Comparative Evaluation of Aqueous and Methanolic Extracts of Spirulina platensis: Antibacterial Efficacy, Cytotoxicity, and Gene Expression Profiling

Abstract

Background: Spirulina platensis is a cyanobacterium with significant bioactive potential. This study evaluated the antibacterial and cytotoxic properties of the aqueous and methanolic extracts and explored associated changes in gene expression. The antibacterial activity of both extracts was tested against Gram-positive (Bacillus cereus, Listeria monocytogenes) and Gram-negative (Klebsiella sp., Salmonella sp., Escherichia coli) bacteria. The cytotoxic effect was determined in human hepatocellular carcinoma (HepG-2) and colorectal adenocarcinoma (Caco-2) cells using the MTT assay after 24 and 48 hours. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) was used to evaluate the expression levels of RAF, LC3, and P53 genes.  Results: At 500 mg/mL, the aqueous extract showed the most potent antibacterial activity, with the largest inhibition zone against L. monocytogenes, followed by Klebsiella sp. Cytotoxicity was time- and concentration-dependent. Cytotoxicity reached its highest level in the aqueous extract after 72 hours, with the 500 µg/mL concentration showing a 77.5% inhibitory effect on HepG-2 cells and the 250 µg/mL concentration showing a 70% inhibitory effect on Caco-2 cells. For the methanolic extract, the maximum cytotoxic effect was observed after 72 hours, with the 125 µg/mL concentration showing a 72% inhibitory effect on HepG-2 cells. Gene expression analysis revealed that both extracts markedly upregulated RAF, P53, and LC3 after 72-hour treatment. The aqueous extract induced the highest P53 expression, while the methanolic extract triggered the most remarkable LC3 upregulation.

Conclusion: Taken together, our findings highlight the differential bioactivity of S. platensis extracts. The aqueous extract demonstrated superior antibacterial and cytotoxic activity. Both extracts could modulate key genes involved in stress response, apoptosis (P53), and autophagy (LC3), suggesting potential mechanisms for their observed effects.