Comparison of methicillin-resistant Staphylococcus
aureus (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 mec
A, but more
(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
A has been transferred between S. aureus
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,
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
for serious MRSA infections may become more limited due to the
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
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
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 (mec
A) 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
A is carried on a mobile genetic element, the staphylococcal
24 hr the broths were sub-cultured onto mannitol salt agar (MSA,
cassette chromosome mec
), 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
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 mec
A 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.
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
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
Joanne Morgan and all staff members of Microbiology, North
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: www.esr.cri.nz poster.pdf
Laboratory Science; and all the participants in AUT and WDHB.
12. Swager, C. Laboratory Manual, Waitemata District Health
13. Heffernan, H, Wheeler, L. Phage typing methicillin resistant S.
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(MRSA). Feb 2007. www.esr.cri.nz
2nd Ed., WB Saunders Co., Philadelphia, 2000.
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2. Upton A, Roberts SA, Milsom P, Morris AJ. Staphylococcal post-
methicillin resistant staphylococcus aureus, 2005. http://www.
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3. Loeb M, Main C, Walker-Dilks C, Eady A. Antimicrobial drugs
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