Q: A student is using a compound light microscope with a 10x ocular lens and a 40x objective lens. What is the total magnification being used, and which formula should be applied to calculate it?

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

Question 1

A student is using a compound light microscope with a 10x ocular lens and a 40x objective lens. What is the total magnification being used, and which formula should be applied to calculate it?

Accepted Answer

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

Question 2

Which of the following best describes the purpose of the streak plate technique in microbiology?

Accepted Answer

To obtain isolated colonies from a mixed culture by diluting bacteria across the surface of an agar plate

Question 3

After performing a Gram stain, a student observes purple cocci in clusters. What does this indicate about the cell wall structure and Gram reaction of the bacteria?

Accepted Answer

The bacteria are Gram-positive with a thick peptidoglycan layer that retains the crystal violet stain

Question 4

A microbiologist is preparing a bacterial smear for staining. Which of the following steps is essential to ensure the cells adhere to the slide and are not washed away during staining?

Accepted Answer

Heat-fixing the smear by briefly passing the slide through a flame

Question 5

During the citrate test, a positive result is indicated by a color change in the medium. What does a positive citrate test tell you about the metabolic capabilities of the bacteria?

Accepted Answer

The bacteria can use citrate as their sole carbon source and convert ammonium phosphate to ammonia, raising the pH

Question 6

A student performs a urea hydrolysis test and observes a bright pink color in the medium. Which enzyme is responsible for this reaction, and what is being produced?

Accepted Answer

Urease; ammonia and carbon dioxide are produced, raising the pH

Question 7

In the S.I.M. test, which reagent is added to detect indole production, and what does a positive result indicate?

Accepted Answer

Kovac's reagent; a positive result (red ring) indicates the organism can degrade tryptophan to indole

Question 8

A phenol red broth test is performed with glucose as the carbohydrate. The broth turns yellow and a gas bubble is observed in the Durham tube. What does this indicate about the metabolism of the bacteria?

Accepted Answer

The bacteria ferment glucose, producing acid (which turns the broth yellow) and gas

Question 9

Which of the following best explains why mineral oil is added to the decarboxylation test medium?

Accepted Answer

To create an anaerobic environment that promotes decarboxylase enzyme activity

Question 10

A microbiologist is testing for catalase activity. After adding hydrogen peroxide to a bacterial colony, vigorous bubbling is observed. What does this indicate, and what is the chemical reaction involved?

Accepted Answer

The bacteria produce catalase, which breaks down hydrogen peroxide into water and oxygen: $2H_2O_2 \rightarrow 2H_2O + O_2$

Question 11

A sample is plated on MacConkey agar. After incubation, colonies appear pink. What does this indicate about the bacteria, and why is MacConkey agar considered both selective and differential?

Accepted Answer

The bacteria are Gram-negative and ferment lactose, producing acid that turns the colonies pink; MacConkey agar is selective for Gram-negative bacteria and differential for lactose fermentation

Question 12

Which of the following best describes the oxygen requirements and growth patterns of a facultative anaerobe in thioglycollate broth?

Accepted Answer

Growth throughout the tube, but denser at the top where oxygen is present

Question 13

A blood smear shows an abnormally high number of eosinophils. Which condition is most likely associated with this finding?

Accepted Answer

Eosinophilia, often due to parasitic infections or allergic reactions

Question 14

You have a tube with a dilution factor of $10^{-3}$. If you plate 100 µL from this tube onto an agar plate (the tube contains 1000 µL), what is the dilution factor of the plate?

Accepted Answer

The plate dilution is $10^{-4}$, calculated as $10^{-3} 	imes \frac{100}{1000} = 10^{-4}$

Question 15

Which of the following best defines the term 'mycelium' in fungi?

Accepted Answer

A mass of hyphae that forms the vegetative part of a fungus

Question 16

Which of the following protozoans is responsible for causing malaria, and what is the form that infects red blood cells?

Accepted Answer

Plasmodium; the trophozoite form infects red blood cells

Question 17

A patient is diagnosed with an infection caused by Candida. Which type of organism is Candida, and what is its typical metabolic strategy?

Accepted Answer

Fungus; it is a saprotroph that absorbs nutrients from organic matter

Question 18

Which of the following best describes the difference between a definitive host and an intermediate host in the life cycle of a helminth?

Accepted Answer

The definitive host harbors the adult, sexually reproducing stage; the intermediate host harbors the larval or asexual stage

Question 19

A microbiologist is counting colonies on a plate to determine the original cell density (OCD) of a bacterial culture. If 50 colonies are counted on a plate inoculated with 0.1 mL of a $10^{-4}$ dilution, what is the OCD in CFU/mL? Use the formula $	ext{OCD} = \frac{	ext{CFU}}{	ext{Volume plated (mL)} 	imes 	ext{Dilution}}$.

Accepted Answer

OCD = $\frac{50}{0.1 	imes 10^{-4}} = 5 	imes 10^{6}$ CFU/mL

Microbiology Lab Study Guide: Microscopy, Staining, Metabolic Tests, Selective Media, and Eukaryotic Microbes

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