Screening and Characterization of Aerobic Xylene-Degrading Bacteria from Gasoline Contaminated Soil Sites Around Gas Stations in Northern Jordan

A B S T R A C T Xylene is frequently released into the environment from biomass. As a consequence of this, its bioaccumulation can cause adverse health effects in humans. The purpose of this study was to screen for aerobic xylene-degrading bacteria from gasoline contaminated soil sites located around gas stations in the city of Al-Mafraq, Jordan. The effects of some physicochemical factors were examined. The 10 g of soil sample were transferred to Stanier?s mineral medium supplemented with 1% m-xylene and incubated at 30?C for 72 h. At least 4 aerobic m-xylene degrading isolates, designated as X1-X4 were identified using biochemical and molecular biology techniques. Isolates X1 and X2 were rod-shape Gram negative, oxidase and catalase positive bacteria. Isolate X3 was a rod-shape and Gram negative bacterium that was catalase positive and oxidase negative. Isolate X4 was a rod-shape, spore forming and Gram positive bacterium that was oxidase and catalase positive. Isolates X1, X2 and X4 showed high similarity to Pseudomonas aeruginosa, Pseudomonas stutzeri and Bacillus firmus, respectively, whereas X3 was a novel species of the genus Citrobacter, similar to Citrobacter amalonaticus. The growth rates of these isolates were slower at 2% m-xylene than at 1% m-xylene. The growth rate was less when the temperature was reduced from 30-25?C, whereas, at 45?C, the growth rate almost completely ceased. The growth rate was higher at pH 6.5 than at pH 5.5 or 8.5. The shortest generation times were found to be 8 h for Bacillus firmus, followed by 9 h for Pseudomonas stutzeri, 10 h for Citrobacter amalonaticus and 11 h for Pseudomonas aeruginosa under 1% m-xylene at 30?C and pH 6.8. In conclusion, we reported for the first time the isolation of four bacterial species with the ability to utilize m-xylene as a growth substrate.