Exercise 19 Capsule Stain PDF

w V'I u 0::: w X w CAPSULE STAIN LEARNING OBJECTIVES------------ 1. Define a glycocalyx and discuss its composition and function. 2. Describe how a capsule can influence survival, adhesion, and pathogenicity. 3. Use a bright light mic...

w V'I u 0::: w X w CAPSULE STAIN LEARNING OBJECTIVES------------ 1. Define a glycocalyx and discuss its composition and function. 2. Describe how a capsule can influence survival, adhesion, and pathogenicity. 3. Use a bright light microscope to view a capsule stain of a bacterial culture. 4. Differentiate between encapsulated and non-encapsulatedorganisms using a bright light microscope. INTRODUCTION-------------- A glycocalyx is a general term used to describe any substance that adheres to the external surface of cells. The glycocalyx is a sticky, gelatinous polymer that is composed of polysac- charide, polypeptide, or both. Generally, this substance is produced inside the cell and is then secreted to the exterior of the cell. Because of the high water content of this polymer, an important function of the glycocalyx is to protect organisms from drying out. Additionally, by virtue of its sticky nature, the glycocalyx helps to retain nutrients for organisms. One form of glycocalyx is a capsule. Although many bacterial species possess capsules, not all species do. A capsule is a thick layer that adheres tightly to organisms and often gives a mucoid appearance to bacterial colonies. The chemical composition of the capsule is unique to each individual organism. Capsules can contribute to bacterial virulence, which is the rel- ative ability of a bacterium to cause disease because they often protect bacteria from the immune response of the host. Scientists have observed a repulsion between encapsulated organisms and human white blood cells and they believe this is due to the strong nega- tive charges in the capsular material. Other researchers believe that a capsule simply causes a bacterium to become too slippery for the white blood cells to ingest during the process of phagocytosis. Regardless of the reason why, it has been documented that encapsulated bacteria are generally more pathogenic, or capable of causing disease, than nonencapsu- lated species, and oftentimes the capsule is the primary determinant of virulence. In addi- tion, some capsules may be composed of chemicals that are similar to those found in the host which helps to provide an immunological disguise for these organisms when in the human body. Finally, capsules also allow bacteria to attach to various surfaces, such as the respiratory tract and the urinary tract. Thus, the presence of a capsule can facilitate attach- ment, the first step in the disease process. THE MICROBES AROUND US 133 EXERCISE19 CAPSULE STAIN A second form of glycocalyx is called a.slime layer, which is also only.in bacteria. This type of glycocalyx is rather loosely attached to the cell wall and its main function is to allow bacteria to adhere to objects in their immediate environment. In addition a slime layer protects the cell against drying out, helps to trap nutrients for the cell, and helps in the formation of biofilms. When bacteria are encased in capsules or a slime layer and form microcolonies attached to surfaces, they are said to be part of a biofilm. Biofilms can form in and on oil pipelines, med- ical devices, sewage treatment equipment, and the human body. Biofilms begin where the fluid glycocalyx layer of bacteria meets a solid surface and consist of this underlying layer with bacteria encased in the exposed portion. The biofilm provides protection for the micro- bial population living within this community by shielding the organisms from antibiotics, disinfectants, and the host immune system. One example of a biofilm is the plaque that forms on our teeth. This sticky layer begins to form soon after we brush our teeth and allows cavity-causing organisms to adhere to teeth, where they can ferment dietary carbohydrates to produce the acids that dissolve tooth enamel. Capsule staining is a very difficult staining procedure. Because of their solubility in water, capsular materials are easily dislodged or removed during the washing steps of staining procedures. Therefore, you will simply observe a pre-stained capsule stain and will not perform the capsule stain on your environmental isolate. Capsules are visible as halos around the bacterial cells. Gt'l e:; TODAY'S PROCEDURE------------ ,. Your graduate teaching assistant has set up a microscope focused on a capsule stained preparation of bacteria. It is each student's responsibility to view this slide. c:-- 2. Be sure to note the difference between the capsular material and the bacterial cell (::- itself. 3. Record all results. = G b: b: t'. G' 134 THE MICROBESAROUND US