Function of Capsule in Bacterial cell
Structure-Function-Pathogenicity RelationshipsMM 1-16
General1. To compare and contrast the Gram-positive and the Gram-negative bacterial cells
2. To develop an understanding of the relationships between cell components and clinical features of disease
3. To explain the bacterial growth curve
4. To familiarize you with immune reactions induced by the bacterial cell
Specific (terms and concepts upon which you will be tested)The bacteria are approximately ten times the size of viruses, ranging from 0.4 um to 2.0 um in size. They assume one of three morphological forms, spheres (cocci), rods (bacilli) or spirals although there is much variation in each group. The morphology of a bacterium is maintained by a rigid cell wall and it is the nature of this cell wall that allows us to divide bacteria into two basic groups, Gram-positive bacteria and Gram-negative bacteria.
It is important to note the differences between the human (eukaryotic) cell and the bacterial (prokaryotic) cell because many of these differences account for disease pathogenesis and it has also been possible to exploit these differences in developing a chemotherapy regimen. In contrast to the human cell, the bacterial cell:
1. May have a capsule. Not all bacterial cells have a capsule but when it is present it is a major virulence factor. The capsule
includes the K-antigen.
2. May have anouter membrane which is the outer surface of the cell or, in the case of encapsulated strains, lies just
underneath the capsule. This has a trilaminar appearance. It contains lipopolysaccharides (LPS). These are known
as endotoxins. They are also the somatic or O-antigen and are used in serological typing of species. These occur only in
3. May have a periplasmic space which lies between the outer membrane and the plasma membrane. This is filled with the
periplasmic gel which contains various enzymes. Again, this occurs only in the Gram-negative bacteria.
4. Has a rigid cell wall made of peptidoglycan (except for the mycoplasma). This cell wall is thick in Gram-positive bacteria and
thin in Gram-negative bacteria. It is the thickness of the peptidoglycan that accounts for the ability/lack of ability to retain the
crystal violet used in the Gram stain.
5. Has a cytoplasmic membrane lacking sterols (except for the mycoplasma). Up to 90% of the ribosomes are attached to
this membrane. It also contains:
- a. The energy-producing cytochrome and oxidative phosphorylation system.
b. The membrane permeability (transport) systems.
c. Various polymer-synthesizing systems.
d. An ATPase.
attachment site for the chromosome.
7. May have a flagellum which arises from the plasma membrane and protrudes through the cell wall. This is the source of the H
antigen which is used in serologic diagnosis. It is also the motility organ and possibly an organ for attachment to a human
cell. It is considered a virulence factor.
8. Has hairlike microfibrils, termed fimbriae or pili, which originate in the plasma membrane and protrude through the cell
wall. They are straighter, thinner and shorter than flagella. The pili contain chemical compounds called adhesins which
allow the cell to bind to specific receptors on various human tissues. This binding gives rise to organ specificity of some
bacterial strains. Fimbriae/pili are major virulence factors.
9. Has ribosomes attached to the plasma membrane and also free in the cytoplasm which have a mass of 70S (the human
ribosome has a mass of 80S). The protein and RNA species in the bacterial ribosome differ from
those in the human ribosome.
10. May have an endospore within the cytoplasm. This is a body that allows the organism to resist adverse conditions.
11. Has a nucleus lacking a nuclear membrane.
12. May have a circular plasmid. This is a small (relative to the chromosome) piece of DNA that often codes for virulence
13. Has a haploid (single) chromosome.
There are many common themes in bacterial pathogenicity related to cell structure of the species. These are based on the presentation to the human body of the bacteria, its parts and its metabolites. When an organism, or more commonly a number of organisms of the same species, enters the human body and encounters no host defenses, it will exhibit a growth curve like the one depicted below for a closed system.
In the lag phase there is an increase in cell size at a time when little or no cell division is occurring. During this phase, there is a marked increase in macromolecular components (many of which are toxic to the human cell), metabolic activity and susceptibility to physical and chemical agents. The lag phase is a period of adjustment necessary for the replenishment of the cell's pool of metabolites to a level commensurate with maximum cell synthesis.
In the exponential orlogarithmic phase, the cells are in a state of balanced growth. During this state, the mass and the volume of the cell increase by the same factor in such a manner that the average composition of the cells and the relative concentrations of the metabolites remain constant. During this period of balanced growth, the rate of increase can be expressed by a natural exponential function.
The accumulation of waste products, exhaustion of nutrients, change in pH, induction of host immune mechanisms and other obscure factors exert a deleterious effect on the culture, resulting in a decreased growth rate. During the stationary phase, the viable cell count remains constant. The formation of new organisms equals the death of organisms in the system.
As the amount of the factors detrimental to the bacteria within the body increase, more bacteria are killed than are formed. During the phase of decline there is a negative exponential phase which results in a decrease in the numbers of bacteria within the system.