Colistin is being considered “the last ditch” treatment in many infections caused by multidrug-resistant GNB clinical isolates, but colistin-resistant (Col-R) strains with different drug resistance mechanisms have appeared worldwide. Hence, it is of great significance to rejuvenate sensitization of clinical Col-R Gram-negative bacteria to colistin.

Colistin is used as the “last line of defense” against multidrug-resistant (MDR) Gram-negative bacteria (GNB). However, improper use of colistin may further lead to an increasing number of colistin-resistant (Col-R) strains worldwide, which greatly limits antibiotic treatment options. In this study, we investigated the antibacterial and antibiofilm activities of naringenin (NG) combined with colistin against Col-R GNB in vitro and in vivo. The checkerboard method and time-kill test showed that NG combined with colistin has better antibacterial activity (FICI < 0.5) compared with NG and colistin alone. Biofilm formation inhibition tests demonstrated that combining the two drugs could inhibit biofilm formation; scanning electron microscopy (SEM) confirmed that the combination of the two significantly reduces the number of cells in the biofilm compared with the drug alone. The in vivo experiment showed that the combination of NG and colistin can improve the survival rate of the Galleria mellonella (G. mellonella) and reduce the microbial load in the mouse thigh infection model. Mechanistically, the combination of NG and colistin synergistically enhances the antibacterial activity and changes the permeability of the bacterial outer membrane. More importantly, cytotoxicity tests showed no cell cytotoxicity of NG in combination with colistin. In conclusion, our data revealed that NG combined with colistin exhibited good synergistic effects in vivo and in vitro, thus providing a new therapeutic option for clinical Col-R GNB infections.

Acinetobacter baumannii is an opportunistic pathogen that is primarily associated with nosocomial infections. With the rise in cases of acquired drug resistance, A. baumannii is gaining resistance to conventional antimicrobial drugs and even to the last line of antibiotics, such as colistin. Hence, the application of the synergistic combination of an antibiotic and a non-antibacterial agent is being contemplated as a new alternative therapeutic approach. Chrysin is a component of honey with anti-inflammatory and antioxidant properties. In this study, we evaluated the antibacterial activity of chrysin in combination with colistin against A. baumannii both in vitro and in vivo, as well as the cytotoxicity of chrysin with or without colistin. Our results revealed that chrysin and colistin exerted synergistic effects against A. baumannii by damaging the extracellular membrane and modifying the bacterial membrane potential. The chrysin/colistin combination group demonstrated an inhibitory effect on biofilm formation. In conclusion, it is expected that the synergy between these drugs can allow the use of a lower concentration of colistin for the treatment of A. baumannii infections, thereby reducing dose-dependent side effects. Thus, a combination therapy of chrysin/colistin may provide a new therapeutic option for controlling A. baumannii infections.

Background Pseudomonas aeruginosa (P. aeruginosa) has been majorly implicated in the infection of burns, wounds, skin, and respiratory tract. Colistin is considered the last line of defense against P. aeruginosa infections. However, colistin is becoming increasingly invalid in treating patients infected with colistin-resistant (COL-R) P. aeruginosa. As one of the disinfectants used for wound infections, acetic acid (AA) offers good antibacterial and antibiofilm activities against P. aeruginosa. This study investigated the effects of AA on COL-R P. aeruginosa in terms of its antibacterial, antibiofilm, and anti-virulence properties and the corresponding underlying mechanisms. Results The antimicrobial susceptibility and growth curve data revealed that 0.078% (v/v) AA exhibited good antibacterial activity against COL-R P. aeruginosa. Subinhibitory concentrations of AA were ineffective in inhibiting biofilm formation, but 4 × and 8 × of the minimum inhibitory concentration (MIC) was effective in removing the preformed biofilms in biofilm-eradication assays. The virulence results illustrated that AA inhibited COL-R P. aeruginosa swimming, swarming, twitching, and pyocyanin and elastase production. The analysis of the potential antibacterial mechanisms of AA on COL-R P. aeruginosa revealed that AA acted by increasing the outer and inner membrane permeability, polarizing the membrane potential, and decreasing the reduction potential in a concentration-dependent manner. The qRT-PCR results revealed that AA may inhibit the virulence of COL-R P. aeruginosa by inhibiting the expression of T3SS-related and QS-related genes. Conclusions AA possesses antibacterial, antibiofilm, and anti-virulence properties that ultimately lead to the alteration of the bacterial membrane permeability, membrane potential, and reduction potential. Our findings indicated that AA is presently one of the effective treatment options for infections. A high concentration of AA (> 0.156% v/v) can be used to sterilize biofilm-prone surgical instruments, for hospital disinfection, and for treating the external wound, whereas a low concentration of AA (0.00975–0.039% v/v) may be used as an anti-virulence agent for adjuvant treatment of COL-R P. aeruginosa, thereby further improving the application value of AA in the treatment of infections.