Plasmid mediated antibiotic resistance in aquaculture travels through conjugation between bacteria, not through fish that move. pRAS2, a conjugative plasmid found in Aeromonas salmonicida, encodes a complete type IV secretion system and uses it to mobilize a second, non conjugative plasmid, pRAS3, into new host cells, while pRAS2 itself stays behind in the donor. The event is too rare to emerge in routine culture, and it needs no antibiotic exposure at all to occur. Why a single 1992 fish isolate still matters Antibiotics are used at scale in aquaculture to treat bacterial infection, and that use is one of the strongest forces pushing antimicrobial resistance (AMR) upward in farmed aquatic…