Knowing the Unknown Identifying microorganisms can provide insight into diagnosing diseases and discovering the most beneficial treatment possible. The purpose of this assignment was to identify an unknown microorganism using biochemical tests and various methods learned in microbiology laboratory classes. In this article I will discuss the processes by which I came to identify my unknown microorganism. For this experiment, I used the unknown number three which I later identified as Staphylococcus epidermis. I concluded that the unknown organism was Staphylococcus epidermis based on numerous laboratory tests which I will discuss in detail in this article. One of the first tests performed was the Gram stain. Gram stain differentiates bacterial organisms based on cell wall structure. Gram-positive cells stain blue to purple while Gram-negative cells stain red to pink (Laboratory Handout; Gram Stain). After performing the Gram stain on my unknown, I concluded that it was a Gram-positive cell due to the purple color when viewed under the microscope. I also determined the shape of the cocci, as evidenced by the spherical shape of the bacterium. In the following image seen with an electron microscope, you can clearly see that the epidermis of Staphylococcus is a Gram-positive coccus (microbiologyglossary.wikispaces.com). Gram-positive cocci are arranged in cluster-like clusters and are nonmotile. They also form cohesive, white, raised colonies ranging from 1 to 2 mm in diameter (vetbact.org). This strain of bacteria is a facultative anaerobic organism, meaning it can grow with or without oxygen. Staphylococcus epidermis grows well on nutrient agar plates with a temperature... center of the paper... notched from the agar plate remaining blue-black. Eosin methylene blue (EMB) agar is a medium used to isolate fecal coliforms and is selective for gram-negative bacteria over gram-positive bacteria. Sucrose and lactose serve as sources of fermentable carbohydrates that encourage growth and help distinguish between fermenting and non-fermenting microbes. Vigorous fermenters of lactose or sucrose will produce quantities of acid sufficient to form the complex dark purple dye that is usually associated with a green metallic sheen. Slow fermenters will produce less acid production and appear brown-pink in color (Lab Handout; EMB). This experiment did not produce any expected color change since two previous tests indicated that my unknown was a gram-positive bacteria and this test is selective for gram-negative bacteria.