Microalgae play a significant role in the development of new products for medical and pharmaceutical research due to their ability to generate different biologically active metabolites. They are target organisms in the search for new antibiotic molecules to deal with antibiotic resistance. In addition, the use of natural antibiotics could satisfy consumer demand to avoid the side effects of chemicals. Our results showed antimicrobial activity of two algal species Spirulina platensis and Chlorella vulgaris against nine human pathogenic bacteria by agar well diffusion method. Seven concentrations of algal extract (10, 50, 100, 150, 200, 250, and 300 mg/ml) were used. It was observed that ethanolic extract of Spirulina platensis was most effective against Streptococcus agalactiae with maximum inhibition zone of 21.6 mm, while the minimum inhibition zone (8.5mm) was found in case of Pseudomonas aeruginosa at concentration 300 mg/ml. In the case of ethanolic extract of Chlorella vulgaris, the inhibition zone was the highest (31.6 mm) against Staphylococcus lentus, while the lowest inhibition zone (20.6 mm) was in case of Staphylococcus aureus at concentration 300 mg/ml. While the concentrations less than 300 mg/ml showed varying inhibition of pathogenic bacteria, some bacterial isolates showed resistance to low concentrations of algal extracts. The results of gas chromatography–mass spectrometry (GC-MS) analysis of the two algal extracts showed that chemical composition analysis consisted of terpenes (monoterpenes and sesquiterpenes)

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