en Microbiology and Anatomy Microbiology and Anatomy Original Research: Basic Science in Medicine Original Research: Basic Science in Medicine 71 samples of milk and dairy products were collected from 18 cities throughout Iran. 93 strains of lactic acid bacteria (LAB) were isolated and assayed against four opportunistic patho gens including E. coli, proteus and klebsiella as gram-negative and S. aureus as a gram-positive organism. All indicators (organisms) were hospital strains. Among isolated bacteria, lactobacilli (37 isolates) had stronger activity against gram-negative pathogens. 37 isolates were lactococci out of which 9 strains were leuconostocs and seven strains were S.jaecalis. S. aureus was more sensitive to lactococci than lactobacilli. Only one isolate of leuconostoc inhibited the growth of gram-negative indicators, but five isolates inhibited S. aureus. The use of bacteriocin-producing lactic acid bacteria in dairy products provides a natural mean of preservation. Lactic Acid Bacteria, Bacteriocin, Growth Inhibition, Antibiosis. 159 163 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-1-449&slc_lang=en&sid=1 FARROKH A. NAKHDJAVANI 20031947532846005406 20031947532846005406 Yes From the Department of Microbiology, Medical Science Faculty, Tarbiat Modarres University, Tehran NASRIN MOAZAMI 20031947532846005407 20031947532846005407 No , the Department of Biotechnology, Iranian Research Organization for Science and Technology, p.o. Box 15815-3338, Tehran HASSAN LAMEA 20031947532846005408 20031947532846005408 No the Food Science and Technology Center, Postgraduate and Advanced Research Branch, Islamic Azad University, P.O. Box 19395-1775, Tehran, Islamic Republic of Iran.