Characteristics of Community Staphylococcus aureus Isolates from Patients with Respiratory Illness – Winter 2003-09

Background: Staphylococcus aureus (SA) is a cause of community-acquired pneumonia (CAP), often associated with winter respiratory viral illness. SA infections have been associated with necrotizing pneumonia and may result in significant morbidity and mortality. Emergence of methicillin-resistant SA (MRSA) CAP led to changes in empiric CAP treatment recommendations; however, limited isolate data were available to inform these recommendations.

Objective: We examined a convenience sample of SA isolates recovered from patients with CAP and respiratory illness for strain characteristics, presence of toxin genes, and antimicrobial susceptibility.

Methods: 87 SA isolates from 86 patients with respiratory illness collected from October through April 2003-09 were characterized by pulsed-field gel electrophoresis (PFGE), PCR for Panton Valentine Leukocidin (PVL) and toxic shock syndrome toxin (TSST) genes, and antimicrobial susceptibility testing to penicillin (PN), erythromycin (EY), clindamycin (CC), chloramphenicol (CH), daptomycin, doxycycline, gentamicin, linezolid, levofloxacin (LV), rifampin, tetracycline (TE), trimethoprim/sulfamethoxazole and vancomycin.  Nearly half (47%) of all isolates were recovered from sterile sites.

Results: Case-patients ranged in age from 7mo to 73y (median 15y), were predominantly male (64%) and 57 (66%) case-patients died. Most isolates (66; 78%), were MRSA.  MRSA isolates exhibited limited strain diversity [Simpson’s Diversity Index (DI) = 0.25] and were typed by PFGE as USA300 (57; 86%), followed by USA400 (5%), USA100 (3%), and USA600, USA800, Group D, or Novel Type (1.5% each).  Methicillin-susceptible (MSSA) isolates (19; 22%) demonstrated more diversity (DI=0.86), and included USA200 (37%), USA300 (21%), USA500, USA600 and USA800 (10.5% each) and USA900, USA1100, Group D or EMRSA15 (5% each). Most MRSA were PVL+ (60; 91%) including 56 USA300 (98%) and all USA400 isolates. One MRSA isolate (USA800) was TSST+. MSSA isolates were occasionally PVL+ (8; 42%), including all USA300. TSST was identified in 7 (37%) MSSA isolates, all USA200. Of all SA isolates, only 9 (10%) had neither toxin gene present. Of MRSA isolates 55 (83%) were non-susceptible (NS) to EY, 21 (33%) were NS to LV and 9 (14%) were NS to CC. Most MSSA isolates (17; 81%) were NS to PN; 8 (38%) were NS to EY, 4 (19%) to CC, 4 (19%) to LV, and 3 (14%) to CH. All SA isolates were susceptible to vancomycin and linezolid.

Conclusions: The majority of SA isolates from community respiratory infections were USA300 MRSA. PVL, often associated with community SA isolates, was common, particularly among USA300 isolates. All isolates were susceptible to multiple antimicrobial agents, including vancomycin and linezolid, the agents recommended for treatment of severe CAP when MRSA is a consideration.