4.5 Conclusive remarks
In conclusion, we show that the intermediate host range plasmid R27, of the HI1A group, transfers horizontally among the microbial communities extracted from three Swedish WWTPs. However, the transfer frequency of R27 was always lower than that of plasmid pB10, of the P1 group. The enriched transconjugant communities were found to be efficient plasmid donors to a model of a pathogenic Enterobacteriaceae strain, revealing that strains in the recipient sewage community are able to acquire and disseminate both plasmids. Our results do furthermore suggest that the consistently higher transfer frequency of pB10 compared to R27 is a plasmid intrinsic trait. We find that R27 has a broad potential host range, but a low host divergence. Whereas pB10 has both a broad potential host range and a high host divergence. We discovered that the vast majority of R27 transfer events were distributed within members of the Enterobacteriaceae family. Nonetheless we also find a subset of transfer events to members of Aeromonadaceae, Burkholderiaceae, Flavobacteriaceae, MoraxellaceaePseudomonadaceae and Shewanellaceae , thus revealing a transfer host range up to grade VI, even across phyla. It is very rare to isolate HI1A plasmids from hosts outside the Enterobacteriales order, thus we speculate that these phylogenetically broad transfer events are short term, consisting of hosts that do not stably maintain the plasmid. However, these hosts may act as intermediate hosts utilized as “plasmid stepping stones” ensuring transfer to compatible hosts within Enterobacteriaceae. These events could additionally also indicate that HI1A plasmids facilitate a horizontal gene flow between Enterobacteriaceae across phyla. Whereas R27 seems to be specialized in its transfer, the transfer of pB10 reflects that of a generalist transfering to the most abundant bacteria in the recipient sewage community. A strategy that could ensure a higher resilience to change in the microbial composition. The enriched transconjugant sewage community, harboring R27 or pB10, were almost completely dominated by a single genus of Enterobacteriaceae or Aeromonadaceae , respectively. Nevertheless, as in our study these families are often found as the core permissive fraction, across diverse environments, suggesting a pivotal role in plasmid dissemination. In a broader context, these results provide new insight to the transfer of resistance in complex communities and emphasize the role of HI1A plasmids as vectors for environmental spread of ARGs. By demonstrating that HI1A plasmids conjugate in sewage communities and ultimately end in a family which comprises a large number highly medical relevant pathogens, our results support previous studies and highlight that WWTPs are facilitating the dissemination of resistance genes.
Table 1. Strains used in this study