Survey of Nasal Carriage of Staphylococcus aureus Essay Example

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Detection of Nasal Carriage of S.aureus Bacteria

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Using MSA as the primary tests and Tube coagulase tests as confirmatory tests for presumptive S.aureus, an experiment was carried out among 163 students in the second semester 2017 in RMIT University to determine the carriage rate of Staphylococcus aureus Bacteria in the snares of student. S.aureus is linked with increased public health cost burden, morbidity and mortality rates. In any given population about 20-30% of S.aureus may asymptomatically hosts S.aureus within their snares making it inhabitable source for endogenous infections and contact transmission

The demand for partially processed foods and natural foods, and ready-to-eat convenient foods as the main reason behind escalating S.aureus rates today as the environment is suitable for toxin-producing bacteria. S.aureus is a fatal and versatile microbe causing the respiratory tract infections, skin and infections of soft tissue characterized with abscesses. S.aureus causes pneumonia, osteomyelitis, scalded skin syndrome, toxic shock syndrome, and infective endocarditis among others. The pathogen versatility has various virulence factors; toxins, are encoded on mobile genetic elements (MGEs) like prophages or plasmids and are transferrable between strains using horizontal gene transfer (HGT). MRSA is the commonest species globally and in Australia. S.aureus strains are antibiotic resistant strains.

MSA results were interpreted as positive if yellow zone around single isolated colonies was formed and presence of growth was evident on salt tolerance. On confirmation of presumptive S.aureus, presence of clotting of plasma was interpreted as positive results. On average 27.61% student tested positive for S.aureus carrier semester 2 while on average in this academic year 25.30% students in RMIT University tested positive for S.aureus carrier.

The study concluded that the percentage was in line with other researchers that about 20-30% of adult population is S.aureus carriers. Further, nasal decolonization of S.aureus is difficult to attain and nose and throat recolonization is easily achieved.

Aims and Significance

An experiment was carried out among 163 RMIT University students in the second semester 2016/2017 academic year to determine the carriage rate of Staphylococcus aureus Bacteria in the snares of student. All students chosen as participants did this experiment; which included nasal swab tests and conformation tests. The experiment did use sterile swab and Mannitol salt agar (MSA) plate. MSA was the primary tests used to isolate presumptive pathogenic staphylococcus species. The tests is suitable for isolating saline tolerance organisms and consume mannitol sugar as energy source (carbohydrate). Tube coagulase test was further used to confirm presumptive S.aureus (RMIT Laboratory Workbook).

Warnke, et al., (2014) S.aureus is a primary cause of pussy bacterial infections of which most of them lead to life-threatening complications. This costs the public health a great deal. Hanssen, et al., (2017) this study is of paramount importance because nasal S.aureus carriage approximately affects 20% -30% in a certain population may asymptomatically hosts S.aureus within their snares, and thus serve as key source for endogenous infections and contact transmissions. Mostly, S.aureus endogenous strain is the main cause of infection. Eliminating and preventing S.aureus carrier state may help in reducing its disease burden. Historical research shows that the pathogen is a risk factor for nosocomial and community-acquired infections.


Schelin, et al., (2011) currently there is an increased demand for unprocessed foods. The war against bacterial foodborne infection comes with a great challenge due to climate change, the food market globalization and human consumption changing patterns. Currently, consumers are looking for less processed natural foods, with little preservatives added, low fat, sugar or salt content, and of high quality and an extended shelf-life. There has been a sharp increase on the demand for ready-to-eat convenient food, with the food industry coming up with new food processing techniques like chilled food, minimally processed and semi-prepared foods in response to these rising demands. Unfortunately convenience foods are excellent environment for toxin-producing bacteria growth like the S.aureus.

Otto, (2014) S.aureus is a fatal and versatile microbe and may cause multiple different human syndromes and diseases. More often it causes the respiratory tract, skin and infections of soft tissue characterized of abscesses. With skin-infection, there are community acquired; while for lung infections are rampant among the infections of nosocomial S.aureus. S.aureus is the commonest among nosocomial pathogens and is linked with high mortality and morbidity. Kobayashi, Malachowa & DeLeo, (2014) S.aureus infections primary defense in the host is the polymorphonuclear leukocytes (neutrophils).host cells produce and contain many antimicrobial agents that may kill the bacteria effectively, although they can also trigger non-specific damage to these host tissues and trigger formation of abscesses. Otto, (2014) S.aureus pneumonia is common among inpatients with underlying conditions like viral infections and immune deficiencies infections. S.aureus may as well lead to other life-threatening and very severe diseases like osteomyelitis, scalded skin syndrome, toxic shock syndrome, and infective endocarditis among others.

S.aureaus versatility as a pathogen arises because it houses various virulence factors of which most of them are encoded on mobile genetic elements (MGEs) like prophages or plasmids and are transferrable between strains using horizontal gene transfer (HGT). With S.aureus, HGT may occur via direct “naked” DNA uptake by genetic competence, conjugation, and phage transduction. Virulence factors of S.aureus are toxins; poisonous substances. These factors in S.aureus are distinct, in that; molecules increase the capacity for the pathogen to extensively cause disease because they are secreted by the organism producing them and directly interfere with the host. During its pathogenesis, S.aureus produces large quantities of secreted, surface bound proteins that provide the pathogen with a mechanism for attaching to the host tissue.

Kobayashi, Malachowa & DeLeo, (2014) despite its high prevalence, S.aureus easily acquire antibiotic resistance characteristics. Since 1940s, S.aureus has acquired penicillin-resistant. Early 1960s reports on methilinn-resistant S.aureus (MRSA) were made rendering MRSA endemic in health today most in all industrialized countries health care. Further, community-associated MRSA was also reported in 1990s and today it has proven to be a big problem in many nations globally including Australia. Although, MRSA infection occur majorly on persons with predisposing risk factors, CA-MRSA may cause infection in healthy individuals. β-lactams, S.aureus antibiotics resistance is the main problem, however, the microbe may develop resistance to many other antibiotics like vancomycin imperative therapeutic agent used for MRSA severe infections. Consequently, MRSA remains a Golden Staph case in Australia.

Warnke, et al., (2014) understanding the S.aureus carriage, sends a strong message to the health professional, patients, food handlers and food manufactures, and the general public that there is serious need to embrace personal, health equipment and food equipment and food and personal hygiene always. Among many methods used for testing S.aureus carriage nasal swab-based screening is the main one used to identify its subcarriers.

Materials and methods

The experiment procedure was done during a practical class. The study included 163 students who were randomly picked among the institution population regardless of gender, race and age (>18 years) and informed of the study objectives. The survey was made to determine the S.aureus bacteria carriage rate in the students’ snares.

Mannitol salt agar-MA was used as it selectively isolates the presumptive staphylococcus pathogenic species. Tube coagulase test was used to confirm the presumptive S.aureus. Presumptive coagulase-positive Staphylococci appear as colonies with bright yellow zones surrounding, while red-purple zones are produced by non-pathogenic staphylococci colonies (RMIT University,). The MSA procedures included:

  • Moisten one sterile swab in sterile saline then swab both the left and right nostrils and culture onto a MSA plate.

  • Use the swab to prepare the primary inoculum

  • Streak dilute in normal way with a flamed loop. Incubate 35c .O2. 44-48h (Oxoid web.)

Biochemical confirmation tests were then used. In this case, Tube coagulase test was the only test used on presumptive S.aureus isolates. The procedure included:



  • sterile citrated rabbit plasma

  • 24 hour broth culture of organism to be tested

  • sterile plastic test tubes

  • 37oC waterbath.


  1. Take 3-4 loopfuls of pure colonies of the test organism and emulsified in freshly prepared citrated rabbit plasma in a test tube

  2. Place in a 37oC waterbath for four hours. 

  3. Check the clotting formation after 4 hours incubation.  

  4. If there is no coagulation after four hours incubation then incubate the test overnight at room temperature (21-24oC).  

During the experiment, control reagent(s) were maintained under sterile status to ensure results validity. Lastly, upon discussing the results with class moderator, isolated presumptive colonies of S. aureus were confirmed via tube coagulase tests (RMIT University, ).

MSA Results Interpretation 

Positive for mannitol utilization
 = Yellow zone around single isolated coloniesNegative for mannitol utilization = No change in media around single isolated colonies

Positive for salt tolerance = presence of growth

Negative for salt tolerance = no growth or poor growth

Tube coagulase tests Result Interpretation

Positive = Clotting of plasma

Negative = No clotting of plasma, plasma remains unclotted.

 Positive tube coagulase test result for Staphylococus aureus after a 24-hour incubation in coagulase plasma. A positive result is indicated by clotting in the tilted tube

As shown in the Table 1 below, there were 45 S.aureus students carriers confirmed via tube test coagulase only as positive this term. From the table in semester one, 39 of 169 students were S.aureus carrier positive students, making this, 23.08% where 169=100%. In semester two 45 of 163 students tested positive S.aureus carrier which is 27.61%; where 163=100%. On average for both semesters, (39+45=84) students tested positive of S.aureus carrier among (169+163=332); when 332=100%, 84=25.30%.

Table 1: S.aureus Carriage Rate among Students in Both Semesters

Survey of Nasal Carriage of Staphylococcus aureus


Positive results were first tested via MSA which was presented by a yellow zone around single isolated colonies. Further, rampant growth was a sign of positive growth as S.aureus is tolerant to salt. The positive results were subsequently discussed with the class moderator and on agreement isolated presumptive colonies of S. aureus were confirmed via tube coagulase tests. In semester 1 the number of students tested positive of S.aureus carrier were 39 of 169 (23.08%) while in semester 2 the number was 45 of 163 (27.61%). On both semesters the average number of students’ positive of S.aureus carrier was 25.30%. According to Hanssen, et al, (2017) in approximates, 20-30% of adult population have persistent S.aureus nasal carriage. This is in line with the findings on this experiment regardless of the semester as the percentage obtained on both semesters and on average lies within this bracket. In semester 1 the rate was 23.08% while in semester 2 this was 27.61%, and 25.03% on average. Hanssen et al., (2017) argues that successful S.aureus nasal decolonization is hard to achieve and S.aureus, recolonizes the throat and the nose with much ease like one week mostly with the same S.aureus genotype. Further, MRSA eradication rate is quite low, and thus, those tested earlier might have passed the pathogen to others or could have been retested during the second trial leading to a higher percentage on a smaller sample than semester 1.


Hanssen, A., Kindlund, B., Stenklev, N.C. et al., 2017. Localization of staphylococcus aureus in tissue from the nasal vestibule in healthy carriers. BMC Microbiol, Vol17, p89.

Kobayashi, S.D., Malachowa, N. and DeLeo, F.R. 2014. Pathogenesis of staphylococcus aureus abscesses. The American Journal of Pathology, Vol.185, No.6, pp1518-1527.

Otto, M. 2014. Staphylococcus aureus toxins. Curr Opin Microbiol, pp32-37.

Oxoid web,

Schelin, J., Wallin-Carlquist, N., Cohn, M.T. et al., 2011. The formation of staphylococcus aureus enterotoxin in food environments and advances in risk assessment. Virulence, Vol.2, no.6., pp580-592.

Warnke, P., Harmack, T., Ottl, P. et al., 2014. Nasal screening for staphylococcus aureus- daily routine with improvement potentials. PLoS One, Vo.9, no.2, pe89667.