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Staphylococcus epidermidis | Microbiology

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Knowing the identity of microorganisms is beneficial because it allows healthcare professionals the ability to determine the organism’s pathogenicity, to prescribe the right antibiotics, as well as the correct dose, and to effectively treat infections and illnesses in the human body.  Microorganisms are also helpful within the human body to aid in the digestion of food and to fight off harmful disease-causing bacteria. To determine the unknown microorganism a series of lab tests, learned in a microbiology lab, were performed. (Laboratory Manual)


An unknown in a test tube labeled 123 was given out at random by the lab instructor. The test tube given out had two types of bacteria, one being gram-positive, and the other being gram-negative. The unknowns have been tested by a series of tests learned in the laboratory; the tests were performed according to the instructions in the laboratory manual. (Laboratory Manual)

The first step in determining the unknowns was to streak the contents from the unknown test tube onto a Nutrient Agar plate to identify two different colonies. The streaking method used was the isolation streak, which is a streak in a zigzag formation, pulling from the previous streak until there are four zigzag streaks on all sides of the plate. The isolation streak was then placed in the incubator for forty-eight hours. When taken out of the incubator a gram stain was performed on two different colonies. The bacteria was then determined as gram-positive or gram-negative, and then streaked onto two separate Nutrient Agar plates to grow a pure culture. After the pure cultures were placed in an incubator to grow the bacteria was re-stained to determine that the initial stain was done correctly. (Laboratory Manual)

The following tests were performed on Unknown 123:

  1. Mannitol
  2. Catalase
  3. Urea
  4. Lactose
  5. Simmons Citrate
  6. Indole
  7. Casein



After isolating a pure culture of gram positive cocci a series of tests, from the unknown identification table, was performed to identify the bacteria. A Mannitol test was performed on the gram positive cocci. The bacterium was grown on a Mannitol Salt Agar plate, which tests for the bacteria’s ability to ferment mannitol. When mannitol is fermented acid is produced changing the color of the agar. This bacterium did not result in the production of acid which determined the gram positive cocci to be negative for mannitol fermentation. The catalase test was then performed on the bacterium to determine if the unknown produces the enzyme catalase. The bacterium tested positive for the production of the enzyme catalase.  The organism was then inoculated into a urea tube to test for the enzyme production of urease. This bacterium was positive for urease production; this was indicated by a change in the color of the test tube from yellow to bright pink. This concluded that the gram positive unknown 123 was Staphylococcus epidermidis, which was confirmed by the instructor. No problems were encountered when testing for the gram positive bacterium.

A series of problems were encountered when isolating the gram negative bacterium. After several contaminated cultures, a pure culture was given out by the instructor to grow and test with. A Simmons Citrate test was performed to see if the unknown could use citrate as a carbon source, the unknown tested negative, as there was no change in the color of the agar. A urea test was performed giving a negative result. The unknown was predicted to be Escherichia coli. This prediction was confirmed to be incorrect by the instructor.The gram negative bacterium was then grown on a Desoxcholate Agar, which is a selective medium, as well as a test for lactose fermentation. This unknown did not ferment lactose, determined by a colorless colony, which concluded the test as negative. The unknown was also grown on a Milk Agar plate to test for the production of casease. This unknown tested negative for the production of casease due to the fact that there was no clearing in the agar plate.  An Indole test was then performed on the unknown to test for the production of Indole. This test was performed by inoculating the gram negative bacteria into a SIM tube, and adding the Indole reagent after incubation. The SIM tube also tests for sulfur reduction and motility. The unknown was positive for Indole production, motility, and sulfur reduction. These results were indicated by a red color change at the top of the tube that indicated Indole production, black coloring that indicated sulfur reduction, and a radiation of growth from the stab mark, which indicated motility. The unknown was determined as Proteus vulgaris, and confirmed by the instructor. (Laboratory Manual)

Staphylococcus epidermidis is part of the normal human flora. Staphylococcus epidermidis is an opportunistic bacterium that is one of the leading nosocomial infections. Newborns, elderly people, and drug users are the most susceptible to this bacterium. Most infections are associated with prosthetic instruments such as catheters and stints. Septicemia and Endocarditis are diseases that are associated with S. epidermidis. Septicemia is usually common in babies with low birth weights. Its pathogenicity is due to the fact that it has the ability to form biofilms. The treatment for infections usually involves antimicrobial therapy and the removal of indwelling devices. (Bukhari)

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  1. Bukhari, Mohammad. “Staphylococcus Epidermidis.” N.p., 27 Sept. 2004. Web. 4 Dec. 2012. <http://web.uconn.edu/mcbstaff/graf/Student%20presentations/S%20epidermidis/sepidermidis.html>.


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