Pseudomonas aeruginosa is ubiquitous in the nature and is one of the most commonly found microorganisms in petroleum-contaminated environments. With versatile metabolic activities, it can be used to produce various industrial and pharmaceutical products. P. aeruginosa is also a clinically important, opportunistic pathogen that causes a variety of infections, particularly in patients with severe burns, cancer, AIDS and cystic fibrosis. An important metabolic trait that supports the efficient adaptation of P. aeruginosa to this wide range of environments is its ability of active denitrification. The bacterium’s properties and respiratory behaviors under different growth rates and dissolved oxygen concentrations (DO) were therefore systematically studied in this research. Continuous cultures of P. aeruginosa (ATCC 9027) were maintained at different DO (0-4.8 mg/L) and dilution rates (D, 0.01, 0.026, 0.06, and 0.13 h-1). Aerobic denitrification was found to function as an electron-accepting mechanism supplementary, instead of competitive, to aerobic respiration. The experimental results suggested that the zero-DO conditions were more favorable for survival of the bacterium. A closer examination revealed that increasing DO enhanced O2 respiration only at extremely low DO (< 0.05 mg/L), beyond which the increasing DO only slightly increased its weak inhibition on denitrification. While O2 was the preferred electron acceptor, the fraction of electrons accepted (and the ATP generated) via denitrification increased with increasing D. Unlike glucose, when hexadecane was used as the sole carbon source, there was a critical DO (0.4 mg/L in this study), below which the system could not reach the steady state. Phosphate concentration appeared to be also very important to the behaviors of culture growing on the hexadecane-based media.
Furthermore, intriguing metabolic fluctuations were observed during the transition from non-aerated batch culture to aerated continuously-fed culture. The phenomenon suggested the complex nature of the microaerobic metabolism by P. aeruginosa as well as the inter-regulation between the two respiratory mechanisms. DO appeared to have a more pronounced effect on nitrite reduction during the transition period than at the chemostat (steady state) eventually reached after the culture adapted to the microaerobic condition. In the transition period, the autoinducer PAI1 or its homologue was found to trigger the inhibition to cell growth and nitrite reduction.
The effect of autoinducer PAI2 on rhamnolipid (RL) production by Pseudomonas aeruginosa was also evaluated by a model developed to describe the production kinetics regulated by the rhl quorum-sensing system. The best-fit model parameters obtained also provided important insights. To complex half of the intracellular RhlR proteins would require 1.61 µM of PAI2, about half of the PAI2 concentration obtained in the stationary-phase culture of wild-type PAO1. On the other hand, to activate the synthesis of rhamnosyltransferase at half of its maximum rate would require the binding of 39% of RhlR with PAI2. The maximum RL production rate of the culture was found 0.042 g/L-h and the fully induced culture would require at least 1.61 h to synthesize the enzyme to the level for producing RL at half of the maximum rate.