عنوان مقاله [English]
Biodegradation of starch used in drilling fluid by many microorganisms, result in reduction of starch concentration in the fluid which leads to fluid efficiency downfall. Present study has been done with the aim of identification of major starch degrading bacteria in the drilling fluid and manner of controling the growth and activity of these organisms. Bacteria peresent in back up drilling fluid,were isolated and amylase activity was studied by starch hydrolysis test. Strain with most starch hydrolysis ability in all three temperatures (37, 45 and 55°C) were selected as superior strain and was identified by 16S rRNA. Also growth curve of superior strain was plotted. 54 bacterial strains include 26 Gram-positive bacillus, 22 Gram-negative bacillus and 6 Gram-positive cocci was isolated. 18 strains from 26 Gram-positive bacillus was sporated. Three strains in all three temperatures(37, 45 and 55°C) had growth and significant amylase activity. Bacteria with the ability to create maximum clear halo zone of amylase were selected as superior strain. The results of 16S rRNA analysis showed 99.8% similarity of superior strain to Bacillus licheniformis. Sporated Gram-positive bacillus belongs to Bacillus jenus like Bacillus licheniformis isolataed in this study was the most active degrading bacteria in well special conditions. Identification of degradation factors with the aim of selection of a suitable biocide in order to prevent biodegredeation is recommended.