A strain of Bacillus licheniformis T5 was isolated from soil contaminated with crude oil due to its efficient degradation of polycyclic aromatic hydrocarbons (PAHs). When subjected to stress metabolism using phenanthrene as a carbon source, significant changes were observed in T5 cells. Infrared spectrum analysis revealed the presence of -C=C- and Ph-O-C (aromatic) groups on the bacterial surface, facilitating the adsorption of PAHs on the phospholipid layer and causing damage to the cell membrane. Scanning electron microscope (SEM) analysis showed the changes of cell morphology, including a large number of folds on the lower surface and the folding of cell membrane. Transmission electron microscope (TEM) observation showed that non-stressed bacteria with adequate nutritional conditions accumulated more lipids. However, the stress group contained more protein. It was found that stress metabolism led to the increase of protein content in T5 cells by 16.4% and the activity of oxidoreductase more than doubled. These physiological and biochemical changes enhance the ability of stressed bacteria to degrade PAHs efficiently, thereby reducing the degradation cycle. The findings offer valuable insights for the remediation of PAHs pollution.

Titanium dioxide nanoparticles (TiO2 NPs) are among the most commonly used nanomaterials and are most likely to end up in soil. Therefore, it is pertinent to study the interaction of TiO2 NPs with soil microorganisms. The present in vitro broth study evaluates the impacts of low-dose treatments (0, 1.0, 5.0, 10.0, 20.0, and 40.0 mg L-1) of TiO2 NPs on cell viability, morphology, and plant growth promoting (PGP) traits of rhizobia isolated from mung bean root nodule. Two types of TiO2 NPs, that is, mixture of anatase and rutile, and anatase alone were used in the study. These TiO2 NPs were supplemented in broth along with a multifunctional isolate (Bradyrhizobium sp.) and two reference cultures. The exposure of TiO2 (anatase+rutile) NPs at low concentrations (less than 20.0 mg L-1) enhanced the cell growth, and total soluble protein content, besides improving the phosphate solubilization, Indole-3-acetic acid (IAA) production, siderophore, and gibberellic acid production. The TiO2 (anatase) NPs enhanced exopolysaccharide (EPS) production by the test rhizobial cultures. The radical scavenging assay was performed to reveal the mode of action of the nano-TiO2 particles. The study revealed higher reactive oxygen species (ROS) generation by the TiO2 (anatase) NPs as compared with TiO2 (anatase+rutile) NPs. Exposure to TiO2 NPs also altered the morphology of rhizobial cells. The findings suggest that TiO2 NPs could act as promoters of PGP traits of PGP bacteria when applied at appropriate lower doses.