#microbiology basics

Microbial Physiology

Which factor is most critical for bacterial doubling time?

Bacterial doubling time influenced multifactorially because growth rate emerges from integrated functioning of transport, catabolism, respiration, biosynthesis. Type of medium richness determines carbon supply glucose versus acetate, amino acids availability bypassing biosynthesis costs, trace elements iron cofactor for cytochromes. Oxygen levels dictate energetics aerobic respiration using NADH dehydrogenase, succinate dehydrogenase, cytochrome bo3 generating proton motive force giving up to 38 ATP per glucose versus anaerobic fermentation 2 ATP dramatically altering mu. Temperature near optimum increases enzyme kcat following Arrhenius, influences membrane fluidity controlling nutrient uptake diffusion, deviating causes heat shock cold shock stress. pH, osmolarity also matter but question highlights three major listed; all contribute synergistically. Regulatory networks integrate signals CRP for carbon, Fnr Arc for oxygen, RpoH for heat to modulate gene expression adjusting metabolism. Therefore accurate prediction generation time requires considering all above factors together rather than single parameter isolation explaining variability observed between laboratory flasks and natural habitats like gut where nutrients oxygen gradients produce differing growth rates.

Ref: Prescott's Microbiology, 11th ed., Chapter 7: Factors controlling doubling time - Medium oxygen temperature.

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Microbial Physiology

In a batch culture, which phase has zero net growth?

In batch culture population dynamics defined by net rate birth minus death. Lag phase net near zero during adaptation, log phase net strongly positive with mu dominant, stationary net zero because division still occurs using scavenged nutrients from lysed cells cryptic growth at rate equal to death rate from oxidative damage, acid stress, toxin accumulation. Viable counts plateau because new cells forming balance those losing colony forming ability. Microscopically heterogeneity high some cells dividing, some filamentous, some entering persistence via toxin antitoxin HipA. Optical density may continue slight rise due to cell size and storage inclusions but CFU flat. Death phase follows when death exceeds birth yielding negative net. Recognizing stationary zero net growth not absence activity crucial for understanding antibiotic tolerance persister cells that survive bactericidal drugs without resistance by dormancy, for interpreting growth curves where plate counts misleading if aggregated, and for designing fed batch strategies where preventing zero net extends productive exponential phase improving biomass yield and recombinant protein expression.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 6: Batch culture - Zero net growth in stationary.

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Microbial Physiology

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.

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Microbial Physiology

Barophiles are microorganisms adapted to:

Barophiles also called piezophiles specialized for high hydrostatic pressure exceeding 10 megapascals up to 110 MPa in hadal trenches. Pressure increases ordering of acyl chains reducing membrane fluidity similar to low temperature. Adaptations include increased proportion unsaturated polyunsaturated fatty acids EPA DHA introducing kinks maintaining fluidity, branched iso anteiso fatty acids, shorter chain length, incorporation cholesterol like molecules and carotenoids. Proteins exhibit reduced void volume due to compact packing, increased multimerization, reduced compressibility. Piezolytes beta hydroxybutyrate accumulate stabilizing. Many isolates Colwellia psychrerythraea, Moritella yayanosii, Shewanella benthica are psychropiezophiles requiring both cold and pressure. Obligate barophiles fail to grow at atmospheric pressure indicating pressure dependent enzyme function, ribosome assembly, and transcription complexes. Study of barophiles informs limits biosphere, pressure sterilization pascalization preserving food nutrients, origins life hypothesis that deep sea vents could be cradle where high pressure stabilizes biomolecules. Their cytochromes and ATP synthases adapted maintaining proton motive force under compression in deep ocean habitats.

Ref: Madigan et al., Brock Biology of Microorganisms, Chapter 19: Barophiles - High hydrostatic pressure adaptation.

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Microbial Physiology

In bacterial growth, water availability is controlled by:

Water availability parameter aw water activity influences growth because intracellular reactions require aqueous medium, turgor pressure drives expansion. aw equals relative humidity 100 percent in equilibrium. Bacteria regulate response to aw change mainly via osmotic pressure sensing osmosensors EnvZ. Hyperosmotic shock water exits cytoplasm plasmolysis outer membrane separates, cytoplasm shrivels, growth arrested. Compatible solutes accumulation potassium glutamate, proline, ectoine, trehalose synthesised or imported via ProP, BetT transporters balances osmolarity without denaturing proteins. Mechanosensitive channels MscL MscS open to release solutes during hypoosmotic downshock preventing lysis. Halophiles require high sodium for protein solvation acidic proteome. Temperature indirectly affects aw through evaporation, oxygen concentration affects respiration not hydration, barometric pressure affects gas solubility. Thus osmotic pressure is primary determinant controlling water availability defining limits in dried foods jerky aw 0.8, salted cod, brines, desert soils. Food industry reduces aw by adding salt sugar to inhibit spoilage microbes and extend shelf life without refrigeration.

Ref: Brock Biology of Microorganisms, 16th ed., Chapter 5: Water activity and osmotic pressure.

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Microbial Physiology

Which type of bacteria thrive at pH below 5.5?

pH adaptation shapes microbial diversity. Acidophiles thrive at pH below 5.5 often below 3, found acid mine drainage Rio Tinto, volcanic pools, fermented foods yogurt, gastric epithelium. Maintaining cytoplasm near neutrality around pH 6 involves highly impermeable tetraether lipid membranes, active proton extrusion via primary P type ATPases and secondary antiporters, cytoplasmic buffering with basic amino acids arginine, acid chaperones HdeA preventing periplasmic protein aggregation, and DNA repair systems resistant to depurination. Membrane proteins reduced proton channels, periplasm buffering. Acidithiobacillus ferrooxidans oxidizes ferrous iron sulfur at pH2 generating ferric iron. Helicobacter pylori survives stomach via urease producing ammonia alkalinizing periplasm. Neutrophiles optimum 5.5 to 8.5 include most pathogens Escherichia coli, alkaliphiles optimum above 8.5. Recognizing acidophily important for food preservation acidification inhibits neutrophilic spoilage, controlling bioreactor pH, understanding dental caries Streptococcus mutans acid tolerant demineralizes enamel, and biomining where acidophiles leach metals from ores and generate acid mine drainage environmental challenges.

Ref: Prescott's Microbiology, 11th ed., Chapter 7: Acidophiles - Growth below pH 5.5.

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