Cellular differentiation is important feature of complex life, during which one cell type transforms into another, in a manner that is often irreversible. The mechanisms that lock in irreversibility during differentiation remain unclear, even in simpler models of cellular differentiation, such as bacterial sporulation. Sporulation occurs in response to nutrient deprivation and results in the formation of highly-resistant, dormant spores, such as those produced by the soil bacterium Bacillus subtilis and human pathogens Clostridiodes difficile and Bacillus cereus that impacts food safety and health. However, the exact mechanism(s) that lock-in the sporulation program remain unknown.   Early in sporulation, the developing spore becomes committed to the sporulation process and cannot return to a vegetative state, even if nutrients become available – this is called commitment. Here we challenge the 20-year-old commitment model in the model organism. Bacillus subtilis and identify three factors that commit the developing spore to sporulation. Using quantitative image analysis, and various phenotypic assays, we show that the absence of these factors and in the presence of nutrients, the developing forespore outgrows and exhibit characteristics consistent with a vegetative-growth. We propose a model whereby commitment to sporulation involves physiological and metabolic changes that prevent the developing spore from resuming vegetative growth in response to nutrient availability, thereby locking the spore into the sporulation pathway.   Collectively, our data provide new insight into the a critical stage of sporulation. This knowledge has the potential to provide the basis for future strategies aimed at minimising the negative impact of spore-forming bacteria.