Q: You are using a compound light microscope with a 10x ocular lens and a 40x objective lens. What is the total magnification, and which formula do you use to calculate it?

Total magnification is 400x, calculated using $$\text{Total Magnification} = \text{Ocular Magnification} \times \text{Objective Magnification}$$

Question 1

You are using a compound light microscope with a 10x ocular lens and a 40x objective lens. What is the total magnification, and which formula do you use to calculate it?

Accepted Answer

Total magnification is 400x, calculated using $$	ext{Total Magnification} = 	ext{Ocular Magnification} 	imes 	ext{Objective Magnification}$$

Question 2

A student observes a broth culture that is turbid throughout with a thin film at the top. What does this growth pattern suggest about the organism's oxygen requirements?

Accepted Answer

The organism is a facultative anaerobe, able to grow with or without oxygen but prefers oxygen.

Question 3

Which of the following best describes the purpose of aseptic technique in microbiology labs?

Accepted Answer

To prevent contamination of cultures, media, and the environment by unwanted microorganisms.

Question 4

During a negative stain, which type of dye is used and what is the resulting appearance of the cells and background?

Accepted Answer

An acidic dye is used; cells appear clear against a dark background.

Question 5

Which step is essential when preparing a bacterial smear from solid media for a simple stain?

Accepted Answer

Mix a small amount of bacteria with a drop of water on the slide before air drying and heat fixing.

Question 6

A Gram stain is performed on a mixed culture. After the decolorization step, Gram-negative cells appear colorless. Which chemical is responsible for this effect?

Accepted Answer

Alcohol or acetone, which dissolves the outer membrane and removes crystal violet-iodine complex from Gram-negative cells.

Question 7

Which of the following best explains why Mycobacterium species are acid-fast?

Accepted Answer

Their cell walls contain high concentrations of mycolic acid, making them resistant to decolorization by acid-alcohol.

Question 8

You are given a mixed culture and asked to isolate a pure colony. Which technique would be most appropriate and why?

Accepted Answer

Streak plate, because it physically separates individual cells to form isolated colonies.

Question 9

A student performs a serial dilution and pour plate to estimate bacterial concentration. If 0.1 mL of a $$10^{-4}$$ dilution yields 50 colonies, what is the original concentration of bacteria per mL?

Accepted Answer

$$5 	imes 10$$^{6}$$ CFU/mL

Question 10

Which of the following best describes the result of a catalase test when hydrogen peroxide is added to a bacterial colony and bubbles are observed?

Accepted Answer

The organism produces catalase, which breaks down $$2H_2O_2 \rightarrow 2H_2O + O_2$ (gas).

Question 11

Which of the following pairs correctly matches a staining method with the type of information it provides?

Accepted Answer

Simple stain: reveals cell morphology and arrangement.

Question 12

You are given a dichotomous key and a Gram-stained unknown. The cells are purple cocci in clusters. Which genus and species is most likely, based on typical lab organisms?

Accepted Answer

Staphylococcus epidermidis

Question 13

Which of the following is a correct step in the negative staining procedure?

Accepted Answer

Mix bacteria with a drop of acidic dye and spread across the slide without heat fixing.

Question 14

A student observes a thioglycollate broth tube with growth only at the bottom. What is the oxygen requirement of the organism?

Accepted Answer

Obligate anaerobe

Question 15

Which of the following best explains why Gram-positive bacteria retain the crystal violet-iodine complex during Gram staining?

Accepted Answer

They have a thick peptidoglycan layer that traps the complex.

Question 16

Which of the following is a correct safety procedure when transporting a microscope?

Accepted Answer

Carry the microscope with both hands: one on the arm and one under the base.

Question 17

Which of the following bacteria is acid-fast and was used in the acid-fast stain exercise?

Accepted Answer

Mycobacterium smegmatis

Question 18

A student performs a Gram stain but forgets the decolorization step. What will be the appearance of Gram-negative bacteria under the microscope?

Accepted Answer

They will appear purple, like Gram-positive bacteria.

Question 19

Which of the following best describes the difference between a simple stain and a negative stain?

Accepted Answer

A simple stain uses a basic dye to color the cells, while a negative stain uses an acidic dye to color the background.

Question 20

Which of the following is a correct interpretation of a catalase-negative result?

Accepted Answer

No bubbles are produced when hydrogen peroxide is added, indicating the organism lacks catalase.

Microbiology Lab Exam 1 Study Guide: Microscopy, Staining, Aseptic Technique, and Bacterial Identification

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