## 4 Discussion  
### Composition of Bacterial and Fungal Communities during Vermicomposting of Sewage Sludge  
The bacterial and fungal communities in vermicompost differed significantly from those in sewage sludge at the phylum level immediately after GAPs, and the differences in composition increased after CAPs. Bacterial communities in vermicompost were mainly composed of the phyla Proteobacteria, Bacteroidetes and Verrucomicrobiota, while the fungal communities were predominated by the phyla Basidiomycota, Mortierellomycota and Ascomycota. Similar results were previously obtained for bacteria and fungi [2], and Proteobacteria and Basidiomycota were also the most abundant phyla. Other vermicomposting studies have found similar bacterial compositions at the phylum level after vermicomposting white, red and distilled grape marc [48; 49; 50], as well as Scotch broom and silver wattle [51; 52].  
The changes in the microbial communities during vermicomposting of sewage sludge were even more pronounced at the genus and ASV levels than at the phylum level. Thus, the composition and abundance of various bacterial and fungal genera were substantially altered. For example, earthworm casts were predominantly populated by the bacterial genera _Aeromonas_, _Chitinibacter_ and _Candidatus Lumbricincola_, as well as by the fungal genera _Apiotrichum_ and _Mortierella_. These findings are similar to those reported in a previously mentioned study [2] but differ regarding the predominant microbial genera found in the previously mentioned vermicomposting studies [44; 51]. Other vermicomposting studies involving the closely related earthworm species _E. ftida_ indicated different bacterial and fungal compositions at the phylum and genus levels following GAPs and CAPs compared to those described here [22; 23; 53]. For instance, the fungal communities were largely dominated by _Rozellomycota_ and unidentified fungal genera, while the bacterial communities were dominated by _Chloroflexy_, _Saccharibacteria_, _Flavobacterium_ and _Thermononas_[22; 23; 53].  
On the one hand, the previously mentioned bacterial taxa were not identified in vermicompost samples, i.e., after CAPs, while in the previous study, the abundance of predominant bacterial genera in the casts only decreased during CAPs [2]. On the other hand, as the fungal genera did not vary during CAPs, our findings are not consistent with those of the previous study, in which the abundance of predominant fungal taxa decreased [2]. Despite the fact that, at coarse taxonomic levels, earthworms increase the similarity of microbial communities of vermicompost independently of ingested substrates [54], this pattern disappeared when the taxonomic resolution was increased. This indicates that the ingested substrate plays a predominant role in the microbial composition of the resulting vermicompost.  
As expected, since most of the bacterial ASVs from sewage sludge did not appear in earthworm samples (cast and vermicompost), most of the HBPs disappeared readily after gut transit. This effect was even increased during vermicomposting. This result is the expected output of vermicomposting, which significantly reduces or completely eliminates pathogens [6]. It is important to note that HBPs made up a small fraction of the bacterial communities in sewage sludge, accounting for less than 5%. None of these harmful bacteria were _E. coli_ or _Salmonella_ spp., which are the only harmful bacteria specified in the European regulations for waste and recycled byproducts [55; 56]. It is also worth noting that all HBPs found belonged to the established category, meaning there were more than three reported cases of infecting humans [30]. The other two HBPs, _B. massiliensis_ and _T. sanguinis_, were categorized as putative (i.e., with less than three reported cases of infecting humans) [30].