en Bacterial Disease Bacterial Disease پژوهشي Original research papers <div style="text-align: justify;">With the emerging of resistant microorganisms to conventional antibiotics, as well as consumers&#39; concerns about the side effects of chemical drugs, the tendency for natural bioactive compounds is increasing. <em>Astacus leptodactylus</em> is considered for export and consumption in Iran, but during its processing a large amount of waste material could be produced. The aim of the present study was to investigate the antimicrobial properties of chitosan extracted from <em>A. leptodactylus</em> caught from Aras Lake. The shrimp was caught from Lake Aras and transferred to the Microbiology Laboratory of the Islamic Azad University of Lahijan close to ice. Chitosan was extracted from shrimp shells, and methanol extract of shrimp tissue was prepared. Antimicrobial activity of chitosan and extracts were evaluated against Gram-positive bacteria <em>Staphylococcus aureus</em> and Gram-negative bacteria <em>Escherichia coli</em> as well as two species of <em>Aspergillus niger</em> and <em>Candida albicans</em>.&nbsp;Extracted chitosan showed a stronger inhibitory activity against <em>S. aureus</em> compared to <em>E. coli.</em> It was also observed that with increasing chitosan concentration, the inhibitory zone diameter increased against both <em>S. aureus</em> (p = 0.001) and <em>E. coli</em> (p = 0.122). It was also observed that the acidic solution of chitosan had a stronger antibacterial activity than aqueous solution of chitosan. The methanolic extract did not show significant effects on the studied microorganisms. Chitosan had a weak antifungal activity, although it showed a greater effect on <em>A. niger</em> than <em>C. albican</em>s<strong>. </strong></div> Astacus leptodactylus, Chitosan, Antibacterial activity, Escherichia coli, Staphylococcus aureus 105 117 http://ijaah.ir/browse.php?a_code=A-10-322-1&slc_lang=en&sid=1 M. Gholampoor 10031947532846002574 10031947532846002574 No Department of Marine Biology, Lahijan Branch, Islamic Azad University, Lahijan, Iran G. Mahmoodi 10031947532846002575 10031947532846002575 No Department of Biology, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran A. Moshfegh moshfeghazam@gmail.com 10031947532846002576 10031947532846002576 No Department of Biology, Lahijan Branch, Islamic Azad University, Lahijan, Iran A. Tehranifard 10031947532846002577 10031947532846002577 Yes Department of Biology, Lahijan Branch, Islamic Azad University, Lahijan, Iran