BIOL 351 Lab Exam 2.docx

BIOL 351 LAB EXAM 2 (UPDATE 2026)
QUESTIONS AND ANSWERS WITH
VERIFIED SOLUTIONS 100%
CORRECT

What creates the black precipitate that is found in the but of a TSI agar slant inoculated
with Salmonella? - CORRECT ANSWES -- H2S production

A gap in the but of the agar of a TSI agar slant after 24 hours of growth indicates
liquefaction by extracellular enzymes. - CORRECT ANSWES -- False

If you inoculate a TSI tube on Monday in lab but don't return until Thursday to read the
results, what impact could this have? - CORRECT ANSWES -- A false negative of both
red slant and red butt could occur, due to peptone digestion after sugar depletion.

In the TSI test, sodium thiosulfate acts as a substrate for the measure of what reaction?
- CORRECT ANSWES -- hydrogen sulfide production

A bacterial species may produce H2S but not turn the butt yellow. - CORRECT
ANSWES -- False

The triple sugar-iron agar medium contains lactose, sucrose, glucose, sodium
thiosulfate, and ferrous sulfate. - CORRECT ANSWES -- True

A specimen was inoculated onto the Triple Sugar Iron (TSI) medium pictured here.
(black in the bottom and yellow agar) - CORRECT ANSWES -- The pH of the agar
decreased following incubation.

Hydrogen sulfide (H2S) was produced.

The bacterium is able to ferment glucose.

A specimen was inoculated onto the Triple Sugar Iron (TSI) medium pictured here.
(No black in the bottom) - CORRECT ANSWES -- The bacterium is able to ferment
glucose.

To conduct this test the butt of the tube should be stabbed.

The bacterium is able to ferment lactose and/or sucrose.

, To conduct this test the slant must be streaked.

Choose all that apply for the following TSIA: Yellow slant and bottom, bubbles in agar -
CORRECT ANSWES -- Acidic slant, Acidic bottom, Gas production

Choose all that apply for the following TSIA:
Red slant, yellow bottom - CORRECT ANSWES -- Alkaline Slant, Acidic bottom

Choose all that apply for the following TSIA:
Yellow slant, bubble on bottom, black agar - CORRECT ANSWES -- Acidic Slant
Acidic Butt
Gas production
H2S Production

Why is it important to stab and streak a TSIA slant? - CORRECT ANSWES -- So that
both the aerobic and anaerobic regions of the media are inoculated

If your TSIA has a red slant and a black butt, is the butt acidic or alkaline? What sugars,
if any, have been fermented? - CORRECT ANSWES -- The butt is acidic because acid
conditions must exist for the thiosulfate reduction to take place. Glucose only has been
fermented.

What is a reversion? In what region does this occur and why? - CORRECT ANSWES --
A reversion is when the media turns from yellow to red due to the breakdown of amino
acids which raises the pH. It can only occur in the slant because it needs an aerobic
environment for the reaction to occur.

Why is timing critical when reading results on a TSIA slant? - CORRECT ANSWES --
With a glucose-only fermenter, all glucose is fermented within the first 12 hours of
incubation and turns the entire medium yellow. If read too early, you would read a false
positive for the fermentation of sucrose and/or lactose because the process of reversion
hasn't begun.

If an organism is able to ferment sucrose, lactose, or both, the supply of these sugars
won't be used up until 24 hours of incubation. Once these sugars are used up, reversion
will begin. If read too late, you would get a false negative for the fermentation of
sucrose/lactose.

If you have a yellow slant and a yellow butt on TSIA, can you tell if the organism can
ferment sucrose only or lactose only? If not which other test can you use to be more
specific about sugar fermentation? - CORRECT ANSWES -- No, this test will not
determine if the organism is only a sucrose or only a lactose fermenter. You would need
to perform the phenol red broth test with the individual sugars to determine which sugar
the organism can ferment.