Anthropogenic soil contamination has become a worldwide environmental problem in the past decades. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants in soil contaminated with crude oil, creosote, and coal tar. They are generated and dispersed into the environment by fossil fuel combustion, wood treatment processes, automobile exhaust, and waste incineration. The effect and fate of PAHs in soil is of great environmental and human health concern because of the carcinogenic, mutagenic, and teratogenic properties of PAHs. They have been frequently found in soils with high concentrations. PAHs present in soil may be absorbed by plants and translocated from roots to shoots, which is the major pathway for toxic organic substances to reach the food chain/web. Because plants form the basis of human and animal food chains, potentially harmful organic contaminants could find their way into human and animal populations via this route. Clearly, understanding the uptake of PAHs by plant and reducing the plant PAH contamination are essential for assessment of both the PAH exposure to humans and other animal species and the risk represented by PAH-contaminated soils.
Endophytic bacteria in plant tissues protect plants from external harsh environments and promote the plant growth. However, there is still little information available heretofore on the endophytic bacteria-influenced uptake and metabolism of PAHs by plants. We proposed that isolation of PAH-degrading endophytic bacteria from plant and colonization of them in the target plants are expected to improve the PAH degradation in plant, thereby reducing the risk of plant PAH contamination. In this study, two pyrene-degrading endophytic bacterial strains, named as BJ03 and BJ05, were isolated from plants grown in PAH-contaminated soils. They were individually identified as Acinetobacter sp. and Kocuria sp. based on the morphology, physiology, and 16S rDNA gene sequence analysis. The degradation characteristics of pyrene by strains BJ03 and BJ05 with different environmental conditions were investigated. It was observed that 65.0% and 53.3% of pyrene in culture solution were degraded by BJ03 and BJ05 at 30 ℃ and 150 r/min in 15 days, respectively. The two strains grew well under the condition of pH 6-9, 25-40 ℃, and NaCl concentrations of 0-15 g/L. BJ03 and BJ05 grew aerobically, and the stronger aeration resulted in their better growth and the faster degradation of pyrene in culture solution. The addition of exotic carbon (C) and nitrogen (N) sources in medium effectively promoted the bacteria growth and pyrene degradation. When sucrose and yeast extract were added as the respective C and N sources, 71.1% and 55.3% of pyrene were degraded by BJ03 and BJ05 within 4 days. BJ03 and BJ05 were observed with different cell surface hydrophobicity. The resistance tests revealed that the obtained two strains were sensitive to tetracycline and rifampicin, but were resistant to a variety of other antibiotics. This study provides new perspectives on the endophytic bacteria-influenced uptake of organic contaminants by plants. Results are valuable for the risk assessment of plant PAH contamination, and are instructive to the management of PAH-contaminated sites.