|MadSci Network: Microbiology|
The best way to answer your question about how fast skin microorganisms grow is to do the experiment you propose to do.
Microbiology manuals can give you the doubling time of specific organisms but part of your question probably involves identifying the diversity of kinds of microbes that live on your body. Without knowing specifically what they are, you can be able to tell from the shape, color and surface texture of the colonies that form that multiple kinds of microbes are growing. There are microbiology atlases that show nice pictures of colony forms that may help narrow down the classes of microbes but actual identification of them might take more technology than you have available to you at this stage in your education.
The skin that covers the outside of your body and the skin that covers your intestines and parts of your body that are inside but connect with the outside normally has microbes on it that are not harmful to you and may perform some functions that are actually good for you like competing with the bad bacteria for nutrients so the bad bacteria are kept from growing. http://www.microbes.info/
Washing too much can actually make your skin more susceptible to invasion by bad bacteria (pathogenic bacteria) because too much washing can cause drying and cracking of the protective epithelial layers. This makes it easy for Staphylococcus aureus or pathogenic streptococci to invade and cause abscesses (Staph) or cellulitis (strep). Cellulitis is a spreading infection in the dermis (under the epidermis) while an abscess is a localized reaction filled with pus. Acne or feruncles (pimples) are small abscesses that form in oil or sweat glands in your skin and may also be caused by Corynobacterium acnes.
On the other hand, washing too little can allow some bacteria to convert the oils and secretions on your skin surface to chemical forms that smell bad. At first, the byproducts of bacterial digestion are only smelly but later on these chemicals can damage the skin and allow bacteria to invade. In the time of the Romans soap was not used for bathing, instead the skin was encouraged to sweat in steam baths and then oils were applied and scraped off with a special scraping tool. Then the person would bathe in hot followed by cold water. This wouldn't be done very often so one bath house for a small community was all that was needed.
Our society in the US is too obsessed with bodily cleanliness and a school
of scientific analyses has concluded that
much cleanliness may be responsible for the increased incidence of
asthma and allergic skin rashes in our people.
In countries where people are less obsessed about cleanliness there is much less asthma even among peoples who are known to be unusually susceptible to these allergic diseases. In the countries where less bathing is done, perfumes and skin creams are popular.
Fungi and yeast also like to live on the skin. For the most part these microbes just like to digest the dead cells that slough off the epidermis every day and some also like the skin oils. You should expect to find some colonies of yeast like candida albicans or fungi of the family trichophyton. Unless you have folds in your skin that stay moist with sweat, the candida species won't invade and cause a localized infection. Trichophyton species, if left to there own resources will cause "athletes foot" and toe nail colonizations. There may be other fungi on your skin that you might not have been aware of. If you have access to a "black light" a kind of ultraviolet light, it will cause the skin fungi to glow in the dark. This is one of the ways doctors can tell if you have a tinea infection (ring worm) or tinea versicolor which causes patchy tan or pale color changes to your skin.
I know of no viruses that would be called normal inhabitants of skin but "warts" are caused by papilloma viruses and "cold sores" are caused by herpes viruses. You won't be able to grow any viruses on your gelatin since these require cell culture systems for isolation.
Do lots of reading after you search for the information you need. This will help you narrow down what you intend to do and that will help you be successful in your research. Good luck with your experiment.
Also, be careful while you are examining your plates. While it is unlikely that you will grow dangerous bacteria, it is possible that the increased number of bacteria that may grow on the gelatine might not be what you want to touch or breath. Wear a long sleeved washable or disposable gown, a surgical mask and rubber gloves when you examine your plates. Also, I am hoping that you will do these experiments under the supervision of one of your teachers in the science department.
Good luck with your experiment. I hope this helps.
Try the links in the MadSci Library for more information on Microbiology.