Action of Kanamycin Against Single and Dual Species Biofilms of Escherichia coli and Staphylococcus aureus

Antibiotic resistance in one of the greatest threats to mankind, and prospects for the discovery of new antibiotic compounds are reduced. Due to their low-level of usage, aminoglycosides seem to have remained active against many pathogens and thus gained a renewed interest. In this work, we tested the effects of kanamycin against single and dual species biofilms of Escherichia coli and Staphylococcus aureus. The minimum inhibitory concentration (MIC), determined by the microdilution method, of E. coli and S. aureus were 4.5 and 3.5 mg/L, respectively. The MIC was 5.0 mg/L for the dual species scenario. Biofilms were formed in 96-wells microtiter plates and their viability and biomass was assessed by alamar blue and crystal violet staining, respectively. Although the viability of sessile bacteria was greatly reduced with kanamycin (65% and 67% for E. coli and S. aureus, respectively), their removal from surfaces was more difficult (< 35%). For dual species biofilms, the highest viability reduction obtained was 70% and the removal percentage was 15%. Kanamycin was more efficient on biofilm killing than on their removal. Total biofilm killing and removal was not achieved even for kanamycin at 10 × MIC and resistance was more pronounced for the dual species biofilms. This highlights the greatest resistance capacity of biofilms when compared with planktonic cells. The overall results suggest that improved therapeutic strategies, including the application of higher doses or the combination with synergistic products, should be applied in biofilm control, particularly in polymicrobial biofilms.