Question 1

Which of the following best explains why immersion oil is used with the 100× objective lens in brightfield microscopy?

Accepted Answer

It reduces light refraction by matching the refractive index of glass, improving resolution.

Question 2

A student is calibrating an ocular micrometer using a stage micrometer. If 50 ocular divisions align with 100 μm on the stage micrometer, what is the calibration factor for one ocular division?

Accepted Answer

$$\frac{100\ \mu m}{50} = 2\ \mu m$$ per ocular division

Question 3

Which of the following statements about Serratia marcescens pigment production is correct?

Accepted Answer

Prodigiosin is produced at room temperature (20–25°C) but not at 40°C due to temperature-regulated gene expression.

Question 4

Why is it essential to flame the mouth of a culture tube both before and after inserting the inoculating loop during a transfer?

Accepted Answer

To kill microorganisms around the tube opening and create an upward convection current that reduces airborne contamination.

Question 5

A student forgets to cool the inoculating loop after flaming and immediately collects bacteria from a slant. What is the most likely outcome?

Accepted Answer

The bacteria will be killed by the hot loop, resulting in little or no growth after transfer.

Question 6

Which of the following best describes the purpose of using a straight stab with an inoculating needle when inoculating a deep?

Accepted Answer

To minimize disruption of the agar and allow for observation of motility and oxygen requirements.

Question 7

Which of the following is NOT a reason why agar is used as a solidifying agent in microbiological media?

Accepted Answer

Agar provides essential nutrients for bacterial growth.

Question 8

A blood agar plate shows clear, colorless zones around colonies. What type of hemolysis is indicated, and what does it mean?

Accepted Answer

Beta hemolysis; complete lysis of red blood cells.

Question 9

During the preparation of media, why is an autoclave set to 121°C, 15 psi, for 15 minutes?

Accepted Answer

To sterilize the media by killing all forms of microbial life, including endospores.

Question 10

Which of the following best defines 'aseptic technique' in the context of microbiology laboratory procedures?

Accepted Answer

A set of practices designed to prevent contamination of cultures, media, and the environment.

Question 11

A student is measuring a bacillus using a calibrated ocular micrometer. If the calibration factor is $$2\ \mu m$$ per division and the cell spans 7 divisions, what is the length of the cell?

Accepted Answer

$$2\ \mu m 	imes 7 = 14\ \mu m$$

Question 12

Why should the cap of a culture tube never be placed on the bench during a transfer procedure?

Accepted Answer

To avoid contaminating the sterile inner surface of the cap with environmental microbes.

Question 13

Which of the following best describes the difference between a broth, a slant, and a deep in microbiological media?

Accepted Answer

Broth is liquid, slant is solidified at an angle, deep is solidified upright.

Question 14

A student observes that after UV exposure, bacterial colonies fail to grow on a plate. What is the most likely explanation for this observation?

Accepted Answer

UV radiation caused the formation of thymine dimers in DNA, preventing replication.

Question 15

Which of the following is a correct calculation for total magnification when using a 10× ocular lens and a 40× objective lens?

Accepted Answer

$$10 	imes 40 = 400	imes$$

Question 16

Which sterilization method is most appropriate for heat-sensitive solutions such as antibiotic stock solutions?

Accepted Answer

Filtration through a membrane filter.

Question 17

Which of the following best describes the function of numerical aperture (NA) in microscopy?

Accepted Answer

NA is a measure of the lens's ability to gather light and resolve fine specimen detail at a fixed object distance.

Question 18

A student is preparing a streak plate and accidentally touches the agar with the loop before it has cooled. What is the most likely result?

Accepted Answer

The hot loop will kill bacteria at the point of contact, resulting in no growth in that area.

Question 19

Which of the following best explains why a slant is used for maintaining stock cultures?

Accepted Answer

The slant provides a large surface area for growth and is less likely to dry out than a plate.

Question 20

Which of the following is the correct sequence for transferring bacteria from a broth culture to a slant using aseptic technique?

Accepted Answer

Flame loop, remove cap, flame tube, obtain inoculum, transfer to slant, flame tube, replace cap, flame loop.

Microbiology Lab Exam Master Checklist: Experiments 1–5

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