairly evenly over the whole
surface of bacteria.
The Bacterial Endospore
A number of gram-positive bacteria can form a special resistant, dormant structure
called an endospore. Endospores develop within vegetative bacterial cells of several
genera: Bacillus and Clostridium (rods). These structures are extraordinarily resistant to
environmental stresses such as heat, ultraviolet radiation, gamma radiation, chemical
disinfectants, and desiccation. Endospores often survive boiling for an hour or more;
therefore autoclaves must be used to sterilize.
Potential contributing factors to spore heat resistance include their thick wall
structures, the dehydration of the spore, and cross linking of the proteins by the calcium
salt of pyridine-2, 6-dicarboxylic acid, both of which render protein denaturing difficult.
Endospore consist from: core, spore wall, cortex, coat, and exosporium. The germination
process occur in three stages: activation, initiation, and outgrowth.
Biofilm
A bacterial biofilm is a structured community of bacterial cells embedded in a selfproduced polymer matrix (Glycocalyx) is a network of polysaccharides extending from
the surface of bacteria and other cells. glycocalyx also aids bacterial attachment to
surfaces of solid objects in aquatic environments or to tissue surfaces. Such films can
develop considerable thickness (mm). The bacteria located deep within such a biofilm
structure are effectively isolated from immune system cells, antibodies, and antibiotics.
Examples of Medically Important Biofilms
1- Following implantation of catheters, cardiac pacemakers, shunt valves, these foreign
bodies are covered by matrix of proteins and the microorganism such as fibrinogen,
fibronectin, vitronectin, or laminin. Staphylococci have proteins on their surfaces with
which they can bind specifically to the corresponding proteins, for example the clumping
factor that binds to fibrinogen and the fibronectin-binding protein. The adhering bacteria
then proliferate and secrete an exopolysaccharide glycocalyx.
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Medical Microbiology
Lecture 3/
Third class/ Dentistry college
2- Certain oral Streptococci (S. mutans) bind to the proteins covering tooth enamel, then
proceed to build a glucan matrix. Other bacteria then adhere to the matrix to form plaque,
the precondition for destruction of the enamel and formation of caries .
3- Oral Streptococci and other bacteria attach to the surface of the cardiac valves to form
a biofilm.
References:
1-Prescott L. M., Microbiology, 5th Edition ,(2002).
2- Kayser, F.H., Medical Microbiology ( 2005).
3- Jawetz, Medical Microbiology ( 2007).
Lecturer
Dr. Zuhair S. Al Sehlawi
For further informations please visit college web site
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