Enhanced antibacterial effect of blue light in combination with an Amazonian tree sap (<i>Croton lechleri</i>)

In the United States, 8.2 million patients suffer from non-healing wounds which are often infected with antibiotic-resistant bacteria. Blue light (BL) and Sangre de Drago (Croton lechleri, SD) have potent mechanisms of antibacterial action through free radical formation and anti-biofilm effect, respectively. The aim of this pilot study was to evaluate the enhanced antibacterial effect of a novel combination treatment consisting of blue light and Sangre de Drago. Preliminary dosimetry measurements for effective SD concentration (5%) and 415-nm blue LED light fluence (125.3 J/cm² with a standard variation of 5 mW) were performed. E. coli K-12 (volume 0.1-mL, concentration 2 × 10⁵CFU/mL) was applied to each of 32 tryptic soy agar (TSA) plates. Inoculated TSA plates were separated into four groups: (1) no treatment (Control), (2) treatment with SD only, (3) treatment with blue light (BL) only, and (4) treatment with both SD and BL. Plates were incubated for 12 h at 37°C. Colony-forming units (CFUs) were analyzed using Image J software and count, size and overal TSA plate coverage were quantified. The median CFU count was highest in the Control group (157.9, interquartile range [IQR]: 112.0-157.9), followed by SD-only (60.5, IQR: 51.6-93.6), BL-only (33.7, IQR: 23.6-45.2), while no bacterial growth was observed in the combination treatment group (0, IQR: 0-0). The median CFU size was largest for control (0.44 mm², IQR: 0.35-0.59 mm²), followed by BL-only (0.28 mm², IQR: 0.19-0.43 mm²) and SD-only (0.16 mm², IQR: 0.11-0.23 mm²). BL-only caused a marked reduction in total CFU count, while the median CFU size was only moderately decreased compared to Control. The significant reduction in CFU count may be due to the bactericidal action of BL on bacteria. Conversely, SD-only caused just a moderate decrease in CFU count but had the largest decrease in median CFU size, indicating a possible strong bacteriostatic mechanism of action by SD. The combination of BL and SD resulted in no bacterial growth. The Bliss independence model demonstrated a Bliss synergy value of 0.04 indicating low synergy between the two treatments, even though its presence was significant (p = 0.001). This initial investigation on the combination treatment using 5% SD and 415-nm BL demonstrates synergy resulting in an enhanced antibacterial effect compared to each treatment alone. Further investigation and validation of these results is required. If validated, this novel combination approach may be translated to clinical practice to help treat chronic wounds infected with antimicrobial-resistant bacteria, using non-traditional antimicrobial agents that bypass the most common bacterial mechanisms of antibiotic resistance.