MRSA Research Center

The University of Chicago Medicine

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For Laboratory Researchers – MRSA Surveillance

What is the purpose of MRSA surveillance at the MRSA Research Center Lab?

To monitor clinical and molecular trends of methicillin-resistant Staphylococcus aureus (MRSA) at a large, urban teaching and tertiary care institution—the University of Chicago Medical Center (UCMC).

How does the MRSA Research Center Lab conduct MRSA surveillance?

For the past five years, the MRSA Research Center Lab has collaborated with the UCMC's Clinical Microbiology Lab to obtain all MRSA strains isolated from patients on a daily basis.  Upon receipt, the MRSA Research Center Lab makes subcultures of the isolates, stores them in a freezer, and warehouses molecular and clinical information from these isolates in a database.  The database includes patient demographic information, date of admission, date of specimen procurement, isolate antibiotic susceptibility, and anatomical source of the culture.  The MRSA Research Center Lab analyzes the clinical and demographic characteristics of patients from whom the isolates were recovered, as well as the isolates' genetic characteristics and antimicrobial resistance profiles.  In a recent MRSA Research Center Lab study, for example, isolates underwent genotyping and susceptibility testing to help define community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA).

How are the profiles of the isolates determined?

Routine susceptibility testing for each isolate is determined in the Clinical Microbiology Lab. All isolates identified by automated testing as susceptible to oxacillin undergo cefoxitin disc susceptibility testing for confirmation. For isolates that test resistant to erythromycin but susceptible to clindamycin, the D-zone test to detect inducible clindamycin resistance is performed, as is a disk-diffusion test for susceptibility to trimethoprim-sulfamethoxazole.  Once the isolates are collected from the Clinical Microbiology Lab, they are logged in and entered into the CA-MRSA Surveillance database, frozen in vials with skim milk, and stored in a -70o C freezer.  On a subset of MRSA isolates, MLST is performed for which clonal complexes are assigned using the eBURST algorithm.  The presence of mecA is assessed by PCR (polymerase chain reaction) and the SCCmec type of each strain is determined by establishing the molecular architecture of the ccr and mec gene complexes.  The presence of lukF-PV and lukS-PV encoding the PVL toxin is assessed by PCR.

How many strains does the MRSA Research Center Lab have?

To date, the MRSA Research Center Lab has collected >11,000 strains of Staphylococcus aureus, both healthcare associated methicillin-resistant Staphylococcus aureus (HA-MRSA) and community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strains (mostly CA-MRSA isolates).

How does the new Illinois MRSA screening law affect what the MRSA Research Center Lab does?

In August 2007, the Governor of Illinois signed two bills mandating that all patients admitted into an ICU be screened for MRSA.  As of September 17, 2007, the UCMC has been screening admissions to all ICUs by PCR.  The swabs from the PCR test that are positive are plated onto blood agar and sent to the MRSA Research Center Lab.  We plan to assess the role of this blood agar-plated swab compared with the PCR.  A plan is underway to use a MRSA screening plate instead of a blood agar plate when plating the positive PCR swabs to enhance MRSA detection.

What's planned for the future?

We plan to compare the molecular typing data from the ICU admission PCRs with the data from MRSA isolates from clinically relevant infective sites.  We also plan to address the prevalence of MRSA backgrounds and type a sample of MSSA isolates to determine if these MRSA MLST types are circulating in Chicago.