Questions: Microbial Growth Kinetics and Population Dynamics

Question 1

Bacteria are transferred from a rich glucose broth to a minimal medium containing only lactose. Compared to transfer within the same glucose medium, what happens to the lag phase and why?

• A. The lag phase is shorter — bacteria grow faster when they must efficiently use a simpler substrate
• B. The lag phase is longer — bacteria must synthesize new enzymes (such as β-galactosidase from the lac operon) to metabolize lactose
• C. There is no lag phase — bacteria adapt instantaneously to any available carbon source
• D. The lag phase is longer — bacteria are dormant while they wait for optimal temperature and pH

Answer: (B)

The lag phase duration reflects how much new cellular machinery must be synthesized to exploit the new environment. Transferring to lactose requires inducing the lac operon to produce β-galactosidase and lactose permease — proteins not expressed when glucose is available. This synthesis takes time, extending the lag phase compared to a glucose-to-glucose transfer. Option D captures a common misconception: lag phase cells are not dormant — they are metabolically active, synthesizing the enzymes, ribosomes, and transport proteins needed for the new conditions.

Question 2

On a semilogarithmic plot (log of viable cell count on the y-axis, time on the x-axis), the exponential growth phase appears as a straight line. What does the slope of this line represent?

• A. The doubling time g — steeper slope means shorter doubling time
• B. The specific growth rate μ — the slope equals μ = ln(2)/g
• C. The carrying capacity K — the y-intercept when the line extrapolates to maximum
• D. The number of generations elapsed — each unit rise equals one generation

Answer: (B)

On a semilogarithmic plot, exponential growth N(t) = N₀e^(μt) becomes a straight line with slope μ (the specific growth rate). The specific growth rate μ relates to doubling time by μ = ln(2)/g ≈ 0.693/g. A steeper slope means faster growth (higher μ, shorter g). The doubling time g is not directly the slope — it is the time interval over which the log increases by ln(2). Understanding the semilog plot is central to calculating growth parameters from experimental data.

Question 3

During the lag phase, bacterial cells are not dormant — they are actively synthesizing proteins, enzymes, and ribosomes needed to exploit the new growth environment.

• T. True
• F. False

Answer: True

The lag phase is a period of intense metabolic activity, not dormancy. Cells are upregulating gene expression to produce the molecular machinery (transport proteins, metabolic enzymes, additional ribosomes) required for the new nutrient conditions. Cell numbers barely change because the cells are not yet dividing at their full rate, but their internal composition is changing dramatically. This is why the lag phase length correlates with how different the new environment is from the previous one — more preparation is needed when conditions differ more.

Question 4

In a batch culture, the stationary phase occurs when bacterial cells have largely stopped dividing due to nutrient depletion.

• T. True
• F. False

Answer: False

Stationary phase is a dynamic equilibrium, not a halt to division. Both growth (new cell divisions) and death continue, but at approximately equal rates — so the net viable cell count stabilizes. Nutrient depletion and waste accumulation slow growth rates, but division doesn't stop entirely until the death phase. During stationary phase, cells also activate stress-response programs and undergo physiological changes (cell shrinkage, thickened walls). The phrase 'completely stopped dividing' overstates what happens.

Question 5

Why can't exponential growth continue indefinitely in a batch culture, and what biological and chemical changes drive the transition from exponential to stationary phase?

Model answer: Exponential growth cannot continue indefinitely because the batch culture environment is finite and closed. As cells consume nutrients, substrate concentrations fall — and the Monod equation (μ = μ_max × [S]/(K_s + [S])) shows that growth rate falls as [S] decreases below K_s. Simultaneously, metabolic byproducts (organic acids, alcohols, CO₂, reduced oxygen) accumulate and become inhibitory or toxic. Together, nutrient limitation and waste accumulation reduce the net growth rate. When growth rate equals death rate, the culture enters stationary phase at the carrying capacity. Eventually, as resources are fully exhausted, death exceeds growth and the death phase begins.

This transition has practical significance: in clinical microbiology, antibiotic susceptibility tests use log-phase cells because stationary-phase cells are physiologically distinct and more resistant. In industrial fermentation, process engineers try to extend the productive exponential phase while controlling the onset of stationary phase. The logistic growth model and the Monod equation both capture mathematically what the four-phase growth curve shows qualitatively.