Nontypeable Haemophilus influenzae (NTHi), an important human respiratory pathogen, frequently causes biofilm
infections. Currently, resistance of bacteria within the biofilm to conventional antimicrobials poses a major obstacle to effective medical treatment on a global scale. Novel agents that are effective against NTHi biofilm are therefore urgently required. In this study, a series of natural and synthetic
chalcones with various chemical substituents were evaluated in vitro for their antibiofilm activities against strong biofilm-forming strains of NTHi. Of the test
chalcones, 3-hydroxychalcone (
chalcone 8) exhibited the most potent inhibitory activity, its mean minimum biofilm inhibitory concentration (MBIC50 ) being 16 μg/mL (71.35 μM), or approximately sixfold more active than the reference
drug,
azithromycin (MBIC50 419.68 μM). The inhibitory activity of
chalcone 8, which is a chemically modified
chalcone, appeared to be superior to those of the natural
chalcones tested. Significantly,
chalcone 8 inhibited biofilm formation by all studied NTHi strains, indicating that the antibiofilm activities of this compound occur across multiple strong-biofilm forming NTHi isolates of different clinical origins. According to antimicrobial and growth curve assays,
chalcone 8 at concentrations that decreased biofilm formation did not affect growth of NTHi, suggesting the biofilm inhibitory effect of
chalcone 8 is non-antimicrobial. In terms of structure-activity relationship, the possible substituent on the
chalcone backbone required for antibiofilm activity is discussed. These findings indicate that 3-hydroxychalcone (
chalcone 8) has powerful antibiofilm activity and suggest the potential application of
chalcone 8 as a new therapeutic agent for control of NTHi biofilm-associated
infections.