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Nasal Carriage of Staphylococcus Aureus in Microbiology Students Report

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Updated: Dec 16th, 2020

Abstract

Staphylococcus aureus is a round-shaped bacterium belonging to the Firmicutes phylum of bacteria. The escalation in the prevalence and gravity of staphylococcal contagions necessitates assessing the burden of asymptomatic carriage of Staphylococcus aureus in the community setting. The aim of this study was to establish the nasal carriage rates of S. aureus in microbiology students at RMIT. Nasal swabs were collected from 766 microbiology students at RMIT between 2013 and 2018. The swabs were inoculated on mannitol salt agar (MSA) and identified by their morphological characteristics. Tube coagulase tests were done to ascertain the identity of the pathogens. About 26.11% (200) samples tested positive for S. aureus. This value was comparable to those found in previous studies conducted in the community, hospital and food industry settings. The experimental nasal carriage rates varied from year to year, which could be attributed to inconsistencies in the streaking methods and biochemical tests. It was concluded that S. aureus was a prevalent normal flora in humans. As a result, it is important to observe high standards of hygiene to lower the risk of contracting infections attributed to the pathogen.

Aim and Significance

The purpose of this study was to conduct a survey of nasal carriage of Staphylococcus aureus in microbiology students at RMIT from 2013 to 2018. Nasal swabs from the subjects were collected and subjected to bacteriological analysis. Carriage of S. aureus plays a vital role in the epidemiology and development of infection. Eradication of carriage is a viable pre-emptive measure in at-risk individuals. Therefore, it is important to determine the carriage of S. aureus as part of developing appropriate measures to reduce the risk of S. aureus infection.

Introduction

S. aureus is an important pathogen as it has been connected to a wide range of infections. This bacterium is commonly found on the skin as well as in the respiratory tract and nose (Kobayashi, Malachowa & DeLeo, 2015). Numerous S. aureus infections happen because of its far-reaching virulence factors. The importance of this bacterium also rests on its high rates of resistance to antimicrobial agents as well as prevalence as a nosocomial disease-causing agent.

S. aureus possesses surface proteins such as laminin and fibronectin, constituents of the extracellular matrix that enhance its attachment to the host. Blood clots as well as epithelial and endothelial surfaces also contain fibronectin, which promotes the attachment of S. aureus to injured tissues via a fibrin-binding protein. Some virulence factors employed by S. aureus include antigens, enzymes and toxins. Capsules and adhesins are examples of antigens, while coagulase, hyaluronidase, nuclease and staphylokinase are examples of enzymes that promote the virulence of S. aureus. Virulent toxins include enterotoxins, P-V leukocidin and alpha, beta and delta toxins (Tong, Davis, Eichenberger, Holland & Fowler, 2015). S. aureus exhibits multifactorial virulence because of the collective action of numerous virulence factors and extracellular and cell wall constituents.

S. aureus causes many human diseases, particularly skin and soft tissue illnesses. Examples of minor skin infections include impetigo, pimples, boils, folliculitis, cellulitis, scalded skin syndrome and carbuncles. In addition, life-threatening disorders caused by S. aureus encompass meningitis, pneumonia, endocarditis, osteomyelitis, bacteraemia, toxic shock syndrome and sepsis (Kobayashi et al., 2015).

S. aureus is responsible for several nosocomial infections that plague health-care settings. This problem is aggravated by antibiotic resistance. Health-care workers are the major carriers of S. aureus. Studies indicate that nasal carriage of S. aureus in health-care staff ranges from 16.8 to 56.1% (Rongpharpi, Hazarika & Kalita, 2013). Moreover, S. aureus carriage has a significant impact on the food industry as indicated by the prevalence of food-borne illnesses. The World Health Organization (WHO) estimates that approximately 30% of the population in developed countries are afflicted by food-borne diseases annually (Rongpharpi et al., 2013). In contrast, 2 million deaths associated with food poisoning occur in developing countries every year. S. aureus may be present on the nose or skin of food handlers and is transferred to humid cooked foods that then become poisonous if unrefrigerated. High carriage rates of S. aureus in the community are linked to the appearance and universal dissemination of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA; Tong et al., 2015).

In Australia, approximately 1962 respiratory infections, 3946 surgical site infections and 1100 hospital-onset S. aureus bacteraemia cases were reported between 2010 and 2016 (Mitchell, Shaban, MacBeth, Wood & Russo, 2017). However, these numbers were considered underestimations because other disorders such as pneumonia and bloodstream infections had not been included in the statistics.

Other researchers have examined S. aureus carriage rates through cross-sectional studies (Bojang et al., 2017; Rongpharpi et al., 2013) and population-based surveys (Pires et al., 2014). A common technique employed in all investigations is obtaining nasal swabs and subjecting them to conventional bacteriological analysis.

Many cases of drug-resistant S. aureus have been reported over the last few years. One example of a reported strain is Methicillin-resistant S. aureus (MRSA), barely a year following the introduction of methicillin. Multiple drug-resistant S. aureus has also been isolated from food items and human nasal mucosa. These strains are a significant public health concern because they easily circulate in the environment.

Materials and Methods

About 766 microbiology students at RMIT were selected to participate in the study from 2013 to 2018. Nasal swabs were obtained from the participants using aseptic methods by rolling an applicator stick dampened with normal saline. A nasal swab from each student was inoculated onto mannitol salt agar (MSA, CM0085; Oxoid, UK) and incubated for 24 hours at 35 to 37 °C (Bojang et al., 2017). Isolates were identified as S. aureus by their growth features on MSA. Gram staining was conducted using conventional procedures (Microbiology Unit RMIT University, 2015). The tube coagulase test was performed as a biochemical test to verify the identity of the isolates (Grando & Paras, 2009).

Results

Yellow colonies with yellow pigmentation were produced in MSA medium, indicating the growth of S. aureus. The bacterial cells stained purple in the Gram staining test. All S. aureus isolates coagulated the plasma in the coagulase test. Among the 766 healthy microbiology students, the overall prevalence of nasal carriage of S. aureus was 200 (26.11%). Table 1 indicates the annual prevalence of S. aureus carriage from 2013 to 2018.

Table 1: S. aureus nasal carriage rates of RMIT Students from 2013 to 2018. Source: T. Istivan, personal communication, 2018.

Year Number positive students (No. of S. aureuscarrier) Total number students tested Percentage S. aureuscarriage rates
2013 39 110 35.45
2014 26 135 19.26
2015 33 113 29.20
2016 47 135 34.81
2017 39 169 23.08
2018 16 104 15.38
 A graph of percentage S. aureus carriage rates for RMIT students from 2013 to 2018.
Figure 1. A graph of percentage S. aureus carriage rates for RMIT students from 2013 to 2018.

Discussion

A nasal carriage prevalence of 26.11% was reported over a six-year period in 766 subjects. These findings were within the range of published data on nasal S. aureus incidence in health-care settings, the food industry and the community. Eke, Eloka, Mgbachi, Nwobodo and Ekpen-Itamah (2015) investigated the nasal carriage of S. aureus among food handlers in Nigeria. About 100 nasal swabs were collected from participants aged 21 to 40 years and evaluated using conventional methods. The prevalence rate was 60%, with males having higher incidence (58%) than females (42%). Rongpharpi et al. (2013) assessed health-care staff in Assam. Samples were collected from 315 subjects between August 2009 and July 2010. The swabs were inoculated on MSA, 10% sheep blood agar medium and MacConkey’s agar. Five biochemical tests were used: catalase test, tube and slide coagulase tests, mannitol fermentation and the modified Hugh and Leifson test (Rongpharpi et al., 2013). About 70 samples (22.22%) were positive for S. aureus.

Three studies checked the nasal presence of S. aureus in the general community with varying outcomes. Pires et al. (2014) studied an urban Brazilian population using 686 subjects aged one year and older. Nasal swabs were grown on Baird Parker agar. Species identification entailed Gram staining and biochemical tests (catalase, coagulase and fermentation of mannitol, trehalose and maltose). The isolates further underwent genotypic classification by Polymerase Chain Reaction (PCR). Younger subjects (average age 28 years) who had reported recent skin infections were more likely to carry the pathogen than their older counterparts. Bojang et al. (2017) conducted a similar investigation in rural Gambia using 1264 nasal swabs from 232 children aged between 5 and 10 years. The samples were analysed using conventional methods (MSA and coagulase test). Prevalence of S. aureus carriage was 25.9%. Chen et al. (2017) tested nasal swabs from 295 volunteers living on a medical campus. The samples were subjected to molecular means of S. aureus identification. Approximately 24.7% of the isolate samples were positive for the bacterium.

Methodological differences that accounted for the differences in the findings of previous studies included the use of assorted biochemical tests and different isolation media (Pires et al., 2014; Rongpharpi et al., 2013). Pires et al. (2014) and Chen et al. (2017) also used molecular methods, known to have high accuracy levels, to confirm their identification. The sampling also differed from one study to another.

The prevalence rates ranged from 22% to 26.11% in this experiment and four other studies. Only the Nigerian study by Eke et al. (2015) reported a higher prevalence of S. aureus than this experiment (60%). However, the method used by Eke et al. (2015) did not differ substantially from those used in this investigation. The high prevalence in this study could be attributed to differences in the distribution of S. aureus in developed and developing countries.

The prevalence of S. aureus at RMIT fluctuated over the years (Figure 1). This observation could be attributed to the lack of standardization of the methods used in swabbing nostrils, streaking techniques and performing confirmatory tests. Antibiotic-resistance strains of S. aureus were not identified in this study because antibiotic sensitivity was not done.

Conclusion

This study determined the nasal carriage of S. aureus in microbiology students at RMIT from 2013 to 2018, which was 26.11%. While this value was within the reported ranges in the community, food industry and hospital settings, these rates indicated a significant risk of contracting S. aureus infections in these three areas. Therefore, the study results reveal the need to observe high standards of hygiene to prevent the transmission of the bacterium and disease development.

References

Bojang, A., Kendall, L., Usuf, E., Egere, U., Mulwa, S., Antonio, M.,… Roca, A. (2017). Prevalence and risk factors for Staphylococcus aureus nasopharyngeal carriage during a PCV trial. BMC Infectious Diseases, 17(1), 588.

Chen, B. J., Xie, X. Y., Ni, L. J., Dai, X. L., Lu, Y., Wu, X. Q.,… Huang, S. Y. (2017). Factors associated with Staphylococcus aureus nasal carriage and molecular characteristics among the general population at a Medical College Campus in Guangzhou, South China. Annals of Clinical Microbiology and Antimicrobials, 16(1), 28.

Eke, S. O., Eloka, C. C. V., Mgbachi, N., Nwobodo, H. A., & Ekpen-Itamah, U. J. (2015). Nasal carriage of Staphylococcus aureus among food handlers and restaurant workers in Ekpoma Edo State, Nigeria. International Journal of Community Research, 4(1), 7-14.

Grando, D., & Paras, C. (2009). G.E.R.M.M. Melbourne, Australia: RMIT University.

Kobayashi, S. D., Malachowa, N., & DeLeo, F. R. (2015). Pathogenesis of Staphylococcus aureus abscesses. The American Journal of Pathology, 185(6), 1518-1527.

Microbiology Unit RMIT University. (2015). Medical microbiology techniques manual. Melbourne, Australia: RMIT University.

Mitchell, B. G., Shaban, R. Z., MacBeth, D., Wood, C. J., & Russo, P. L. (2017). The burden of healthcare-associated infection in Australian hospitals: A systematic review of the literature. Infection, Disease & Health, 22(3), 117-128.

Pires, F. V., de Souza, M. D. L. R., Abraão, L. M., Martins, P. Y., Camargo, C. H., & Fortaleza, C. M. C. B. (2014). Nasal carriage of Staphylococcus aureus in Botucatu, Brazil: A population-based survey. PloS One, 9(3), e92537.

Rongpharpi, S. R., Hazarika, N. K., & Kalita, H. (2013). The prevalence of nasal carriage of Staphylococcus aureus among healthcare workers at a tertiary care hospital in Assam with special reference to MRSA. Journal of Clinical and Diagnostic Research: JCDR, 7(2), 257-260.

Tong, S. Y., Davis, J. S., Eichenberger, E., Holland, T. L., & Fowler, V. G. (2015). Staphylococcus aureus infections: Epidemiology, pathophysiology, clinical manifestations, and management. Clinical Microbiology Reviews, 28(3), 603-661.

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