Lab Exam Flashcards
(40 cards)
Motility Test
Motility Agar deeps are used to test motility. There is a pink dye within the media to show growth in the agar. A sterile needle is used to inoculate this media rather than a loop.
Positive Motility Test
fanning out away from the stab line. This can also make the tube look cloudy.
(Flagella present.)
Negative Motility Test
No fanning out from the stab line. (No flagella likely.)
Catalase Test
Aerobic respiration results in the formation of hydrogen ions (H+) which are converted to toxic hydrogen peroxides (H2O2) as part of the electron transport chain. The Catalase enzyme breaks down the peroxides into non-toxic H2O and O2. This enzyme is needed in order for a cell to exist in the presence of oxygen. Therefore, most bacteria are catalase positive with the exception of bacteria that cannot carryout aerobic respiration (i.e. obligate anaerobes).
Catalase Test Reagent
Hydrogen peroxide
Catalase Test Positive
Bubble formation after placing a drop of hydrogen peroxide on the
colony on a glass slide. (Catalase enzyme present.)
Catalase Test Negative
No bubble formation after placing a drop of hydrogen peroxide on
the colony on a glass slide. (No catalase enzyme detected.)
Nitrate Reduction Test
The ability to reduce nitrate to nitrite is commonly used to identify bacterial unknowns. The
media contains peptone and beef extract to support growth, as well as potassium nitrate. If the
organism can reduce the nitrate, it will react with the reagent.
Nitrate Reduction Test Reagents
PABA, a white powder.
Positive Result for Nitrate (NO3-) → Nitrite (NO2-)
Yellow, pink or red color change after the addition of the Paba.
Negative Result for Nitrate (NO3-) → Nitrite (NO2-)
No color change after addition of the Paba.
Glucose Test
Specifically tests for GLUCOSE breakdown, just because an organism breaks down one sugar doesn’t mean it breaks them all down. The potential outcomes are (+/+), (+/-), or (-/-). The first + or - is sugar breakdown (color change). The second + or - is gas production.
Durham tube
A small tube inside the larger tube of a glucose test. It is upside down and is there to catch any gas that is produced by the organism’s breakdown pathway.
Glucose Test: (+,+)
Yellow tube with gas bubble. (Glucose breakdown, gas produced)
Glucose Test: (+,-)
Yellow tube without gas bubble. (Glucose breakdown, no gas produced)
Glucose Test: (-,-)
Red tube, without gas bubble. (No glucose breakdown, no gas produced)
Differential Media
Colonies may be different colors based off of some
characteristics.
Selective Media
Media may inhibit growth of certain bacterial species.
Mannitol Salt Agar (MSA)
It selects for salt tolerant (or Gram positive) organisms, and differentiates based off the fermentation of mannitol. Growth (selection) is the first + or - and the color change (differential) is the second + or -.
Mannitol Salt Agar Outcome: (+,+)
Growth of organism, yellow agar color. (Salt tolerant organism, can ferment mannitol.)
Mannitol Salt Agar Outcome: (+,-)
Growth of organism, pink agar color. (Salt tolerant organism, cannot ferment mannitol.)
Mannitol Salt Agar Outcome: (-,-)
No growth of organism, pink agar color. (Not salt tolerant.)
MacConkey Agar
Identities lactose fermenting, Gram-negative enteric organisms, and for inhibiting growth of Gram-positive organisms (selective). Bacterial colonies that can ferment lactose turn the medium any variation of red (differential). This red color is due to the pH
indicators response to the acidic environment created by fermenting lactose. Organisms that do not ferment lactose do not cause a color change.
MacConkey Agar Outcome: (+,+)
Growth of organism, pink color change. (Gram negative organism, can ferment lactose.)
