Egyptian Journal of Natural Toxins, Vol. (4), 1-21 Jun. 2007. * Corresponding author: Dr. Hesham Abdulla; e-mail: hesham_abdulla@hotmail.com TOXIGENICITY OF AIRBORNE ACTINOMYCETES FROM THE CHURCH OF SAINT KATHERINE MONASTERY, SOUTH SINAI, EGYPT Hesham Abdulla 1* , Sahar El-Shatoury 1 , Nahla S. El- Shenawy 2 , Hekmate Morshedy 1 and Ahmed Dewedar 1 1 Botany Department and 2 Zoology Department, Faculty of Science, Suez Canal University, Ismailia, Egypt (Received: 15/8/2006 Accepted: 18/9/2006) ABSTRACT ctinomycetes were isolated from the indoor air of the church of Saint Katherine Monastery during different visiting hours to identify its possible adverse health effects. Fifty air samples collected over one year by using automated air sampler were plated on four different media. Average counts of bacteria, actinomycetes and fungi were 487, 65 and 90 cfu/m 3 , respectively. Fifty-six morphologically different actinomycete isolates were recovered during this study. Tentative identification of the isolates indicated predominance of genus Streptomyces. All isolates were screened for antibiotic resistance, antimicrobial activity and cytotoxicity. A prevailing Streptomyces strain (S.9) showed resistance to six of the eleven tested antibiotics and demonstrated antimicrobial activity against four different reference and clinical microbial cultures. Crude organic extract of S.9 metabolites exhibited 100% mortality in Artemia salina with threshold median lethal concentration (LC 50 ) of 0.28 mg/l after 6 hours. Mouse bioassay was performed to evaluate toxicity of S.9 crude extract using intraperitoneal exposure for 48 hours. S.9 caused severe damage in the plasma membrane, increase in the oxidative stress and damage of erythrocytes, which could be the mechanism behind its effect on the biochemical and hematological parameters. S.9 crude metabolites also caused reduction in levels and activities of antioxidant markers including total protein, glutathione and uric acid as compared with control group and resulted in significant increase in hepatic lipid peroxidase levels. This study highlights the importance of monitoring indoors microbial air quality, A