MICROBIOLOGY LAB EXAM SET 3

PDF Download

MICROBIOLOGY LAB EXAM SET #3

Actual Qs and Ans - Expert-Verified Explanation -Guaranteed passing score -8 Questions and Answers

-Format: Multiple-choice / Flashcard

Question 1: Number of viable yeast cells=

Answer:

CFUs x dilution factors Question 2: What range of numbers of colonies are considered statistically valid?

Answer:

30-300

-If the CFUs are greater than 300, there is a probability that overcrowding on the plate could have inhibited some cells from growing.-Less than 30 CFUs could involve a sampling error and an underestimate of numbers.

Question 3: Percent Viability

Answer:

Number of viable yeast cells per ml/ Direct count x 100%

Question 4: Does a microscope count determine alive or dead cells?

Answer:

Both

Question 5: Enumeration of bacteria

Answer:

The yeast Saccharomyces cerevisiae is used instread of bacteria.

• Count the number of yeast cells in six small squares and calculate the average number of

cells/square. Count six squares that are all next to each other.-Each small square accurately represents a volume of 1/(4 x 10^6) mls of liquid in the counting chamber.

-To calculate the number of cells per ml, use the equation:

No. cells per ml suspension= Average x (4 x 10^6) (10^1)

-The 10^1 is also factored in because the original sample was diluted 10:1

Question 6: What is the most common method for determining bacterial numbers in a sample?

Answer:

Standard plate or viable count.-A sample is diluted in a series of dilution blanks.-The dilutions are then plated onto media and the number of colonies are counted after incubation for 24-48 hours.-It is assumed that the bacterial cells are diluted to an end point where a single cell divides giving rise to a visible colony on a plate.-The number of bacteria in the original sample is determined by multiplying the number of colonies by the dilution factor.-However, the assumption that a colony represent a single cell is not always correct because cells in a chain, such as Streptococcus, will also give rise to a colony on a plate.-Because of the uncertainty in how many actual cells form a colony, count by the SPC are colony forming units (CFUs)

Question 7: Does the SPC method determine alive or dead cells?

Answer:

Alive or viable

Question 8: How is the SPC (standard plate count) method biased?

Answer:

Because specific conditions and media are used and these factors may exclude some bacteria in the count.-Example: the SPC would severely underestimate the numbers of bacteria in a soil sample because the conditions and the medium used for the count probably favor heterotrophs that grow aerobically at neutral pH values. These conditions do not allow for the growth of anaerobes, chemolithotrophs or bacteria that may grow at extremes of pH.