Cell Growth and Plate Counts Quiz

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Questions and Answers

During which growth phase are cells observed to be maximum in size?

Lag Phase

Stationary Phase

Decline Phase

Log Phase (correct)

What characteristic is observed in cells during the Stationary Phase?

Cells have no intracellular granules.

Cells are actively dividing.

Cells exhibit uniform staining.

Cells are Gram variable and irregularly staining. (correct)

What is the purpose of performing serial dilutions?

To ensure colony counts remain between 25 – 250 on a plate. (correct)

To increase the biomass in the original culture.

To measure all cells present in the original sample.

To produce a concentrated culture for growth.

What is a likely issue if a Petri plate contains more than 250 colonies?

Overcrowding preventing some cells from developing. Signup and view all the answers

What occurs when a sample with 10,000 bacteria per milliliter is plated without dilution?

There would be too many colonies to count accurately. Signup and view all the answers

What is a major disadvantage of the pour plate method?

It may damage heat-sensitive microorganisms. Signup and view all the answers

Which group of microorganisms grows optimally at temperatures between 25˚C to 40˚C?

Mesophiles Signup and view all the answers

When performing a plate count using the spread plate method, what characteristic of islands is crucial for diagnostic purposes?

The distinctive appearance of the colonies. Signup and view all the answers

What is a characteristic of extreme halophiles regarding their environmental adaptation?

They adapt well to high salt concentrations. Signup and view all the answers

Which of the following statements about acidophiles is true?

They can tolerate low pH levels. Signup and view all the answers

What happens to bacterial cells under high osmotic pressure?

They experience plasmolysis. Signup and view all the answers

Which phase of bacterial growth is characterized by adjusting to new environments without significant cell division?

Lag phase Signup and view all the answers

In a serial dilution, if a sample is diluted 1:10 and then plated, how many colonies might be expected from 100 bacteria in the original sample?

100 colonies Signup and view all the answers

What is the primary characteristic of the lag phase in bacterial growth?

Cells are actively metabolizing but not dividing. Signup and view all the answers

What technique involves diluting a sample to obtain a countable number of colonies?

Serial dilution Signup and view all the answers

In the pour plate method, what happens to the bacteria during the plating process?

Bacteria are mixed into molten agar and allowed to solidify. Signup and view all the answers

Which of the following accurately describes the definition of generation time in bacterial growth?

Time required for a population to double. Signup and view all the answers

During which growth phase does rapid population increase occur?

Log phase Signup and view all the answers

Which method allows for estimating the number of viable bacteria in a sample by creating a dilution series?

Serial dilution Signup and view all the answers

What is a significant factor that can influence bacterial generation time?

Environmental conditions like temperature Signup and view all the answers

In the spread plate technique, how are bacteria typically deposited onto the agar?

With a spreader to evenly distribute. Signup and view all the answers

Study Notes

Association of Growth and Cell Changes

Lag Phase: Phase where maximum cell size is reached towards the end, preparation for division occurs.
Log Phase: Characterized by smaller cells that stain uniformly, rapid cell division happens.
Stationary Phase: Cells become Gram variable and exhibit irregular staining due to intracellular granules; sporulation occurs.
Decline Phase: Involution forms become common as resources deplete.

Plate Counts

Measures viable cell counts; results reported as colony-forming units (CFU).
Visible colonies require about 24 hours or more to develop.
Serial dilution is performed to ensure colony counts are between 25 and 250 to avoid overcrowding on plates.

Serial Dilutions

Example: A milk sample with 10,000 bacteria/ml would yield too many colonies if 1 ml is plated.
Dilution process starts by mixing 1 ml of the milk sample with 9 ml of water, reducing the concentration to 1,000 bacteria/ml.
Further dilution (1 ml into 9 ml of water) reduces concentration to 100 bacteria/ml, yielding a countable number of colonies.

Pour Plates and Spread Plates

Plate count methods include the pour plate and spread plate techniques for isolating colonies.

Pour Plates: Disadvantages

Heat-sensitive microorganisms may be damaged during the melting of agar, failing to form colonies.
Differential media may require distinct colony appearances on the surface, which is problematic in pour plates.

Filtration

Employed when bacterial quantities are minimal, allowing for concentration and isolation of bacteria.

Physical Requirements: Temperature

Psychrophiles: Grow optimally at -10°C to 20°C.
Mesophiles: Thrive between 10°C to 50°C, optimal growth at 25°C to 40°C.
Thermophiles: Prefer 40°C to 70°C with a peak around 50°C to 60°C.
Psychrotrophs: Spoilage organisms, grow from 0°C to 30°C.
Hyperthermophiles/Extreme Thermophiles: Optimal growth at 65°C to 110°C.

Physical Requirements: pH

Most bacteria flourish at pH 6.5-7.5; very few survive below pH 4.
Acidophiles: Remarkably tolerant to acidic conditions.

Physical Requirements: Osmotic Pressure

High osmotic pressure can lead to plasmolysis, causing cell shrinkage due to water loss.
Extreme Halophiles: Thrive in high salt concentrations, with some requiring it for growth.

Growth Curve: Bacterial Growth

Defined as an increase in bacterial numbers rather than cell size, measured directly or indirectly.

Bacterial Division

Binary Fission: The primary method of asexual reproduction, yielding two daughter cells.
Budding: Asexual reproduction creating outgrowths or buds from mature cells; separation occurs when the bud reaches parent cell size.

Generation Time

Refers to the time required for a cell to divide and for the population to double.
Most bacteria have a generation time of 1-3 hours, while some species may take up to 24 hours.

Phases of Growth

Bacterial growth is influenced by the nutritional environment, leading to an increase in cell numbers under suitable conditions.

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Description

Test your knowledge on the phases of cell growth and the significance of plate counts in microbiology. This quiz covers key concepts such as the lag, log, stationary, and decline phases of cell changes, as well as methods for measuring viable cells. Perfect for students studying microbiology or cellular biology.