Microbiological responses to chemical stress are important in the context of engineered systems as well as the natural environment. This study investigated the impact of chemical stressors on microbiological metabolism at the population level, using model chemicals 2,4-dinitrophenol (DNP), pentachlorophenol (PCP), and N-ethylmaleimide (NEM). Biological activity of a pure culture of Pseudomonas aeruginosa was measured in batch systems, with and without stressors at sub-lethal concentrations.

Stressor DNP, between 49 and 140 mg L^-1, and PCP, at 15 and 38 mg L^-1, caused decreases in biomass growth yields, but did not inhibit substrate utilization rates. These effects increased with stressor concentrations, showing as much as a 10% yield reduction at the highest DNP concentration. This suggests that a portion of carbon and energy resources are diverted from growth and used in stress management and protection. At higher concentrations, DNP, between 300 and 700 mg L^-1, and PCP at 85 mg L^-1 caused decreases in growth yields and substrate utilization rates. This suggests an inhibition of both anabolism and catabolism. Stressor NEM was the most potent, inhibiting biological activity at concentrations as low as 2.7 mg L^-1.

The second part of this study investigated whether bacterial populations developed resilience to chemical stressors under multiple exposures. Two serial exposures to stressors DNP, PCP and NEM, within the time scale of the experiments and the concentration ranges tested, did not affect metabolism. Three serial exposures to stressor DNP elicited very different responses at different concentrations. At the highest DNP concentration of 1200 mg/L, no biological activity was observed. At low DNP concentrations of 400 mg/L, multiple exposures led to an increase in stress, indicating that the population did not develop resilience. Also, at the first exposure, the initial biomass concentration was not reduced, indicating the population retained its strong and weak members. At the intermediate concentrations of 800 and 900 mg/L, the stress decreased, indicating that the population developed resilience. The initial biomass concentration was reduced, indicating that a toxic effect of the stressor had eliminated a fraction of the weaker organisms, leaving behind a more resilient population. Differences in resilience between organisms are expected even in a pure culture, because the physiological conditions of the organisms vary.

These findings indicate that below a certain concentration, multiple exposures to a chemical stressor reduce resilience of the population. Above a certain concentration, a single exposure to the stressor is sufficient to stop biological activity. Within these two concentrations exists a range in which the toxic effects of the stressor eliminate the weaker organisms, leaving behind a more resilient population. These findings will ultimately be useful in better monitoring and management of biological treatment operations and contaminated natural systems.