Pathogen species and capsule expression shape opportunistic transfer and the recipient response during plasmid RP4 conjugation

Conjugative plasmids drive bacterial evolution and niche adaptation, yet how their active acquisition reshapes host transcription remains poorly understood. Most studies focus on stable plasmid carriage, overlooking the dynamic transcriptional changes during conjugation itself. Here, we investigate the active conjugation of RP4, a prototypical broad-host range plasmid, to E. coli, and K. pneumoniae recipients. We observe that an immediate, host- and surface factor-dependent transcriptional response occurs. This includes activation of non-SOS stress pathways, motility, exopolysaccharide production, anaerobic respiration, and metabolic adaptation. These responses do not inhibit conjugation, suggesting they serve to maintain host homeostasis, particularly of envelope encoded functions. Capsule expression prevents these responses and inhibits RP4 activation by physically blocking donor-recipient contact. Unexpectedly, RP4 can bypass the capsular barrier through opportunistic transfer into a sub-population of phenotypically thin-capsulated cells. These findings show how RP4, the hosts species, and surface architecture shape the active conjugation transcriptional landscape, which may have implications for plasmid dissemination and bacterial evolution.