Antibiotic discovery has saved the lives of many people from most infectious diseases.[1] But antibiotic abuse has led to a dramatic increase in antimicrobial resistance (AMR), and treating intractable infections has become increasingly challenging, posing a serious threat to public health.[2] Bacterial infection was the second leading cause of human death in 2019, accounting for nearly 7,700,000 people deaths worldwide, and 4,500,000 deaths were associated with bacterial AMR. If antibiotic resistance is not curbed, this number could rise to 10 million and cumulative global economic losses could reach about $100 trillion annually by 2050.[3], [4] Epidemiological investigations showed that MRSA and VRE are the most common nosocomial infection worldwide.[5] People infected with MRSA were 64% more likely to die than other infections, with more than 100,000 attributable deaths annually.[6] VRE caused at least 5,400 deaths in the United States each year, and the Centers for Disease Control (CDC) ranked it as the second leading nosocomial infectious agent.[7] Vancomycin and ampicillin, as well as daptomycin and linezolid, which have been marketed in recent years, are clinically indicated for the treatment of MRSA and VRE infections, respectively, and they have low drug resistance.[8], [9], [10], [11], [12] The World Health Organization has listed bacterial resistance as one of the top 10 public health threats to humanity. How to overcome AMR has become a challenging scientific problem that deserves high attention and research.[13] The discovery of new antibiotics lags far behind the rapid evolution and spread of antibiotic resistance.[14] Therefore, the development of novel efficient and non-resistant antibacterial drugs is a vital and urgent need.[15]

Nowadays, natural products (NPs) derived from microorganisms, animals, and plants represent an important source of biologically active molecules against bacteria treatments.[16] Drug resistance might be caused by the co-evolution of antibiotics from microorganisms and pathogens, therefore, antibiotics derived from microorganisms may coexist with drug resistance.[17] However, plants have accumulated highly diverse secondary metabolites after a long period of co-evolution and developed mechanisms to survive in harsh environments co-existing with pathogenic microorganisms.[18] Flavonoids from plant kingdoms with different resources, structures, mechanisms and targets should not be easy to induce drug-resistant bacteria.[19] Natural flavonoids have demonstrated excellent activity against a range of clinically important pathogens and enhance the antimicrobial activity of existing antibiotics.[20] However, owing to the limited resources and structures of natural flavonoids to meet the growing demand for antibiotic resistance, their derivatives might contribute to the development of pharmacologically acceptable antimicrobials.[21]

The rational design of effective antibacterial agents is essential to address the resistance problem. Our research group has long been engaged in the study of NPs against drug-resistant bacteria and found many plant NPs were not easily resistant due to their unique structure.[[22], [23], [24], [25], [26], [27], [28], [29], [30]] Among them, natural flavonoids exhibited excellent potential against multidrug-resistant (MDR) bacteria activity.[29], [30] We investigated 162 natural flavonoids against both MRSA and VRE bioactivity, indicating the importance of the presence of a lipophilic group on the C-3 of flavonoids. Scheme 1 was designed and synthesized to obtain 42 lipophilic flavonoids. It has been shown that the lipophilic structure promotes penetration of the bacterial membrane, which requires charge neutralization for deep permeation.[31] The hydrophilic cation interacts with the electrostatic charge of the negatively charged bacterial membrane and has a stronger ability to disrupt the membrane.[21], [32] Therefore, Scheme 2 was designed by further adding cationic hydrophilic groups at C2' and C4' to obtain 21 amphiphilic flavonoid derivatives. Fig. 1 demonstrates the strategy for the synthesis of amphiphilic flavonoid derivatives. The greater detail evaluation of flavonoid derivatives in vitro and in vivo on MRSA and VRE.

In short, structure-activity relationship (SAR) analysis of natural flavonoids and their derivatives suggested that both the chain lipophilic group at C-3 and cationic hydrophilia function group were important to the antibacterial bioactivity. We preliminarily investigated the antibacterial mechanism of biofilm and cell membrane against MRSA and VRE to avoid resistance development.