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#growth phases

9 public questions tagged with this topic.

Microbial Physiology

During which phase does sporulation occur in spore-forming bacteria?

Endospore formation among Firmicutes is strategy for persistence when environmental conditions deteriorate, occurring specifically upon entry to stationary phase. Triggered by nutrient limitation, high cell density quorum peptides Phr, ComX, accumulation of guanosine tetraphosphate ppGpp produced by RelA SpoT. Phosphorelay KinA to KinE autophosphorylate and transfer to Spo0F to Spo0B to Spo0A, master transcription factor. Phosphorylated Spo0A activates genes for asymmetric polar septum via SpoIIE phosphatase dephosphorylating SpoIIAA leading to FtsZ relocation. Forespore compartment trapped chromosome only one third initially, translocated by SpoIIIE DNA motor. Engulfment proceeds by mother cell membrane phagocytosing forespore, then cortex peptidoglycan between double membranes, germ cell wall, dipicolinic acid calcium complex dehydrating core, SASPs binding DNA protecting from UV. Mother cell lysis releases mature spore. This lengthy ATP demanding program requires still functional metabolism, so initiation must occur early stationary, not death, when energy reserves remain sufficient to complete spore assembly and resist environmental insults.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 7: Endospore formation during stationary phase.

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Microbial Physiology

The bacterial growth curve represents the number of live cells over time when cultured in:

Laboratory bacterial growth curve plotting viable number versus time is obtained from synchronous population in closed liquid batch culture, typically nutrient broth shaken for aeration. Solid agar shows colonies spatially separated, diffusion limited, colony counts reflect single time point not temporal kinetics. In broth, aliquots removed at intervals, optical density at 600 nm measures light scattering correlated to biomass and serial dilutions plated counting colony forming units representing live cells. Curve reveals four phases lag adaptation, log exponential where nutrients exceed Ks Monod constant, stationary zero net growth, death decline. Liquid allows uniform distribution of nutrients, oxygen, quorum molecules acyl homoserine lactones, ensures representative sampling. Continuous culture chemostat extends exponential by fresh medium inflow. Solid medium suited for isolation and enumeration but not for dynamic growth modeling. Recognizing that growth curves derive from liquid batch culture is essential for interpreting specific growth rate, carrying capacity, yield coefficients in physiology experiments, antibiotic time-kill curves, and industrial fermentation scale up where broth conditions are strictly controlled.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 6: Bacterial growth curve - Batch liquid culture.

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Microbial Physiology

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.

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