1119. Invasive Staphylococcus aureus Infections: a Large Role for MSSA
Session: Poster Abstract Session: MRSA/VRE Epidemiology
Friday, October 9, 2015
Room: Poster Hall
  • mltk_IDWeek 2015_final.pdf (326.1 kB)
  • Background:

    Staphylococcus aureus (SA) is a key pathogen in healthcare and community settings. The MN Dept of Health conducts active surveillance for invasive (i) MRSA infections as part of the CDC Emerging Infections Program. Due to decreased burden of iMRSA in recent years, particularly hospital onset (HO) cases, we assessed the clinical and molecular epidemiology of iMSSA.


    Between 8/1/2014 and 1/31/2015, cases of SA isolated from a normally sterile site were reported from 2 counties, case isolates submitted, and medical records reviewed. Isolate identification was confirmed, and broth microdilution susceptibility testing and pulsed-field gel electrophoresis characterization were performed. Cases were defined epidemiologically as HO (culture obtained ≥4 days after admission); healthcare associated, community onset (HACO, cases have ≥1 healthcare-associated risk factor); and community associated (CA). 


    There were 387 cases of iSA (MRSA: 34.9%, MSSA: 65.1%). The % HACO and CA differed by resistance status (%HACO: MRSA: 69.6%, MSSA: 56.4%; p=0.01; %CA: MRSA: 22.2%, MSSA: 36.5%, p=0.01). The 90-day case fatality ratio (CFR) was 17.8% (CFR as inpatient: 9.3%, post-discharge CFR within 90 days post culture: 8.5%). Neither CFR nor median length of stay (MRSA: 8, MSSA: 7 days) differed by resistance status. Nominal differences in infection types were seen for pneumonia (MRSA: 14.8%, MSSA: 8.7%, p=0.07) and septic arthritis (MRSA: 15.6%, MSSA: 23.8%, p=0.06). Of 341 case isolates (MRSA: 36.4%, MSSA: 63.6%) characterized, differences in USA groups were found (Table). iMRSA case isolates were less likely than iMSSA to be susceptible to clindamycin (MRSA: 50.8%, MSSA: 93.1%, p<0.01), erythromycin (MRSA: 14.5%, MSSA: 75.1%, p<0.01) and levofloxacin (MRSA: 29.0%, MSSA: 91.7%, p<0.01).


    iMSSA accounted for most iSA in this surveillance; mortality is high and similar to iMRSA. Most iSA was HACO, although CA iSA was more commonly MSSA than MRSA. iMRSA isolates were more likely to be resistant to other antibiotics and in clonal groups USA100 or 300, whereas iMSSA isolates were more clonally diverse. Additional insight into iMSSA risk factors would be useful to better inform prevention and control strategies for iSA. 


    USA group

    % of MRSA

    % of MSSA














    Undefined (multiple unrelated pulsed-field patterns not part of a USA group)




    Mackenzie Koeck, MPH1, Kathryn Como-Sabetti, MPH1, Anita Glennen, MT(ASCP)2, David Boxrud, MS3, Ginette Dobbins, BS3, Isaac See, MD4 and Ruth Lynfield, MD, FIDSA1, (1)Minnesota Department of Health, St. Paul, MN, (2)Microbiology, Minnesota Department of Health, St Paul, MN, (3)Public Health Laboratory, Minnesota Department of Health, St. Paul, MN, (4)Division of Healthcare Quality Promotion, Centers for Disease Control and Prevention, Atlanta, GA


    M. Koeck, None

    K. Como-Sabetti, None

    A. Glennen, None

    D. Boxrud, None

    G. Dobbins, None

    I. See, None

    R. Lynfield, None

    Findings in the abstracts are embargoed until 12:01 a.m. PDT, Wednesday Oct. 7th with the exception of research findings presented at the IDWeek press conferences.