Turmeric Root and Its Bioactive Ingredient Curcumin Effectively Neutralize SARS-CoV-2 In Vitro

Abstract

Severe Acute Respiratory Syndrome Coronavirus Type 2 (SARS-CoV-2) is the causative agent of the coronavirus disease 2019 (COVID-19). The availability of effective and well-tolerated antiviral drugs for the treatment of COVID-19 patients is still very limited. Traditional herbal medicines elicit antiviral activity against various viruses and might therefore represent a promising option for the complementary treatment of COVID-19 patients. The application of turmeric root in herbal medicine has a very long history. Its bioactive ingredient curcumin shows a broad-spectrum antimicrobial activity. In the present study, we investigated the antiviral activity of aqueous turmeric root extract, the dissolved content of a curcumin-containing nutritional supplement capsule, and pure curcumin against SARS-CoV-2. Turmeric root extract, dissolved turmeric capsule content, and pure curcumin effectively neutralized SARS-CoV-2 at subtoxic concentrations in Vero E6 and human Calu-3 cells. Furthermore, curcumin treatment significantly reduced SARS-CoV-2 RNA levels in cell culture supernatants. Our data uncover curcumin as a promising compound for complementary COVID-19 treatment. Curcumin concentrations contained in turmeric root or capsules used as nutritional supplements completely neutralized SARS-CoV-2 in vitro. Our data argue in favor of appropriate and carefully monitored clinical studies that vigorously test the effectiveness of complementary treatment of COVID-19 patients with curcumin-containing products.

Keywords: COVID-19; Curcuma longa; SARS-CoV-2; antiviral; curcumin; herbal medicine; turmeric root.

Figure 1

Neutralization of SARS-CoV-2 by aqueous turmeric root extract, curcumin-containing nutritional supplement capsules, and curcumin. Decreasing concentrations of aqueous turmeric root extract (1:8–1:1024 dilution), nutritional supplement capsules (468.8–3.7 µg/mL), and curcumin (125–1 µg/mL) were pre-incubated with 100 TCID₅₀ of SARS-CoV-2 for one hour and subsequently added to confluent Vero E6 cells. After 48 h, cells were stained with crystal violet and analyzed for cytopathic effects using transmitted light microscopy. The experiment was performed three times independently. Representative images are displayed. NC = negative control (medium); PC = positive control (100 TCID₅₀ SARS-CoV-2); scale bar = 200 µm.

Figure 2

Dose-dependent antiviral activity of aqueous turmeric root extract, curcumin-containing nutritional supplement capsules, and curcumin against SARS-CoV-2. Decreasing concentrations of aqueous turmeric root extract (1:8–1:1024 dilution) (A), nutritional supplement capsules (468.8–3.7 µg/mL) (B), and curcumin (125–1 µg/mL) (C) were pre-incubated with 100 TCID₅₀ of SARS-CoV-2 for one hour. Subsequently, each dilution of virus–herb suspensions was incubated on confluent Vero E6 cells grown on a 96-well plate. After 48 h, cells were stained with crystal violet and analyzed regarding cytopathic effects. The half-maximal effective concentration (EC₅₀) was calculated by nonlinear regression using GraphPad Prism. The cytotoxic effect of various concentrations of aqueous turmeric root extract, nutritional supplement capsules, and curcumin toward Vero E6 cells was determined by Orangu Cell Counting Solution (Cell guidance systems) after 48 h. Cell viability was normalized to untreated control cells. The experiment was performed three times independently. Error bars represent the standard error of the mean (SEM).

Figure 3

Neutralization of SARS-CoV-2 by aqueous turmeric root extract, curcumin-containing nutritional supplement capsules, and curcumin on a human cell line assessed by an in-cell ELISA (icELISA)-based neutralization test (icNT). (A) Decreasing concentrations of aqueous turmeric root extract (1:16–1:128 dilution), nutritional supplement capsule content (468.8–58.6 µg/mL), or curcumin (125–15.6 µg/mL) were pre-incubated with 5000 plaque-forming units (PFU) of SARS-CoV-2 for one hour. Subsequently, mixtures were added to human Calu-3 cells and incubated for 24 h. After incubation with a SARS-CoV-2 N-specific primary antibody and peroxidase-labelled secondary antibody, the enzyme reaction was visualized by adding tetramethylbenzidine. Absorbance was measured with a microplate multireader at OD450. Statistical analysis was performed with one-way analysis of variance (ANOVA) and Dunnett’s multiple comparison test using GraphPad Prism. ** p < 0.01; *** p < 0.001; and **** p < 0.0001; error bars represent the standard error of the mean (SEM). NC = negative control (medium); PC = positive control (5000 PFU SARS-CoV-2). (B) The cytotoxic effect of various concentrations of aqueous turmeric root extract, nutritional supplement capsule, and curcumin toward Calu-3 cells was determined by Orangu™ Cell Counting Solution (Cell guidance systems) after 24 h. The cell viability was normalized to untreated control cells. Error bars represent the SEM.

Figure 4

Dose-dependent activity of curcumin on SARS-CoV-2 RNA genome copy numbers. Decreasing concentrations of curcumin (125–1 µg/mL) were pre-incubated with 100 TCID₅₀ of SARS-CoV-2 for one hour and subsequently added to confluent Vero E6 cells. After 48 h, cell culture supernatants were harvested, and the genomic SARS-CoV-2 RNA was quantified via RT-qPCR, using primer targeting the viral M or N gene. Cells infected with 100 TCID₅₀ of SARS-CoV-2 served as positive control. The experiment was performed three times independently. The half-maximal effective concentration (EC₅₀) was calculated by nonlinear regression using GraphPad Prism.