FIGURE 10.

BifA regulated-biofilm formation is a primary mechanism to defend exogenous H₂O₂ exposure and closely correlates to cellular oxidative stress. Mn²⁺ ion and H₂O₂ represent 2 types of environmental factors, a trace nutrient substrate and an oxidative stress, respectively. Four genes that encode MCO, Mn²⁺ transporter, SOD and BifA were assayed for their expression during bacterial growth following exposure to the two environmental factors Mn²⁺ ion and H₂O₂ on the growth, Mn²⁺ acquisition, anti-oxidative system and biofilm formation in P. putida MnB1. Exogenous Mn²⁺ promoted Mn²⁺ uptake and acquisition. An increase in intracellular Mn²⁺ may act as nutrient substrate promoting microbial growth. Alternatively, Mn²⁺ may function as a cofactor for important enzymes, such as SOD, which scavenges ROS. Mn oxide, catalyzed by MCO is a barrier to protect from invasion, heavy metal and oxidative damage. Mn²⁺ may target Mn²⁺-responsive regulators to inhibit biofilm formation by BifA up-regulation. Decrements in intracellular anti-oxidative system and the inhibition of biofilm formation suggests that Mn²⁺ favors the microbial growth but suppresses biofilm development. Exogenous H₂O₂ triggers intracellular ROS over-production, but does not significantly affect the proliferation of P. putida MnB1. Bio-degradation of exogenous H₂O₂ and quick recovery of proliferative capacity at high H₂O₂ levels are due to CAT activity. In contrast to the activation of anti-oxidative system, promotion of the biofilms by down-regulation of BifA gene or the blockade of BifA activity suggests that biofilm formation may be a primary mechanism by which P. putida MnB1 defends unfavorable environmental factors. Therefore, BifA-mediated biofilm formation may be more sensitive than anti-oxidative system in response to Mn²⁺ ion and H₂O₂ in P. putida MnB1. The sensitivity of BifA-mediated biofilm formation may highlight its role in the adaption of microbes to environment stress. MCO, Manganese oxidase; MntABC, ABC-Type manganese transporter; SOD, Superoxide dismutase; CAT, Catalase; PDE, Phosphodiesterase; ROS, Reactive oxygen species.