Gram-negative bacteria are resistant to many hydrophobic antibiotics (such as penicillin G) owing to the highly hydrophilic saccharide part of lipopolysaccharide in the cell membrane, whilst most hydrophilic antibiotics (such as ampicillin) are more freely diffused into cells through aqueous porins. In this study, we investigated the possibility of altering the membrane permeability of Escherichia coli with tricyclic cationic compounds, such as the non-antibiotic drug promethazine. We also established the activity of these compounds as modifiers of antibiotic resistance in bacteria by a fluoro-luminometric approach. According to the results, promethazine has no bacteriostatic effect on E. coli at concentrations <64 microg/mL. However, promethazine at these concentrations in combination with penicillin G produced a significant synergistic activity against E. coli. Specifically, a constant promethazine concentration of 32 microg/mL in combination with penicillin G concentrations of 16-128 microg/mL suppressed the growth and viability of bacteria and converted penicillin G-resistant cells to being susceptible to this antibiotic with a minimum inhibitory concentration of 128 microg/mL. In contrast to penicillin G, the efficacy of ampicillin was apparently not increased in the presence of promethazine, suggesting that promethazine directly affects the membrane permeability of bacteria or alternatively inhibits the function of efflux pumps. In conclusion, we conclude that exposing E. coli cells to a given antibiotic in combination with promethazine can increase the susceptibility of bacteria; this effect is reliably assessed on a real-time basis using kinetic fluoro-luminometric measurements.