4.3 Barriers to plasmids transfer and the core permissive
fraction
We observe a high degree of strain specificity for plasmid uptake within
genera (figure 4a). Consistent with this, Maher & Taylor found that the
uptake of different HI1A plasmids vary highly at the species level
(Maher & Taylor, 1993).
Likewise, another study also found that the capability of receiving
plasmids is highly variable and strain specific, even within a single
genus (Li et al., 2018). We
speculate that this phenomenon can be attributed to a lack of host
encoded defense systems e.g. R/M, CRISPR-Cas systems, etc. Moreover,
other barriers to plasmid transfer that could cause strain-specific
levels of permissiveness are plasmid incompatibility and plasmid entry
exclusion mechanisms (Novick,
1987). Recently, it has been reported that a fundamental function of
the diverse plasmid-encoded type IV CRISPR-Cas system is to target and
eliminate competing plasmids
(Crowley et al., 2019;
Pinilla-Redondo et al., 2020).
We identify 40 shared ASVs (supplementary figure 5), which we define as
the core permissive fraction. Within the core permissive fraction of our
study, we do find ASVs representing the seven families identified in the
overall pool of transconjugants: Aeromonadaceae, Burkholderiaceae, Enterobacteriaceae, Flavobacteriaceae, Moraxellaceae, Pseudomonadaceae and Shewanellaceae. The families Aeromonadaceae, Pseudomonadaceae and Shewanellaceae are even found at
relative abundances above 3% for both R27 and pB10 (figure 4b).
Previous studies have identified Aeromonadaceae ,Enterobacteriaceae , Moraxellaceae and Pseudomonadaceae as part of the core permissive fraction, which
extends across diverse environments
(Klümper et
al., 2015; Li et al., 2018; Musovic et al., 2014; Pinilla-Redondo et
al., n.d.). Of special interest are Enterobacteriaceae and Aeromonadaceae , as these two families have been found to
represent a large fraction of the core permissive fraction in WWTPs
(Li et al., 2018).
Suggesting further importance in the core-permissive fraction of these
two families is the observation that a enrichment of R27 and pB10
transconjugants resulted in the domination of a single genus of Serratia (Enterobacteriaceae) or Aeromonas (Aeromonadaceae ), respectively (Figure 5b). Thus we also show
that members of the core-permissive fraction are fully capable of
further carrying and disseminating plasmids. Given the positive impact a
plasmid can have under selective conditions, and conversely, the
parasitic burden it may represent when it does not grant advantages, it
is not unimaginable that some bacterial families have evolved to embrace
plasmids, while other families have evolved to categorically reject
them. We speculate that a central feature of the core permissive
fraction could be a lack of plasmid-targeting host defenses. Altogether,
the families Aeromonadaceae, Enterobacteriaceae, Moraxellaceae, Pseudomonadaceae and Shewanellaceae seem to play a central role in the dissemination of ARGs via plasmids
and our work suggest that HI1A and P1 plasmids are included in this
network of conjugation within the sewage microbiome.