In vitro activity of sitafloxacin (STFX) and various oral antimicrobial agents against bacterial isolates recovered from clinical specimens between January and December 2009, at different healthcare facilities in Japan was evaluated. A total of 1,620 isolates including aerobic and anaerobic organisms was available for the susceptibility testing using the microbroth dilution methods recommended by Clinical Laboratory Standard Institute.

The minimum inhibitory concentration of STFX at which 90% of isolates (MIC₉₀) was 0.06𝜇g/mL for methicillin-susceptible Staphylococcus aureus and was equal to that of garenoxacin (GRNX), 2 times lower than that of moxifloxacin (MFLX), and 8 times lower than that of levofloxacin (LVFX). STFX inhibited the growth of all the isolates of Streptococcus pneumoniae at 0.06𝜇g/mL or less. The MIC₉₀s of STFX ranged from 0.03 to 0.06𝜇g/mL and were 1 to 2 times lower than those of GRNX, 2 to 4 times lower than those of MFLX, and 16 to 32 times lower than those of LVFX. Against Streptococcus pyogenes, the MIC₉₀ of STFX was 0.06𝜇g/mL and was 2 times lower than that of GRNX, 4 times lower than that of MFLX, and 32 times lower than that of LVFX. The MIC₉₀ of STFX was 0.25𝜇g/mL for Enterococcus faecalis, and was 2 times lower than those of GRNX and MFLX, and 8 times lower than that of LVFX. The MIC₉₀ of STFX for E. coli was 2𝜇g/mL, and the MIC₉₀s of other 10 species of Enterobacteriaceae which were the lowest values of the quinolones tested ranged from 0.03 to 1𝜇g/mL. The MIC₉₀ of STFX for Pseudomonas aeruginosa isolates recovered from urinary infections was 8𝜇g/mL and was 16 times lower than those of GRNX, MFLX and LVFX. The MIC₉₀ of STFX for P. aeruginosa isolates recovered from respiratory infections was 2𝜇g/mL and was 32 times lower than those of GRNX and MFLX, and 16 times lower than that of LVFX. STFX inhibited the growth of all the isolates of Haemophilus influenzae at 0.004𝜇g/mL or less, and was 2 to 4 times lower than those of GRNX, 8 times lower than those of MFLX, and 4 times lower than those of LVFX. The MIC₉₀ of STFX was 0.008𝜇g/mL for Moraxella catarrhalis, and was 2 times lower than that of GRNX, 8 times lower than those of MFLX and LVFX. The MIC₉₀s of STFX ranged from 0.015 to 0.12𝜇g/mL for all the species of anaerobic bacteria and were the lowest values of all the antimicrobial agents tested.

In conclusion, the activity of STFX against Gram-positive cocci was comparable or superior to those of GRNX, MFLX and LVFX. STFX showed the most potent activity against Gram-negative bacteria and anaerobic bacteria of all the antimicrobial agents tested in this study.

The activity of antibacterial agents against aerobic Gram-positive cocci (26 species, 1022 strains) and anaerobic bacteria (23 species, 184 strains) isolated from clinical specimens in 2006 at 16 clinical facilities in Japan were studied using either broth microdilution or agar dilution method. The ratio of methicillin-resistant strains among Staphylococcus aureus and Staphylococcus epidermidis was 53.0% and 65.8%, suggesting that resistant strains were isolated at high frequency. Vancomycin (VCM) and quinupristin/dalfopristin (QPR/DPR) had good antibacterial activity against methicillin-resistant S. aureus and methicillin-resistant S. epidermidis, with MIC₉₀s of ≤2𝜇g/mL. The ratio of penicillin (PC) intermediate and resistant strains classified by mutations of PC-binding proteins among Streptococcus pneumoniae was 87.6%. Ceftriaxone, cefpirome, cefepime, carbapenem antibiotics, VCM, teicoplanin, linezolid(LZD) and QPR/DPR had MIC₉₀s of ≤1𝜇g/mL against PC-intermediate and resistant S. pneumoniae strains. Against all strains of Enterococcus faecalis and Enterococcus faecium, the MICs of VCM and TEIC were under 2𝜇g/mL, and no resistant strain was detected, suggesting that these agents had excellent activities against these species. 10.9% of E. faecalis strains or 3.5% of E. faecium strains showed intermediate or resistant to LZD. 24.4% of E. faecium strains showed intermediate or resistant to QPR/DPR. Against all strains of Clostridium difficile, the MIC of VCM were under 1𝜇g/mL, suggesting that VCM had excellent activity against C. difficile. Carbapenems showed good activity against Peptococcaceae, Bacteroides spp., and Prevotella spp. However since several strains of Bacteroides fragilis showed resistant to carbapenems and the susceptibility of this species should be well-focused in the future.

We determined MICs of antibacterial agents against 1280 clinical strains of aerobic Gram-negative bacteria (19 genus or species) isolated at 16 Japanese facilities in 2006. MICs were determined using mostly broth microdilution method and antibacterial activity was assessed. Strains producing extended-spectrum 𝛽-lactamases (ESBL) accounted for 3.7% of Escherichia coli, 2.7% of Klebsiella spp., and 11.4% of Proteus spp. Notably, 18.8% of Proteus mirabilis was found to produce ESBL higher than 16.7% in 2004. This result was higher extremely than other species. Among Haemophilus influenzae, only 1.2% produced 𝛽-lactamase and 62.8% that increased compared with 57.7% in 2004, were 𝛽-lactamase-negative ampicillin-resistant strains when classified by penicillin-binding protein 3 mutation. Although few antibacterial agents against Pseudomonas aeruginosa have potent activity, only three agents—doripenem, ciprofloxacin, and tobramycin—showed an MIC₉₀ of 4µg/mL. Of all P. aeruginosa strains, 5.7% were resistant to six or more agents of nine antipseudomonal agents, a decrease compared to 8.7% in 2004. Against other glucose-non-fermentative Gram-negative bacteria, the activity of most antibacterial agents was similar to that in 2004.