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Microbiology Unknown Lab Report | Staphylococcus aureus

By at January 3, 2013 | 5:07 am | Print


Microbiology Unknown Lab Report 

Samantha Moellenhoff

Microbiology, Fall 2012



With the constant debate in society about microorganisms, it is important to distinguish their identities. (There are many reasons as to why this is important such as knowing what agent is causing a disease and the correct treatment is determined, to knowing which microbes are beneficial to a person and important to body functioning.) The purpose of this study was to identify an unknown bacterium by applying all methods that were previously conducted and learned in the microbiology laboratory class.


An unknown test tube labeled 120 was given out by the lab instructor. The goal at this point was to determine two separate unknown bacterium, one Gram positive and one Gram negative. The procedures performed consisted of sterile technique in addition to being followed as stated in the referenced course laboratory manual, unless otherwise noted.

The first procedure performed was an isolation of the unknown bacteria with the goal of obtaining a pure culture. This was conducted by streaking the unknown out on a Trypticase Soy Agar (TSA) plate using the T streak method described in the manual. From that, the plates were incubated for two days and the bacterium grew. The bacteria were studied based on their physical characteristics and recorded. Two distinct different colonies grew. Each colony was isolated to grow purely on its own TSA plate. One TSA was labeled culture “A” and the other culture “B”. Gram stains were performed on each culture. It was critical to follow each step correctly while using sterile technique because from this test not only is the stain determined but also the shape of the cells. Culture “A” was determined to be a Gram positive cocci and culture “B” was determined to be a Gram negative rod. After the Gram stain was determined, specific biochemical tests were performed. The biochemical tests performed were chosen based on the identification table that was given from the lab instructor. The flow charts on the following page list these tests and results for the Gram positive and Gram negative bacteria.

All of the following tests were performed on Gram positive unknown:

  1. Mannitol Salt Agar
  2. Nitrate
  3. Catalase

All of the following tests were performed on Gram negative unknown:

  1. Simmons Citrate
  2. Mannitol Salt Agar
  3. Urea
  4. MacKonkey’s Agar


RESULTS – (this section contains flowcharts)                      



After several different biochemical tests, it was concluded that the Gram positive in unknown 120 was Staphylococcus aureus. The bacterium was grown on a TSA agar plate for use of additional tests, gram stain reaction determined that the unknown was a gram positive cocci. The culture was then inoculated onto a Mannitol Salt Agar plate. The growth and color change to yellow indicated Mannitol fermentation because acid was produced. This indicated that the unknown was either Staphylococcus aureus or Enterococcus faecalis. Acid fermentation is a characteristic of pathogenic Staphylococci. Then, a Nitrate broth test was performed. This test had a positive reaction meaning the reduction to nitrites occurred. This indication meant the unknown was Staphylococcus aureus. A Catalase test was the performed for clearification. The Catalase test had a positive reaction meaning the bacteria produced the enzyme catalase confirming that the unknown bacterium was Staphylococcus aureus. 

Staphylococcus aureus is in the Staphylococcaceae family. It is usually found in nasal passageways but can also be found on skin, in the oral cavity and gastrointestinal tract. It can be carried as part of the normal bacterial flora. Staphylococcus aureus is commonly known because of MRSA (methicillin-resistant Staphylococcus aureus) which is an epidemic in the medical world because of its antibiotic resistance. S aureus can become pathogenic through tissue invasion and toxin pollution. Vancomycin is the antibiotic usually prescribed because of resistance to penicillin.   



  1. McDonald, V., Thoele, M., Salagiver, B., & Gero, S. (2011). Lab Manual for General Microbiology: Bio 203. N.p.: St. Louis Community College at Meramec.
  2. Tolan Jr., R. W. (2012, July 9). Staphylococcus Aureus Infection . In Medscape Reference . Retrieved December 2, 2012, from
  3. Todar, K. (n.d.). Staphylococcus aureus and Staphylococcal Disease. In Todar’s Online Textbook of Bacteriology . Retrieved December 3, 2012




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  1. […] S.aureus was first discovered in 1880 by Sir Alexander Ogston, a surgeon in Scotland.  He saw this bacterium in pus inside an abscess in a knee joint. Twenty percent of the human population are carriers of S aureus, which can be found on the skin and inside the nasal passages. S. aureus is the most common species to cause Staphylococcus infections.  It can cause many illnesses, from minor skin infections, to life threatening diseases such as meningitis, pneumonia, osteomyelitis, endocarditis, toxic shock syndrome, and sepsis (Todar, 2012). It is estimated that some 500,000 patients in American hospitals contract a staphylococcal infection each year.  It is often the cause of postsurgical wound infections and one of the five most common causes of nosocomial infections (Todar, 2012). […]

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