A total of 4829 isolates were collected from 87 sites in 40 US states. Isolates included S. aureus (n = 1695), CoNS (n = 1475, including S. epidermidis [n = 1119]), S. pneumoniae (n = 474), H. influenzae (n = 586), and P. aeruginosa (n = 599). Of the isolates collected, 1886 (39.1%) originated from 32 sites in the Midwest, 1167 (24.2%) from 14 sites in the West, 1143 (23.7%) from 20 sites in the Northeast, and 633 (13.1%) from 21 sites in the South (Fig. 1). In vitro MIC90s and resistance profiles by geography are presented in Tables 1, 2, and 3.
Compared with other antibiotics, S. aureus and CoNS isolates, especially the respective MR subsets, showed notable in vitro resistance to azithromycin and the fluoroquinolones (Tables 1 and 2). Among S. pneumoniae isolates, resistance was observed for azithromycin and penicillin, whereas resistance was low overall among P. aeruginosa isolates and negligible among H. influenzae isolates. Of the fluoroquinolones tested, besifloxacin, a chlorofluoroquinolone for which susceptibility breakpoints are not available, had the lowest MIC90 against staphylococcal (including MR isolates) and streptococcal isolates. Newer fluoroquinolones (besifloxacin, moxifloxacin, and gatifloxacin) generally had lower MIC90s compared with older fluoroquinolones (ofloxacin, ciprofloxacin, and levofloxacin). Ciprofloxacin had the lowest MIC90 against P. aeruginosa and, along with gatifloxacin, the lowest MIC90 against H. influenzae.
Among S. aureus and CoNS, 621 and 717 isolates were MR (MRSA and MRCoNS), whereas 1074 and 758 isolates were MS (MSSA and MSCoNS), respectively. Resistance to methicillin varied by geographic region among both S. aureus and CoNS isolates (p ≤ 0.006; Fig. 2). Among S. aureus isolates, the proportions of MRSA isolates were 48.5, 40.1%, 36.0%, and 24.4% in the South, Midwest, Northeast, and West, respectively, with pairwise differences observed between the South and Northeast and between the West and all other regions (Fig. 2A). The proportions of MRCoNS isolates were 53.8% in the Midwest, 51.1% in the South, 44.3% in the Northeast, and 44.1% in the West, with significant pairwise differences found between the Midwest and both the Northeast and West (Fig. 2B).
Analysis of the overall mean percentage of resistance showed variations based on the geographic region for S. aureus (p < 0.001), S. pneumoniae (p < 0.001), and P. aeruginosa (p = 0.005), despite low overall resistance for P. aeruginosa (Fig. 3). Among S. aureus isolates, mean [standard error (SE)] percentage of resistance was highest in the South [28.1% (1.5%)] and lowest in the West [16.8% (1.1%); Fig. 3A]. Among S. pneumoniae isolates, mean (SE) percentage of resistance was 14.5% (1.0%), 11.9% (1.8%), 9.9% (1.4%), and 7.6% (1.3%) in the Midwest, South, Northeast, and West, respectively, with pairwise differences observed between the Midwest and both the Northeast and West (Fig. 3B). For P. aeruginosa isolates, the mean (SE) percentage of resistance was 8.5% (1.1%), 5.4% (1.3%), 3.6% (1.6%), and 2.9% (1.4%) in the Midwest, Northeast, South, and West, with pairwise differences observed between the Midwest and both the South and West (Fig. 3C). No regional differences in overall mean resistance rates were observed among CoNS (Fig. 3D) or H. influenzae isolates (both p > 0.05; Fig. 3E).