Potent intracellular antibacterial activity of a marine peptide-N6NH and its D-enantiomer against multidrug-resistant Aeromonas veronii.

Aeromonas veronii can cause a variety of diseases such as sepsis in humans and animals. However, there has been no effective way to eradicate A. veronii. In this study, the intracellular antibacterial activities of the C-terminal aminated marine peptide N6 (N6NH) and its D-enantiomer (DN6NH) against A. veronii were investigated in macrophages and in mice, respectively. The result showed that DN6NH with the minimum inhibitory concentration (MIC) of 1.62 μM is more resistant to cathepsin B than N6NH (3.23 μM). The penetration percentages of the cells treated with 4-200 μg/mL fluorescein isothiocyanate (FITC)-DN6NH were 52.5-99.6%, higher than those of FITC-N6NH (27.0-99.1%). Both N6NH and DN6NH entered macrophages by macropinocytosis and an energy-dependent manner. DN6NH reduced intracellular A. veronii by 34.57%, superior to N6NH (19.52%). After treatment with 100 μg/mL DN6NH, the levels of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-1β were reduced by 53.45%, 58.54%, and 44.62%, respectively, lower than those of N6NH (15.65%, 12.88%, and 14.10%, respectively); DN6NH increased the IL-10 level (42.94%), higher than N6NH (7.67%). In the mice peritonitis model, 5 μmol/kg DN6NH reduced intracellular A. veronii colonization by 73.22%, which was superior to N6NH (32.45%) or ciprofloxacin (45.67%). This suggests that DN6NH may be used as the candidate for treating intracellular multidrug-resistant (MDR) A. veronii. KEY POINTS: • DN6NH improved intracellular antibacterial activity against MDR A. veronii. • DN6NH entered macrophages by micropinocytosis and enhanced the internalization rates. • DN6NH effectively protected the mice from infection with A. veronii.