Interpretation of the results from metabolic assays (Lab)

Question 1

What does it mean when halos form around the colonies in a starch hydrolysis test?

Answer 1

Amylase is produced and the organism is hydrolyzing starch

Question 2

What does it mean when no halos form around the colonies in a starch hydrolysis test?

Answer 2

No amylase is present. No starch hydrolysis

Question 3

What does it mean when the tube is blue in an oxidative-fermentation test?

Answer 3

The organism can’t use glucose and is using the peptone instead, which produces alkaline ammonia

Question 4

What does it mean when the aerobic tube is yellow, but the anaerobic tube is green in an oxidative-fermentation test?

Answer 4

The organism does glucose oxidation only in the presence of O2

Question 5

What does it mean when the both the aerobic and anaerobic tubes are yellow in an oxidative-fermentation test?

Answer 5

The organism does glucose fermentation with or without O2

Question 6

What does it mean when the tube is yellow in a carbohydrate fermentation test?

Answer 6

The organism is fermenting the carbohydrate and producing acid

Question 7

What does it mean when the tube is red in a carbohydrate fermentation test?

Answer 7

The organism is not fermenting the carbohydrate and is using the peptone instead, producing alkaline ammonia

Question 8

What does it mean when the tube is red on the top and yellow in a carbohydrate fermentation test?

Answer 8

The organism was fermenting the carbohydrate, produced a bunch of acid, and used it up, then starting using the peptone in aerobic conditions and created ammonia, which changed the phenol red back to red

Question 9

What does it mean when the tube is red in a methyl red test?

Answer 9

The organism is not producing acid, so it does not do mixed acid fermentation

Question 10

What does it mean when the tube is yellow in a methyl red test?

Answer 10

The organism is doing mixed acid fermentation and is producing large amounts of acid

Question 11

What does it mean when the tube is red in a Voges-Proskauer test?

Answer 11

The organism is doing butanediol fermentation and is producing acetoin

Question 12

What does it mean when the tube is yellow in a Voges-Proskauer test?

Answer 12

The organism is not producing acetoin

Question 13

What does it mean when the agar is green in a citrate test?

Answer 13

The organism can’t use citrate for fermentation, so doesn’t have citrate lyase and can’t grow

Question 14

What does it mean when the agar is blue in a citrate test?

Answer 14

The organism has citrate lyase and can grow using citrate for fermentation

Question 15

What does it mean when the bacterial smear is stringy in a KOH test?

Answer 15

Gram-negative. The membrane got destroyed and the DNA got released

Question 16

What does it mean when the bacterial smear doesn’t do anything in a KOH test?

Answer 16

Gram-positive. The PG layer protected the membrane so the DNA stays in the cell

Question 17

What does it mean when the test strip turns blue in an oxidase test?

Answer 17

Cytochrome C is present and ADMA oxalate got oxidized and turned blue

Question 18

What does it mean when the test strip stays colourless in an oxidase test?

Answer 18

Cytochrome C is absent

Question 19

What does it mean when the tube is yellow in an ONPG test?

Answer 19

Beta-galactosidase is present and lactose is being broken down

Question 20

What does it mean when the tube is colourless in an ONPG test?

Answer 20

No beta-galactosidase and no lactose breakdown

Question 21

What does it mean when the tube is red in an indole test?

Answer 21

Tryptophanase is present and indole is being produced

Question 22

What does it mean when the tube is yellow in an indole test?

Answer 22

No tryptophanase and no indole

Question 23

What does it mean when colonies are yellow on MacConkey agar?

Answer 23

They’re enteric and gram-negative, but don’t do lactose fermentation

Question 24

What does it mean when colonies are pink on MacConkey agar?

Answer 24

Enteric and gram-negative and do lactose fermentation (acid caused the dye to be absorbed)

Question 25

What does it mean when colonies are pink with pink haloes on MacConkey agar?

Answer 25

Enteric, gram-negative, and strong lactose fermenters that caused the bile salts to precipitate around the colonies

Question 26

What does it mean when the organism doesn’t grow on MacConkey agar?

Answer 26

Is gram-positive and can’t tolerate the crystal violet, or is non-enteric

Question 27

What does it mean when colonies are metallic green on EMB agar?

Answer 27

Gram-negative and do mixed acid fermentation, which caused the eosin to turn black

Question 28

What does it mean when colonies are fish-eye on EMB agar?

Answer 28

Gram-negative and do some sort of fermentation that isn’t mixed acid, so they produce less acid and less eosin turns black

Question 29

What does it mean when colonies are light pink on EMB agar?

Answer 29

Gram-negative and don’t do lactose fermentation, but do absorb the eosin

Question 30

What does it mean when the organism doesn’t grow on EMB agar?

Answer 30

Gram-positive

Question 31

What does it mean when bubbles are produced in a catalase test?

Answer 31

Catalase is present and the organism is part of the Micrococcaceae

Question 32

What does it mean when no bubbles are produced in a catalase test?

Answer 32

Catalase is absent and the organism is a Streptococcaceae

Question 33

What does it mean when gas is produced in a nitrate reduction test?

Answer 33

Positive for nitrate reduction. The organism is producing NO, N2O, or N2 from reducing nitrate

Question 34

What does it mean when the tube turns red after adding sulfanillic acid and alpha-naphthylamine in a nitrate reduction test?

Answer 34

Positive for nitrate reduction. The organism is reducing nitrate to nitrite

Question 35

What does it mean when the tube turns red after adding the zinc in a nitrate reduction test?

Answer 35

Negative for nitrate reduction. Nitrate is still present in the tube so the organism isn’t using it

Question 36

What does it mean when the tube turns yellow after adding the zinc in a nitrate reduction test?

Answer 36

Positive for nitrate reduction. Nitrate is being reduced to some other byproduct that isn’t detected

Question 37

What does it mean when the agar is discoloured green or brown around the colonies on blood agar?

Answer 37

Alpha-hemolysis. The organism produces hemolysins that reduce Hb but don’t lyse red blood cells

Question 38

What does it mean when the agar is cleared around the colonies on blood agar?

Answer 38

Beta-hemolysis. The organism produces hemolysins that completely lyse red blood cells

Question 39

What does it mean when the agar isn’t cleared or discoloured around the colonies on blood agar?

Answer 39

Gamma-hemolysis. The organism doesn’t produce hemolysins

Question 40

What does it mean when colonies are yellow on mannitol salt agar?

Answer 40

The organism can tolerate high salt and ferments mannitol

Question 41

What does it mean when colonies are pink on mannitol salt agar?

Answer 41

The organism can tolerate high salts but can’t ferment mannitol

Question 42

What does it mean when the organism doesn’t grow on mannitol salt agar?

Answer 42

It can’t tolerate high salt