Gram-negative bacteria must coordinate remodeling of the peptidoglycan cell wall with invagination of the outer membrane to preserve envelope integrity during growth and division. The conserved Tol-Pal system has been implicated in coordinating these processes, yet its physiological contribution to envelope organization remains unclear and may depend on environmental context. Here, we examined the role of Tol-Pal in coordinating envelope remodeling in Acinetobacter baumannii across distinct growth environments. Loss of Tol-Pal did not cause a major population growth defect, and septal peptidoglycan incorporation remained largely preserved under standard laboratory growth conditions. In contrast, under specific environmental conditions-including nutrient-rich media, altered osmotic conditions, and host-like environments-Tol-Pal deficiency disrupted the spatial organization of cell division and cell morphology. Tol-Pal mutants also exhibited modest but reproducible reductions in outer membrane barrier robustness and decreased fitness in environmental and host-associated contexts. Together, these findings demonstrate that Tol-Pal is not an essential component of the core division machinery but instead contributes to the coordinated organization of the Gram-negative envelope under conditions that impose additional physiological demands. More broadly, our results highlight how environmental context can reveal conditional roles for conserved envelope systems that are not apparent during standard laboratory growth.
Olea-Ozuna, R. J., Furlan, B., Tiwari, S., Gong, H., Hunt-Serracin, A. C., Whalen, M., Massidda, O., Dillon, N. A., Boll, J. M.
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