In which growth phase do bacteria prepare for sporulation?
Sporulation is energy expensive differentiation reserved for conditions where vegetative growth cannot continue. Throughout exponential phase nutrients abundant, ribosome content high, cells divide via FtsZ divisome and elongasome MreB. As population density rises and carbon nitrogen phosphate become scarce, quorum peptides ComX CSF and nucleotide second messengers ppGpp c-di-GMP accumulate, sensed by KinA KinB KinC histidine kinases. Kinases activate phosphorelay transferring phosphate from Spo0F to Spo0B to Spo0A master regulator. Phosphorylated Spo0A binds 0A boxes activating over 120 genes initiating asymmetric septation near pole governed by SpoIIE mediated FtsZ relocation. Sigma factor cascade follows compartment specific: SigF in forespore, SigE in mother cell governing engulfment where mother membrane migrates surrounding forespore, then SigG and SigK driving cortex peptidoglycan synthesis, dipicolinic acid calcium complex dehydrating core, and proteinaceous coat crosslinked by transglutaminase. Trigger occurs precisely as cultures enter stationary phase because sufficient ATP and intact chromosomes remain to complete multi-hour morphogenesis before total starvation collapses membrane potential and biosynthetic capacity required for successful spore maturation.
Ref: Prescott's Microbiology, 11th ed., Chapter 7: Bacterial sporulation - Stationary phase Spo0A phosphorelay.