The use of antibiotics has greatly improved the treatment of bacterial infections; however, its abuse and misuse has led to a rapid rise in multidrug-resistant (MDR) bacteria. Therefore, the search for new antimicrobial strategies has become critical. Natural flavonoids, a class of widely existing phytochemicals, have gained significant research interest for their diverse biological activities and antibacterial effects on various drug-resistant bacteria. This review summarizes the latest research progress on flavonoids, with a particular focus on several flavonoids exhibiting certain antibacterial activity, and explores their antibacterial mechanisms, including disruption of cell membranes and cell walls, inhibition of proteins and nucleic acids, interference with signal transduction, suppression of efflux pump activity, and inhibition of biofilm formation and virulence factor production. Additionally, we have reviewed the synergistic combinations of flavonoids with antibiotics, such as the combination of quercetin with colistin or EGCG with tetracycline, which significantly enhance therapeutic efficacy.

Background: The natural hosts ofShigellaare typically humans and other primates, but it has been shown that the host range ofShigellahas expanded to many animals. AlthoughShigellais becoming a major threat to animals, there is limited information on the genetic background of local strains. The purpose of this study was to assess the presence of virulence factors and the molecular characteristics ofS. flexneriisolated from calves with diarrhea.Results: Fifty-fourS. flexneriisolates from Gansun, Shanxi, Qinghai, Xinjiang and Tibet obtained during 2014 to 2016 possessed four typical biochemical characteristics ofShigella. The prevalences ofipaH,virA,ipaBCD,ial,sen,set1A,set1Bandstxwere 100 %, 100 %, 77.78 %, 79.63 %, 48.15 %, 48.15 and 0 %, respectively. Multilocus variable number tandem repeat analysis (MLVA) based on 8 variable number of tandem repeat (VNTR) loci discriminated the isolates into 39 different MLVA types (MTs), pulsed field gel electrophoresis (PFGE) based onNotI digestion divided the 54 isolates into 31 PFGE types (PTs), and multilocus sequence typing (MLST) based on 15 housekeeping genes differentiated the isolates into 7 MLST sequence types (STs).Conclusions: The findings from this study enrich our knowledge of the molecular characteristics ofS. flexnericollected from calves with diarrhea, which will be important for addressing clinical and epidemiological issues regarding shigellosis.

Bacterial drug resistance is becoming an increasingly serious problem, and the development of antibacterial synergists is urgently needed. Combining existing antibiotics with promising nonantibiotic agents is one strategy that has been shown to be effective at overcoming the widespread emergence of antibiotic-resistant pathogens. In this study, we investigated the antibacterial activities and mechanism of naringenin (NG) combined with amikacin (AMK) against multidrug-resistant Escherichia coli (E. coli). We first measured the fractional inhibitory concentration (FIC) of NG combined with antibiotics via the checkerboard method. The results indicated that the combination of NG and AMK had a synergistic effect on E. coli ATCC 25922 and E. coli C7F3. In addition, this synergistic effect was verified by time-kill assays. Moreover, scanning electron microscopy (SEM) was used to observe cell morphology. The results showed that the cell wall of E. coli was destroyed. Furthermore, we assessed the leakage of alkaline phosphatase (AKP), K⁺, and protein. The extracellular AKP activity increased after the combinational group of 1/2MIC NG and 1/2MIC AMK, suggesting an impairment in cell wall permeability. An increase in the leakage of intracellular K⁺ and protein indicated an increase in cell inner membrane permeability. These results revealed that NG and AMK inhibited E. coli by damaging cell walls and membranes. In addition, PI uptake rapidly increased after treatment with NG and AMK. Confocal laser scanning microscopy (CLSM) revealed that NG caused cell wall and cell membrane damage in E. coli. In summary, our results provide a new strategy for responding to the development of E. coli drug resistance.

The widespread presence of antibiotics resistance genes in pathogenic bacteria can cause enormous problems. Food animals are one of the main reservoirs of intestinal pathogens that pose a potential risk to human. Analyzing the epidemiological characteristics and resistance patterns of Shigella flexneri in calves is necessary for animal and human health. A total of 54 Shigella flexneri isolates, including six serotypes (1a, 2a, 2b, 4a, 6 and Xv), were collected from 837 fecal samples obtained from 2014 to 2016. We performed pulsed-field gel electrophoresis (PFGE) and applied the restriction enzyme NotI to analyze the genetic relatedness among the 54 isolates and to categorize them into 31 reproducible and unique PFGE patterns. According to the results of antimicrobial susceptibility tests, all 26 Shigella flexneri 2a serotypes were resistant to cephalosporin and/or fluoroquinolones. The genes bla TEM-1 , bla OXA-1 , and bla CTX-M-14 were detected in 19 cephalosporin-resistant S. flexneri 2a isolates. Among 14 fluoroquinolone-resistant isolates, the aac(6′)-Ib-cr gene was largely present in each strain, followed by qnrS (5). Only one ciprofloxacin-resistant isolate harbored the qepA gene. Sequencing the quinolone resistance determining regions (QRDRs) of the fluoroquinolone-resistant isolates revealed two point mutations in gyrA (S83 L, D87N/Y) and a single point mutation in parC (S80I). Interestingly, two gyrA (D87N/Y) strains were resistant to ciprofloxacin. The current study enhances our knowledge of Shigella in cattle, although continual surveillance is necessary for the control of shigellosis. The high level of cephalosporin and/or fluoroquinolone resistance in Shigella warns us of a potential risk to human and animal health.

Natural plant-derived compounds with broad-spectrum antimicrobial activity have become an effective strategy against multidrug-resistant bacteria. The present study was designed to compare the antibacterial activity of six chlorogenic acid (CA) isomers extracted from stevia and investigated the underlying antibacterial mechanisms involved. The results indicated that isochlorogenic acid C (ICAC) exhibited the strongest antibacterial activity against the tested bacteria, especially E. coli, at a 2 mg/mL minimum inhibitory concentration (MIC) and 8 mg/mL minimum bactericidal concentration (MBC). At the MBC, ICAC inhibited 72.66% of the clinical multidrug-resistant strains. Scanning electron microscopy (SEM) revealed that ICAC induced considerable morphological alterations in E. coli ATCC25922 and C4E2. The significant increase in the activity of extracellular alkaline phosphatase (AKP) indicated that ICAC damages the permeability of the bacterial cell wall. Additionally, the intracellular membrane (IM) permeability and the content of lipopolysaccharide (LPS), a main component of the outer membrane (OM), were determined. The significant decrease in LPS content and increased leakage of intracellular proteins and K⁺ from E. coli indicated that ICAC could induce the exfoliation of OM and disrupt IM permeability, resulting in the loss of barrier function. The uptake of propidium iodide (PI), a compromised cell membrane nucleic acid stain, and confocal laser scanning microscopy (CLSM) further demonstrated that ICAC disrupted IM integrity. Moreover, the bactericidal effect and damage to bacterial microstructural function occurred in a dose-dependent manner. These data demonstrate that ICAC has excellent antibacterial activity and is a promising approach for overcoming the antibiotic resistance of pathogenic bacteria.

Rutin is a widely known plant secondary metabolite that exhibits multiple physiological functions. The present study focused on screening for synergistic antibacterial combinations containing rutin, and further explored the mechanisms behind this synergy. In vitro antibacterial test results of rutin showed that the ranges of minimum inhibitory concentration (MIC) and Minimum bactericidal concentration (MBC) are 0.125–1 and 0.125–2 mg/mL, respectively. However, rutin and amikacin have a significant synergistic effect, with a fractional inhibitory concentration index (FICI) range of 0.1875–0.5. The time bactericidal curve proved that the combination of rutin and amikacin inhibited bacterial growth within 8 h. Scanning electron microscopy (SEM) revealed that a low-dose combination treatment could disrupt the cell membrane of Escherichia coli (E. coli). A comprehensive analysis using alkaline phosphatase (AKP), K⁺, and a protein leakage assay revealed that co-treatment destroyed the cell membrane of E. coli, resulting in the significant leakage of AKP, intracellular K⁺, and proteins. Moreover, confocal laser scanning microscopy (CLSM) and red–green cell ratio analysis indicated severe damage to the E. coli cell membrane following the co-treatment of rutin and amikacin. This study indicates the remarkable potential of strategically selecting antibacterial agents with maximum synergistic effect, which could significantly control antibiotic resistance.