JJ Deng, GG Xiao, Y Zhu, W Zhou, CM Wan

Abstract

Objectives: To describe the Staphylococcus aureus (S. aureus) nasal carriage as well as its antibiotic resistance patterns among Tibetan healthy school children in Songpan County in Southwest China. Methods: Nasal swabs of both anterior nares were collected from healthy school children. The identification of S. aureus was based on the colony morphology, biochemical tests and the results of Slidex Staph Plus kit. The antibiotic resistance profiles of the isolates were determined according to standard methods. Isolated strains were tested with the polymerase chain reaction assays to detect the mecA gene. Results: 673 Tibetan healthy children were included in this study, and S. aureus was detected in 16 of 673 nasal samples (2.4%). The resistance of isolated strains to penicillin, erythromycin, clindamycin, and gentamicin was 87.5% (14/16), 56.2% (9/16), 12.5% (2/16), and 18.8% (3/16), respectively. No strains were resistant to cefoxitin or vancomycin, and mecA gene was not detected within these isolates. Conclusion: The methicillin-sensitive S. aureus (MSSA) as well as methicillin-resistant S. aureus (MRSA) nasal carriage rate seemed to be very low among healthy school children in the Tibet Plateau.

Introduction

Staphylococcus aureus (S. aureus) is a known coloniser in humans, and the anterior nares are the most common colonisation site. S. aureus is also a frequent cause of clinically important infections ranging in severity from superficial skin and soft-tissue abscesses to invasive disease and even death.¹ Colonising strains are often similar to those isolates from the infected tissue.² Furthermore, the emergence of community acquired methicillin-resistant S. aureus (MRSA) has become an important challenge for the treatment of staphylococcal infections due to its high virulence and emerging antibiotic resistance of this kind of S. aureus.³ Nasal carriage of S. aureus has been regarded as being associated with the infections caused by S. aureus in the host. A better understanding of the prevalence of nasal carriage of S. aureus could be helpful for devising appropriate measurements to control diseases caused by this organism. The objectives of this study were to investigate the prevalence of nasal carriage of S. aureus among Tibetan healthy children and to determine the antibiotic resistance profiles of these isolates.

Materials and Methods

This study was conducted between September and October 2008 in Songpan County at altitude between 2700 and 3000 meters with a population density of about 7.8 people per square kilometer. In this region, the medical resources are rather scanty. The traditional Tibetan medicine is prevalent, and antibiotics such as penicillin are used sometimes, whereas most of other antibiotics are not commonly used in the population. In this study, we calculated the minimum sample size required to accurately assess the S. aureus carriage as 242 based on the expected prevalence of nasal colonisation (30%),² with a 95% confidence interval (design effect, 3%).

Six hundred and seventy-three Tibetan healthy children aged between 7 and 18 years old from three primary and secondary schools in Songpan County were recruited. Children receiving antibiotic therapy or requiring admission in the recent 4 weeks and those with current skin infections were not included.

Samples were collected by twice rotating a sterile cotton swab pre-wetted with sterile saline in the vestibule of both anterior nares of the participants after obtaining oral informed consent.

Nasal swabs were transported to the laboratory using the Sterile Amies Agar Gel Transport Swab (Copan, Italy) and inoculated on the mannitol salt agar (Oxoid Ltd, Basingstoke, United Kingdom) and Columbia blood agar (bioMérieux, France) within 6 h. Then the inoculations were incubated for 24 h at 37^oC. The identification of S. aureus was based on the colony morphology, positivity of catalase and tube coagulase, and the results of Slidex Staph Plus kit (bioMérieux, France). MRSA identification and antibiotic susceptibility of the isolates were performed according to the Clinical Laboratory Standards Institut guidelines,⁴ with the following antibiotics: penicillin, erythromycin, clindamycin, gentamicin, cefoxitin and vancomycin. Inducible resistance to clindamycin was tested by the double disk diffusion test (D test).⁴

All the isolated S. aureus strains were detected for mecA gene by the polymerase chain reaction assays to identify a 162 bp target product. Primers used were mecA P4 5'-TCCAGATTACAACTTCACCAGG-3' and mecA P7 5'-CCACTTCATATCTTGTAACG-3'.⁵

Chi-square tests were performed to detect the statistical significance (5%) between groups with SPSS 13.0 for windows (SPSS Inc., Chicago, IL, United States of America). The Ethics Committee of West China Second Hospital approved this study.

Results

Six hundred and seventy-three Tibetan healthy children including 390 (57.9%) boys and 283 (42.1%) girls participated in the study. There were 245 (36.4%), 277 (41.2%) and 151 (22.4%) children in the age group of 7-13, 13-15 and 15-18 years, respectively. S. aureus strains were isolated from 16 (2.4%) children including 9 boys and 7 girls without significant difference with regard to sex (p=0.889). Of the 16 S. aureus strains, 7 strains (43.7%) was isolated from children between 7 and 13 years of age, 5 (31.3%) from children between 13 and 15 years of age, and 4 (25.0%) from children between 15 and 18 years of age. The antibiotic resistance profiles of the isolated S. aureus strains are shown in Table 1. No isolated strains were resistant to vancomycin or cefoxitin, suggesting no MRSA strain was detected. Inducible clindamycin resistance (positive D test) was found in 2 out of the 9 erythromycin-resistant isolates. No mecA gene was detected among the isolated S. aureus strains.

Discussion

Colonisation by S. aureus is considered to be associated with the S. aureus infections in human body. And studies of the carriage of S. aureus can be of great importance. A few studies have investigated the carriage rate of S. aureus in the child population in the community of the low-altitude regions of mainland China,^6,7 suggesting that the overall carriage rate of S. aureus was about 18% and MRSA accounted for about 6% of the isolated S. aureus strains. However, data about the carriage rate of S. aureus in the child population on the highland of China are scarce.

This study aimed to assess the prevalence of nasal carriage of S. aureus within healthy children living in the Tibet Plateau and the antibiotic resistance profiles of these isolated strains. To our knowledge, this is the first study reported in this specific population in this region.

Our study showed that the nasal carriage rate of S. aureus among Tibetan healthy children was 2.4%. This rate was fairly low compared with previously reported rate of 30% among healthy persons.^2,8 Two studies performed among school children aged from 5 to 15 years old reported the S. aureus carriage rate of 16.1% and 52.3%, respectively.^9,10 Another study found the S. aureus carriage rate of 31.1% among healthy children aged between 5 and 20 years old.¹¹ Our team assessed the prevalence of S. aureus nasal carriage within healthy school children aged between 7 and 18 years old in Chengdu, and the results showed that the carriage rate was 18.2% between 2005 and 2007, and 18.1% between 2008 and 2010.⁷ The extremely low S. aureus carriage rate in this specific population seems to be related to the highland environment where these people live in. As children from areas of better economic status such as Chengdu could get better medical care and receive antibiotic therapy frequently, the low carriage rate within the Tibetan children may also be associated with the socioeconomic status and possibly the race of people. Nonetheless, this relationship needs further determination.

The resistance of the isolated S. aureus strains to some antibiotics often used in the empirical treatment of S. aureus infections was slightly higher than that reported in some other studies,^9-10,12,13 whereas no isolates were found resistant to cefoxitin or vancomycin. It seemed that although the nasal carriage rate of S. aureus in this study population was low, once the S. aureus strains were isolated, these strains might be highly resistant to some antibiotics usually used in the clinical situation. In this situation, some other antibiotics such as cefoxitin and vancomycin may be used. However, compared with our study in Chengdu,⁷ the resistance of the isolated strains to the antibiotics assessed in this study was relatively lower, which may result from the low level of antibiotic consumption. As the number of the isolated S. aureus strains is very limited in this study, further studies are needed to determine the antibiotic resistance profiles of this kind of organism more precisely in this region.

No MRSA strains were detected in this study, in agreement with the results of detection of mecA gene which allowed the S. aureus strains to be resistant to antibiotics such as oxacillin. Two studies conducted among school children aged between 5 and 15 years old reported the nasal carriage rate of MRSA of 0.5% and 3.9%, respectively.^9,10 One study conducted among school children aged between 7 and 18 years old in Chengdu by our team found that the prevalence of MRSA was about 6%.⁷ The results of this study indicated that the prevalence of MRSA in the community of this region was still extremely low. Although there's few data about the prevalence of MRSA among paediatric patients in the hospital of this area, a few reports showed that the isolation rate of MRSA in adult patients of Tibetan highland was lower than that of other low altitude areas of mainland China.¹⁴ What's more, we found no mecA gene in these isolated S. aureus strains, which is different from the results of other studies previously reported.^1,8,10 The low carriage rate of MRSA in this population may be related to the fact that antibiotics are not commonly used in these people and the specific environment. But as the number of participants in this study is limited, the prevalence of MRSA in this region needs to be further determined.

Conclusion

This is a preliminary study which showed that the S. aureus and MRSA nasal carriage rate seemed to be still low among healthy school children in the Tibet Plateau. However, as there's only a small group of participants included in this study, larger studies are needed to further determine the carriage rate of S. aureus among this population. Further studies can be performed to investigate why the nasal carriage of S. aureus is lower among this population than that from other parts of the world, which may provide new ways to reduce S. aureus nasal carriage in order to control the S. aureus infections.

Funding

This work was supported by the Key Basic Research Program of the Ministry of Science and Technology of China (No. 2006FY110300). The funders had no role in the study design, data acquisition and analysis, decision to publish, or preparation of the manuscript.

Declaration of Interest

None

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