Carbon metabolism characteristics of quorum quenching bacteria Rhodococcus sp. BH4 determine the bioaugmentation efficiency under different carbon source conditions

Carbon sources are critical factors influencing bacterial bioaugmentation, however, the underlying mechanisms, particularly the metabolic characteristics of bioaugmented bacteria remain poorly understood. The bioaugmented bacterium Rhodococcus sp. BH4 secretes the quorum quenching (QQ) enzyme QsdA to disrupt the quorum sensing (QS) in the activated sludge (AS) process, reducing AS yield in-situ. This study investigated the carbon metabolic characteristics of BH4 and explored the effects on bioaugmentation with different influent carbon sources. Because of the absence of glucose-specific phosphoenol phosphotransferase system (PTS), BH4 prefers sodium acetate to glucose. However, the lactones produced during extracellular glucose metabolism enhance BH4 qsdA expression. Moreover, BH4 possess carbon catabolite repression (CCR), acetate inhibits glucose utilization. BH4 microbeads were added to reactors with different carbon sources (R1: sodium acetate; R2: glucose; R3: a mixture of sodium acetate and glucose) for in-situ AS yield reduction. During operation, AS reduction efficiency decreased in the following order: R1 > R3 > R2. R2 and R3 microbeads exhibited similar QQ activity to R1, with less BH4 biomass at 5 d. C-13 labeling and Michaelis-Menten equation showed that, due to differences in the competitiveness of carbon sources, R1 BH4 obtained the most carbon, whereas R2 BH4 obtained the least carbon. Moreover, acetate inhibited glucose utilization of R3 BH4. Transcriptome analysis showed that R1 BH4 qsdA expression was the lowest, R2 BH4 was the most serious form of programmed cell death, and the R3 BH4 PTS pathway was inhibited. At 10 d, R1 BH4 biomass and microbead QQ activity were higher than that in R3, and the R2 BH4 lost viability and QQ activity. This study provides new insights into bioaugmentation from the perspectives of carbon source competitiveness, carbon metabolism pathways, and CCR.