Q: What is the correct formula to calculate the original density of bacteria in a sample?

$$\text{CFU/mL} = \frac{\# \text{colonies counted}}{\text{volume plated} \times \text{dilution factor}}$$

Question 1

Which Biosafety Level (BSL) is appropriate for handling organisms that pose a moderate potential hazard to personnel and the environment, and is the designation for most teaching laboratories?

Accepted Answer

BSL-2

Question 2

Why must media be autoclaved before use in microbiology experiments?

Accepted Answer

To sterilize the media by killing all living organisms, including spores

Question 3

What is the correct formula to calculate the original density of bacteria in a sample?

Accepted Answer

$$	ext{CFU/mL} = \frac{\# 	ext{colonies counted}}{	ext{volume plated} 	imes 	ext{dilution factor}}$$

Question 4

Which of the following stains is considered a basic stain commonly used in microbiology labs?

Accepted Answer

Crystal violet

Question 5

After performing a Gram stain, what color will Gram-negative cells appear under the microscope?

Accepted Answer

Red or pink

Question 6

Which of the following best describes the morphology and arrangement seen in the second image provided?

Accepted Answer

Staphylococcus

Question 7

What is the primary stain used in the Ziehl-Neelsen acid-fast staining procedure?

Accepted Answer

Carbol fuchsin

Question 8

Which group of UV radiation is most detrimental to bacterial growth?

Accepted Answer

UVC

Question 9

In the disk-diffusion test, antibiotic X has a zone of inhibition of 2 cm and antibiotic Y has a zone of inhibition of 4 cm. Which antibiotic is more effective against the tested bacteria?

Accepted Answer

Antibiotic Y

Question 10

Which of the following best describes the third image provided (red rods with green ovals)?

Accepted Answer

Endospore stain showing bacilli with spores

Question 11

What is the purpose of the steam in the endospore staining protocol?

Accepted Answer

To drive the primary stain into the endospores

Question 12

Which of the following best describes an obligate anaerobe?

Accepted Answer

Cannot grow in the presence of oxygen

Question 13

Which of the following is the correct definition of prevalence in epidemiology?

Accepted Answer

The total number of cases of a disease in a population at a given time

Question 14

Which of the following best describes the fourth image provided (blue cocci and pink rods)?

Accepted Answer

Gram stain showing Gram-positive cocci and Gram-negative bacilli

Question 15

What is the main difference between disinfection and antiseptics?

Accepted Answer

Disinfection is used on inanimate objects, antiseptics are used on living tissue

Question 16

Which of the following is a characteristic of a countable plate in a standard plate count assay?

Accepted Answer

Contains between 30 and 300 colonies

Question 17

Which genus of bacteria can be detected using the acid-fast staining procedure?

Accepted Answer

Mycobacterium

Question 18

What is the purpose of the KOH test in bacterial identification?

Accepted Answer

To differentiate between Gram-positive and Gram-negative bacteria based on cell wall lysis

Question 19

Which of the following best describes a facultative anaerobe?

Accepted Answer

Can grow with or without oxygen

Question 20

Which of the following is the correct order of reagents used in the Gram staining procedure?

Accepted Answer

Crystal violet, iodine, alcohol, safranin

MCB 2010L Practical Review – Microbiology Study Guidance

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