Question 1

Identify the structure labelled as isolated colonies in the streak plate method. The diagram shows a Petri dish with a streaked agar surface where bacterial cells are deposited in decreasing density to obtain isolated colonies at the end of the streak.

Accepted Answer

Isolated colonies formed at the end of the streak where individual bacterial cells have grown into separate colonies — The streak plate method involves spreading a microbial sample over the agar surface in a pattern that dilutes the cells. Near the end of the streak, isolated colonies appear because individual cells have been separated sufficiently to grow independently. These isolated colonies are used to obtain pure cultures.

Question 2

In the pour-plate method, what is the purpose of diluting the original microbial sample several times before plating?

Accepted Answer

To reduce the population sufficiently to obtain separate colonies — Diluting the microbial sample reduces the number of cells in the volume plated so that individual cells can grow into isolated colonies. This is essential for obtaining pure cultures.

Question 3

Which of the following steps is essential in the spread-plate technique to achieve a pure culture?

Accepted Answer

Using a sterile L-shaped glass spreader to evenly spread the sample — In the spread-plate technique, a small volume of diluted bacterial mixture is spread evenly over the agar surface using a sterile L-shaped glass spreader to obtain isolated colonies.

Question 4

Which term describes microorganisms that grow optimally between 20°C and 45°C?

Accepted Answer

Mesophiles — Mesophiles are microorganisms that grow best at moderate temperatures, typically between 20°C and 45°C.

Question 5

Explain why fungi tend to dominate bacteria in environments with slightly acidic pH.

Accepted Answer

Fungi generally prefer slightly acidic conditions for growth, whereas most bacteria prefer neutral or slightly alkaline conditions. Therefore, in environments with acidic pH, fungi outcompete bacteria and dominate. — Fungi have a wider tolerance for acidic pH and thrive in such environments, while bacteria's growth is inhibited by acidity. This ecological preference allows fungi to dominate in acidic conditions.

Question 6

Match the types of microorganisms with their oxygen requirements:

1. Aerobes
2. Obligate anaerobes
3. Facultative anaerobes
4. Aerotolerant anaerobes

Accepted Answer

— Aerobes require oxygen for survival. Obligate anaerobes cannot tolerate oxygen. Facultative anaerobes can survive with or without oxygen. Aerotolerant anaerobes do not use oxygen but are not inhibited by it.

Question 7

Why is carbon dioxide provided as bicarbonate in the growth medium of autotrophic microorganisms?

Accepted Answer

Autotrophic microorganisms use carbon dioxide as their carbon source. Providing carbon dioxide as bicarbonate in the growth medium supplies the necessary carbon for their growth and metabolism. — Bicarbonate acts as a soluble source of carbon dioxide which autotrophs can assimilate during photosynthesis or other carbon fixation processes, facilitating their growth in culture.

Question 8

Describe the four distinct phases of the microbial growth curve.

Accepted Answer

The microbial growth curve has four phases:
(a) Lag phase: Cells adapt to new environment; no increase in cell number.
(b) Exponential (log) phase: Cells divide at maximum rate; population doubles at regular intervals.
(c) Stationary phase: Growth ceases; cell division balances cell death.
(d) Death (decline) phase: Cells die due to nutrient depletion and toxic waste accumulation. — The lag phase is the period where cells prepare for division by synthesizing enzymes and molecules. During the exponential phase, cells divide rapidly, and growth rate is constant. In the stationary phase, nutrient limitation or waste accumulation halts net growth. Finally, in the death phase, cells die faster than they divide due to unfavorable conditions.

Question 9

Calculate the specific growth rate ($\mu$) and doubling time ($t_d$) for a bacterial culture that increases from $10^{4}$ cells/mL to $10^{8}$ cells/mL in 2 hours during exponential growth.

Accepted Answer

Specific growth rate = 4.606 hr⁻¹, Doubling time = 0.15 hr — Given:
X₀ = 10⁴ cells/mL
Xₜ = 10⁸ cells/mL
Time, t = 2 h
Formula:
μ = 2.303 × (log Xₜ - log X₀) / t
= 2.303 × (8 - 4) / 2
= 4.606 hr⁻¹
Doubling time, t_d = 0.693 / μ = 0.693 / 4.606 = 0.15 h
Answer: μ = 4.606 hr⁻¹, t_d = 0.15 h
Note: Students often forget to use logarithms or use natural logs instead of base 10 logs.

Question 10

What is the significance of the lag phase in microbial growth?

Accepted Answer

Lag phase is the initial period after inoculation where cells do not divide but synthesize enzymes and molecules required for growth. It allows adaptation to new environment before exponential growth begins. — During lag phase, cells prepare for division by producing necessary cellular components. The duration depends on the condition of the inoculum and the medium. This phase is crucial for successful growth.

Question 11

During which phase of microbial growth is the population most uniform in chemical and physiological properties, making it ideal for biochemical studies?

Accepted Answer

Exponential (log) phase — During the exponential phase, cells divide at a constant and maximal rate, making the population homogeneous in terms of growth and metabolism. This phase is ideal for biochemical and physiological studies.

Question 12

Explain why the growth curve becomes horizontal during the stationary phase.

Accepted Answer

The growth curve becomes horizontal in the stationary phase because the rate of cell division equals the rate of cell death, resulting in a constant number of viable cells. This is due to nutrient depletion and accumulation of toxic wastes. — As nutrients become limited and waste products accumulate, cells stop dividing at the same rate as they die, leading to a plateau in population size called the stationary phase.

Question 13

Which of the following factors can lead to the death phase in microbial growth?

Accepted Answer

Nutrient deprivation and accumulation of toxic wastes — The death phase occurs due to unfavorable environmental conditions such as lack of nutrients and build-up of toxic metabolic wastes, leading to decline in viable cell numbers.

Question 14

What is the generation time ($t_d$) in microbial growth?

Accepted Answer

Doubling time / Generation time — Generation time or doubling time is the time required for a microbial population to double in number during exponential growth.

Question 15

Calculate the number of generations (n) in 4 hours if the doubling time of a bacterial culture is 30 minutes.

Accepted Answer

8 generations — Given:
Time, t = 4 hours = 240 minutes
Doubling time, t_d = 30 minutes
Formula:
n = t / t_d = 240 / 30 = 8 generations
Answer: 8 generations
Note: Students sometimes confuse hours and minutes; always convert units properly.

Microbial Culture

Microbial Culture — Study Notes

6.1 HISTORICAL PERSPECTIVE

6.2 NUTRITIONAL REQUIREMENTS AND CULTURE MEDIA

6.3 STERILISATION METHODS

Practice Questions — Microbial Culture

All 13 Chapters in Biotechnology