infection control and hospital epidemiologyaugust 2007, vol. 28, no. 8
Prevalence of Nasal Colonization Among Patients
With Community-Associated Methicillin-Resistant
Staphylococcus aureus Infection and Their Household Contacts
Uzma Zafar, MD; Leonard B. Johnson, MD; Michel Hanna, MD; Kathleen Riederer, BS, MT; Mamta Sharma, MD;
Mohamad G. Fakih, MD; Muthayipalayam C. Thirumoorthi, MD; Rand Farjo, MD; Riad Khatib, MD
aureus (CA-MRSA) infection and their household contacts.
To evaluate the prevalence of colonization among patients with community-associated methicillin-resistant Staphylococcus
15, 2004, to February 20, 2006.
Prospective, observational laboratory study of nasal colonization among patients and their household members from September
setting. A 600-bed, urban, academic medical center.
Fifty-one patients who presented with CA-MRSA infections and 49 household members had cultures of nasal swab specimens
and 10 (20%) of 49 household members were colonized with MRSA. An additional 5 patients (10%) and 12 household members (24%)
were colonized with methicillin-susceptible Staphylococcus aureus. Most MRSA isolates (95%; infective and colonizing) carried the staph-
ylococcal cassette chromosome mec type IV complex, and 67% represented a single clone, identical to USA 300. Of the colonized household
members, 5 had isolates related to the patients’ infective isolate.
Skin and soft-tissue infections were seen in 50 patients (98%) and 2 household members. Twenty-one (41%) of 51 patients
than rates reported among the general population. Among colonized household members, only half of the MRSA strains were related to
the patients’ infective isolate. Within the same household, multiple strains of CA-MRSA may be present.
The frequency of CA-MRSA colonization among household members of patients with CA-MRSA infections is higher
Infect Control Hosp Epidemiol 2007; 28:966-969
From the Department of Internal Medicine, St. John Hospital and Medical Center (U.Z., L.B.J., M.H., K.R., M.S., M.G.F., R.F., R.K.), and the Department
of Internal Medicine, Wayne State University School of Medicine (L.B.J., M.S., M.G.F., M.C.T., R.K.), Detroit, Michigan.
Received October 10, 2006; accepted January 25, 2007; electronically published June 19, 2007.
? 2007 by The Society for Healthcare Epidemiology of America. All rights reserved. 0899-823X/2007/2808-0012$15.00.DOI: 10.1086/518965
The epidemiology of methicillin-resistant Staphylococcus au-
reus (MRSA) is changing, with an increasing frequency of
community-associated MRSA (CA-MRSA) causing predom-
inantly skin and soft tissue infections and necrotizing pneu-
monias.1These infections have been associated with the
spread of well-characterized strains with preserved suscepti-
bility to non–b-lactam antibiotics and carriage of Staphylo-
coccus cassette chromosome (SCC) mec type IV complex and
genes encoding Panton-Valentine leukocidin toxin.1
Colonization with S. aureus is a known risk factor for sub-
sequent infections.2,3Prior studies have identified the anterior
nares as the most consistent site of S. aureus colonization.4
A recent survey of S. aureus nasal carriage in the US popu-
lation in 2001-2002 showed that the national S. aureus col-
onization prevalence was 32.4%; MRSA colonization was un-
common (0.8%).5S. aureus may also be transmitted from
person to person, especially in the setting of close physical
contact.6Although family outbreaks of CA-MRSA infection
have been reported to cause severe disease,7only limitedstud-
ies have systematically estimated the spread of MRSA among
household contacts.8The prevalence of nasal colonization
with CA-MRSA among members of the households of in-
fected patients is unknown. We evaluated the frequency of
nasal colonization among patients who presented with CA-
MRSA infections and their household contacts.
We conducted a prospective, observational study to assess the
prevalence of nasal colonization of S. aureus among patients
who presented to St. John Hospital and Medical Center (De-
spread of ca-mrsa nasal colonization among household contacts 967
Household Members From Whom Staphylococcus aureus Was
Demographic and Clinical Characteristics of Patients and
(n p 51)
(n p 49)
Age, mean ? SD, years
Relative risk (95% CI) for
SCC mec type, % of isolates
38.4 ? 13.6
2.30 (1.06-5.00) 0.48 (0.25-0.83)
cause characteristics of household members were not alwaysrecorded,values
were calculated for those with available information. CI, confidenceinterval;
MRSA, methicillin-resistant S. aureus; MSSA, methicillin-susceptible S. au-
reus; NA, not applicable.
aValues were calculated for patients colonized with S. aureus only (P p
Data are no. (%) of individuals, unless otherwise specified. Be-
troit, MI) with infections due to CA-MRSA and their house-
hold members from September 15, 2004, to February 20,
2006. Patients were identified through prospective monitor-
ing of culture findings by the microbiology laboratory. All
patients with MRSA isolates were screened. Inclusion criteria
were infection, defined as isolation of MRSA from a patient
who presented with a localized infection in an outpatient
setting or within 48 hours of admission; community onset
of infection, defined as no admission to a short-term or long-
term healthcare facility within the last 2 years and no receipt
of antibiotics active against S. aureus within the past year;
and agreement to participate, confirmed by signing an in-
formed consent document. Household members were given
$25 if they agreed to participate. Patient information was
collected, including age, race, sex, and number of household
members. Data were entered into an Excel file. Statistical
analysis was performed using the computer software program
SPSS, release 10 (SPSS).
Infective S. aureus isolates were recovered from specimens
sent to the clinical microbiology laboratory for culture. Nasal
swab specimens were obtained from both anterior nares of
consenting patients and household members on the day of
enrollment and were cultured on trypticase soy agar with 5%
sheep blood (Remel). Identification was confirmed on the
basis of catalase production and results of Staphaurex latex
agglutination tests (Remel). Susceptibility testing of infective
isolates was performed using Vitek 2 (bioMe ´rieux) and meth-
icillin resistance confirmed by growth on Mueller-Hinton
agar that contained 6 mg/mL of oxacillin (Remel). Vanco-
mycin resistance was evaluated with 6 mg/mL of vancomycin
(Remel) for MRSA isolates; no resistant isolates were iden-
tified. All MRSA (infective and nasal) isolates had DNA ex-
tracted using CHEF Genomic DNA Plug Kits (Bio-Rad Lab-
oratories), and DNA specimens were digested with SmaI
restriction endonuclease (Invitrogen). Pulsed-field gel elec-
trophoresis (PFGE) was performed using CHEF-DR III (Bio-
Rad) with switch times of 1-20 seconds at 6 V/cm for 21
hours. Gels were stained with ethidium bromide, destained
in water, and photographed with ChemiImager 4000 (Alpha
Innotech). The resulting band patterns for MRSA isolates
were visually compared and interpreted using standardguide-
lines.9A sample of MRSA strain USA 300, a common strain
associated with CA-MRSA infections in North America and
our community, was also analyzed for comparison.10,11A rep-
resentative isolate from each PFGE pattern was subjected to
SCC mec typing, using the protocol of Zhang et al.12
The study was approved by the St. John Hospital and Med-
ical Center institutional reviewboard. Colonizationstatuswas
reported to the patient and household members when asked.
Education and counselingwereprovided;however,noroutine
attempts were made to decolonize patients.
During the 18-month period from September 2004 through
February 2006, a total of 54 patients met our selection criteria
and were asked to participate in the study. Three refused to
participate, and the remaining 51 patients were enrolled. Six
of the infective isolates were not available for genetic analysis.
The mean age (?SD) of the patients was
most (69%) were male and either African American or white.
Two infections occurred in the same households during the
study period. All but 1 patient presented with skin and soft
tissue infection. The time between CA-MRSA identification
and culture of nasal swab specimens was 0-86 days (median,
1 day; interquartile range, 0-3 days). Of the 51 patients, 18
(35%) reported having no household members. There were
80 household members associated with the remaining 33 pa-
tients. Forty-nine household members (61%) from 22 house-
holds agreed to participate. The results of nasal cultures are
given in the Table. The patients had highratesofcolonization,
as anticipated. A high rate was also found among household
members. Methicillin-susceptible S. aureus colonization was
significantly more common among household members.
PFGE analysis of 76 isolates (45 infective and 31 coloniz-
years;38.4 ? 13.6
968infection control and hospital epidemiology august 2007, vol. 28, no. 8
munity-associated methicillin-resistant Staphylococcus aureus iso-
lates. Lanes 1 and 10, l ladder; lanes 2 and 3, a patient-household
pair with a 1-band difference; lanes 6 and 7, another patient-house-
hold pair with indistinguishable band patterns; and lanes 4, 5, 8,
and 9, other patient isolates. The band pattern is indistinguishable
in lanes 3 and 5-7 (strain A); 1 band is different (indicating relat-
edness) in lanes 2, 8, and 9; and more than 3 bands are different
(indicating unrelatedness) in lane 4.
Pulsed-field gel electrophoresis of representative com-
ing) revealed that 68 (89%) were related: 51 isolates had
indistinguishable patterns, with a 1-2–band difference for 17
isolates. This PFGE pattern (hereafter, “strain A”) was iden-
tical to that of strain USA 300. Four additional unrelated
patterns, each of which differed from the pattern of strain A
by more than 3 bands, were detected among the remaining
8 isolates. Of 17 patients from whom both infecting and
colonizing isolates were recovered, 16 (94%) had identical
strains. The remaining patient was colonized with an unre-
lated strain. The isolates from the 10 colonized household
members (from 8 different households) were related to the
corresponding patients’ infective isolate (all of which were
strain A) in 5 instances: 4 had indistinguishable patterns, and
the pattern for 1 differed by 1 band (Figure). The remaining
5 isolates were unrelated. Nearly all isolates had SCC mec
type IV, and 1 had SCC mec type V.13The risk for MRSA
colonization among household members was highest for par-
ents of the patient (60%; risk ratio [RR], 3.77 [95% con-
fidence interval (CI), 1.40-10.10];
spouses (27%; RR, 1.46 [95% CI, 0.47-4.53]) and children
or siblings (19%; RR, 0.56 [95% CI, 0.15-2.06]). The number
of MRSA-colonized household members limits our ability to
draw meaningful conclusions.
Attempts at performing follow-up cultures 6 months later
were limited, because a large number of individuals were lost
to follow-up. Of the 5 patients and 4 household members
who were colonized and underwent follow-up testing, 3 (1
), followed by
P p .05
patient and 2 household members) remained colonized with
the same strain originally detected on culture.
Nasal colonization with S. aureus is a well-established risk
factor for subsequent S. aureus infection.2,3The risk appears
to be higher for patients colonized with CA-MRSA. A recent
prospective study evaluated S. aureus colonization rates
among 812 military personnel and the subsequent risk for
developing soft tissue infection. Although only 3% of indi-
viduals were initially colonized with CA-MRSA, the relative
risk of subsequent infection was 10.7, compared with indi-
viduals colonized with methicillin-susceptible S. aureus.14Ad-
ditionally, the recent increase in infections attributed to the
clonal spread of CA-MRSA strains may suggest an increased
risk of infection among individuals colonized with CA-
MRSA. The high frequency of soft tissue infections due to
the USA 300 MRSA strain confirms its strong pathogenicity.9
The source of most CA-MRSA cases is not always apparent.
Patients may acquire infection after contact sports or other
athletic activities.15Additionally, in our study, nasal coloni-
zation was not documented in some patients with infection,
suggesting colonization at different body sites or an exoge-
nous source. In fact, to our knowledge, the rate of nasal
colonization among patients with CA-MRSA infections at the
time of clinical disease has not been previously reported. In
our study, we found nasal colonization in 41% of the patients.
We also found that the incidence among household contacts
was significantly higher than that reported for the general
population,5,14,16suggesting either frequent spread or a com-
mon source of exposure among household contacts. Other
outbreaks of invasive CA-MRSA infection among families
have also been reported, suggesting intrafamilial spread or
spread among close contacts.7The frequency of colonization
among household contacts is comparable to that reported
elsewhere (14.5%) among contacts of nosocomial MRSA.8No
evidence based on findings from this study is available to
indicate that CA-MRSA strains are transmitted more easily
than other MRSA strains. In addition, direct spread did not
account for all household colonization, because only half of
the colonized household members carried the same strain as
the patient. Of the 5 household members colonized with
strains different from the patient isolate, 4 carried SCC mec
type IV. Thus, multiple strains of CA-MRSA may colonize a
The natural history of CA-MRSA colonization is not well
established. In their study among military personnel, Ellis et
al.14noted a decrease from 3% to 1.6% in colonization rates
during an 8-10–week period. Our efforts to study rates of
persistent colonization were limited by patient follow-up.
Larger studies should be undertaken to evaluate persistence
of colonization among patients and perform household sur-
veillance, to determine the likelihood of recurrent infection.
Our study has several limitations. The strict entry criteria
spread of ca-mrsa nasal colonization among household contacts969 Download full-text
for patients with CA-MRSA infection limited our total num-
ber of participants. In addition, more than one third of study
participants did not have household members, and only 61%
of household members chose to participate. Because the nose
was the only site from which specimens were obtained for
culture, the number of colonized patients and household
members is likely underestimated. A recent study that eval-
uated sites of MRSA colonization among patients admitted
to the hospital found that cultures of swab specimens ofintact
skin, the rectum, and axillae identified an additional 27% of
individuals colonized with MRSA.17The addition of cultures
of rectal swab specimens to routine cultures of nasal speci-
mens in that study resulted in identification of 95% of col-
onized individuals. In addition, cultures of throat specimens
have recently identified another area of frequent colonization
with S. aureus.18Future studies that evaluate CA-MRSA col-
onization should include cultures of throat and rectal spec-
imens to better estimate the frequency of colonization. In
addition, the baseline frequency of MRSA colonization in our
community is not known, and thus a comparison with rates
of colonization among household members is not available.
Three other large studies demonstrated rates of CA-MRSA
colonization among nonhospitalized individuals.5,14,16
As reports of intrafamilial spread of CA-MRSA infections
increase, identification of individuals from the community at
risk for MRSA infection will be increasingly important. Rec-
ognition of the household spread of strains of CA-MRSA may
affect antibiotic selection in the appropriate clinical scenario.
Hospital and Medical Center Graduate Medical Education.
Potential conflicts of interest.
forPfizer, Wyeth, andAventis.All otherauthorsreportnoconflictsofinterest.
This study was supported by a grant from the St. John
L.B.J. reports being on the speaker’sbureau
Address reprint requests to Leonard B. Johnson, MD, 19251MackAvenue,
Suite 340, Grosse Pointe Woods, MI 48236 (email@example.com).
Presented in part: 45th International Conference on Antimicrobial Agents
and Chemotherapy; Washington, DC; December 16-19, 2005 (Abstract
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