PCPDTBT Conjugated Polymer Nanoparticles for Photothermal Inactivation of Multidrug-Resistant mcr‑1-Positive Escherichia coli

Abstract

Antimicrobial resistance (AMR) poses a global health challenge, threatening the effectiveness of the current treatment of bacterial infections. The emergence of plasmid-mediated resistance, notably the mcr-1 gene in Escherichia coli (E. coli), further complicates therapeutic options by conferring resistance to colistin, a last-resort antibiotic. This study explores the potential of conjugated polymer nanoparticles made of poly-[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT NPs) for antimicrobial photothermal therapy (PPT) against multidrug-resistant, mcr-1-positive E. coli strains. PCPDTBT NPs were synthesized through nanoprecipitation and characterized for their photothermal response upon near-infrared (NIR) laser irradiation at 806 nm (1.13 W). The nanoparticles exhibited efficient absorption in the NIR range and generated substantial photothermal heating sufficient for bacterial inactivation. PCPDTBT NPs maintained their photothermal performance without degradation across multiple irradiation cycles. Photoinactivation assays confirmed PCPDTBT NPs' (17 mg L^-1) ability to significantly reduce bacterial viability, particularly against mcr-1-positive E. coli. Scanning electron microscopy (SEM) images confirmed pronounced damage to bacterial cells following photothermal treatment. Overall, PCPDTBT NPs are highly promising as standalone agents for PPT against antibiotic-resistant pathogens, indicating their potential for future therapeutic strategies.

1

(a) Schematic representation of the nanoprecipitation process; (b) scanning electron microscopy (SEM) image of PCPDTBT nanoparticles; (c) hydrodynamic diameter distribution of PCPDTBT NPs; (d) particle diameter distribution determined from SEM image analysis.

2

(a) Experimental setup for photothermal analysis; (b) absorption spectrum of PCPDTBT NPs (17.0 mg L^–1). The arrow indicates the absorbance peak at 806 nm; (c) temperature as a function of time upon near-infrared irradiation (806 nm) for PCPDTBT nanoparticles (17.0 mg L^–1) dispersed in aqueous solution and aqueous solution with Tween 20.

3

(a) Temperature of PCPDTBT NP solutions (17.0 mg L^–1) as a function of time under 806 nm irradiation at laser powers (0.22, 0.52, 0.81, and 1.13 W); (b) solution temperature after 60 min of irradiation as a function of PCPDTBT NP concentration at 1.13 W (data from (a)); (c) temperature as a function of time of PCPDTBT NPs at different concentrations (0, 4.25, 8.50, and 17.0 mg L^–1), irradiated at 806 nm irradiation at 1.13 W; (d) temperature of solutions after 60 min of irradiation extracted from data presented in (c); (e) cyclic photothermal heating of PCPDTBT NP solution (17.0 mg L^–1) irradiated at 806 nm with 1.13 W over eight heating–cooling cycles.

4

(a) Growth of E. coli (ATCC 25922) and (b) mcr-1 positive E. coli (CCBH 23595) colonies in Petri dishes. Mean colony-forming unit values (CFU mL^–1) for (c) E. coli (ATCC) and (d) mcr-1 positive E. coli (CCBH 23593), as a function of 806 nm irradiation time (1.13 W), when exposed to PCPDTBT NPs (17.0 mg L^–1). SEM images of E. coli exposed to PCPDTBT NPs (17.0 mg L^–1): (e) nonirradiated and (f) after infrared irradiation (806 nm, 1.13 W) for 30 min. (*) indicates statistically significant difference at a 95% confidence level (p < 0.05) and (#) indicates the absence of bacterial colonies.