Question 1

A microbiologist inoculates a bacterium into phenol red glucose broth. After 24 hours, the broth turns yellow and a gas bubble is observed in the Durham tube. Which of the following best explains these results?

Accepted Answer

The bacterium ferments glucose, producing acid and gas as byproducts.

Question 2

Which enzyme is responsible for the clearing observed around bacterial growth on starch agar after the addition of Gram's iodine?

Accepted Answer

Amylase

Question 3

A student performs a methyl red test on an unknown bacterium. After adding methyl red reagent, the broth turns red. What does this indicate about the metabolic pathway of the bacterium?

Accepted Answer

The bacterium carries out mixed acid fermentation, producing stable acids.

Question 4

Which of the following best describes the selective and differential properties of Mannitol Salt Agar (MSA)?

Accepted Answer

MSA is selective due to high NaCl concentration and differential due to mannitol and phenol red.

Question 5

A blood agar plate shows a clear, transparent zone around a bacterial colony. What type of hemolysis is this, and which organism is most likely responsible?

Accepted Answer

Beta hemolysis; Staphylococcus aureus

Question 6

A laboratory test shows a black precipitate in SIM medium after inoculation with a bacterial sample. Which enzyme is responsible for this result, and what is the chemical reaction involved?

Accepted Answer

Cysteine desulfurase; $$\mathrm{Cysteine} \rightarrow \mathrm{H}_2\mathrm{S} + \mathrm{Pyruvate} + \mathrm{NH}_3$$

Question 7

Which of the following statements correctly distinguishes Staphylococcus from Streptococcus in the laboratory?

Accepted Answer

Staphylococcus is catalase positive and forms clusters; Streptococcus is catalase negative and forms chains.

Question 8

A patient presents with a wound infection. Laboratory tests show Gram-positive cocci in clusters, beta hemolysis on blood agar, and a positive coagulase test. Which organism is most likely responsible?

Accepted Answer

Staphylococcus aureus

Question 9

Which test would you use to distinguish Streptococcus pyogenes (Group A) from other beta-hemolytic streptococci, and what result would you expect?

Accepted Answer

Bacitracin susceptibility; any size zone of inhibition indicates S. pyogenes

Question 10

A microbiologist is testing for the presence of catalase in a bacterial isolate. Which of the following reactions and observations would confirm a positive result?

Accepted Answer

$$2\mathrm{H}_2\mathrm{O}_2 \xrightarrow{\mathrm{catalase}} 2\mathrm{H}_2\mathrm{O} + \mathrm{O}_2$$; bubbles observed upon addition of hydrogen peroxide

Question 11

Which of the following white blood cells is most likely to be elevated in a patient with a parasitic infection, and what is its normal percentage in the blood?

Accepted Answer

Eosinophils; 1-5%

Question 12

A laboratory technician observes a greenish discoloration around colonies on a blood agar plate. What type of hemolysis is this, and what does it indicate?

Accepted Answer

Alpha hemolysis; partial destruction of red blood cells

Question 13

Which of the following best describes the epidemiological significance of Mary Mallon ('Typhoid Mary') in the context of infectious disease transmission?

Accepted Answer

She was an asymptomatic carrier of Salmonella typhi, spreading typhoid fever through poor hygiene as a cook.

Question 14

A bacterial isolate is inoculated onto Simmon's citrate agar. After incubation, the slant turns blue. What does this indicate about the bacterium?

Accepted Answer

It can utilize citrate as its sole carbon source, producing alkaline byproducts.

Question 15

Which of the following is a correct pairing of a test, its enzyme, and a positive result?

Accepted Answer

Urea hydrolysis; urease; hot pink color in broth

Question 16

A patient’s blood smear shows an abnormally high percentage of lymphocytes. Which of the following conditions is most likely associated with this finding?

Accepted Answer

Viral infection

Question 17

Which of the following best defines a pandemic?

Accepted Answer

An infectious disease that spreads to one or more continents.

Question 18

A microbiologist is testing for phenylalanine deamination. After adding ferric chloride to the slant, the medium turns green. What does this indicate?

Accepted Answer

The bacterium produces phenylalanase, deaminating phenylalanine.

Question 19

Which of the following is the correct order of steps for interpreting a positive indole test?

Accepted Answer

Inoculate tryptose broth, incubate, add Kovac’s reagent, observe red ring at top.

Microbiology Lab Study Guide: Physiological Tests, Staphylococcus & Streptococcus, White Blood Cells, and Epidemiology

Study Guide - Practice Questions

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