The version-1 bioRxiv preprint of Cheng et al., later published as "Efficient plasmid transfer via natural competence in a microbial co-culture" (Molecular Systems Biology, 2023), showed in a synthetic two-species community that plasmid transfer from Escherichia coli donors to naturally competent Bacillus subtilis recipients via natural transformation is strongly enhanced in co-culture, controlled by donor RecA-dependent plasmid multimerization and SOS-driven extracellular-DNA release, efficient from live, dead, or antibiotic-inhibited donors, and reduced at high donor lysis rates. The peer-reviewed version reports that cell-cell aggregation is associated with growth inhibition rather than demonstrated to enhance HGT, and that chromosomal DNA transfer was below detection. What remains open is: (i) the molecular mechanisms and genetic determinants of the observed inter-species physical association between E. coli and B. subtilis and whether donor-recipient contact modulates transformation efficiency beyond its effect on DNA availability; (ii) the quantitative dependence of transformation efficiency on plasmid features, donor stress-response state, lysis rate, and single-cell heterogeneity, and whether these determinants generalize to other species pairs, plasmid types, and community contexts; and (iii) the in-situ identification of donor and recipient taxa engaged in competence-mediated horizontal gene transfer in natural communities, the rates at which it occurs, and its modulation by environmental perturbations such as antibiotic exposure.

Background

Horizontal gene transfer (HGT) — the movement of genetic material between organisms rather than from parent to offspring — is a dominant force in bacterial evolution, most consequentially in the spread of antibiotic-resistance genes. Three mechanisms mediate bacterial HGT: conjugation, which requires direct cell-cell contact and donor-encoded transfer machinery; transduction by bacteriophages; and natural transformation, in which a bacterium in the physiological state of natural competence takes up extracellular DNA (eDNA) from its environment and can stably maintain it. Natural competence is widespread across Gram-positive and Gram-negative bacteria and, unlike conjugation, does not require a living donor: any lysed or DNA-secreting cell can serve as a DNA source. The classic paradigm treats transformation efficiency as recipient-controlled — set by the regulation of competence development and by ambient eDNA levels — with donors as passive contributors.

Cheng et al. (Molecular Systems Biology, 2023; bioRxiv preprint posted October 2020) tested this paradigm in a synthetic two-species community: E. coli donors carrying a plasmid with a B. subtilis chromosomal-integration cassette, co-cultured with engineered B. subtilis recipients in which the master competence regulator ComK was inducibly expressed. Time-series quantification of species abundance, eDNA release, and transformant yield showed that plasmid release and HGT efficiency are substantially enhanced in co-culture relative to monocultures and depend on the initial densities of both species. Mechanistically, the donor proved to be a key determinant: plasmids induce the SOS response (the bacterial DNA-damage stress regulon, which includes cell-death programs) in E. coli, and RecA (the central bacterial homologous-recombination protein) promotes plasmid multimerization — concatemeric plasmid forms that transform B. subtilis more efficiently — alongside SOS-associated eDNA release. HGT was efficient from live, heat-killed, or translation-inhibited (chloramphenicol-treated) donors, but experimentally increased donor lysis reduced plasmid transfer because released genomic DNA and nucleases compete with and degrade plasmid DNA; chromosomal DNA transfer from E. coli to B. subtilis was below detection in the same system. The two species physically aggregate in co-culture, an interaction associated with reduced B. subtilis growth, and bioinformatic searches found E. coli plasmid replication origins in wild-type Bacillus genomes, raising biocontainment concerns. Related observations elsewhere — that antibiotic stress can directly induce transformability in Streptococcus pneumoniae (Prudhomme et al., 2006), and that Staphylococcus aureus in mixed biofilms acquires SCCmec (staphylococcal chromosomal cassette mec, the mobile genetic element carrying the methicillin-resistance determinant) from living donor cells at high frequency (Maree et al., 2024) — underscore that donor physiological state and community context shape transformation, yet the underlying determinants remain poorly characterized. Metagenomic methods that link mobile genetic elements to host taxa at strain resolution, such as bacterial Hi-C (proximity-ligation metagenomics, in which physically associated DNA is ligated and sequenced to reconstruct host-vector networks), now permit donor-recipient networks to be inferred in complex communities (e.g., Kent et al., 2020; Robinson et al., 2025), but such associations do not identify the transfer mechanism or measure competence-specific transfer rates.

Problem Statement

Determine the molecular mechanisms and ecological determinants that control the efficiency of HGT via natural competence in microbial communities, building on the E. coli (donor)–B. subtilis (recipient) co-culture system of Cheng et al. (2023). The source discussions pose three distinct open directions — each an open question in its own right rather than a stage of a single programme with a joint completion boundary:

(1) Contact mechanism. What molecular mechanisms and genetic determinants mediate the physical association (aggregation) observed between E. coli donor and B. subtilis recipient cells in co-culture, and does donor-recipient contact per se modulate natural-transformation efficiency in addition to its consequences for eDNA availability and recipient growth? (The version-1 preprint reported that spatial proximity of the two species substantially enhanced plasmid transfer; that claim was not carried into the published version, which reports aggregation together with reduced B. subtilis growth. Neither version resolves the mechanism of the association.)

(2) Quantitative determinants and generalization. How do plasmid features (e.g., replication origin, SOS-inducing capacity, multimerization propensity), donor physiological state (lysis rate, single-cell SOS-response heterogeneity), and recipient competence heterogeneity quantitatively determine eDNA release and HGT efficiency? Does this donor-side control — established for one E. coli–B. subtilis pair and a small set of plasmids — generalize to other bacterial species pairs, plasmid types, and community contexts?

(3) In-situ ecology of competence-mediated transfer. In natural microbial communities, which taxa act as donors and recipients in competence-mediated HGT, at what rates does it occur, and how do environmental perturbations such as antibiotic exposure modulate these events? Approaches may combine time-series strain-resolved sequencing with proximity-ligation (Hi-C) metagenomics and network inference, but must distinguish competence-mediated transfer from conjugation and transduction.

The design of anti-HGT interventions, mentioned in the source's discussion as an application, is an open-ended engineering aspiration and is not part of the scientific core of this problem.

References

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