forspore

A forespore is the nascent, daughter cell that forms during the sporulation process of certain Gram-positive bacteria, notably Bacillus and Clostridium species. It represents the smaller compartment created by an asymmetric division and is eventually converted into a mature endospore, a highly resistant dormant cell.

During sporulation, nutrient limitation initiates a regulatory cascade, often beginning with the phosphorylation of Spo0A, which activates genes required for asymmetric septation. This division produces a small compartment, the forespore, and a larger mother cell. The mother cell then engulfs the forespore, surrounding it with two membranes. In the ensuing stages, the forespore begins to develop protective layers, including a peptidoglycan cortex and a protein coat, with expression regulated by compartment-specific sigma factors. In Bacillus subtilis, for example, sigmaF becomes active in the forespore, followed by sigmaG, while sigmaE acts in the mother cell, followed by sigmaK.

Structural maturation involves dehydration of the forespore core and assembly of protective layers, yielding a highly resistant endospore. The mother cell ultimately lyses, releasing the mature spore into the environment. The forespore thus serves as the precursory compartment that sequesters the genome and essential machinery during dormancy and is transformed into a metabolically dormant, resilient endospore upon completion of coat and cortex formation.

Germination can occur when favorable conditions return, triggering rehydration, resumption of metabolism, and return to vegetative growth. The forespore concept is central to understanding bacterial sporulation and the remarkable durability of endospores.