The Concentration of Staphylococcus Aureus in the General Public


The objective of this experiment was to determine the concentration of staphylococcus aureus in the general public.

In this experiment, a series of confirmation tests were performed. Before these tests, strains from the nasal cavity were initially cultured in Mannitol salt agar, a selective media responsible for enhancing the growth of Staphylococcus strains. The tests that followed were: gram staining, catalase test, and coagulase test consecutively. However, caution ought to be taken in the penultimate test in that it should be done promptly to yield genuine results. These tests confirmed the presence of pathogenic strains of Staphylococcus aureus in less than 40% of each of the two population lots. These results were consistent with the theoretical range reserved for the general public.


The objective of this experiment was to identify and consequently determine the carriage rate of Staphylococcus aureus in the nasal cavities of students.


Microorganisms tend to colonize our body surfaces and nostrils without causing any infection in our bodies. One such common microorganism that inhabits the nasal cavity is Staphylococcus aureus. Though inconsequential when in minute quantities, if many, these colonies in food industries as well as clinical settings can impact fatally the terminal end users. Hence, caution ought to be taken by service providers such that microbial screening is preceded to determine the presence of the same (Madigan 18). Concentration ranges of beyond 10-40% and 50-70% for the general public and hospital personnel populations respectively represent a health hazard (Madigan 20).

Materials/ reagents

Mannitol salt agar plate, sterile swabs, incubator, hydrogen peroxide, nutrient agar plate, coagulate tube, specimen (students), and a streaking loop.


Using sterile swabs, left and right nostrils were swabbed after which they were cultured in Mannitol salt agar (MSA) – a selective media. This was then followed by streaking of dilute using a flamed loop after which it was aerobically incubated for 36 hours under temperatures of 350 C.

With the aid of a demonstrator, the presence of opaque colonies encompassed within bright yellow zones was investigated. If the test would turn out positive, gram staining and catalase test would follow after which plate coagulase test would eventually follow if the previous tests would turn out positive. Noteworthy, in case of limitation in the number of colonies required to perform the above tests, the colonies would be sub-cultured in nutrient agar to obtain enhanced pure colonies. All the positive results were later recorded for analysis to determine the concentration of the group tested.


The investigation revealed the presence of Staphylococcus aureus since all the tests turned out positive for the students who tested positive. The concentrations per population tested are as shown in table 1 below which also gives the concentrations for the previous years.

Table 1: of concentration of Staphylococcus aureus in student population.

Year Bundoora Campus City Campus % S. aureusconcentration Bundoora Campus % S. aureusconcentration City Campus
No. Positive
Total No.
No. Positive
Total No.
2010 16 71 52 121 22.5% 43.0%
2011 14 80 32 140 17.5% 22.9%
2012 40 104 47 135 38.5% 34.8%


The objective of this experiment was to determine the concentration of Staphylococcus aureus in student populations theoretically thought to fall within the 10%-40% bracket. The experimental results gave consistent results which coincided with the theoretical range. In comparison with the previous years, the current year’s concentrations were very high. Beyond the theoretical range, there would be high chances of cross-contamination that would eventuate in food poisoning in case the students come in contact with food. This can be averted by the use of protective clothing when handling food, and also one should avoid touching the nasal cavities and face while handling food.

Initially, MSA was used because it is both a selective as well as a deferential media. Hence, it inhibits the growth of nonstaphylococcus strains while at the same time enhancing the growth of pathogenic staphylococci cultures. The essence of doing gram staining was to establish the identity of staphylococcus aureus which is a gram-positive bacterium. As expected, the results turned positive confirming that the bacterium has a thick peptidoglycan wall responsible for retaining the violet stain observed. The catalase test performed was vital in distinguishing staphylococcus from streptococcus. The former is catalase-positive as opposed to the latter, and as expected the results turned positive.

Eventually, the coagulase test performed was to substantiate the real identity of the strain. This is so because not all gram-positive and catalase-positive strains are pathogenic cocci (Pankey 3). Coagulase-positive strains (where S. aureus falls) can protect themselves from phagocytes, antimicrobials, and other immune responses. They achieve this by engaging the fibrils of the plasma to form a protective layer owing to their exoenzyme coagulase. As a caution, the terminal test should be done promptly to obtain genuine results. Noteworthy, the incubation temperatures needed to be at 350 C. since it is the optimal temperature for maximum performance of the bacterium enzymes. Also, this is close to the body temperature which is the habitat of these microorganisms (Pankey 9).


The objective of the experiment, which was to determine the concentration of staphylococcus aureus in a students’ population was achieved. The results obtained were consistent with the theoretical range of 10-40% reserved for the general public.

Works Cited

Madigan, Micheal. Brock Biology of Microorganisms. Philadelphia: Lippincott Williams & of South Carolina, 2005. Print.

Pankey, George. “Streptococcus pneumoniae and Staphylococci”. Columbia: University Wilkins. 2005. Print.

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