In this work, biosurfactant production by Bacillus subtilis, Pseudomonas aeruginosa, and Bacillus cereus has
been compared. These microorganisms were isolated from agricultural and oil polluted area of Iran. Maximum
biosurfactant production reached to about 1.6 g/l and liquid surface tension decreased from 72 to 25 mN/m
using B. subtilis. All screened bacteria exhibited a good ability to emulsify the crude oil. Biosurfactant of
B. subtilis attained an emulsion index of 75% for crude oil which was significantly greater than other strains.
Stability studies were carried out under the extreme environmental conditions as find normally in the oil
reservoir such as high temperature, pH and salinity. Results showed an excellent resistance of all produced
biosurfactants to retain their surface-active properties at extreme conditions. It was found that the
biosurfactants show a good stability around pH of 4 but at lower pH, they will harmfully be affected. Lesser
precipitation has been observed at low pH for biosurfactant produced by B. subtilis. It has also been observed
that biosurfactants from three isolated bacteria were able to support the condition up to 25 g/l salinity.
B. cereus biosurfactant was even stable at the higher salinity. Regarding temperature, all produced
biosurfactants demonstrated a good stability in the temperature up to 120 °C. Oil displacement experiments
in micromodel with kerosene showed around 25% recovery of residual oil after water flooding when the
biosurfactant of B. subtilis was added. These results suggest that biosurfactants of these indigenous isolated
strains are appropriate candidates for enhanced oil recovery with a preference to biosurfactant of B. subtilis