- A bacterial culture has been maintained in the stationary phase for 48 hours in a sealed flask. Compared to cells taken from the mid-log phase, which set of characteristics would be most expected in these stationary-phase cells?
Answer: Decreased cell size, condensed nucleoid, and increased resistance to thermal and oxidative stress.
Stationary phase is a stress-response state triggered by nutrient limitation and/or waste accumulation. Cells undergo significant physiological changes to promote long-term survival. These include a reduction in cell size (reductive division), condensation of the nucleoid, and the expression of a suite of stress-response genes (e.g., RpoS-regulated genes in E. coli). These changes confer cross-protection against various insults, including heat, oxidative stress, and osmotic shock. In contrast, log-phase cells (A, D) are large, rich in ribosomes, and actively replicating, making them more sensitive to stress and certain antibiotics.
- A culture of Lactobacillus, an aerotolerant anaerobe, is growing exponentially via fermentation in a complex medium. The culture is then vigorously aerated. How will the growth curve most likely be affected immediately following the introduction of oxygen?
Answer: The growth rate will remain relatively unchanged, but the culture may enter stationary phase sooner due to oxidative stress.
Aerotolerant anaerobes can survive in the presence of oxygen but do not use it for respiration; they rely exclusively on fermentation. Therefore, introducing oxygen will not increase the growth rate (A, B are incorrect). However, oxygen can still be harmful by generating reactive oxygen species (ROS). While these organisms have enzymes like superoxide dismutase to cope with some ROS, high levels of aeration can cause oxidative stress that damages cellular components, potentially slowing or stopping growth and leading to an earlier onset of stationary or death phase. They are not obligate anaerobes, so they are not immediately killed by oxygen (D is incorrect).
- A researcher generates a growth curve by measuring the optical density (OD) of a liquid culture over time. The curve appears normal through the log and stationary phases, but then shows a significant decrease in OD during the late stationary and death phases. This decrease in OD is most likely caused by:
Answer: the widespread activation of autolysins, leading to cell lysis and a decrease in turbidity.
Optical density is a measure of the turbidity or light scattering of the culture. The primary cause of a decrease in OD during the death phase is cell lysis. Many bacteria activate autolytic enzymes (autolysins) that degrade their own peptidoglycan, causing the cells to burst. This process reduces the number of intact, light-scattering particles in the suspension, thereby lowering the turbidity and the measured OD. While cell shrinking (A) occurs, it typically does not cause a large decrease in OD. Settling (B) and refractive index changes (D) are possible but less likely to be the primary cause of a significant, consistent decline phase.
- In a long-term stationary phase culture, the viable cell count may remain stable due to 'cryptic growth'. This phenomenon is best described as a state where:
Answer: the rate of cell division is balanced by the rate of cell death, with dividing cells feeding on nutrients released by lysed cells.
Cryptic growth describes a dynamic equilibrium in a stationary phase culture. Some cells in the population die and lyse, releasing their cellular components (nutrients) into the medium. These released nutrients are then scavenged by the remaining viable cells, allowing them to undergo a limited number of divisions. This results in a balanced state where the growth of some cells is fueled by the death of others, leading to a relatively constant total viable cell count over a period.
- A microbiologist aims to study the effect of a specific growth-limiting nutrient on bacterial gene expression. Which cultivation method would be most suitable for maintaining a culture in a steady state of exponential growth at a defined, sub-maximal rate?
Answer: A chemostat with a controlled dilution rate and a defined limiting nutrient.
A chemostat is a continuous culture system designed for this exact purpose. By continuously adding fresh medium (with one nutrient at a limiting concentration) and removing culture fluid at a constant rate (the dilution rate), the system reaches a steady state. In this state, the growth rate of the culture equals the dilution rate, and both the cell density and nutrient concentration remain constant. This allows for precise control of the growth environment and physiological state of the cells, which is not possible in a batch culture where conditions are constantly changing.
- During exponential growth, the mean growth rate constant (k) of a bacterial population is determined to be 0.5 generations per hour. What is the generation time (g) of this bacterium?
Answer: 2.0 hours
The mean growth rate constant (k) is the number of generations per unit time. The generation time (g) is the time required for one generation (time per generation). The two values are reciprocals of each other: g = 1/k. Given k = 0.5 generations/hour, the generation time g = 1 / (0.5 generations/hour) = 2.0 hours/generation.
- A researcher tracking a Legionella pneumophila population in a water sample finds that after several weeks, the number of Colony Forming Units (CFU/mL) on agar plates drops to zero. However, staining with a fluorescent viability probe (which stains cells with intact membranes green and damaged ones red) reveals a large population of green-staining cells. This discrepancy suggests the Legionella population has:
Answer: entered a viable but non-culturable (VBNC) state.
The VBNC state is a survival strategy where bacteria are metabolically active and have intact cell membranes (as indicated by the green viability stain) but are unable to undergo division and form colonies on standard lab media. This perfectly explains why the plate count (CFU) is zero while direct viability assays show that live cells are still present. Legionella does not form endospores (A). The viability stain rules out that the cells are dead (B). While competition is possible (D), the VBNC state is a well-documented phenomenon that directly accounts for the specific discrepancy observed between culturability and viability.
- A culture of Lactococcus lactis, a bacterium that performs homolactic fermentation, is grown in a liquid medium with a high concentration of glucose but with very low buffering capacity. Which parameter is most likely the first to become limiting and trigger the onset of the stationary phase?
Answer: The decrease in extracellular pH due to lactic acid production.
Homolactic fermentation produces lactic acid as its primary end product. In a medium with poor buffering capacity, the accumulation of this acid will cause a rapid and significant drop in the external pH. Most bacteria, including L. lactis, have an optimal pH range for growth. Once the pH drops below a critical threshold, it will inhibit the function of essential enzymes and disrupt membrane transport, thereby halting growth and initiating the stationary phase. This often occurs long before the primary carbon source (glucose) is fully depleted.
- Four parallel cultures are initiated in identical fresh minimal media. The inocula are taken from four different source cultures as described below. Which inoculum is expected to produce a culture with the longest lag phase?
I. From a mid-log phase culture grown in the same minimal medium.
II. From a late stationary phase culture grown in the same minimal medium.
III. From a mid-log phase culture grown in a rich, complex medium.
IV. From a refrigerated slant (4°C) of the same minimal medium composition.
Answer: Inoculum IV
The length of the lag phase depends on the physiological state of the inoculum and the degree of change in the environment. Inoculum I is already actively growing in the same medium and will have the shortest lag. Inoculum II comes from a stationary phase, requiring time for repair and synthesis of growth machinery. Inoculum III comes from a rich medium and will need to synthesize enzymes for pathways that were previously unnecessary, resulting in a lag. Inoculum IV is the most stressed; not only is it likely in a stationary-like state due to age, but it has also been stored at a cold temperature, which slows all metabolic processes and may have caused cold-shock damage. It will require the longest period of adjustment to repair damage, synthesize enzymes, and prepare for growth at the optimal temperature.
- A student monitoring bacterial growth with a spectrophotometer observes that the semi-log plot of OD vs. time begins to curve and flatten at an OD of ~1.0, even though a parallel viable plate count shows that the cells are still increasing in number exponentially. What is the most common reason for this discrepancy between the two measurement methods?
Answer: The relationship between cell number and optical density becomes non-linear at high cell densities due to multiple light scattering events.
Spectrophotometry relies on the principle that OD is directly proportional to the concentration of light-scattering particles (cells). However, this linear relationship only holds true at lower cell densities. At higher densities (typically OD > 0.8-1.0), light scattered by one cell is likely to be re-scattered by other cells before it reaches the detector. This 'multiple scattering' effect causes the measured OD to be lower than the actual cell concentration, leading to a flattening of the curve. The plate count, which measures the number of colony-forming units, is not subject to this artifact and will continue to show exponential growth until a true biological limit is reached.
- Which statement most accurately describes the population dynamics during the death phase of a typical bacterial growth curve?
Answer: Cell death is typically a logarithmic process, but the rate often decelerates over time due to the survival of resistant persister cells.
The death phase is generally modeled as a first-order kinetic process, meaning a constant fraction of the population dies per unit time. This results in an exponential decrease, which appears as a straight line with a negative slope on a semi-log plot. However, this is an idealization. In many real cultures, the curve 'tails off', meaning the death rate slows down. This is often attributed to population heterogeneity, specifically the presence of a small subpopulation of 'persister cells' that are phenotypically more resistant to the lethal conditions, allowing them to survive for much longer.
- A classic sigmoidal growth curve accurately represents population dynamics for planktonic bacteria in a well-mixed batch culture. Why is this model generally inappropriate for describing the total biomass accumulation in a bacterial biofilm?
Answer: Biofilms create nutrient and oxygen gradients, resulting in spatially heterogeneous growth rates throughout the structure.
When comparing bacterial growth patterns, you need to distinguish between planktonic (free-floating) bacteria in liquid cultures versus sessile bacteria in biofilms. The classic sigmoidal growth curve applies to well-mixed systems where all bacteria experience uniform conditions, but biofilms create fundamentally different microenvironments.
Answer A is correct because biofilms develop complex three-dimensional structures that create steep gradients of nutrients, oxygen, and waste products. Bacteria at the surface have abundant access to nutrients and oxygen, while those deeper in the biofilm experience increasingly limited resources. This spatial heterogeneity means different regions of the biofilm grow at dramatically different rates - some areas may be in exponential phase while others are in stationary phase or even declining. The overall biomass accumulation becomes much more complex than a simple sigmoidal curve can represent.
Answer B is wrong because biofilm growth isn't strictly linear - it can still show exponential characteristics in certain regions and phases. Answer C is incorrect because biofilm cells continue to divide; they don't become terminally differentiated like some eukaryotic cells. Answer D is false because mature biofilms don't maintain constant biomass - they continue to grow, shed cells, and reshape their structure dynamically.
Remember that biofilms are the predominant lifestyle for bacteria in nature. When you encounter biofilm questions, always consider how the three-dimensional structure creates microenvironments that differ drastically from the uniform conditions assumed in planktonic growth models. This principle applies to antibiotic resistance, metabolic activity, and growth patterns.
- Two batch cultures of a chemoheterotroph are grown in a minimal medium where the carbon source is limiting. Culture A is given 1 g/L of glucose, and Culture B is given 1 g/L of succinate. The growth rate (slope of the log phase) is identical for both cultures. However, the final optical density in the stationary phase is significantly higher for Culture A. What is the best explanation for this observation?
Answer: The organism can generate more ATP and biosynthetic precursors per gram of glucose than per gram of succinate.
The final cell density (yield) is determined by the total amount of biomass that can be synthesized from the limiting nutrient. This is a function of the energy (ATP) and carbon skeletons generated from its metabolism. Glycolysis and subsequent oxidation of glucose yield more ATP per gram than the oxidation of succinate, which enters the TCA cycle directly. Therefore, even though the growth rates are the same (ruling out D), more cell mass can be produced from 1 gram of glucose, resulting in a higher carrying capacity (final OD). Transport efficiency (A) would affect the growth rate, which is stated to be identical.
