Pathogenic bacteria including Escherichia coli and Salmonella sp. are the major causative agents of endometritis and can cause infertility in livestock animals. Antibiotics are commonly used to terminate bacterial infections, but the development of bacterial antibiotic resistance is often encountered. Nanotechnology associated with silver nanoparticles has been highlighted as an alternative anti-bacterial agent, and pegylated silver-coated single-walled carbon nanotubes have high anti-bacterial effects and are non-toxic to human and murine cells in vitro. Here we verified whether a real-time bioluminescence monitoring system could be an alternative tool to assess anti-bacterial effects of nanotubes in a noninvasive approach. Escherichia coli and Salmonella sp. were transfected with plasmids containing constructs for luciferase enzyme (LuxCDABE) and substrate (luciferin) to create self-illuminating bioluminescent bacteria. Pathogens were grown in LB broth at 37°C, adjusted to 107 cfu mL−1, and placed in 96-well plates for treatments. Pegylated (pSWCNTs-Ag) and non-pegylated (SWCNTs-Ag) nanotubes were prepared and added to culture wells at various concentrations (31.25–125 µg mL−1). The control group corresponded to bacteria without nanotubes (0 µg mL−1). Anti-bacterial effects of nanotubes were determined every 10 min until 1 h, then every 30 min up to 6 h incubation through optical density (600 nm) measurements and bioluminescence imaging (BLI) and quantification using an IVIS system. Optical density and BLI data were compared at each time-point using 2-way ANOVA, with P < 0.05 set for significance. Bioluminescence signals emitted by both bacteria stains appeared within 10 min of incubation. Thereafter, control bacteria showed exponential growth that was detected as early as 25 min post-incubation. Bioluminescence imaging revealed dose-dependent anti-bacterial activities of both pSWCNTs-Ag and SWCNTs-Ag on each E. coli and Salmonella sp. (P < 0.05). Contrary to BLI, the OD values did not always reflect bacteria concentrations, and varied according to nanotube concentrations. No significant differences in anti-bacterial activities were revealed between pSWCNTs-Ag and SWCNTs-Ag based on OD values during 6 h of incubation (P > 0.05); meanwhile, pSWCNTs-Ag nanotubes exhibited stronger anti-bacterial effects than SWCNTs-Ag during the same period using BLI (P < 0.05). In summary, we confirmed previous reports showing dose-dependent eliminations of pathogenic bacteria by silver nanotubes. Pegylated nanotubes exhibited high anti-bacterial activity compared to non-pegylated nanotubes. Bioluminescence imaging system revealed superior resolution to enable precise investigation of anti-bacterial kinetics of silver nanotubes. This feature could be useful for the study of bacterial infections that impair livestock fertility.
Work was supported by USDA-ARS Biophotonics Initiative grant #58-6402-3-018.
Probing the toxicity mechanism of multiwalled carbon nanotubes on bacteria