Micrococcus is an ubiquitous genus from the phylum Actinomycetota which can be found in harsh environments exposed to extreme conditions. Megaplasmids have been identified in Micrococcus strains and thought to play a role in mediating resistance against, e.g., UV irradiation, dryness, hypersalinity, high concentrations of heavy metals and different macrolid antibiotics.

Horizontal gene transfer may be important to disseminate these survival factors. Natural transformation, i.e. the uptake of naked DNA from the environment is a known trait for Micrococcus strains, while conjugation of linear plasmids has not been shown for this genus before.

This study presents compelling evidence for the transfer of a linear invertron-type megaplasmid from the original host to a recipient Micrococcus strain and further transfer to another Micrococcus strain in a DNAse-insensitive and competence genes-independent way.

Mating experiments were performed after growth in full medium on minimal medium plates for 48 h with and without DNAse I. The donor strain harbored the linear plasmid which contains an erythromycin resistance gene, whereas the kanamycin resistant recipient strain was incapable of DNA uptake via natural transformation due to a mutation of an essential competence gene. In addition, control experiments were performed with isolated plasmids to show that cells on selective plates were not transformed by natural transformation.

Transconjugants were selected on full medium plates containing both antibiotics, and were extensively checked via PCR, while the presence of the megaplasmid was also verified using PFGE. To determine the conjugation frequency, the CFU of recipient cells were determined on plates containing only kanamycin. Notably, the addition of DNAse I did not impede the transfer of the linear plasmid to recipient cells.

This study uncovered a novel mechanism for horizontal gene transfer of linear plasmids between Micrococcus strains, which awaits further investigation in the future.