The outer membrane of Gram-negative bacteria is one of the major factors contributing to the development of antibiotic resistance, resulting in a lack of effectiveness of several hydrophobic
antibiotics.
Plantaricin A (PlnA) intensifies the potency of
antibiotics by increasing the permeability of the bacterial outer membrane. Moreover, it has been proven to bind to the
lipopolysaccharide of Escherichia coli via electrostatic and hydrophobic interactions and to interfere with the integrity of the bacterial outer membrane. Based on this mechanism, we designed a series of PlnA1 analogs by changing the structure, hydrophobicity, and charge to enhance their membrane-permeabilizing ability. Subsequent analyses revealed that among the PlnA1 analogs, OP4 demonstrated the highest penetrating ability, weaker cytotoxicity, and a higher therapeutic index. In addition, it decelerated the development of antibiotic resistance when the E. coli cells were continuously exposed to sublethal concentrations of
erythromycin and
ciprofloxacin for 30 generations. Further in vivo studies in mice with
sepsis showed that OP4 heightens the potency of
erythromycin against E. coli and relieves
inflammation. In summary, our results showed that the PlnA1 analogs investigated in the present study, especially OP4, reduce the intrinsic antibiotic resistance of Gram-negative pathogens and expand the
antibiotic sensitivity spectrum of hydrophobic
antibiotics in Gram-negative bacteria. IMPORTANCE Antibiotic resistance is a global health concern due to indiscriminate use of
antibiotics, resistance transfer, and intrinsic resistance of certain Gram-negative bacteria. The asymmetric bacterial outer membrane prevents the entry of hydrophobic
antibiotics and renders them ineffective. Consequently, these
antibiotics could be employed to treat
infections caused by Gram-negative bacteria, after increasing their outer membrane permeability. As PlnA reportedly penetrates outer membranes, we designed a series of PlnA1 analogs and proved that OP4, one of these
antimicrobial peptides, effectively augmented the permeability of the bacterial outer membrane. Furthermore, OP4 effectively improved the potency of
erythromycin and alleviated inflammatory responses caused by
Escherichia coli infection. Likewise, OP4 curtailed antibiotic resistance development in E. coli, thereby prolonging exposure to sublethal
antibiotic concentrations. Thus, the combined use of hydrophobic
antibiotics and OP4 could be used to treat
infections caused by Gram-negative bacteria by decreasing their intrinsic antibiotic resistance.