en Anti-microbial agents, resistance and treatment protocols Anti-microbial agents, resistance and treatment protocols Original article Original article <b><span style="font-size:10.0pt"><span style="line-height:107%"><span new="" roman="" style="font-family:" times="">Introduction:</span></span></span></b><i><span style="font-size:10.0pt"><span style="line-height:107%"><span new="" roman="" style="font-family:" times=""> Pseudomonas aeruginosa, </span></span></span></i><span style="font-size:10.0pt"><span style="line-height:107%"><span new="" roman="" style="font-family:" times="">a Gram-negative bacterium, is the cause of infections in immunocompromised individuals, resulting in various conditions, including pneumonia, and urinary tract, skin, and bloodstream infections. This pathogen produces tissue-destructing toxins, leading to significant morbidity and mortality in affected patients. Conventional antibiotics, including rifampicin and colistin, are often ineffective in treating <i>P. aeruginosa</i> infections due to the emergence of bacterial resistance within affected communities. Alternatively, algae have been explored as a promising source for controlling pathogenic bacteria. In this study, we investigated the antibacterial effects of ethanol and aqueous extracts of <i>Spirulina platensis</i> against <i>P. aeruginosa.</i><b> Methods:</b> Ethanol and aqueous extracts of <i>S. platensis</i> were prepared at concentrations of 0.125, 0.25, 0.5, and 1 mg/mL. The antibacterial effect of <i>Spirulina</i> blue-green algae against <i>P. aeruginosa</i> was conducted using a disc diffusion test in an LB medium with 7-mm wells. We measured the inhibition zone and statistically analyzed the data by comparing the means using the Duncan multiple range test. <b>Results:</b> The ethanol extract of <i>S. platensis</i> significantly inhibited the growth of <i>P. aeruginosa. </i>Furthermore, applying the ethanol extract of <i>S. platensis</i> at a concentration of 1 mg/mL resulted in the largest inhibition zone (20 mm) compared to the control. In contrast, the <i>S. platensis</i> aqueous extract did not significantly inhibit <i>P. aeruginosa</i> growth<i>.</i> <b>Conclusions:</b> The ethanol extracts of <i>S. platensis</i> algae exhibited a significant antibacterial effect against <i>P. aeruginosa</i>. This alga represents a promising source of antibacterial metabolites, which could be a suitable alternative to common antibiotics. Further investigations are necessary to identify and purify the specific antibacterial substance in <i>S. platensis</i>. </span></span></span><br> &nbsp; Disk diffusion test, Ethanol extraction, Pseudomonas aeruginosa, Secondary metabolites 117 122 http://jommid.pasteur.ac.ir/browse.php?a_code=A-10-482-1&slc_lang=en&sid=1 Amir Ghaffar Shahriari shahriari.ag@gmail.com 0000-0003-1772-9060 Yes Department of Agriculture and Natural Resources, Higher Education Center of Eghlid, Eghlid, Iran Mehdi Zarei mehdizarei@shirazu.ac.ir 0000-0001-5199-2618 No 1Department of Agriculture and Natural Resources, Higher Education Center of Eghlid, Eghlid, Iran; 2Department of Soil Science, School of Agriculture, Shiraz University, Iran Afsaneh Mohkami amohkami@uk.ac.ir 0009-0006-7234-9166 No Research and Technology Institute of Plant Production, Shahid Bahonar University of Kerman, Kerman, Iran Ali Niazi niazi@shirazu.ac.ir 0000-0002-9902-9422 No Institute of Biotechnology, School of Agriculture, Shiraz University, Iran