In vitro antibacterial screening of six proline-based cyclic dipeptides in combination with β-lactam antibiotics against medically important bacteria.

The in vitro synergistic antibacterial activity of six proline-based cyclic dipeptides [cyclo(D-Pro-L-Leu), cyclo(L-Pro-L-Met), cyclo(D-Pro-L-Phe), cyclo(L-Pro-L-Phe), cyclo(L-Pro-L-Tyr), and cyclo(L-Pro-D-Tyr)] in combination imipenem and ceftazidime was investigated in the present manuscript. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the cyclic dipeptides were compared with those of the standard antibiotics (imipenem and ceftazidime). The synergistic antibacterial activities of cyclic dipeptides with imipenem or ceftazidime were assessed using the checkerboard and time-kill methods. The results of the present study showed that the combined effect of six cyclic dipeptides with imipenem predominantly recorded synergistic interaction (FIC index <0.5), whereas combination of certain cyclic dipeptides with ceftazidime recorded additive. The time-kill curve showed that the growth of the test bacteria was completely attenuated after 12-24 h of treatment with a 50:50 ratio of proline-based cyclic dipeptides and antibiotics. These synergistic effects have a potential role in delaying the development of resistance as the antibacterial activity is achieved with the very low concentrations of cyclic dipeptides and antibiotics. The cytotoxicity of cyclic dipeptides was tested against VERO cell line (African green monkey kidney cell line), and no cytotoxicity was recorded for cyclic dipeptides up to 100 μg/mL. These findings suggest that combination of cyclic dipeptides and antibiotics might be a good strategy for the individualization of novel templates for the development of new antimicrobial agents or combinations of drugs for antimicrobial chemotherapy. Moreover, these combinations may lead to the development of a new and vital antimicrobial combination against the infections caused by pathogenic bacteria. The in vitro synergistic activity of cyclic dipeptides with antibiotics against medically important bacteria is reported here for the first time.