Practice question

When batch cultures exhaust carbon, nitrogen or phosphate and accumulate organic acids like acetate lowering pH, growth rate decelerates and matches death rate producing plateau termed stationary phase. This phase triggers global reprogramming governed by alternative sigma factor sigmaS encoded by rpoS in Escherichia coli and master regulator Spo0A in Bacillus subtilis. Cells activate stringent response via RelA sensing uncharged tRNA and SpoT synthesizing alarmones ppGpp and pppGpp which inhibit stable RNA promoters and redirect transcription to survival genes. Adaptations include highly resistant endospore formation via asymmetric septation, engulfment and cortex synthesis in spore-formers, storage of carbon as glycogen and polyhydroxybutyrate inclusions, synthesis of secondary metabolites including antibiotics, toxins and siderophores to scavenge resources and inhibit competitors, upregulation of catalase KatE and DNA-protecting Dps ferritin. Nutrient levels are low, net growth zero, maximum per-cell metabolic rate occurred earlier in mid-log where ribosomes most active. Hence sporulation and secondary metabolite production typify stationary phase survival strategy not growth.