Reduced ability to detect surface-related biofi lm bacteria after antibiotic exposure under in vitro conditions

Background and purpose — Antibiotic treatment of patients before specimen collection reduces the ability to detect organisms by culture. We investigated the suppressive effect of antibiotics on the growth of non-adherent, planktonic, and surface-related bio- fi lm bacteria in vitro by using sonication and microcalorimetry methods. Patients and methods — Biofi lms of Staphylococcus aureus, S. epidermidis, Escherichia coli, and Propionibacterium acnes were formed on porous glass beads and exposed for 24 h to antibiotic concentrations from 1 to 1,024 times the minimal inhibitory con- centration (MIC) of vancomycin, daptomycin, rifampin, fl ucloxa- cillin, or ciprofl oxacin. The beads were then sonicated to dislodge biofi lm, followed by culture and measurement of growth-related heat fl ow by microcalorimetry of the resulting sonication fl uid. Results — Vancomycin did not inhibit the heat fl ow of staphy- lococci and P. acnes at concentrations ≤ 1,024 μg/mL, whereas fl u- cloxacillin at > 128 μg/mL inhibited S. aureus. Daptomycin inhib- ited heat fl ow of S. aureus, S. epidermidis, and P. acnes at lower concentrations (32–128 times MIC, p < 0.001). Rifampin showed inconsistent results in staphylococci due to random emergence of resistance, which was observed at concentrations ≤ 1,024 times MIC (i.e. 8 μg/mL). Ciprofl oxacin inhibited heat fl ow of E. coli at ≥ 4 times MIC (i.e. ≥ 0.06 μg/mL). Interpretation — Whereas time-dependent antibiotics (i.e. vancomycin and fl ucloxacillin) showed only weak growth sup- pression, concentration-dependent drugs (i.e. daptomycin and ciprofl oxacin) had a strong suppressive effect on bacterial growth and reduced the ability to detect planktonic and biofi lm bacte- ria. Exposure to rifampin rapidly caused emergence of resistance. Our fi ndings indicate that preoperative administration of antibi- otics may have heterogeneous effects on the ability to detect bio- fi lm bacteria.