Question 1

Which of the following best describes the function of a serological pipette in the microbiology lab?

Accepted Answer

It is used to transfer and measure variable volumes of liquid, with graduations down to the tip, allowing for partial delivery.

Question 2

A student is preparing a Gram stain. After applying the primary stain and mordant, they forget to use the decolorizer. What is the most likely result when observing both Gram-positive and Gram-negative bacteria under the microscope?

Accepted Answer

Both Gram-positive and Gram-negative bacteria will appear purple.

Question 3

Which of the following best explains why EMB (Eosin Methylene Blue) agar is considered both selective and differential?

Accepted Answer

It contains eosin and methylene blue dyes that inhibit Gram-positive bacteria (selective) and differentiates lactose fermenters by color change (differential).

Question 4

A patient presents with a sore throat, fever, and a red rash. A Gram-positive coccus in chains is isolated from a throat swab. Which pathogen is most likely responsible, and what is its Gram reaction and shape?

Accepted Answer

Streptococcus pyogenes; Gram-positive coccus in chains

Question 5

Which of the following equations correctly represents the calculation of the number of new cases in the next generation of an epidemic, given $$R_0$$ and the current number of cases $$C$$?

Accepted Answer

$$C_{next} = C 	imes R_0$

Question 6

A microbiologist is testing a bacterial isolate for catalase activity. After adding hydrogen peroxide, bubbles are observed. What is the correct interpretation and the chemical reaction involved?

Accepted Answer

The organism is catalase-positive; $$2H_2O_2 \rightarrow 2H_2O + O_2$ (bubbles formed).

Question 7

Which of the following best distinguishes a propagated epidemic from a common source epidemic?

Accepted Answer

A propagated epidemic involves person-to-person transmission over time, while a common source epidemic results from a single contaminated source.

Question 8

A microbiologist is using a thioglycolate broth to test a bacterium's oxygen requirements. Growth is observed only at the bottom of the tube. What is the most likely classification of this organism?

Accepted Answer

Obligate anaerobe

Question 9

Which of the following statements about the use of oil immersion with a light microscope is correct?

Accepted Answer

Oil immersion increases the numerical aperture and reduces light refraction, improving resolution at high magnification.

Question 10

A sample is streaked on Mannitol Salt Agar (MSA). After incubation, yellow colonies with a yellow halo are observed. What does this indicate about the organism?

Accepted Answer

The organism is salt-tolerant and ferments mannitol, producing acid.

Question 11

Which of the following best describes the difference between defined and complex media?

Accepted Answer

Defined media contain exact amounts of pure chemicals, while complex media contain ingredients of unknown composition such as peptones or extracts.

Question 12

A microbiologist performs a coagulase test on a Staphylococcus isolate. The plasma clots after incubation. What is the correct interpretation?

Accepted Answer

The organism is coagulase-positive and likely to be Staphylococcus aureus.

Question 13

During an acid-fast stain, which step is responsible for differentiating acid-fast from non-acid-fast bacteria, and what is the primary stain used?

Accepted Answer

Decolorization with acid-alcohol; primary stain is carbol fuchsin.

Question 14

A student is asked to identify a bacterium that is Gram-negative, rod-shaped, ferments lactose with a metallic green sheen on EMB agar, and is commonly found in the human gut. Which organism is most likely?

Accepted Answer

Escherichia coli

Question 15

Which of the following is the correct sequence of steps in the Gram stain procedure?

Accepted Answer

Crystal violet, iodine, alcohol/acetone, safranin

Question 16

A patient is diagnosed with botulism. Which bacterial pathogen is responsible, and what is a unique feature of this organism?

Accepted Answer

Clostridium botulinum; produces endospores

Question 17

A microbiologist is testing for nitrate reduction. After adding nitrate reagents A and B, the broth turns red. What does this indicate about the organism's metabolism?

Accepted Answer

The organism reduced nitrate to nitrite.

Question 18

Which of the following is a critical safety precaution when working with live bacterial cultures in the lab?

Accepted Answer

Wear gloves and lab coat, disinfect work surfaces before and after use, and never eat or drink in the lab.

Question 19

A student is asked to prepare a bacterial smear from an agar plate. Which of the following is the correct first step?

Accepted Answer

Add a drop of water to the slide.

Question 20

A patient is infected with Yersinia pestis. Which of the following best describes its transmission and reservoir?

Accepted Answer

Transmitted by flea bites; reservoir is rodents.

lab exam 1 - Microbiology Lab Practicum Midterm Study Guide

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