Microbiology Lab Final

Question 1

What is the spherical shape of bacteria

Answer 1

cocci

Question 2

What is the rod shaped bacteria

Answer 2

bacilli

Question 3

What color will gram positive be under the microscope

Answer 3

purple

Question 4

What color will gram negative be under the microscope

Answer 4

pink

Question 5

What are the steps and purpose for each part of the gram stain

Answer 5

1.) Crystal violet: primary stain
2.) Grams Iodine: mordant(intensifies)
3.) Alcohol: decolorizer
4.) Safranin: counterstain

Question 6

What are four sources of error for gram staining

Answer 6

1.) Picking up colony with hot needle or heat fixing too long
2.) Decolorizing for too short or long
3.) Leaving stain on for too long
4.) Smear too thick/too much bacteria

Question 7

What are the steps for a catalase test

Answer 7

1.) Place one or two colonies on a slide with inoculating loop
2.) Drop 1 drop of hydrogen peroxide over it
3.) Observe for bubbling

Question 8

What is the interpretation of results for the catalase test

Answer 8

positive: bubbling
negative: no bubbling

Question 9

What are three sources of error for catalase test

Answer 9

1.) reversing the order of procedure can give false positive reactions if you put hydrogen peroxide on first
2.) Cultures more than 24 hrs old may give false negative reactions
3.) Improper storage of hydrogen peroxide can give false negative

Question 10

What is the interpretation of results for the mannitol salt agar

Answer 10

• positive for mannitol fermentation: yellow colonies
• negative for mannitol fermentation: red colonies

Question 11

What is a source of error for the mannitol salt agar

Answer 11

heavy growth can cause mannnitol fermenting organism to revert to peptone use, giving false negative

Question 12

Why do some organisms change color during mannitol salt agar test

Answer 12

Because the presence of mannitol in the medium allows organisms that ferment it to turn color

Question 13

What is the interpretation of results for the bile esculin agar

Answer 13

• positive: blackening of agar plate
• negative: no blackening of agar plate

Question 14

What are the two sources of error for the bile esculin agar

Answer 14

1.) If large inoculum is used, S. viridian can give a false positive reaction
2.) H2S producing bacteria may show a blackening of the agar

Question 15

Why does organisms turn color in the bile esculin agar

Answer 15

Organisms that are positive can split the esculin and release glucose into the medium, the ferric citrate allows color change to black with the ability to hydrolyze esculin

Question 16

What is the interpretation of results for the Bacitracin test

Answer 16

• positive: area of inhibition immediately surrounding the bacitracin disc
• negative: growth up to the bacitracin disc

Question 17

What are the two sources of error for the bacitracin test

Answer 17

1.) Only beta-hemolytic strep should be used, others may give false positive
2.) heavy inoculum may cause colonies to grow to the disc producing false positive results

Question 18

What is the interpretation of results for the optochin test

Answer 18

• positive: area of inhibition greater than 14 mm in diameter immediately surrounding the optochin disc
• negative: no inhibition or area of inhibition that is less than 14 mm

Question 19

What is the source of error for the optochin test

Answer 19

1.) Only alpha-hemolytic strep should be used, others may give false positive

Question 20

List the steps for the latex agglutination test

Answer 20

1.) Drop 1 drop of the staph latex reagent into a circle
2.) Use sterile loop to transfer two colonies into the circle and spread in liquid to cover entire circle
3.) gently rock card from side to side and observe agglutination for up to 20 seconds

Question 21

What is the interpretation of results for the staph latex agglutination test

Answer 21

• positive: agglutination within 20 sec
• negative: no agglutination in 20 sec

Question 22

What is a source of error for the staph agglutination test

Answer 22

1.) False negative or false positive results can occur if inadequate amounts of culture or reagent are used

Question 23

How do you perform the oxidase test step by step

Answer 23

1.) Place two drops of oxidase reagent on the filter paper
2.) The colony to be tested is picked up with a wooden stick and smeared over moist area

Question 24

What is the interpretation of results for the oxidase test

Answer 24

• positive: intense deep purple hue within 5-10 seconds
• negative: absence of coloration or coloration later than 20 secs

Question 25

What are 4 sources of error for oxidase test

Answer 25

1.) not using fresh reagents may give false positive
2.) the bacteria should not be grown on selective or differential media
3.) Recommended to use colonies that are 18-24 hrs old, older colonies will produce weaker results
4.) Any color changes after 20 secs should be disregarded

Question 26

What enzyme is the citrate test looking for

Answer 26

citritase

Question 27

What is the interpretation of results for the citrate test

Answer 27

• positive(alkaline): medium has growth or medium has growth and turns blue
• negative(no growth): medium remains green

Question 28

What are the steps for the indole test

Answer 28

1.) Place two drops of the indole reagent onto filter paper
2.) Pick up one colony with the applicator stick and smear it into the indole reagent
3.) Observe for a blue to blue/green color change within 2 minutes

Question 29

What is the interpretation of results for the indole test

Answer 29

• positive: pink/red
• negative: no color

Question 30

What are the two source of error for the indole test

Answer 30

1.) allow at least two minutes for the absence of a reaction before considering it negative
2.) Colonies should not be test from media containing dyes, dye carryover may lead to false results

Question 31

What are the carbohydrates that are in the TSI agar

Answer 31

glucose, lactose, and sucrose

Question 32

What is the interpretation of results for the TSI test

Answer 32

• K/A: an alkaline/acid(red slant/yellow butt) reaction
• A/A: an acid/acid(Yellow slant & butt) reaction
• K/K: an alkaline/alkaline(red slant & butt) reaction
• g: gas production: if theres space in between

Question 33

What is the interpretation of results for the urea test

Answer 33

• 4+: all dark pink/large amount of urease
• 3+: dark pink most of way down butt
• 2+: dark pink involves entire slant
• 1+: dark pink halfway down slant or pale pink over whole slant
• negative: all yellow or peach, no urease

Question 34

What is the interpretation of results for the macconkey agar

Answer 34

• lactose fermenter: red or bright pink
• lactose nonfermenter: clear

Question 35

What is the interpretation of the results for the EMB agar

Answer 35

positive: green or dark shimmer
negative: light pink or clear colony

Question 36

What sugars are in the EMB agar

Answer 36

lactose and sucrose