## 4 Conclusions  
This study demonstrated that among the eight bacterial strains isolated from sewage sludge through NP enrichment cultures, _S. putrefaciens_ CN17, _A. faecalis_ CN8, and _B. safensis_ CN12 showed the highest degradation rates of 4-n-NP, with _B. safensis_ CN12 achieving complete degradation (100%) and _S. putrefaciens_ CN17 and _A. faecalis_ CN8 achieving 74% and 75%, respectively. Despite these high degradation rates, the TPs produced during the process, namely 4-(6-oxo-1,1-dimethylheptyl)phenol, 4-nonyl-4-hydroxycyclohexan-2,5-dienone, and 4-nonyl-4-hydroxycyclohex-2-enone, were found to be more toxic than the parent compound. Genetic analysis of _B. safensis_ CN12 revealed the presence of key genes, such as multicopper oxidase _cotA_ and ring-cleaving dioxygenase _mhqO_, which are involved in the degradation and mineralization of aromatic compounds. These genes were significantly upregulated during 4-n-NP biodegradation, underscoring their essential role in the process. _B. safensis_ CN12 was able to degrade 4-n-NP in solution, but it was not able to achieve mineralization. However, mineralization was achieved when 4-n-NP was adsorbed onto sewage sludge and treated in the presence of the bioavailability enhancer HPBCD (which facilitated the extraction of 4-n-NP from the sludge), achieving over 33% mineralization. Through mineralization, a large number of the TPs produced were removed from the sewage sludge, thereby reducing its toxicity. In conclusion, while bioaugmentation strategies offer promise for bioremediation, it is crucial to consider the possible formation of toxic TPs. However, as demonstrated in this study, the interaction between the degrading bacterium and the indigenous microbiota in contaminated environments can lead to a synergistic effect that enhances contaminant mineralization and promotes more effective environmental remediation. Future studies should be carried out to determine the presence of such TPs as well as the toxicity of the final system, since only a few studies have paid attention to the toxicity changes due to their presence resulting from metabolic pathways. To deepen the understanding of the mechanisms underlying the synergistic promotion of NP mineralization by microbial consortia, studies about the effects of bioaugmentation on the indigenous microbial community should be carried out by integrating metagenomic sequencing approaches. This would allow for a detailed characterization of the endogenous microbial community dynamics during the degradation process, providing direct evidence of shifts in microbiota composition and the potential establishment of functional synergies among microbial groups. Furthermore, such analyses could offer insights into the ecological impacts of the biodegradation process, particularly regarding the observed increase in toxicity, and help assess whether additional post-treatment steps are necessary to mitigate environmental risks. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15126408/s1](https://www.mdpi.com/article/10.3390/app15126408/s1)]([https://www.mdpi.com/article/10.3390/app15126408/s1](https://www.mdpi.com/article/10.3390/app15126408/s1)), Figure S1. Percentages of 4-n-NP degraded by the isolated bacterial strains after 30 days of incubation in the absence and presence of glucose (I g L-1); Figure S2. Chemical structures of 4-n-NP and the transformation products (TPs) detected during its degradation in solution by various bacterial strains isolated from sewage sludge; Figure S3. Subsystem category distribution of major protein-coding genes of B. safensis CN12 as annotated by the RAST annotation server. The bar chart shows the subsystem coverage in percentage (the blue bar corresponds to the percentage of proteins included). The pie chart shows the percentage distribution of the 26 most abundant subsystem categories; Table S1. Physico-chemical properties of sludge sample used (mean of four replicates). Standard deviations are in parentheses; Table S2.