Journal of Infection and Public Health (2013) 6, 196—201
Nasal carriage and antimicrobial susceptibility of Staphylococcus aureus among medical students at the HRH Princess Maha Chakri Sirindhorn Medical Center, Thailand: A cross sectional study Arucha Treesirichod ∗, Sumalee Hantagool, Olarn Prommalikit Department of Pediatrics, Faculty of Medicine, Srinakharinwirot University, Bangkok, Thailand Received 27 September 2012 ; received in revised form 9 December 2012; accepted 15 December 2012
KEYWORDS Staphylococcus aureus; Medical students; Nasal carriage; Antimicrobial susceptibility
Summary Objective: To determine the epidemiology of the nasal carriage of Staphylococcus aureus and its susceptibility pattern among preclinical medical students at the HRH Princess Maha Chakri Sirindhorn Medical Center, Srinakharinwirot University. Methods: Nasal swabs were taken from 128 preclinical medical students prior to working at the hospital. Susceptibility testing of S. aureus was performed using Kirby Bauer’s disc diffusion method. Results: Of the 128 participants, 38/128 (29.7%; 95% confidence interval [CI] = 21.8%, 37.6%) were carriers of S. aureus. No methicillin-resistant S. aureus was detected by the cefoxitin disk diffusion test. Resistance of S. aureus to erythromycin, clindamycin, tetracycline, chloramphenicol and fusidic acid was observed at the following rates: 63.2% (95% CI; 47.8%, 78.5%), 63.2% (95% CI; 47.8%, 78.5%), 34.2% (95% CI; 19.1%, 49.3%), 2.6% (95% CI; −2.5%, 7.7%) and 2.6% (95% CI; −2.5%, 7.7%), respectively. There was no statistically significant correlation between nasal carriage of S. aureus and possible risk factors. Conclusions: The prevalence of asymptomatic nasal carriage of S. aureus was higher than reported by previous literature in Thailand, and S. aureus isolates exhibited relatively high resistance to erythromycin and clindamycin. © 2013 King Saud Bin Abdulaziz University for Health Sciences. Published by Elsevier Ltd. All rights reserved.
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Corresponding author at: Department of Pediatrics, Faculty of Medicine, Srinakharinwirot University, HRH Princess Maha Chakri Sirindhorn Medical Center, Rangsit-Nakhonnayok Road, Ongkharak, Nakhonnayok 26120, Thailand. Tel.: +66 815935232. E-mail address: Trees
[email protected] (A. Treesirichod).
Introduction Staphylococcus aureus is a common pathogen in both healthy individuals and individuals with underlying diseases. Rates of nasal colonization with
1876-0341/$ — see front matter © 2013 King Saud Bin Abdulaziz University for Health Sciences. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.jiph.2012.12.004
Nasal carriage and antimicrobial susceptibility of Staphylococcus aureus S. aureus have been reported to range from 6.3 to 17.8% and 18.2 to 43.8% in the general population and healthcare workers, respectively [1—5]. Vulnerable populations of carriers who are prone to infection are children in daycare, members of sport teams, prisoners and soldiers. The anterior nares are ecological mediums for S. aureus and other microorganisms, such as Propionibacterium spp., Corynebacterium spp., Enterobacter spp., coagulase-negative Staphylococci (CoNS), Streptococcus pneumoniae and Moraxella catarrhalis [6,7]. In addition to these bacteria, methicillin-resistant S. aureus (MRSA) is also found in the nasal vestibules of both healthy persons and healthcare workers [3,5,8,9]. Transmission from person to person or from healthcare workers to patients is a major concern. Nasal carriage of invasive human pathogens has been associated with increased incidences of infection and morbidity. A study of the risks and outcomes of nosocomial S. aureus bacteremia in nasal carriers concluded that at least 80% of S. aureus bacteremia was identical to a nasal carrier strain revealed as endogenous by genotyping [10]. Nasal colonization of S. aureus was also relevant in the occurrence of impetigo in children [11]. In healthy subjects during long-term followup, approximately 20% of subjects were identified as persistent carriers, 60% were intermittent carriers, and the remaining subjects almost never tested positive for S. aureus [12]. Over the past decade, reports of MRSA in community have increased in persons without healthcare-associated risk factors. This has been challenged by the emergence of communityassociated MRSA (CA-MRSA). The presence of CA-MRSA was confirmed by the presence of the smaller methicillin-resistant element known as the staphylococcal cassette chromosome mec (SCCmec) type IV and the Panton-Valentine leukocidin (PVL), along with susceptibility to non -lactam antimicrobial agents; healthcare-associated MRSA (HA-MRSA) strains were identified by the presence of the larger SCCmec element types I, II or III [13,14]. A report on MRSA in healthy Thai adults revealed a 1% prevalence of MRSA in nasal carriage and noted that these isolates were positive for SCCmec type II, which is associated with the strain found in healthcare personnel [3]. Currently, there have been few reports in Thailand on the prevalence of nasal colonization and the antibiotic susceptibility patterns of S. aureus and MRSA in the community; particularly absent are studies in medical students prior to conducting their clinical rounds in hospitals where the risk of colonization by MRSA is the highest [3,15]. Therefore, this study focused on determining the
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epidemiology of nasal carriage of S. aureus among preclinical medical students and its antibiotic susceptibility patterns.
Materials and methods Study design and subjects This study was designed as a cross-sectional study. Subjects who enrolled were all healthy, third-year, preclinical medical students at Srinakharinwirot University, HRH Princess Maha Chakri Sirindhorn Medical Center, Thailand. Nasal swabs were taken in March 2012, prior to beginning their work at the hospital. Exclusion criteria included respiratory infections requiring hospitalization and/or a current skin infection up to 4 weeks before nasal swab collection. Demographic data were collected, including age, gender, underlying diseases, antibiotic usage in the last 4 weeks, previous hospitalization, hand washing habits, bathing habits and sharing of personal belongings. These data were recorded for the assessment of risk factors. This study was reviewed and approved by the appropriate institutional review boards and ethics committee of the HRH Princess Maha Chakri Sirindhorn Medical Center. All participants provided letters of consent before enrolling in this study.
Sample collection and antimicrobial susceptibility testing Samples collected from both anterior nasal vestibules were transported using Stuart’s transport media. Collection was performed using a moist cotton swab with a rotating technique in both anterior nasal vestibules. Microorganisms were isolated by culturing swab samples on 5% sheep blood agar, chocolate agar, mannitol salt agar and MacConkey’s agar; growth was examined after 24—48 h. Colony-forming morphology on culture plates, Gram staining, catalase production and tube coagulase tests were used to identify the type of microorganisms that grew on each medium type. Susceptibility testing of S. aureus was performed using Kirby Bauer’s disc diffusion method according to the Clinical and Laboratory Standards Institute (CLSI) guidelines from 2012 [16]. S. aureus ATCC 25923 was used as a control strain. The following antibiotics were used: ciprofloxacin (5 mcg), gentamycin (10 mcg), chloramphenicol (30 mcg), co-trimoxazole (25 mcg), tetracycline (30 mcg), erythromycin (15 mcg), clindamycin (2 mcg), fosfomycin (50 mcg), fusidic acid
198 (10 mcg) and linezolid (30 mcg). Inducible clindamycin resistance was detected by the double disk diffusion test (D test), which was conducted by placing clindamycin and erythromycin disks 15 mm apart [16]. The cefoxitin disk diffusion test was used for detection of MRSA strains according to CLSI guidelines from 2012 [16].
Statistical analysis All data were analyzed using SPSS version 11.5. The prevalences of S. aureus and MRSA were estimated with 95% confidence intervals. The correlation between each independent variable was determined using the Chi-square test for significance. Statistical significance was identified as P < 0.05.
Results A total of 128 participants enrolled in the study, with a mean age of 20.9 ± 0.9 years old; most of the subjects were female (58.6%). All participants were colonized with microorganisms in their nasal vestibules. S. aureus was isolated from 38 participants, yielding a colonization rate of 29.7% (95% confidence interval [CI]; 21.8%, 37.6%). Within this cohort, bacterial growth was characterized as numerous, moderate, few and rare on plate cultures corresponding to 17.2% (95% CI; 10.7%, 23.7%), 3.1% (95% CI; 0.1%, 6.1%), 6.3%
A. Treesirichod et al. (95% CI; 2.1%, 10.4%) and 3.1% (95% CI; 0.1%, 6.1%) of S. aureus-colonized patients, respectively. The overall rate of nasal colonization by S. aureus accompanied by other microorganisms is summarized in Fig. 1. In this study, coagulasenegative Staphylococci, Corynebacterium spp., Klebsiella pneumoniae, Citrobacter koseri, Enterobacter spp., Klebsiella oxytoca and Proteus spp. were isolated in 76.6% (95% CI; 69.2%, 83.9%), 10.2% (95% CI; 4.9%, 15.4%), 7.0% (95% CI; 2.6%, 11.5%), 3.9% (95% CI; 0.5%, 7.3%), 3.1% (95%CI; 0.1%, 6.1%), 0.8% (95% CI; −0.7%, 2.3%) and 0.8% (95% CI; −0.7%, 2.3%) of participants, respectively. In participants with Klebsiella spp. isolates, two out of 10 participants had a history of antibiotic use up to 4 weeks prior to sample collection. These were identified as ofloxacin for the treatment of cystitis and amoxicillin for the treatment of acne. In this study, we detected no MRSA strains by the cefoxitin disk diffusion test. There was no statistically significant correlation between nasal carriage of S. aureus and the documented demographic risk factors (gender, underlying diseases, antibiotic usage in last 4 weeks, previous hospitalization, hand washing habits, bathing habits and the sharing personal of belongings, P > 0.1). The susceptibility profile analysis of S. aureus isolates is shown in Fig. 2. Resistance of S. aureus to erythromycin, clindamycin, tetracycline, chloramphenicol and fusidic acid was observed for 63.2% (95% CI; 47.8%, 78.5%), 63.2% (95% CI; 47.8%, 78.5%), 34.2% (95% CI; 19.1%, 49.3%), 2.6% (95% CI; −2.5%, 7.7%) and 2.6% (95% CI; −2.5%, 7.7%) of isolates, respectively. No resistance
Figure 1 Overall nasal colonization in participants. (CoNS; coagulase-negative Staphylococci) (a) S. aureus with Klebsiella spp. (n = 1), S. aureus with Corynebacterium spp. (n = 1). (b) S. aureus with CoNS and Klebsiella spp. (n = 1). (c) CoNS with Corynebacterium spp. (n = 10), Klebsiella spp. (n = 6), Citrobacter koseri (n = 4), Enterobacter spp. (n = 3) and Proteus spp. (n = 1). (d) CoNS with Klebsiella spp. and Corynebacterium spp. (n = 1), CoNS with Klebsiella spp. and Citrobacter koseri (n = 1), CoNS with Proteus spp. and Corynebacterium spp. (n = 1).
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Figure 2 The susceptibility patterns of the 38 S. aureus isolates from healthy, third-year, preclinical medical students at Srinakharinwirot University.
was detected against gentamycin, ciprofloxacin, co-trimoxazole and linezolid. Intermediate sensitivity of S. aureus to fosfomycin was observed in 2.6% (95% CI; −2.5%, 7.7%) of participants. Inducible clindamycin resistance was also identified in one of 24 erythromycin-resistant isolates.
Discussion S. aureus has long been recognized as a virulent pathogen and a leading cause of infection in both children and adults. The rapid emergence of MRSA over the past several years was observed in both community- and healthcare-associated populations, complicating prospective MRSA treatments. Data on the global prevalence of nasal colonization with S. aureus and MRSA has been reported [1—5,17]. In Thailand, there have been few studies of S. aureus carriage published in the literature. This study demonstrated that 29.7% of healthy, third-year, preclinical medical students at Srinakharinwirot University carried S. aureus, and there was no MRSA carriage that could be indicated as a community group. In comparison to a previous report, Kitti et al. reported prevalences of 15% for S. aureus and 1% for MRSA nasal carriage in healthy young Thai adults. These MRSA carriers were associated with healthcare risk factors and contained SCCmec type II. At Siriraj hospital, a tertiary care center, S. aureus prevalence was 66.9% in hospitalized patients; of these, MRSA and CA-MRSA were identified in 41.5% and 0.9% of patients, respectively [15]. A study conducted on a population of preclinical medical students in China also reported a prevalence of 23.1% for S. aureus and 9.4% for MRSA nasal carriage with SCCmec type IVa [18]. This
MRSA carriage may be contributing to the spread of MRSA between the community and hospital populations. In this study, the cefoxitin disk diffusion test was used to screen for MRSA because cefoxitin is a potent inducer of the mecA regulatory system [19]. CLSI guidelines from 2012 recommended the cefoxitin disk diffusion test for the detection of MRSA. However, confirmation of CA-MRSA or HAMRSA requires identification of the specific SCCmec type, which has been bacterially contained. Antimicrobial resistance in S. aureus has become an increasingly prevalent problem [20]. In this study, antibiotic susceptibility tests revealed that S. aureus remained sensitive to most antibiotics, but there was a high rate of resistance against erythromycin and clindamycin. We also observed inducible clindamycin resistance in this study. The reported prevalence of erythromycin-resistant S. aureus varies between previous studies. The rates of erythromycin-resistant S. aureus were 26.7—29.9%, varying according to different situations [3,21]. These high resistance rates are concerning in patients with staphylococcal infections who are allergic to penicillin. Simultaneously, susceptibility test data showed that 100% of the S. aureus isolates from this study were sensitive to ciprofloxacin, gentamycin, linezolid and co-trimoxazole. This test also revealed the high susceptibility of S. aureus to chloramphenicol, fosfomycin and fusidic acid. The antimicrobial susceptibility profile of S. aureus has been different in several previous reports and depended on the resistance profiles in both the community and healthcare worker groups [22,23]. Therefore, this data should be considered for inclusion in epidemiologic datasets for both areas.
200 We observed that nasal microorganisms in our study were gram-negative bacteria. In particular, Klebsiella spp. and C. koseri, which are usually found as nosocomial pathogens, were isolated from participants with histories of antibiotic use up to 4 weeks prior to sample collection. Recently, extended-spectrum -lactamase in gram-negative bacteria has also become a major problem [24]. The potential role of nasal colonization as a reservoir for future infections remains unclear. Most of them could be associated with the transmission of microorganisms and the cause of invasive diseases, especially in immunocompromised hosts. To control the spread of bacteria, it is important to understand the epidemiological relationship between these organisms. Further study on the role of this type of microorganism colonization is needed. While various demographic risk factors were explored, there were no statistically significant correlations between S. aureus and the risk factors. However, the routine of health sanitation is still recommended, especially in healthcare workers. In the long-term follow up study, no participants were persistent carriers of S. aureus [12]; carrier status changed and decreased over time. Thus, we will conduct a follow-up study to determine the conversion rates of colonization of S. aureus and MRSA in medical students after working at the hospital as healthcare workers who could have potential high risks for future MRSA colonization. In summary, the prevalence of asymptomatic nasal carriage of S. aureus was higher than noted in previous studies in Thailand. Those same studies reported high resistance to erythromycin and clindamycin. Further studies on the longitudinal course of colonization and the role and cost effectiveness of nasal eradication in carriers should be performed.
Conflict of interest Funding: No funding sources. Competing interests: None declared. Ethical approval: Not required.
Acknowledgements We would like to thank our colleagues and participants at the HRH Princess Maha Chakri Sirindhorn Medical Center. This research was supported by the Faculty of Medicine, Srinakharinwirot University, Thailand.
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