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Journal of Clinical Microbiology, July 2006, p. 2423-2427, Vol. 44, No. 7
0095-1137/06/$08.00+0 doi:10.1128/JCM.00254-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Comparisons of Community-Associated Methicillin-Resistant Staphylococcus aureus (MRSA) and Hospital-Associated MSRA Infections in Sacramento, California
Hsin Huang,
Neil M. Flynn,*
Jeff H. King,
Caroline Monchaud,
Margaret Morita, and
Stuart H. Cohen
Department of Infectious Disease, Department of Internal Medicine, University of California, Davis, Medical Center, Sacramento, California 95817
Received 3 February 2006/
Returned for modification 28 March 2006/
Accepted 13 May 2006

ABSTRACT
Methicillin-resistant
Staphylococcus aureus (MRSA) has long
been a common pathogen in healthcare facilities, but in the
past decade, it has emerged as a problematic pathogen in the
community setting as well. A retrospective case series study
of patients from whom MRSA was isolated from December 1, 2003,
through May 31, 2004, was conducted at the University of California,
Davis, Medical Center. Patient data were collected from electronic
medical records and traditional chart reviews to determine whether
MRSA acquisition was likely to have been in the community or
in the hospital. Antimicrobial susceptibility testing and pulsed-field
gel electrophoresis (PFGE) were performed for all confirmed
isolates. Skin and soft tissue were the most common infection
sites for all MRSA patients. Among the 283 MRSA infections,
127 (44.9%) were defined as community-associated (CA)-MRSA.
Ninety-six percent of the CA-MRSA isolates were susceptible
to clindamycin. Double-disk diffusion tests were performed to
examine inducible clindamycin resistance by erythromycin induction
on both CA and hospital-associated (HA) clindamycin-susceptible
and erythromycin-resistant isolates. Ten percent (17 of 183)
were positive. Most CA-MRSA isolates were identified by PFGE
as a unique strain, genotype USA300, which was not genetically
related to the predominant genotype, USA100, in the HA-MRSA
isolates. Injecting drug users accounted for 49% of CA-MRSA
infections but only 19% of the HA-MRSA infections (odds ratio,
4.2; 95% confidence interval, 2.4 to 7.4). Our study shows that
a single clone of CA-MRSA accounts for the majority of infections.
This strain originated in the community and is not related to
MRSA strains from healthcare settings. Injecting drug users
could be a major reservoir for CA-MRSA transmission.

INTRODUCTION
The first methicillin-resistant
Staphylococcus aureus (MRSA)
case was reported in the United Kingdom in 1961 (
15), shortly
after methicillin was introduced into clinical practice. Seven
years later, after the resistant strain had become widespread
in Japan, Europe, and Australia, the first case of MRSA in the
United States was described in 1968 (
2). Traditionally, MRSA
has been considered a major nosocomial pathogen in healthcare
facilities, but in the past decade, it has been observed emerging
in the community as well. The first case of community-associated
MRSA (CA-MRSA) infection in the United States was reported in
1980 (
21). More-widespread identification of CA-MRSA in the
United States began in the 1990s, following the report of CA-MRSA
infections among four children (
5). Patients with CA-MRSA infections
have often lacked risk factors known for patients with hospital-associated
MRSA (HA-MRSA) infections, which include recent hospitalization,
dialysis, nursing-home residence, and other co-morbid conditions
such as diabetes, chronic renal failure, and chronic pulmonary
diseases which bring them into contact with healthcare settings.
CA-MRSA has also been found to be composed of more-diverse clonal
groups than HA-MRSA and to usually contain a unique
SCCmec type
IV DNA element (
13). Clusters of CA-MRSA infection have been
described among aboriginals in Australia (
18), rural Native
American communities in the United States (
14), prisoners (
7),
sports players (
6), children (
12), and injecting drug users
(IDUs) (
21). The substantial increase in CA-MRSA infections
has increased the challenge of selecting empirical antimicrobial
treatments in outpatient settings. Previous studies have reported
that in the United States, the prevalence of CA-MRSA infections
varies from 76% among MRSA skin and soft tissue infection (SSTI)
isolates in Alaska (
1) to 12% of all MRSA infections in Minnesota
(
20). These reports prompted us to review the current epidemiology
of MRSA in Sacramento, California, a metropolitan area of approximately
1.2 million people. In this study, we describe and compare the
characteristics and MRSA strains of patients treated for CA-MRSA
and HA-MRSA infections at the University of California, Davis,
Medical Center (UCDMC) from December 1, 2003 to May 31, 2004.

MATERIALS AND METHODS
Case ascertainment and definition.
This is a retrospective case series study. Samples from adult
patients (both inpatients and outpatients) with MRSA infections
newly identified in the microbiology laboratory at the UCDMC
were collected from December 1, 2003, through May 31, 2004.
None of these was obtained as a "screening" or "surveillance"
culture for MRSA. The UCDMC is a tertiary referral center serving
primary care, emergency, and hospitalization needs of the majority
of medically uninsured and indigent patients in Sacramento county.
The average inpatient census is approximately 450 patients,
and its outpatient services experience 2,270 visits per day.
Duplicate isolates collected from the same patient were excluded.
HA-MRSA infection was defined as occurring in a patient whose
MRSA isolate was cultured more than 48 h after admission, who
had a history of hospitalization, surgery, dialysis, or residence
in a long-term healthcare facility within 6 months prior to
the culture date, or who had an indwelling intravenous line,
catheter, or any other percutaneous medical device present at
the time the culture was taken. Patients with none of the above
conditions were classified as having CA-MRSA infection. Patients
who had had an MRSA-positive isolate prior to the study period
were excluded from the study.
Data collection.
Information was extracted from the electronic medical records and by traditional chart review and recorded on a standard data collection sheet. Data obtained about the study subjects included basic demographics, reason for admission, medical history (underlying diseases), medication history, sites of MRSA infection, culture site, length of hospital stay, social history, and isolate characterization (e.g., antimicrobial susceptibility and molecular typing results) (11). Antimicrobial administration within 30 days before the study was recorded as well.
Characterization of isolates.
All MRSA cultures were confirmed in the UCDMC microbiology laboratory. Susceptibility testing was performed by the Sceptor system microtiter dilution method (Becton-Dickinson, Franklin Lakes, N.J.). Susceptibility to cefazolin, clindamycin, ciprofloxacin, erythromycin, gentamicin, oxacillin, rifampin, tetracycline, trimethoprim-sulfamethoxazole, and vancomycin was determined. Oxacillin was used for methicillin susceptibility testing. The results were categorized according to the guidelines of the National Committee for Clinical Laboratory Standards (now Clinical Laboratory Standards Institute) and confirmed with Denka latex agglutination. The double-disk diffusion test (D-test) (23) was performed on all MRSA isolates whose antimicrobial susceptibility patterns were clindamycin susceptible and erythromycin resistant. The test was used to estimate the proportion of inducible macrolide-lincosamide-streptogramin B (iMLSB) resistance.
Molecular typing of MRSA strains was done by pulsed-field gel electrophoresis (PFGE) with SmaI restriction endonucleases at Stanford University, using a method previously published (10). For visual strain analysis, isolates were considered different strains if their PFGE patterns differed by
4 bands (25).
Statistical analysis.
SAS version 8.1 software was used for statistical analysis (SAS Institute, Cary, N.C.). Descriptive analysis (univariate analysis) was employed in investigating the distributions of variables between the HA and CA groups. Categorical variables between the two groups were compared by means of the chi-square test or Fisher's exact test if 20% of the expected values were smaller than five. Continuous variables were analyzed using the two-tailed t test. A P value of <0.05 was considered statistically significant with chi-square distribution. Analysis of variance was used for three-group comparisons.

RESULTS
Within the 6-month study period, 283 out of 328 patients with
individual MRSA-positive isolates were eligible for our study
by the inclusion criteria. The proportion of MRSA infections
among all
S. aureus isolates was 42% during the study period.
In our data, 156 (55.1%) met the definition of HA-MRSA infections
and the rest; 127 patients (44.9%), were classified as CA. The
basic demographics of the MRSA patients are shown in Table
1.
The CA-MRSA group had significantly different distributions
from the HA-MRSA group with respect to age, length of hospital
stay, ethnicity, and insurance status (
P < 0.01 for all comparisons).
Among those whose occupation was documented, 67 (64%) CA-MRSA
patients were not employed, as were 36 (29%) in the HA-MRSA
group (
P < 0.001).
View this table:
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|
TABLE 1. Basic demographics of patients with CA-MRSA versus HA-MRSA infections from whom samples were collected within a 6-month period
|
A significant difference between the two groups was in the sites
of MRSA infection (Table
2). Skin and soft tissue were the most
common infection sites among all subjects (174 of 283, 61%),
especially among CA patients (109 of 127, 85.8%). We compared
underlying conditions in the patients with CA-MRSA to those
with HA-MRSA. Besides dermatological conditions, diabetes (
P = 0.0023), chronic renal disease (
P < 0.001), and cancer
(
P = 0.0012) were the most common underlying conditions observed
in the HA-MRSA group. Of a total of 253 patients whose drug
use histories were recorded, nearly half of CA-MRSA patients
(53 of 108, 49.1%) were IDUs compared with only 18.6% (27 of
145) of the HA-MRSA group (odds ratio [
O
]
= 4.2;
P < 0.001; 95% confidence interval (CI), 2.4 to 7.4).
Among HA-MRSA patients with SSTI, the isolates were more likely
to be susceptible to clindamycin (
O
= 2.7; 95% CI, 1.4 to 5.2;
P = 0.003) and tetracycline (
O
= 0.17; 95% CI, 0.05 to 0.54;
P = 0.001)
than were isolates associated with other sites of infection.
Molecular typing.
The results of PFGE identified seven clonal groups among our isolates, with 28 distinct subtype patterns (Table 3). One PFGE clonal type, designated clonal group USA300, accounted for 87% (108 of 124) of CA-MRSA isolates but only 33% (48 of 47) of HA-MRSA isolates (P < 0.001). Another PFGE clonal group, group USA100, predominated only in the HA-MRSA group, accounting for 47% (69 of 147) of HA isolates. Further, among the HA isolates, USA300 appeared predominantly in patients with a history of injecting drug use (16 of 26, 69%), in contrast to the non-IDU group (26 of 110, 23.6%; P = 0.009).
Antimicrobial susceptibility patterns.
The prevalence of resistance to each antimicrobial tested is
presented in Table
4, which includes the D-test-positive results
as well. CA-MRSA isolates were more likely to be susceptible
than were HA-MRSA isolates with respect to ciprofloxacin (
P < 0.001) and clindamycin (
P < 0.001).
One hundred eighty-three isolates were analyzed by the D-test
to evaluate for iMLSB resistance. Seventeen (10.2%) of them
were positive.
The results of comparing antimicrobial patterns between CA-MRSA USA300 and HA-MRSA USA300 are shown in Table 5. Even though similar with respect to molecular strain, HA-MRSA USA300 had significantly higher prevalence of resistance to ciprofloxacin (P = 0.023). A higher proportion of HA-MRSA USA300 isolates were resistant to clindamycin than were CA-MRSA USA300 isolates, almost reaching statistical significance level (P = 0.072). By comparing susceptibility results between HA-MRSA USA300 and HA-MRSA non-USA300 types (including USA100, USA400, USA800, and other, unique strains), we found that the non-USA300 nosocomial strains were more likely to be resistant than USA300 types to ciprofloxacin and clindamycin but less likely to be resistant to tetracycline (P < 0.01 in all comparisons).

DISCUSSION
This study demonstrates that a high proportion (45%) of MRSA
patients identified in our institution had CA-MRSA infections.
Our findings confirmed our clinical impression that CA-MRSA
had emerged in our community. Overall, CA-MRSA infection was
unlikely to result in prolonged hospitalization in our community.
Most of these CA-MRSA infections were of the skin and soft tissue
types, which responded quickly to wound care (incision and drainage)
when indicated and to outpatient oral antimicrobial therapy.
The distribution of MRSA infection sites for CA-MRSA and HA-MRSA
groups was consistent with those of previous studies (
18,
20).
CA-MRSA isolates were more susceptible to multiple antibiotics such as ciprofloxacin and clindamycin. Nevertheless, our results showed an unusually high prevalence of resistance to erythromycin (93%), in contrast to other reports of 69% in Alaska (1) and 61% in the San Francisco urban poor study (9). Further, unlike other studies, in our data the prevalence of MRSA strains expressing iMLSB was relatively low (10.2%) (16), suggesting that clindamycin remains one option for effective antimicrobial treatment in our community.
Our data demonstrate that USA300 was not only the most common strain among all MRSA isolates circulating among CA-MRSA patients but was also highly related to SSTI. This strain was not genetically related to the common nosocomial strain in HA-MRSA isolates, USA100. This finding and antimicrobial susceptibility patterns support the conclusion that CA-MRSA infection is not a nosocomial strain which originated in local healthcare facilities, but a distinct clone that has developed and is being propagated within the community.
It has been well documented (4, 22) that skin and soft tissue infections, such as abscesses and cellulitis, are directly related to injecting drug use. Other studies (3, 8, 17, 24) have reported that IDUs were commonly colonized or infected with S. aureus, anaerobes, and facultative gram-positive cocci, with high rates of recurrent infection, particularly among those who were homeless. Although some investigators excluded IDUs as subjects (cases) for studying CA-MRSA infection (14, 19), one study (9) conducted in San Francisco indicated that injecting drug use may be responsible for increasing MRSA colonization in the urban poor community. Similarly, given the fact that USA300 (predominant in the community) has high prevalence in IDUs of the CA-MRSA group, IDUs may be an important population that contributes significantly to the spread of clonal CA-MRSA. Injecting drug users frequently have SSTI and could transmit the organism to non-IDUs by close personal contact, serving as a significant community reservoir for CA-MRSA. Frequent skin puncture, poor injection site hygiene, syringe reuse, lack of personal hygiene knowledge and availability, and sharing needles are among the factors that facilitate CA-MRSA colonization/infection among IDUs.
The PFGE results showed that 33% of HA-MRSA isolates were USA300. There are two likely reasons for this finding. First, the organism may actually have been nosocomially transmitted. That the strain is no longer limited in the community but has spread into the hospital indicates the severity of CA-MRSA infection. This hypothesis is further supported by the finding that the USA300 isolates we found in the HA group exhibit an antimicrobial susceptibility pattern intermediate between those of CA USA300 and the HA non-USA300 types. This finding raises the concern that CA and HA strains may exchange genetic material, resulting in an organism uniquely adapted to produce aggressive SSTI-like CA-MRSA strains which carry the Panton-Valentine leucocidin gene as well as possessing resistance to multiple antimicrobial agents, like current HA strains. Such a development would further complicate efforts at limiting the impact of nosocomially associated S. aureus infections. Alternatively, the organism was colonizing the patient on admission but was identified more than 48 h after admission.
There were several limitations to our study. This is a healthcare-based retrospective case series study. Thus, we were unable to estimate the true prevalence of CA-MRSA infection in the general population or in the IDU population. Second, although medical charts were carefully reviewed, in the absence of personal interviews there is a risk of misclassifying MRSA acquisition due to lack of a detailed history of hospital-related exposures and failure to elicit an accurate history of injecting drug use, especially for those patients whose charts were incomplete. This would tend to bias the study toward underestimating injecting drug use. Third, due to limited resources, we did not test for the existence of the Panton-Valentine leucocidin gene harbored among CA-MRSA isolates. Overall, our data demonstrate a high proportion of CA-MRSA isolates, suggesting that the face of MRSA has changed in both epidemiological and microbiological features and in both community and hospital. HA-MRSA patients acquired their infections while under intensive treatment for other underlying diseases, resulting in additional diverse sites of infection and additional diverse clonalities. In this study, we not only strengthen the hypothesis that injecting drug use is contributing significantly to the increasing incidence of CA-MRSA but also discovered that the CA-MRSA strain has already disseminated into the hospital and has probably adopted multiresistant genes from the hospital strains.

ACKNOWLEDGMENTS
We thank Tom Watson for laboratory data collection.
There is no conflict of interest existing for any of the listed authors.
There was no financial support for this study.

FOOTNOTES
* Corresponding author. Mailing address: Department of Internal Medicine, University of California, Davis, Medical Center, 4150 V Street, PSSB, G500, Sacramento, CA 95817. Phone: (916) 734-3815. Fax: (916) 734-7766. E-mail:
nmflynn{at}ucdavis.edu.


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Journal of Clinical Microbiology, July 2006, p. 2423-2427, Vol. 44, No. 7
0095-1137/06/$08.00+0 doi:10.1128/JCM.00254-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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