Abstract
The need for alternative strategies to fight bacteria is evident from the emergence of antimicrobial resistance. To that respect, photodynamic antimicrobial chemotherapy steadily rises in bacterial eradication by using light, a photosensitizer and oxygen, which generates reactive oxygen species that may kill bacteria. Herein, we report the encapsulation of 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyrin into acetylated lignin water-dispersible nanoparticles (THPP@AcLi), with characterization of those systems by standard spectroscopic and microscopic techniques. We observed that THPP@AcLi retained porphyrin’s photophysical/photochemical properties, including singlet oxygen generation and fluorescence. Besides, the nanoparticles demonstrated enhanced stability on storage and light bleaching. THPP@AcLi were evaluated as photosensitizers against two Gram-negative bacteria, Escherichia coli and Pseudomonas aeruginosa, and against three Gram-positive bacteria, Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecalis. THPP@AcLi were able to diminish Gram-positive bacterial survival to 0.1% when exposed to low white LED light doses (4.16 J/cm2), requiring concentrations below 5 μM. Nevertheless, the obtained nanoparticles were unable to diminish the survival of Gram-negative bacteria. Through transmission electron microscopy observations, we could demonstrate that nanoparticles did not penetrate inside the bacterial cell, exerting their destructive effect on the bacterial wall; also, a high affinity between acetylated lignin nanoparticles and bacteria was observed, leading to bacterial flocculation. Altogether, these findings allow to establish a photodynamic antimicrobial chemotherapy alternative that can be used effectively against Gram-positive topic infections using the widely available natural polymeric lignin as a drug carrier. Further research, aimed to inhibit the growth and survival of Gram-negative bacteria, is likely to enhance the wideness of acetylated lignin nanoparticle applications.
Highlights
Antimicrobial resistance (AMR) upraise is one of the greatest challenges that modern medicine and chemistry are facing
When these reactive oxygen species (ROS) are directed against microorganisms, the process is addressed as photodynamic antimicrobial chemotherapy (PACT) (Wainwright, 2019)
In photodynamic therapy (PDT) addressed against cancer, desired photosensitizers are molecules with strong absorption bands near the infrared range (700–900 nm), which coincides with the skin transparent wavelengths, and permit light to reach deeper through the skin (Josefsen and Boyle, 2012)
Summary
Antimicrobial resistance (AMR) upraise is one of the greatest challenges that modern medicine and chemistry are facing. AMR alone is expected to cause 10 million deaths by 2050, with an accumulative cost of 100 trillion USD (O’Neill, 2016) This prediction only accounts for developed countries, and this number is expected to be higher and still to be determined, with the greatest impact on developing countries. PDT is the conjugation of light and a photosensitizer molecule, generating reactive oxygen species (ROS) from either molecular oxygen in the media (Type II mechanism) or a substrate (Type I mechanism) (Josefsen and Boyle, 2012). When these ROS are directed against microorganisms, the process is addressed as photodynamic antimicrobial chemotherapy (PACT) (Wainwright, 2019). PACT applications are actively pursued by several research groups, with applications in dentistry as a complement of systemic antibiotic treatments (de Freitas et al, 2016; Bechara Andere et al, 2018), as a non-invasive treatment against Helicobacter pylori (Baccani et al, 2019), and even as an environment-friendly alternative for active food packaging (i.e., biodegradable coatings for strawberries disinfection), food disinfection (i.e., curcumin derivatives for lettuce and mung beans disinfection), and other agronomical applications (i.e., porphyrinic insecticides and pesticides) (Riou et al, 2014; Buchovec et al, 2016; Glueck et al, 2017; Martinez et al, 2017)
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