Antibacterial activity of colistin is resurrected by Stephania suberosa Forman extract against colistin-resistant Enterobacter cloacae

Stephania suberosa Forman extract synergistically inhibits ampicillin- and vancomycin-resistant Enterococcus faecium

Increasing antibiotic resistance in enterococci is among the most serious public health problems worldwide. The new naturally occurring antibacterial agents were explored. This study, therefore, investigated the antibacterial potential of Stephania suberosa extract (SSE) and its synergism with ampicillin (AMP) or vancomycin (VAN) against AMP- and VAN-resistant Enterococcus faecium. Disc diffusion assay revealed that SSE inhibited E. faecium DMST 12829, 12852, 12970, and a reference strain of Enterococcus faecalis ATCC 29,212 in a dose-dependent manner. The minimum inhibitory concentration (MIC) of SSE against all E. faecium isolates was 0.5 mg/mL. E. faecium DMST 12,829 and 12,852 were highly resistant to AMP, as indicated by high MIC values, and E. faecium DMST 12,829 and 12,970 were resistant to VAN. Enterococcus spp. were killed by SSE at the minimum bactericidal concentrations (MBCs) ranging from 0.5 to 4 mg/mL. Checkerboard determination showed that SSE plus AMP and SSE plus VAN combinations exhibited synergistic interaction against E. faecium isolates. The killing curve assay of E. faecium isolates confirmed the antibacterial and synergistic activities of combined agents by dramatically reducing the viable counts compared to a single agent. Scanning electron microscope elucidated the cell damage and abnormal cell division. Enterococcal proteases were also inhibited by SSE. These findings support that SSE could reverse the activity of AMP and VAN. Moreover, it can synergistically inhibit AMP- and VAN-resistant E. faecium. Our combined agents could be attractive candidates for developing new combinatorial agents to resurrect the efficacy of antibiotics for treating AMP- and VAN-resistant E. faecium infections.

Antimicrobial and Antibiofilm Activities of Citral Against Carbapenem-Resistant Enterobacter cloacae

Citral that is produced in the essential oils of plants is an isomeric mixture of geranial and neral. Few recent studies exhibited robust antibacterial activity of citral against several pathogens. However, little is reported about effects of citral on carbapenem-resistant Enterobacter cloacae (CREC). The purpose of this study was to assess antibacterial and antibiofilm activities of citral against CREC. Here, minimum inhibitory concentrations (MICs) of citral against CREC were determined by the agar dilution method. Antibacterial mode of citral against CREC was elucidated by evaluating changes in intracellular adenosine triphosphate (ATP) concentration, intracellular pH (pH_in), membrane potential, membrane integrity, and cell morphology. In addition, CREC cell damage within biofilms was examined by confocal laser scanning microscopy (CLSM). The results showed that the MIC of citral against CREC was 1000 μg/mL. Citral inhibited CREC growth and destroyed its cell membrane integrity, as measured by the decrease of intracellular ATP, pH, and membrane potential as well as distinctive deformation in cellular morphology. CLSM images demonstrated that citral could inactivate CREC cells within biofilms. These results revealed that citral exhibits potent antibacterial and antibiofilm activity against CREC, and thus has potential to be used as natural food preservatives to control CREC-associated infection spread.

In Vitro Antibacterial Activity and Mechanism of Vanillic Acid against Carbapenem-Resistant Enterobacter cloacae

Vanillic acid (VA) is a flavoring agent found in edible plants and fruits. Few recent studies exhibited robust antibacterial activity of VA against several pathogen microorganisms. However, little was reported about the effect of VA on carbapenem-resistant Enterobacter cloacae (CREC). The purpose of the current study was to assess in vitro antimicrobial and antibiofilm activities of VA against CREC. Here, minimum inhibitory concentrations (MIC) of VA against CREC was determined via gradient diffusion method. Furthermore, the antibacterial mode of VA against CREC was elucidated by measuring changes in intracellular adenosine triphosphate (ATP) concentration, intracellular pH (pH_in), cell membrane potential and membrane integrity. In addition, antibiofilm formation of VA was measured by crystal violet assay and visualized with field emission scanning electron microscopy (FESEM) and confocal laser scanning microscopy (CLSM). The results showed that MIC of VA against E. cloacae was 600 μg/mL. VA was capable of inhibiting the growth of CREC and destroying the cell membrane integrity of CREC, as confirmed by the decrease of intracellular ATP concentration, pH_in and membrane potential as well as distinctive variation in cellular morphology. Moreover, crystal violet staining, FESEM and CLSM results indicated that VA displayed robust inhibitory effects on biofilm formation of CREC and inactivated biofilm-related CREC cells. These findings revealed that VA exhibits potent antibacterial activity against CREC, and thus has potential to be exploited as a natural preservative to control the CREC associated infections.