Biswas, K.; Brenner, N.; Salman, H.

Exponential accumulation of cell size and highly expressed proteins is observed in many bacterial species at the single cell level. Exponential rates exhibit cycle-by-cycle fluctuations and correlation across components - different proteins and cell size. In such balanced growth, homeostasis of all variables is maintained simultaneously. In this study, we examine the phenomenological features of growth-rate variability and present a theoretical framework to explain them and the emergence of multi-variable homeostasis. Our findings suggest that homeostasis results from the emergence of a high-dimensional dynamic attractor supporting balanced exponential growth. The stability of this attractor leads to a decay in instantaneous growth rate noise throughout the cell cycle, aligning with empirical findings. We also correctly predict that cells with higher growth rates experience a faster decay in growth rate noise. Surprisingly, our analysis identifies noise generated by uneven cell division as the primary source of variability in growth rates. The theory offers a clear explanation for many observations, validated against extensive single-cell data. The emergence of homeostasis spontaneously from dynamic interactions suggests that specific control mechanisms correcting deviations from a target may be unnecessary.