Rethinking the plasmid paradox: when plasmid costs do not affect fitness.

Plasmids frequently impose measurable fitness costs on their bacterial hosts, yet they remain abundant across clinical and environmental microbiomes. This apparent contradiction, known as the plasmid paradox, has traditionally been explained through mechanisms such as horizontal gene transfer, compensatory evolution, addiction systems, and fluctuating selection. Here we suggest that part of the paradox may arise from implicit physiological assumptions embedded in most empirical measurements-specifically, the assumption that growth rate is a direct proxy for fitness and that plasmid burden necessarily reduces it. We argue that these assumptions may not hold under many ecological conditions. We formalize cell division time as the maximum of several required cellular modules, including cytoplasmic biosynthesis and membrane or envelope synthesis. If plasmid carriage primarily increases cytoplasmic demand, its cost will be expressed only when cytoplasmic processes constitute the dominant bottleneck for growth. When other modules limit division, plasmid-associated burdens may be physiologically real yet evolutionarily silent. More broadly, equating fitness with maximal exponential growth rate overlooks well-established growth-survival trade-offs in bacteria, suggesting that plasmid costs measured under optimized laboratory conditions may systematically overestimate ecological selection against plasmid carriage.