How denitrifiers defense ciprofloxacin: Insights from intracellular and extracellular stress response.

Overuse of antibiotics has led to their existence in nitrogen-containing water. The impacts of antibiotics on bio-denitrification and the metabolic response of denitrifiers to antibiotics are unclear. We systematically analyzed the effect of ciprofloxacin (CIP) on bio-denitrification and found that 5 mg/L CIP greatly inhibited denitrification with a model denitrifier (Paracoccus denitrificans). Nitrate reduction decreased by 32.89 % and nitrous oxide emission increased by 75.53 %. The balance analysis of carbon and nitrogen metabolism during denitrification showed that CIP exposure blocked electron transfer and reduced the flow of substrate metabolism used for denitrification. Proteomics results showed that CIP exposure induced denitrifiers to use the pentose phosphate pathway more for substrate metabolism. This caused a substrate preference to generate NADPH to prevent cellular damage rather than NADH for denitrification. Notably, despite denitrifiers having antioxidant defenses, they could not completely prevent oxidative damage caused by CIP exposure. The effect of CIP exposure on denitrifiers after removal of extracellular polymeric substances (EPS) demonstrated that EPS around denitrifiers formed a barrier against CIP. Fluorescence and infrared spectroscopy revealed that the binding effect of proteins in EPS to CIP prevented damage. This study shows that denitrifiers resist antibiotic stress through different intracellular and extracellular defense strategies.

Efficient aerobic denitrification without nitrite accumulation by Pseudomonas mendocina HITSZ-D1 isolated from sewage sludge.

A highly efficient aerobic denitrifying microbe was isolated from sewage sludge by using a denitrifier enrichment strategy based on decreasing carbon content. The microbe was identified as Pseudomonas mendocina HITSZ-D1 (hereafter, D1). Investigation of the conditions under which D1 grew and denitrified revealed that it performed good growth and nitrate removal performance under a wide range of conditions. In particular, D1 rapidly removed all types of inorganic nitrogen without accumulation of the intermediate products nitrite and nitrous oxide. Overall, D1 showed a total nitrogen removal efficiency >96% at a C/N ratio of 8. The biotransformation modes and fates of three typical types of inorganic nitrogen were also assessed. Moreover, D1 had significantly higher denitrification efficiency and enzyme activities than other aerobic denitrifying microbes (Paracoccus denitrificans, Pseudomonas aeruginosa, and Pseudomonas putida). These results suggest that D1 has great potential for treating wastewater containing high concentrations of nitrogen.