An Unknown Microorganism Using Biochemical Tests

Knowing the Unknown

Identifying microorganisms can provide information on 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 that were learned in my microbiology laboratory classes. In this paper, I will discuss the processes of how I came to identify my unknown microorganism.

For this experiment, I utilized unknown number three which I later identified as Staphylococcus epidermis. I concluded that the unknown organism was Staphylococcus epidermis based on numerous tests performed in the laboratory which I will discuss in detail throughout this paper. One of the first tests performed was the Gram Stain. The Gram Stain differentiates bacterial organisms according to their cell wall structure. Gram-positive cells will stain blue to purple whereas Gram-negative cells will stain red to pink (Lab Handout; Gram Stain). Upon 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 to be cocci as evidenced by the spherical shape of the bacterium. In the following picture viewed with an electron microscope, you can clearly see that Staphylococcus epidermis is a Gram-positive cocci (microbiologyglossary.wikispaces.com). The Gram- positive cocci are arranged in grape-like clusters and are non-motile. They also form cohesive, white and raised

colonies ranging from 1–2 mm in diameter (vetbact.org). This strain of bacteria is a facultative anaerobic organism, which means that it can grow with or without oxygen. Staphylococcus epidermis grows well on nutrient agar dishes with a temperatur...

... middle of paper ...

...denced by the agar plate remaining blue-black in color.

Eosin methylene blue agar (EMB) is a medium used to isolate fecal coliforms and is selective for gram-negative bacteria against gram-positive bacteria. Sucrose and lactose serve as fermentable carbohydrate sources which encourage growth and allow one to differentiate between fermenting and non-fermenting microbes. Vigorous fermenters of lactose or sucrose will produce quantities of acid sufficient to form the dark purple dye complex which is usually associated with a green metallic sheen. Slow fermenters will produce a smaller amount of acid production and appear brown-pink in color (Lab Handout; EMB). This experiment resulted in no color change which was expected as two previous tests indicated that my unknown was a gram- positive bacteria and this test is selective for gram-negative bacteria.