#bacterial growth curve

Microbial Physiology

What is the primary reason for bacterial death in the death phase?

Transition to death phase reflects inability to maintain energy homeostasis and repair. Prolonged incubation exhausts fermentable carbon, electron acceptors, phosphate, amino acids, halting ATP generation via respiration and substrate level phosphorylation. Concomitantly toxic end products accumulate: organic acids acetate and lactate lower pH below 5, alcohols ethanol, ammonia, hydrogen sulfide inhibit cytochrome oxidases, and reactive oxygen species superoxide and H2O2 from autooxidation damage iron sulfur clusters. Oxidative lesions cause DNA strand breaks, protein carbonylation, membrane lipid peroxidation increasing permeability. Proteolysis and autolysis via peptidoglycan hydrolases Atl, LytA release intracellular content. Without ATP, chaperones DnaK GroEL cannot refold, SOS response fails, cells lyse. Survivors enter viable but nonculturable state or sporulate. Decline follows first-order kinetics, exponential decay constant kd reflects medium conditions. Understanding accumulation of wastes explains why dilution into fresh medium resuscitates growth and why industrial fed batch strategies remove or neutralize toxic byproducts to extend productive lifespan and improve yield.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 6: Death phase - Toxic metabolites accumulation.

View Details

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.

View Details

Microbial Physiology

Which phase is characterized by exponential bacterial growth?

Log phase also termed exponential phase represents balanced growth state where every cellular constituent doubles at identical specific rate. Under saturating nutrients and aeration, DNA replication initiates synchronously via DnaA ATP activation at oriC, elongation by PolIII, segregation by MukBEF SMC complex, and cytokinetic ring FtsZ with FtsA ZipA divides cells with constant interdivision time. Specific growth rate mu maximal, defined by mu equals ln2 over generation time g. Equation dN over dt equals mu N integrated yields N equals N0 times e to mu t, producing straight line on semi-log plot. Ribosome fraction highest, RNA to DNA ratio elevated, cell size uniform and most sensitive to inhibitors targeting peptidoglycan synthesis penicillin, protein synthesis tetracycline. Generation time for Escherichia coli in rich medium 20 minutes, Mycoobacterium tuberculosis 18 hours. Studying log phase ideal for physiology because global regulation governed by housekeeping sigma RpoD, stress sigma RpoS inactive, providing steady state transcriptome useful for mutation rate, proteomics, and cell cycle analyses without starvation artifacts.

Ref: Prescott's Microbiology, 11th ed., Chapter 7: Exponential phase and balanced growth.

View Details

Microbial Physiology

In which phase of the bacterial growth curve do bacteria adapt to their environment?

After transfer to fresh medium, cells display temporarily static numbers while metabolic machinery adapts, called lag phase. Stationary phase inoculum carries oxidative damage, aggregated proteins, low rRNA pool, condensed nucleoids, and altered membrane fatty acids. Cells sense new environment via two component systems PhoP PhoQ, EnvZ OmpR, induce specific sugar transporters, amino acid biosynthesis operons, and pentose phosphate pathway to generate NADPH. Ribosomal RNA operons rrn rapidly upregulated, increasing ribosome content, tRNAs charged, ATP pools rebuilt via substrate level phosphorylation. DNA lesions repaired by UvrABC, RecA. Morphologically cells enlarge but septation delayed until threshold protein mass reached, following Cooper Helmstetter model. Duration varies inversely with inoculum fitness: young exponential inoculum shows minimal lag, old stressed shows prolonged lag while proteases ClpXP degrade misfolded proteins. High density does not extend lag per se. This preparatory period ensures that when division resumes, biosynthetic capacity matches environment preventing abortive replication and wasted resources optimizing fitness.

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 6: Lag phase adaptation.

View Details

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.

View Details

Microbial Physiology

Which of the following occurs in the stationary phase of bacterial growth?

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.

Ref: Prescott's Microbiology, 11th ed., Chapter 6: Stationary Phase, Sporulation and Secondary Metabolites.

View Details

Microbial Physiology

Which phase of the bacterial growth curve is also known as the acclimatization phase?

Batch growth exhibits four phases in closed culture: lag, log, stationary and death. Lag phase immediately after inoculation into fresh medium appears flat by cell number but represents highly active metabolic acclimatization. Cells sense new nutrient composition, pH, osmolarity and oxygen via EnvZ-OmpR and chemoreceptors, induce necessary transporters and catabolic operons such as lac when lactose present, repair oxidative damage accumulated in stationary-phase inoculum via base excision repair, and massively synthesize ribosomes, tRNAs and aminoacyl-tRNA synthetases to prepare for rapid protein synthesis. DNA replication begins but division lags, so population count remains constant while cell size, RNA to DNA ratio and protein synthesis rate increase markedly. Duration depends on inoculum age, size and medium shift magnitude. Log phase follows with exponential doubling at maximal rate, stationary arises when growth equals death due to nutrient depletion, death when viability declines. Acclimatization synonym accurately describes lag and controlling its length helps optimize fermentation start-up and antibiotic susceptibility assays where log-phase cells are targeted by wall-active agents.

Ref: Prescott's Microbiology, 11th ed., Chapter 6: Lag Phase as Acclimatization Phase of Growth Curve.

View Details