Q: A student is asked to calculate the number of bacteria per mL in a sample. They count 80 colonies on a plate made from a $$1/1000$$ dilution, and they plated $$0.1 mL. $$What is the correct calculation?

$$8.0 \times 10$$^{5}$$ cells/mL

Question 1

A student is asked to convert 0.00035 to scientific notation. Which of the following is the correct representation?

Accepted Answer

$$3.5 	imes 10$$^{-4}$$

Question 2

Which of the following is the correct metric conversion for 2500 milliliters (mL) to liters (L)?

Accepted Answer

$$2.5 L$$

Question 3

Why is it important to incubate agar plates upside down during bacterial culture?

Accepted Answer

To prevent condensation from dripping onto the agar surface and spreading colonies

Question 4

A microbiologist is using a phase contrast microscope to observe live, unstained bacterial cells. What is the main advantage of phase contrast over bright field microscopy in this scenario?

Accepted Answer

Phase contrast increases contrast in transparent specimens without staining, allowing observation of live cells

Question 5

Which of the following best describes the function of immersion oil when using the oil immersion objective on a microscope?

Accepted Answer

It increases the resolution by reducing light refraction between the slide and the objective lens

Question 6

A student prepares a bacterial smear and immediately heat fixes it without air drying. What is the most likely consequence?

Accepted Answer

The cells will be distorted or lysed due to rapid evaporation and boiling

Question 7

Which of the following is a correct description of a bacterial colony?

Accepted Answer

A visible mass of cells derived from a single parent cell

Question 8

Which of the following is an example of a differential stain?

Accepted Answer

Gram stain

Question 9

A microbiologist is using the KOH method to determine the Gram type of a bacterium. What result would indicate a Gram-negative organism?

Accepted Answer

The formation of a viscous, stringy material when mixed with KOH

Question 10

Which of the following best explains why lysozyme is more effective against Gram-positive bacteria than Gram-negative bacteria?

Accepted Answer

Gram-positive bacteria have a thick peptidoglycan layer exposed to lysozyme, while Gram-negative bacteria have an outer membrane that protects the peptidoglycan

Question 11

A student is asked to calculate the number of bacteria per mL in a sample. They count 80 colonies on a plate made from a $$1/1000$$ dilution, and they plated $$0.1 mL. $$What is the correct calculation?

Accepted Answer

$$8.0 	imes 10$$^{5}$$ cells/mL

Question 12

Which of the following best describes a control variable in a scientific experiment?

Accepted Answer

A variable that is kept constant to ensure that the effect of the independent variable can be measured accurately

Question 13

Which of the following arrangements describes 'streptococci'?

Accepted Answer

Chains of spherical cells

Question 14

A Gram stain is performed on a bacterial sample, but the culture is more than 48 hours old. What is a likely result?

Accepted Answer

Gram-positive bacteria may appear Gram-negative due to weakened cell walls

Question 15

Which of the following best describes the mechanism of action of penicillin?

Accepted Answer

It inhibits the synthesis of peptidoglycan, weakening the bacterial cell wall

Question 16

A student observes a microorganism under oil immersion and measures its length as 5 ocular units. If each ocular unit equals $$2 \ \mu m$$, what is the actual length of the organism?

Accepted Answer

$$10 \ \mu m$$

Question 17

Which of the following is a correct example of a basic dye and its use?

Accepted Answer

Crystal violet, used for simple staining of bacterial cells

Question 18

Which of the following best explains why fever temperature can enhance the effect of lysozyme and ampicillin on Gram-negative bacteria?

Accepted Answer

Higher temperatures can increase membrane permeability, allowing lysozyme and ampicillin better access to the peptidoglycan layer

Question 19

Which of the following best describes a negative stain?

Accepted Answer

A staining technique where the background is stained, leaving the cells colorless

Question 20

A scientist is designing an experiment to test the effect of a new antibiotic. Which of the following would be the best example of an experimental control?

Accepted Answer

A group of bacteria treated with a placebo instead of the antibiotic

Microbiology Lab Exam 1 Study Guide: Core Concepts and Techniques

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