- To demonstrate the "transient" versus "normal" flora of the skin.
- To introduce host defenses.
- To demonstrate the effects of washing/disinfecting.
Human skin is an ecological niche, just as the bovine rumen or a hot sulfur spring are distinct niches. The skin is bathed with secretions from glands below the surface. Eccrine and apocrine sweat glands excrete fluids containing small amounts of nitrogenous substances and lactic acid. Each hair follicle is associated with a sebaceous gland which secretes sebum, a substance containing lipids and fatty acids which prevents hair from becoming brittle and forms a protective film over the skin surface. These acids lower the pH of the skin surface to a range of pH 3-5, which is inhibitory to some organisms. Fatty acids are toxic to some organisms and play a role in the development of the skin's normal flora. While skin is supplied with adequate nutrients, the toxic effect of the low pH and fatty acids, combined with the periodic drying of the skin makes skin a hostile environment for many organisms. Skin, like any ecological niche, has a "normal flora" of microorganisms that are able to survive and multiply, while its "transient flora" is comprised of organisms that are constantly in contact with the skin but do not survive there for very long. The skin's normal flora consists primarily of Gram positive bacteria of the genera Staphylococcus, Micrococcus and Propionibacterium, certain viruses and yeasts and the follicle mite.
In this experiment, the bacterial flora from the hand will be demonstrated before and after washing with soap. There may appear to be an increase in numbers after washing. Washing removes the outermost layer of the skin which is composed of loose, scale-like squamous epithelial cells and the transient flora, exposing the normal flora which is lodged around and under these cells and in the hair follicles.
Materials per pair of students:
- 3 Trypticase Soy Agar (TSA) Plates
- Soap, water, paper towels (Any kind of soap will do, but one that is not particularly antibacterial will give more dramatic results.)
- A small beaker of antiseptic or disinfectant (this can vary from group to group)
- A marking pencil
- Alcohol pads can be used to disinfect.
- No-water antibacterial hand cleansers are also popular.
- Divide each TSA plate in half by marking a line across the back with a marking pencil. Read the rest of the protocol and label plates appropriately before starting.
- Each student should inoculate one half of one plate by gently rubbing an unwashed index finger across the surface of the agar in the appropriate region of a plate.
- Each student should scrub the finger used in step two with the soap provided. Try not to clean the other hand. Blot the cleaned finger to dry and rub the finger over the other half of the plate.
- Each student should then immerse the index finger of the unwashed hand in the antiseptic or disinfectant provided for 1-2 minutes as directed by the instructor. Blot the finger dry and rub it over one half of the remaining TSA plate. (Two students will share this plate.)
- Incubate the TSA plates at 37¡C until the next lab meeting.
- Note the number and appearance of the colonies that grew on each plate. Record results. Compare your plates to those of other groups.
(each student) (shared plate)