Question 1

Which of the following best describes a pure culture in microbiology?

Accepted Answer

A culture in which all organisms are descendants of the same organism.

Question 2

Why is it important to sterilize the inoculating loop between each sector during the streak plate method?

Accepted Answer

To ensure that each sector contains a reduced number of bacteria, leading to isolated colonies.

Question 3

A student streaks a plate but observes no isolated colonies in any sector. What is the most likely reason?

Accepted Answer

The loop was not sterilized between sectors.

Question 4

Which medium would you use to selectively grow gram-positive bacteria from a mixed culture?

Accepted Answer

Phenylethyl Alcohol Agar (PEA)

Question 5

On MacConkey agar, which result indicates a strong lactose fermenter?

Accepted Answer

Pink/red colonies with a pink precipitate or cloudiness in the agar.

Question 6

Which of the following is a differential component of MacConkey agar?

Accepted Answer

Lactose

Question 7

A plate shows yellow, water-insoluble colonies on PEA. Which organism is most likely present?

Accepted Answer

Micrococcus luteus

Question 8

Why must agar plates be incubated agar-side up?

Accepted Answer

To prevent condensation from dripping onto the agar surface and spreading colonies.

Question 9

Which of the following best describes the pour plate method?

Accepted Answer

A bacterial sample is mixed with melted agar and poured into a petri dish.

Question 10

A student observes bright pink/red colonies without a pink precipitate on MacConkey agar. What does this indicate?

Accepted Answer

Weak lactose fermentation

Question 11

Which of the following is a general-purpose medium that supports the growth of both gram-positive and gram-negative bacteria?

Accepted Answer

Trypticase Soy Agar (TSA)

Question 12

What is the main purpose of using selective media in microbiology?

Accepted Answer

To inhibit the growth of certain bacteria while allowing others to grow.

Question 13

Which of the following is a common mistake that prevents obtaining well-isolated colonies on a streak plate?

Accepted Answer

Not sterilizing the loop between sectors.

Question 14

Which of the following organisms would NOT grow on Phenylethyl Alcohol Agar (PEA)?

Accepted Answer

Escherichia coli

Question 15

A student observes colorless colonies on MacConkey agar. What does this indicate about the organism?

Accepted Answer

It does not ferment lactose.

Question 16

Which of the following best explains why you should pick only the top portion of a colony when transferring to a new medium?

Accepted Answer

To avoid picking up contaminants from the agar surface.

Question 17

Which of the following is true about differential media?

Accepted Answer

They allow for the distinction between different bacteria based on biochemical reactions.

Question 18

Which of the following is a correct sequence for obtaining a pure culture from a mixed population?

Accepted Answer

Streak plate for isolation, pick a single colony, transfer to sterile medium.

Question 19

Which of the following best describes the function of crystal violet and bile salts in MacConkey agar?

Accepted Answer

They inhibit the growth of gram-positive bacteria.

Question 20

A microbiologist observes irregular, raised, non-pigmented colonies on TSA. Which organism is most likely present?

Accepted Answer

Escherichia coli

Microbiology Lab 3 Study Guide

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