Abstract
Algal alkaloids are widely used for their pharmacological properties as antimicrobial agents. This study determined the antibacterial activities of algal alkaloid-rich extracts against isolates of multidrug-resistant Staphylococcus aureus and enterohaemorrhagic Escherichia coli (EHEC) O157, as well as the probable mode of action underlying their antibacterial effect. The total alkaloids were extracted from two Libyan brown algae, namely Sargassum hornschuchii and Cystoseira compressa and tested against six different isolates from the bacteria mentioned above using the agar-well diffusion method, and their mode of action on isolates was evaluated by several bacterial physiological indicators, including intracellular potassium ion efflux and nucleotide leakage. Also, the extracts' hemolytic activity was assessed as an indicator of their cytotoxicity on red blood cells. Although not to the same extent, both alkaloid extracts presented antibacterial activities against all tested isolates with no evidence of bacterial regrowth. The alkaloid extract from S. hornschuchii exerted the best effect on bacteria growth with minimum inhibitory concentration values ranging between 125 and 500 mg/mL. The results showed that the alkaloid extracts significantly induced a distinct release of nucleotide and potassium ions out of the cell membrane, indicating that they cause a change in the fluidity or permeability or both of the cell membrane. Moreover, the results revealed that there were very low cytotoxic effects. Therefore, algal alkaloids may contribute to the development of potential antibacterial agents in the future.