Gram-negative bacteria maintain strict lipid asymmetry across their outer membrane. The Mla system exports mislocalized phospholipids back to the inner membrane, keeping the outer leaflet clean. This spatial separation is considered fundamental to membrane function.
Ekiert and colleagues found that Acinetobacter baumannii has a second system. ClsO is a cardiolipin synthase that sits on the outer membrane surface. When the Mla system fails and phospholipids accumulate where they should not be, ClsO synthesizes cardiolipin directly into the outer leaflet. This restores membrane function.
The restoration works by breaking the rule. Cardiolipin in the outer membrane violates the asymmetry that the Mla system was maintaining. ClsO does not fix the asymmetry. It achieves the goal of the asymmetry — a functional barrier — through means that the asymmetry was supposed to prevent. The backup system works by doing the thing the primary system was designed to stop.
This is not a failure of compartmentalization. It is a second strategy for the same objective. The Mla system maintains membrane function through lipid separation. ClsO maintains membrane function through lipid transformation. When one path closes, the cell takes the other. The fact that the second path violates the first path's logic is irrelevant because both paths lead to the same destination: a membrane that resists bile salts and antibiotics.
Loss of ClsO sensitizes cells to both. The backup is load-bearing. ClsO homologs are conserved across Proteobacteria, which means this is not a quirk of one species but a widespread strategy. Bacteria carry a contingency plan that operates by opposite principles from the primary system, ready for when the primary system fails. The backup does not restore the original state. It substitutes a different state that serves the same function.