In vitro interference of β-lactams with biofilm development by prevalent community respiratory tract isolates

Interference of cefditoren (CDN) and amoxicillin/clavulanic acid (AMC) with biofilm production was studied using 11 Streptococcus pneumoniae isolates with minimum inhibitory concentrations (MICs) ranging from 0.015 μg/mL to 0.5 μg/mL for CDN and from 0.06 μg/mL to 2 μg/mL for AMC (except for one isolate with an AMC MIC of 8 μg/mL) and 5 Haemophilus influenzae isolates with MICs of 0.03–0.06 μg/mL for CDN and 0.5–16 μg/mL for AMC. Slime production was assessed in antibiotic-free medium and with 0.03 μg/mL CDN or 1/0.5 μg/mL AMC by measuring the optical density at 450 nm (OD450). Significantly lower mean OD450 values were obtained for S. pneumoniae with antibiotics compared with controls (CDN, 0.088 vs. 0.118, P = 0.003; and AMC, 0.095 vs. 0.112, P = 0.003), with significant correlation between both antibiotics (r = 0.752; P = 0.008). Percent reduction in OD450 values was higher for CDN compared with AMC (24.02% vs. 15.92%; P = 0.008). For H. influenzae, significantly lower mean OD450 values were obtained with CDN compared with controls (0.083 vs. 0.096; P = 0.043) but not with AMC (0.086 vs. 0.095; P = 0.08). Comparing percent reductions in S. pneumoniae versus H. influenzae for each antibiotic, no differences were found for AMC (15.92% vs. 9.40%; P = 0.36), with a tendency for CDN (24.02% vs. 13.79%; P = 0.069). Different β-lactams may have different capabilities of interfering with S. pneumoniae biofilm development when tested under the same experimental conditions.