Synergy Between Histone-Derived Antimicrobial Peptides And Conventional Antibiotics

The rapid emergence of multidrug-resistant bacteria in recent years has been a significant medical concern, rendering conventional antibiotics ineffective and necessitating the development of new drugs and therapeutic strategies. This study investigated the potential synergistic effects of a series of histone-derived antimicrobial peptides (HDAPs) and antibiotics on selected gram-positive and gram-negative bacterial strains: Escherichia coli, Acinetobacter baylyi, Enterobacter aerogenes, Staphylococcus epidermidis, Staphylococcus aureus, and Bacillus subtilis. Two membrane-translocating HDAPs, Buforin II (BFII) and DesHDAP1, and two membrane-permeabilizing HDAPs, DesHDAP2 and DesHDAP3A, were examined along with several conventional antibiotics that span a range of activities: 1) agents that inhibit the synthesis of the bacterial cell wall (ampicillin, cephalexin); 2) those that function through inhibiting nucleic acid replication (levofloxacin, rifampicin) and 3) those that function by inhibiting translation (kanamycin, tetracycline). We chose these antimicrobial agents to represent different mechanisms of action in order to assess the effects of these differences on synergy between agents. After determining the relative antimicrobial efficacies and minimum inhibitory concentrations (MICs) of each agent using microdilution and radial diffusion assays respectively, checkerboard assays were designed and performed to determine their combinatory effects in vitro. Time-dependent growth curves were also collected for each strain under each combination. When combined at subinhibitory concentrations, membrane-translocating HDAPs and cell wall-inhibiting antibiotics were generally synergistic. Membrane-translocating HDAPs exhibited mixed results of synergy and additive killing effects with cell wall-inhibiting antibiotics. Interestingly, antagonistic effects were observed for most cell translation-inhibiting antibiotics with all peptides, particularly for combinations involving kanamycin. with these particular results, further investigation and additional studies with a broader range of combinations should be conducted to better understand these patterns in different AMP-antibiotic combinations and identify synergy in specific combinations.