#bacterial culture

Microbial Physiology

Which of the following is not a factor affecting bacterial growth?

Microbial growth influenced by physicochemical variables reflecting physiology. Temperature controls enzyme catalysis activation energy, membrane fluidity, protein thermostability, cardinal minima optima maxima distinct per species. Oxygen availability categorizes obligate aerobes requiring cytochrome oxidases and superoxide dismutase, obligate anaerobes lacking detoxification killed by oxygen, facultative anaerobes switching via Fnr ArcAB, microaerophiles needing low oxygen. pH affects proton motive force, amino acid charge, solubility nutrients. Water activity aw controlled by osmotic solute

Ref: Madigan et al., Brock Biology of Microorganisms, 16th ed., Chapter 7: Factors affecting growth - Mitochondria absent.

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Microbial Physiology

A bacterial culture was diluted 1000-fold, and 0.1 mL of this sample was plated. If 100 colonies formed, what was the or

Viable count estimation via plate counting requires accounting for dilution magnitude and volume plated to infer original concentration. Only 30 to 300 colonies per plate statistically reliable. General formula colony forming units per mL equals colonies observed divided by product dilution factor fraction and volume plated in milliliters. If culture diluted 1000 fold meaning dilution factor 1 times ten to minus three, and 0.1 mL aliquot plated yielding 100 colonies, denominator equals ten to minus three times 0.1 equals ten to minus four. One hundred over ten to minus four equals one million

Ref: Prescott's Microbiology, 11th ed., Chapter 7: Viable count - CFU/mL = colonies/(dilution×volume).

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Microbial Physiology

What is the primary reason for an extended lag phase in bacterial cultures?

Extended lag phase not caused by high density per se but by requirement for extensive macromolecular synthesis before replication can resume. Cells transferred from old, stationary, or differently composed medium lack enzymes for new carbon source utilization, transport permeases, central metabolic dehydrogenases, and contain degraded ribosomes, oxidized proteins, and damaged DNA. They must induce transcription of catabolic operons lac, mal via cAMP CRP, synthesize ribosomal proteins rps, rpl, produce tRNA, generate ATP via substrate level phosphorylation, repair chromosome via Uvr, Rec system

Ref: Prescott's Microbiology, 11th ed., Chapter 7: Extended lag - Enzyme synthesis requirement.

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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 Mo

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

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Microbial Physiology

The CFU/mL of a bacterial culture is calculated using the formula:

Colony forming units per milliliter provides standard measure of viable culturable bacteria assuming each colony derives from single viable unit capable of growth on agar. To achieve statistically reliable counts, serial tenfold dilutions are prepared in diluent and fixed aliquot such as 0.1 mL is spread onto nutrient agar via spread plate using sterile spreader or mixed into molten agar via pour plate. After appropriate incubation colonies are counted; ideal range 30 to 300 avoids overcrowding and sampling error. Because only diluted fraction was plated, conversion to original density require

Ref: Prescott's Microbiology, 11th ed., Chapter 6: CFU/mL Calculation and Dilution Factor Method.

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