Comparison of methicillin-resistant Staphylococcus
(MRSA) carriage rate in the general
population with the health-worker population
Zena H Fadheel1, BSc, BMLS; Laboratory Technician
Holly E Perry2, MApplSc (Hons);
Programme Leader Bachelor of Medical Laboratory Science
Ross A Henderson1, FRACP, FRCPA, PHD;
Clinical Director Laboratory and Consultant Haematologist
1North Shore Hospital, Waitemata District Health Board; 2
Auckland University of Technology, Auckland

The origins of the major MRSA strains are still poorly understood. The emergence of antibiotic resistance in bacteria is becoming a It has been proposed that all MRSAs were descended from a widespread problem and a major health issue. Methicillin resistant single ancestral S. aureus strain that acquired mecA, but more Staphylococcus aureus (MRSA) is becoming increasingly frequent recent studies show that some MRSAs are very divergent, in hospitals. In this study we compared the carriage rate of MRSA in 100 health workers at North Shore Hospital with the carriage mecA has been transferred between S. aureus families (2,4). rate of MRSA in the general population (100 staff members and students of Auckland University of Technology) and found a In the past three decades, MRSA has become wide spread in many prevalence of MRSA in the health workers of 4%, but none in the hospitals (2) and S. aureus (including MRSA) is commonly found in general population group. The implication of MRSA carriage in two main carriage sites, the nose (20%) and the perineum (3%). The skin, including the hands, can be transiently contaminated (5). The major form of spread is hand borne transmission (2).
Key words: Staphylococcus aureus, MRSA, methicillin, prevalence,
health workers
Hospital infection control staff need to limit the spread of MRSA for several reasons. There have recently been reports of strains of MRSA that have intermediate resistance to vancomycin. This is an important concern since the already limited treatment options Introduction
for serious MRSA infections may become more limited due to the Staphylococcus aureus is a virulent bacterium that can cause increase in resistance to vancomycin. Limiting the transmission of serious infections including skin and soft tissue infections, wound MRSA might reduce the potential for these strains to spread (6). infection, bacteremia, pneumonia, and endocarditis (1). It is an organism that is renowned for its potential to acquire resistance to Another concern is the simultaneous spread of MRSA and antimicrobial agents. In 1961 there were reports from the United vancomycin-resistant enterococci (VRE), possibly resulting in the Kingdom of S. aureus that had acquired resistance to methicillin transfer of the vancomycin-resistance gene from VRE to MRSA, (methicillin-resistant S. aureus) (2). The clinical significance of rendering MRSA fully resistant to vancomycin. The first such isolate oxacillin-resistant (methicillin-resistant) S. aureus is heightened was detected in the United States in 2002 (7). The cost of treating by the fact that these isolates are usually resistant to other an MRSA infection is another concern because vancomycin, anti-staphylococcal agents such as lindamycin, erythromycin, the antibiotic most commonly used to treat MRSA infection is tetracycline, and sometimes trimethoprim/sulphamethoxazole, with the exception of vancomycin (4). Although oxacillin-resistant staphylococci appear susceptible in vitro to other β-lactam agents, Epidemiological studies have shown that since the mid 1990s, the (such as the cephalosporins) these are clinically ineffective. incidence of MRSA has been increasing in New Zealand (6). In 2006 Therefore, all oxacillin-resistant staphylococci are reported as the incidence of MRSA amongst hospital patients and staff was resistant to all β-lactam agents, including cephalosporins, β-lactam/ approximately 0.17% (10). Amongst S. aureus isolates in N.Z, 7% β-lactamase inhibitor combinations, and imipenem (2). were resistant to oxacillin/methicillin in 2005. Currently nearly 90% of S. aureus isolates are penicillin-resistant. There are four main strains of MRSA, each of which has Methicillin and other semi-synthetic penicillins were successful distinguishing characteristics (11). In New Zealand, different strains in treating penicillin-resistant S. aureus until the 1980s, when of MRSA can become epidemic in different geographical regions methicillin resistance emerged (3). Methicillin is no longer (10). EMRSA -15 is predominately isolated in hospitals, whereas commercially available, and in many laboratories testing for WSPP is found more frequently in the community.
methicillin resistance has been replaced by oxacillin and/or cefoxitin. Cefoxitin gives clearer endpoints because it is a better The aim of this study was to compare the carriage rate of MRSA inducer of the mecA gene (2). The genetic basis of methicillin in 100 health workers at North Shore Hospital with the carriage resistance in MRSA is the acquisition of mecA gene, that renders rate of MRSA in a general population of 100 staff members and MRSA resistant to all β-lactam antibiotics (2,3).
students of the Auckland University of Technology.
The methicillin resistance gene (mecA) encodes a methicillin-resistant penicillin-binding protein (PBP2a) that is not present in susceptible Nasal swabs from 200 participants were collected, and inserted strains and is believed to have been acquired from a distantly related directly into 7% salt broth for 24 hr incubation at 37°C (12). After species. MecA is carried on a mobile genetic element, the staphylococcal 24 hr the broths were sub-cultured onto mannitol salt agar (MSA, cassette chromosome mec (SCCmec), of which four forms have been Fort Richards, USA)) and examined after a further 24 hr and 48 hr described that differ in size and genetic composition. for yellow colonies. Any yellow colonies on MSA plates had a DNA test and purity plate on Columbia human blood (Fort Richards, Table 2. Characteristics of the MRSA positive nurses
USA). DNA positive isolates had a coagulase test performed. All presumptive S. aureus isolates (coagulase positive, DNA positive yellow colonies) were subjected to testing with MRSA screen slide latex agglutination kit (Pro-Lab Diagnostic) and sensitivities by disc diffusion on Mueller-Hinton agar. The antibiotics tested were penicillin, cefoxitin, erythromycin, tetracycline, clindamycin, ciprofloxacin, gentamycin, fucidic acid, rifampicin, mupirocin and vancomycin. Zone size for determining sensitivity or resistance is shown in Table 1. Isolates of S. aureus showing resistance to penicillin and cefoxitin were considered positive for MRSA in this study. Previous history for MRSA was determined where available. If a new isolate demonstrated resistance to more than penicillin and cefoxitin, a slope was sent to ESR for phage typing to assist in strain identification. PCR typing was not performed. Infection control nurses were notified Positive participants were treated and followed up by infection control and occupational health nurses. The Waitemata DHB Discussion
protocol for MRSA positive staff is as follows: This study, although small in size, found that 4% of health workers Nasal: apply bactroban/fucidic acid to nostrils twice a day in the Waitemata DHB hospital carried MRSA, compared to none of the healthy volunteers in the wider Auckland community. Our Body wash: chlorhexidine 4% washes (shower) daily x 5 figure of 4% is higher than the national reported incidence of 0.17% amongst hospital patients and staff (10), but compares Hair wash: chlorhexidine 4% hair washes x per week 3 with studies conducted on patient cohorts in the United Kingdom, sets of swabs to be taken 48 hr apart.
where MRSA incidence ranged between 1.6% and 5.3% (15). Follow up: swabs taken monthly for 6 months, then 6 However, our study is novel, because we compared carriage rate of MRSA in New Zealand health workers with the general community. We could not find any published studies with which to directly Due to the small number of positive tests, statistical analysis was not carried out. The study was approved by the Northern Regional Ethics Committee.
Detecting or identifying a MRSA can be done in several ways. Detecting the presence of mecA gene using PCR is the gold Table 1. Interpretation of antibiotic sensitivities
standard for identification and confirmation of MRSA isolates (13). Detection of the altered protein PBP2a using commercially available MRSA screen slide latex agglutination kits is a highly specific and sensitive method. In this test, latex particles sensitized with a monoclonal antibody against cell wall PBP2a specifically react with methicillin-resistant staphylococci to cause agglutination. In this study, the latex agglutination, together with antibiotic sensitivity by disc diffusion was used. Several studies (5,11,14) have shown that cefoxitin is superior to oxacillin in the detection of MRSA, and cefoxitin was used in our study. It is interesting to note that the first MRSA positive health worker was sensitive to ciprofloxacin as EMRSA-15 strains are often resistant to ciprofloxacin (11). The concern is that these health workers could transmit MRSA to vulnerable patients. Patients are at higher than normal risk of acquiring S. aureus infection particularly as the in-patient population tends to be older, sicker and weaker, making them more vulnerable to infection. Results
The main finding of our study was that the carriage rate of MRSA in
Various strategies exist for controlling the spread of MRSA within health workers was 4% compared to 0% in the control population. healthcare settings. Preventative measures include laboratory The cohorts were well matched for age while there were more surveillance and screening for MRSA (5), promoting careful hand females than males in both groups. Average age of the health washing with soap and water rather than the antibacterial gels workers and control population were 42 yr and 40 yr respectively in common use, gowning and gloving by staff and eradication while there were 85 females in the health workers group and 64 of MRSA from colonized people (decolonization therapy). Most institutions use a combination of these strategies. Potential side effects associated with the use of eradication therapy include the All four individuals in whom MRSA was isolated were nurses development of further antibiotic resistance or the possibility with more than 2 years of clinical experience. Three of the MRSA of adverse reaction to the antibiotic. Although clinical trials of positive nurses were strain EMRSA-15 and one WSPP1. All four eradication therapy in colonized healthcare workers (healthy subjects showed resistance to cefoxitin and penicillin, three showed adults) exist, in practice healthcare workers are not always additional resistance to erythromycin, two additional resistance to systematically screened for MRSA and offered eradication therapy ciprofloxacin and one additional resistance to clindamycin. One of (3). In contrast, many hospital patients are routinely screened and the four nurses had previously isolated MRSA. Three of the four offered antibiotics or drugs if they are found to be colonized (5,9). nurses were treated with the standard Waitemata DHB protocol Currently at WDHB all staff members have a pre-employment nasal and subsequently showed negative results for MRSA (Table 2).
swab to detect MRSA but no further screening if the staff member tests negative. Patients admitted to the hospital will be screened if they are perceived to be at higher risk of MRSA carriage. The results from this study suggest a small, but significant MRSA 5. Centers for Disease Control and Prevention. Information about carriage in health workers that could be transmitted to vulnerable MRSA for healthcare personnel (updated October 10, 2007).
patients. This does raise the question whether all health workers should be screened at regular intervals for MRSA, as well as the 6. Wenzel RP. Prevention and Control of Nosocomial Infections, 2nd Ed., Lippincot Williams & Wilkins, Philadelphia, 1993.
7. Centers for Disease Control and Prevention. Staphylococcus Acknowledgments:
aureus resistant to vancomycin – United States, 2002. MMWR We would like to acknowledge the following: Morb Mortal Wkly Rep 2002; 51: 565-7.
Colin Swager, Team Leader Microbiology, North Shore Hospital; 8. Utah department of health, Staphylococcus aureus with VISA/ Joanne Morgan and all staff members of Microbiology, North VRSA. Shore Hospital; Dr. Roger Whiting, Acting Head of School, AUT ; Dr. Paul Henriques and Jim Clark, AUT; Dr. Jocelyn Peach, Director 9. Waitemata Health. MRSA Information Sheet for Staff / of Nursing & Midwifery, WDHB; Rachel Haggerty, Peter Pike and Andrea McLoid, previous managers, WDHB; Jane Sherard, Maori 10. Heffernan H, Wheeler L. Annual survey of methicillin resistant Advisor, WDHB; Pat Chainey and Dr. Tim Dare, Northern Regional staphylococcus aureus (MRSA). ESR Annual Report, 2005. Ethics Committee; Lorraine Neave and Dr. Wayne Miles, Knowledge Centre; Infectious control and occupational health nurses, WDHB; 11. Heffernan H, Blackmore T, Wheeler L, Davies H. Surveillance of the Editor and two anonymous referees, NZ Journal of Medical MRSA in New Zealand: poster.pdf Laboratory Science; and all the participants in AUT and WDHB.
12. Swager, C. Laboratory Manual, Waitemata District Health References
13. Heffernan, H, Wheeler, L. Phage typing methicillin resistant S. 1. Mahon CR, Manuselis G. Textbook of Diagnostic Microbiology, aureus (MRSA). Feb 2007. 2nd Ed., WB Saunders Co., Philadelphia, 2000.
14. CDC DHQP Guidelines. Laboratory detection of oxacillin 2. Upton A, Roberts SA, Milsom P, Morris AJ. Staphylococcal post- methicillin resistant staphylococcus aureus, 2005. http://www.
sternotomy mediastinitis: five year audit. ANZ J Surg 2005; 75: 15. Anwar R, Botchu R, Viegas M, Animashawun Y, Shashidhara 3. Loeb M, Main C, Walker-Dilks C, Eady A. Antimicrobial drugs S, Slater GJ. Preoperative methicillin-resistant staphylococcus for treating methicillin-resistant staphylococcus aureus aureus (MRSA) screening: an effective method to control MRSA infections on elective orthopedics wards. Surg Pract 4. Deplano A, Mendonca R, De Ryck R, Struelens M. External quality assessment of molecular typing of Staphylococcus aureus isolates by a network of laboratories. J Clin Microbiol 2006; 44: 3236-44.


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