Upconversion nanoparticles conjugated with curcumin as a photosensitizer to inhibit methicillin-resistant Staphylococcus aureus in lung under near infrared light

Abstract

Curcumin has phototoxic effects on bacteria under <450 nm irradiation, but it is unstable in vivo and cannot exert effects on deep tissues. Near infrared light (NIR) is harmless to the body and has stronger penetration than visible light. In order to improve the effects of curcumin, upconversion nanoparticles conjugated with curcumin (UCNPs-curcumin) are designed to upconvert NIR to the excitation wavelength of curcumin. UCNPs-curcumin were synthesized using polyethyleneimine to combine curcumin and UCNPs, based on typical composition of lanthanide nitrates Re(NO)3 (Y:Yb:Er=78%:20%:2%) linked by ethylenediaminetetraacetic acid in sodium fluoride (NaF) matrix, to upconvert NIR to 432 nm light. The product was characterized by size distribution, thermogravimetric analysis, differential scanning calorimetry, and scanning electron microscopy. Growth inhibition of methicillin-resistant Staphylococcus aureus (MRSA) was not only measured in vitro but also investigated on MRSA-induced pneumonia in mice. The results showed that curcumin was covered by UCNPs to form stable nanoparticles whose average size was 179.5 nm and zeta potential was -33.7 mV in normal saline. The UCNPs-curcumin produced singlet oxygen, which reaches a stable level after 30 minutes of irradiation, and took effect on MRSA through bacterial cytoplasm leakage. They alleviated MRSA-induced pneumonia and reduced bacterial counts in lungs with 980 nm irradiation (0.5 W/cm(2)) on chests of mice. It is confirmed that the UCNPs-curcumin in lungs are activated under NIR irradiation and strengthen their antibacterial effects on MRSA. This research provides a new type of NIR photosensitizer, which plays an important role in phototoxic effects of curcumin in deep tissues under NIR.

Keywords: curcumin; methicillin-resistant Staphylococcus aureus; near infrared light; phototoxicity; upconversion nanoparticle.

Figure 1

The schematic chart of the upconversion nanoparticles conjugated with curcumin (UCNPs-curcumin) inhibiting methicillin-resistant Staphylococcus aureus (MRSA)-induced pneumonia in mice.

Figure 2

Scanning electron micrograph of the upconversion nanoparticles conjugated with curcumin at 5,000× (A) and 20,000× (B) amplification.

Figure 3

Possible structure of the upconversion nanoparticles conjugated with curcumin by polyethyleneimine (PEI).

Figure 4

Differential scanning calorimetric (A) and thermogravimetric (B) chart of the upconversion nanoparticles conjugated with curcumin (UCNPs-curcumin).

Figure 5

Singlet oxygen production of the samples under 980 nm irradiation (0.5 W/cm²) at different times. Singlet oxygen was measured by bleaching of p-nitrosodimethylaniline with a decrease in the absorbance of 440 nm. Data are presented as mean ± standard deviation in three repetitions. Note: **P<0.01, compared with curcumin and upconversion nanoparticles (UCNPs).

Figure 6

Emission spectra of the upconversion nanoparticles (UCNPs) excited by 980 nm laser and absorption spectra of curcumin.

Figure 7

Inhibition of the samples on the growth of methicillin-resistant Staphylococcus aureus (MRSA) with 980 nm laser irradiation (0.5 W/cm²) for 30 minutes. Data are presented as mean ± standard deviation in five repetitions. Notes: ^aP<0.01, compared with upconversion nanoparticles (UCNPs); ^bP<0.01, compared with curcumin.

Figure 8

Cytoplasmic leakage of methicillin-resistant Staphylococcus aureus affected by the samples in different concentrations with 980 nm irradiation (0.5 W/cm², 30 minutes). It was measured in the supernatant of bacterial culture with an increase in the absorbance at 260 nm. Data are presented as mean ± standard deviation $(\overline{x}\pm s\mathrm{n}=5)$. Note: **P<0.01, compared with curcumin and upconversion nanoparticles (UCNPs).

Figure 9

Effects of the treatments on bacterial density (A) and cytokines (B) in lungs of mice. Normal saline (NS), negative (Neg), amoxicillin (Amoxil), curcumin (Cur), upconversion nanoparticles (UCNPs) 45 mg/kg intravenous (IV); U–C H, U–C M, and U–C L are UCNPs-curcumin with IV injection at high dose (45 mg/kg), middle dose (15 mg/kg), and low dose (5 mg/kg). Mice, except the normal group, were intranasally administered with methicillin-resistant Staphylococcus aureus (1×10⁶ colony-forming units [CFU]/mL), treated by drugs and 980 nm irradiation (0.5 W/cm², 30 minutes) on the chest for 3 days, and then euthanized. Lungs were taken for bacterial counting and determination of tumor necrosis factor (TNF)-α and interleukin (IL)-1β by enzyme-linked immunosorbent assay. Data are presented as mean ± standard deviation $(\overline{x}\pm s\mathrm{n}=8)$. Notes: ^aP<0.01, compared with negative group; ^bP<0.01, compared with normal group.