The primary role of the stationary phase in bacterial growth is:
Upon entry into stationary phase nutrient exhaustion does not equate to dormancy but triggers extensive differentiation for long term survival. Genome wide expression reprogrammed by ppGpp alarmone binding RNA polymerase, sigma factor switch from RpoD to RpoS general stress sigma encoded by rpoS. RpoS regulon includes oxidative protection catalase HPII, exonuclease, DNA protection Dps compacting chromosome sequestering iron preventing Fenton chemistry, acid resistance Gad system glutamate decarboxylase, trehalose synthesis. Secondary metabolism activated nonribosomal peptide synthetases, polyketide synthases producing antibiotics bacitracin, streptomycin, prodigiosin to suppress competitors scavenging remaining nutrients. For sporeformers Spo0A phosphorelay initiates sporulation cascade costing ATP but ensuring durable spore with cortex dipicolinic acid calcium dehydrating core, SASPs protection. Biofilm formation curli fimbriae, cellulose, Pel polysaccharides. Energy production via maintenance metabolism using endogenous polyhydroxybutyrate, glycogen. Rapid fission not feature. This productive stationary physiology explains why antibiotic yields peak post exponentially and why industrial fermentations harvest secondary metabolites at stationary, also importance for persistence in natural environments where starvation predominates and competition fierce.
Ref: Prescott's Microbiology, 11th ed., Chapter 7: Stationary phase role - Sporulation and secondary metabolites.