Microbiology Lab Questions

Question 1


You are reading culture plates today and identifying pathogens that grow from patient samples. 

Which two pieces of information are absolutely critical in determining which additional tests must be done to identify the pathogen?


a) Appearance on charcoal agar

b) Endospore Stain

c) Gram Stain

d) Oxygen sensitivity


Question 2


A 10 year old has a wound on the arm that the physician suspects is infected.

Upon culture, you see small white colonies growing on blood agar as well as chocolate agar.

You gram stain the colonies to find that they are gram positive cocci.

Which test will you perform first?


a) catalase

b) indole

c) lactose fermentation

d) coagulase


Question 3


You have isolated catalase positive gram positive cocci from a wound culture.

Which test will you perform next?


a) coagulase

b) P disc, containing optichin

c) A disc, containing bacitracin


Question 4


You are working on a sputum culture.

You see mucoid alpha hemolytic colonies that number many more than the normal flora present.

The gram stain of the colonies shows gram positive cocci that are in lancet shaped pairs.

Which test will you do?


a) P disc, containing optichin

b) A disc, containing bacitracin

c) motility

d) acid fast staining


Question 5


You are working up a throat culture.  

Standard procedure in plating throat cultures in your lab includes dropping an A disc near the initial inoculum in the first quandrant when streaking the specimen.

You examine the blood agar plate and see moderate normal flora and many beta hemolytic colonies that do not grow up to the A disc.  

The beta hemolytic colonies are catalase negative gram positive cocci.

Which pathogen is in this throat culture?


a)Streptococcus agalactiae

b) Staphylococcus aureus

c) Streptococcus pyogenes

d) Staphylococcus epidermidis


Question 6


You are working up a male genital culture.

You see no growth on the blood agar plate but small colonies growing on the chocolate agar plate. The gram stain shows gram negative cocci in pairs.

Which test will you do next?


a) oxidase

b) motility

c) catalase

d) indole


Question 7


You are working up a cerebrospinal fluid culture.

You find colonies growing on blood agar as well as chocolate agar.

The colonies are oxidase positive gram negative cocci.

The colonies ferment glucose and maltose but not sucrose or lactose.

You identify the pathogen as


a) Neisseria meningitidis

b) Haemophilus influenzae

c) Streptococcus pneumoniae

d) Neisseria gonorrhoeae


Question 8


You are working up a urine culture.

You see >100 colonies that are gray and flat on the blood agar plate and >100 colonies that are bright pink on the MacConkey agar.

The IMViC results are Indole positive, Methyl Red positive, Vogues-Proskauer negative, Citrate negative.

You have identified the pathogen as


a) Citrobacter freundii

b) Escherichia coli

c) Enterobacter aerogenes

d) Proteus vulgaris


Question 9


You are working up a stool culture.

On MacConkey agar you see many bright pink colonies and many clear colonies.

Which colonies are potential pathogens that require further testing?


a) Clear colonies, non lactose fermenters

b) Bright pink colonies, non lactose fermenters

c) Bright pink colonies, lactose fermenters

d) Clear colonies, lactose fermenters


Question 10


DNA technology is useful in the identification of :


a) pathogens that are unable to be grown readily on artificial lab media. 

b) pathogens that are no longer alive in the patient sample,

c) species that cannot be differentiated by conventional testing.

d) All of the above.


Question 11


You are preparing a sample of DNA from an unknown colony of bacteria.

After adding digestion buffer and incubating for the time suggested by the manufacturer, you centrifuge the sample.

The DNA is found:


a) stuck to the gel in the tube.

b) stuck to the sides of the tube.

c) in the pellet in the bottom of the tube.

d) in the supernatant in the tube.


Question 12


Which of the following is not true of the Polymerase Chain Reaction?


a) PCR is facilitated by a heat labile DNA polymerase.

b) PCR is a method of replicating DNA in a test tube.

c) PCR can facilitate the detection of DNA that is too low to detect by other methods.


Question 13


Why are dATP, dCTP, dTTP and dGTP added to a PCR reaction tube?


a) They catalyze the polymerase.

b) They buffer the mixture.

c) They allow the DNA in the sample to anneal.

d) They provide the building blocks of DNA.


Question 14


Why are universal 16S rDNA primers used in your experiment?


a) They will anneal to highly conserved areas of the gene that encodes bacterial 16S rRNA.

b) They will anneal to unique sequences of genes encoding 16S rRNA in specific bacteria.






Question 15


If universal primers are used to amplify DNA in a PCR reaction, then the PCR product must be sequenced to determine the bacteria that the DNA belongs to.





Question 16


How is the PCR product separated from the PCR mixture at the completion of the reaction?


a) Perform electrophoresis in an agarose gel, stain the gel and cut the band corresponding to the PCR product from the gel.

b) Pour the PCR mixture into a commercially prepared DNA microconcentrator column and follow the manufacturer’s directions to adhere and elute the PCR product from the column.

c) Both of the above procedures may be used.

d) Neither of the above procedures may be used.


Question 17


Your PCR product was sequenced by a method known as Cycle Sequencing.

Which of the following statements is false?


a) An automatic sequencer performs electrophoresis and reads the tagged DNA pieces, providing a read out of the nucleotide bases comprising the DNA sequence of the fragment being tested

b) Cycle sequencing is done in a PCR machine.

c)Tagged terminator nucleotides facilitate the creation of a series of nested DNA sequences of different length.

d) Cycle sequencing can be completed in just one test tube.


Question 18


The National Library of Medicine has a databank called GenBank that has deposited in it the DNA sequences of numerous genes isolated from known bacterial species.






Question 19


You obtained the following BLAST data from your sample:

99.9% Enterobacter sakazakii

95.2% Enterobacter aerogenes

93.7% Enterobacter cloacae

The pathogen in your sample is:


a)Enterobacter sakazakii

b)Enterobacter aerogenes

c)Enterobacter cloacae

d)Enterobacter species