Bacteria - Capsules and slime layers | badz.info
The glycocalyx is a thick outer covering of the plasma membrane. It is composed of strands of sugars and proteins bound together. The result is. A typical bacteria cell(prokaryote)is made up of three outer membranes The outer most is badz.info middle one is cell wall and the inner. An extensive, tightly bound glycocalyx adhering to the cell wall is called a capsule. In this section on Prokaryotic Cell Anatomy we are looking at the various . Click on this link, read the description of Streptococcus pneumoniae, and be able.
Capsules and slime layers Many bacterial cells secrete some extracellular material in the form of a capsule or a slime layer. A slime layer is loosely associated with the bacterium and can be easily washed off, whereas a capsule is attached tightly to the bacterium and has definite boundaries. Capsules can be seen under a light microscope by placing the cells in a suspension of India ink. The capsules exclude the ink and appear as clear halos surrounding the bacterial cells.
Capsules are usually polymers of simple sugars polysaccharidesalthough the capsule of Bacillus anthracis is made of polyglutamic acid. Capsules can protect a bacterial cell from ingestion and destruction by white blood cells phagocytosis.
Bacterial Structure in Relationship to Pathogenicity
While the exact mechanism for escaping phagocytosis is unclear, it may occur because capsules make bacterial surface components more slippery, helping the bacterium to escape engulfment by phagocytic cells. The presence of a capsule in Streptococcus pneumoniae is the most important factor in its ability to cause pneumonia. Mutant strains of S. The association of virulence and capsule formation is also found in many other species of bacteria.
Chemokines The enzymes and proteins listed above serve to reinforce the glycocalyx barrier against vascular and other diseases.
Another main function of the glycocalyx within the vascular endothelium is that it shields the vascular walls from direct exposure to blood flow, while serving as a vascular permeability barrier.
In microvascular tissue, the glycocalyx serves as a vascular permeability barrier by inhibiting coagulation and leukocyte adhesion. Leukocytes must not stick to the vascular wall because they are important components of the immune system that must be able to travel to a specific region of the body when needed.
In arterial vascular tissue, the glycocalyx also inhibits coagulation and leukocyte adhesion, but through mediation of shear stress-induced nitric oxide release. Another protective function throughout the cardiovascular system is its ability to affect the filtration of interstitial fluid from capillaries into the interstitial space. Certain stimuli that cause atheroma may lead to enhanced sensitivity of vasculature.
Initial dysfunction of the glycocalyx can be caused by hyperglycemia or oxidized low-density lipoproteins LDLswhich then causes atherothrombosis.
In microvasculature, dysfunction of the glycocalyx leads to internal fluid imbalance, and potentially edema. In arterial vascular tissue, glycocalyx disruption causes inflammation and atherothrombosis. One particular study used an isolated perfused heart model designed to facilitate detection of the state of the vascular barrier portion, and sought to cause insult-induced shedding of the glycocalyx to ascertain the cause-and-effect relationship between glycocalyx shedding and vascular permeability.
Hypoxic perfusion of the glycocalyx was thought to be sufficient to initiate a degradation mechanism of the endothelial barrier. The study found that flow of oxygen throughout the blood vessels did not have to be completely absent ischemic hypoxiabut that minimal[ clarification needed ] levels of oxygen were sufficient to cause the degradation. Shedding of the glycocalyx can be triggered by inflammatory stimuli, such as tumor necrosis factor-alpha.
Whatever the stimulus is, however, shedding of the glycocalyx leads to a drastic[ clarification needed ] increase in vascular permeability. In the case of Vibrio cholerae, the vibrios apparently swim laterally into the intestinal mucosa to avoid being flushed out by the peristaltic action of the gut.
Flagella are antigenic, and therefore, vulnerable to attack by host antibody molecules. Liefson's flagellar stain CDC.FUNCTION OF GLYCOCALYX
Bacterial flagella are below the resolving power of the light microscope. In order to be visualized, the bacteria must be reacted with a stain that precipitates along the flagellar filaments, which increases their effective diameter to the point of resolution. Fimbriae and Pili are interchangeable terms used to designate short, hair-like structures on the surfaces of bacterial cells. Fimbriae are shorter and stiffer than flagella, and slightly smaller in diameter.
Like flagella, they are composed of protein. A specialized type of pilus always called a pilusthe F or sex pilus, mediates the transfer of DNA between mating bacteria, but the function of the smaller, more numerous common pili is quite different. Inasmuch as many bacteria are able to exchange genes for virulence by means of conjugation, the sex pilus which confers the ability to conjugate, may well play a role in the their assembly of virulence determinants.
Common pili or fimbriae are often involved in adherence attachment of bacterial cells to surfaces in nature.
In medical situations, they are major determinants of bacterial virulence because they allow pathogens to attach to colonize tissues and, sometimes, to resist attack by phagocytic white blood cells.
As surface structures on the bacterial cell, the functions of fimbriae overlap with those of capsules discussed below. Fimbriae are also antigenic and secretory antibodies IgA will often block bacterial colonization, while circulating antibodies IgG or IgM will opsonize bacterial cells for phagocytosis. Electron micrograph by David M. Phillips, Visuals Unlimitedwith permission.
This pathogen utilizes its fimbriae in order to initially colonize the urethral or cervical epithelium. Most bacteria contain some sort of a polysaccharide layer outside of the cell wall or outer membrane.
In a general sense, this layer is called a capsule. A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall.
A less discrete structure or matrix which embeds the cells is a called a slime layer. Slime layers are equivalent to biofilms below A type of capsule found in bacteria called a glycocalyx is a thin layer of tangled polysaccharide fibers which is a observed on the surface of cells growing in nature.
Some microbiologists consider all types of exopolysaccharides to be glycocalyx.