When starved, the Gram-positive bacterium Bacillus subtilis forms durable spores for
survival. Sporulation initiates with an asymmetric cell division, creating a large mother cell and a
small forespore. Subsequently, the mother cell membrane engulfs the forespore in a phagocytosislike
process. However, the force generation mechanism for forward membrane movement remains
unknown. Here, we show that membrane migration is driven by cell wall remodeling at the leading
edge of the engulfing membrane, with peptidoglycan synthesis and degradation mediated by
penicillin binding proteins in the forespore and a cell wall degradation protein complex in the
mother cell. We propose a simple model for engulfment in which the junction between the septum
and the lateral cell wall moves around the forespore by a mechanism resembling the ‘template
model’. Hence, we establish a biophysical mechanism for the creation of a force for engulfment
based on the coordination between cell wall synthesis and degradation.