Tests for Identifying Unknowns
Bacteria produce acidic products when they ferment certain carbohydrates. The
carbohydrate utilization tests are designed to detect the change in pH which
would occur if fermentation of the given carbohydrate occurred. Acids lower the
pH of the medium which will cause the pH indicator (phenol red) to turn yellow.
If the bacteria do not ferment the carbohydrate then the media remains red. If
gas is produced as a by product of fermentation, then the Durham tube will have
a bubble in it.
The carbohydrate tests we perform are the:
Glucose (Dextrose) test
All carbohydrate test media should be inoculated with the transfer loop.
Fermentation results from left to right:
Left tube shows less acid formation than far right tube, but gas is still made
Center shows no carbohydrate utilization to produce acid or gas.
Right tube shows acid was produced as evidenced by the yellow color, and gas
was made (look at the bubble in the Durham tube)
Tests for the ability of bacteria to convert citrate (an intermediate of the
Kreb’s cycle) into oxaloacetate (another intermediate of the Kreb’s cycle). In
this media, citrate is the only carbon source available to the bacteria. If it
can not use citrate then it will not grow. If it can use citrate, then the
bacteria will grow and the media will turn a bright blue as a result of an
increase in the pH of the media. To inoculate this slant, use the transfer
Left tube is a negative result. Right tube is a positive result.
This media is used to test if bacteria can digest the protein gelatin. To
digest gelatin, the bacteria must make an enzyme called gelatinase. To
inoculate this media, use a transfer needle to stab the gelatin. After
incubating the inoculated media for at least 48 hrs, transfer the tube into a
refrigerator. The tube should be completely chilled prior to observation. If
the media is solid after refrigeration then the test is negative (the bacteria
did not digest gelatin). If the media is liquefied even after refrigeration,
then the test result is positive…the bacteria is able to digest gelatin.
The 'Serratia marcescens' on the left is positive for gelatinase production, as
evidenced by the liquidation of the media. The 'Salmonella typhimurium' on the
right is negative, as evidenced by the solidity of the media.
This test is used to detect the enzyme amylase, which breaks down starch. After
incubation the plate is treated with Gram’s iodine. If starch has been
hydrolyzed (broken down) then there is a reddish color or a clear zone around
the bacterial growth; if it has not been hydrolyzed then there is a black/blue
area indicating the presence of starch. Simply use inoculating loop to spread
bacteria onto plate surface. After the bacteria have grown, you add a few drops
of Gram’s iodine to the plate and look for the color immediately after adding
This test is done to determine if bacteria can breakdown the amino acid
tryptophan into indole. SIM media or TSB (tryptic soy broth) is inoculated
using a transfer needle. After incubating the bacteria for at least 48 hours,
Kovac’s reagent is added to the media to detect if indole has been made by the
bacteria. The development of a red/pink layer on top of the media is a positive
result (the bacteria can breakdown tryptophan to form indole). Failure to see a
red layer is a negative result (indole was not formed from tryptophan).
The tube on the left with the red ring is positive for indole production while
the tube on the right shows a negative result.
The tube on the left is negative and the tube on the right is a positive
MRVP (methyl red-Vogues Proskauer):
This test is used to determine two things. The MR portion (methyl red) is used
to determine if glucose can be converted to acidic products like lactate,
acetate, and formate. The VP portion is used to determine if glucose can be
converted to acetoin.
These tests are performed by inoculating a single tube of MRVP media with a
transfer loop and then allowing the culture to grow for 3-5 days. After the
culture is grown, about half of the culture is transferred to a clean tube. One
tube of culture will be used to conduct the MR test, the second tube serves as
the VP test.
MR (methyl red) test:
Methyl red is added to the MR tube. A red color indicates a positive result
(glucose can be converted into acidic end products such as lactate, acetate,
and formate. A yellow color indicates a negative result, glucose is converted
into neutral end products.
VP (Vogues Proskauer) test: First alpha-napthol (also called Barritt’s reagent
A) and then potassium hydroxide (also called Barritt’s reagent B) are added to
the VP tube. The culture should be allowed to sit for about 15 minutes for
color development to occur. If acetoin was produced then the culture turns a
red color (positive result); if acetoin was not produced then the culture
appears yellowish to copper in color (a negative result).
Triple sugar Iron (TSI) - & Hydrogen sulfide production (H2S): Looks at
fermentation of glucose, lactose, and sucrose and checks if hydrogen sulfide is
produced in the process. Basically a pH indicator will change the color of the
media in response to fermentation…where that color change occurs in the tube
will indicate what sugar or sugars were fermented. The presence of a black
color indicates that H2S was produced. In this media, H2S reacts with the
ferrous sulfate in the media to make ferrous sulfide…which is black. To
inoculate, use a needle to stab agar and then uses a loop to streak the top
In addition to TSI media, SIM media can be used to determine if H2S is
produced. A black color in the SIM medium following inoculation and incubation
indicates that H2S is made by the bacteria.
does not ferment either lactose or sucrose
ferments lactose and/or sucrose
no fermentation of glucose
some fermentation of glucose has occurred, acid has been produced
Seen as cracks in the agar, bubbles, or the entire slant may be pushed out of
the tube. (Caution:these gassy fermenters may have bacteria close to the
H2S has been produced
From left to right:
Red slant and red butt, no black color= no fermentation of glucose, sucrose or
lactose. No Hydrogen sulfide produced
Red slant and black butt= no lactose or sucrose fermentation, H2S has been
Red slant with yellow butt= no lactose or sucrose fermentation, lactose is
fermented, no H2S has been produced
Yellow slant, yellow butt and black coloration= Lactose, sucrose and glucose
fermented, and H2S has been produced
Yellow slant, yellow butt and lifting and/or cracking of media, no black
coloration= Lactose, sucrose and glucose fermented, H2S has not been produced
but gas has been produced
Yellow slant, yellow butt and no lifting and/or cracking of media, no black
coloration= Lactose, sucrose and glucose fermented, H2S has not been produced
nor has gas been produced
This test is used to detect the enzyme urease, which breaks down urea into
ammonia. Ammonia is a base and thus will raise the pH of the media if it is
present. This change in pH is indicated by a pH indicator called phenol red
which is present in the media. A color change from yellow to bright pinkish-red
is positive; lack of color change is a negative result. Inoculate the liquid
media with a transfer loop.
The tube on the left is a positive reaction; the tube in the middle is a
negative reaction and the tube on the right in an un-inoculated control.
This test is can be used to detect the enzyme catalase. This enzyme is
responsible for protecting bacteria from hydrogen peroxide (H2O2) accumulation,
which can occur during aerobic metabolism. If hydrogen peroxide accumulates, it
becomes toxic to the organism. Catalase breaks H2O2 down into water and O2. To
perform the catalase test simply smear a small amount of the test organism onto
the lid of a Petri plate/culture dish. Then add a drop of hydrogen peroxide to
the smear. If bubbles become visible (these would be the O2 bubbling up) then
the test is positive and you can conclude that the organism makes catalase. A
lack of bubbles indicates the absence of catalase. *Note, most aerobic organism
Bubbling upon the addition of hydrogen peroxide is indicative of the presence
of catalase for this organism.
To perform this test simply swab some of your test culture into one of the
boxes on an oxidase dry slide. If a color change to purple or blue is evident
at 30 seconds-1 minute then the result is positive. It is important that the
test is read by one minute to ensure accurate results (avoid false negatives
and false positives). This laboratory test is based on detecting the production
of the enzyme cytochrome oxidase by Gram-negative bacteria. It is a hallmark
test for the Neiserria. It is also used to discriminate between aerobic
Gram-negative organisms like Pseudomonas aeruginosa and other
Additional Non-Biochemical Tests:
The motility test is not a biochemical test since we are not looking at
metabolic properties of the bacteria. Rather, this test can be used to check
for the ability of bacteria to migrate away from a line of inoculation thanks
to physical features like flagella. To perform this test, the bacterial sample
is inoculated into SIM or motility media using a needle. Simply stab the media
in as straight a line as possible and withdraw the needle very carefully to
avoid destroying the straight line. After incubating the sample for 24-48 hours
observations can be made. Check to see if the bacteria have migrated away from the
original line of inoculation. If migration away from the line of inoculation is
evident then you can conclude that the test organism is motile (positive test).
Lack of migration away from the line of inoculation indicates a lack of
motility (negative test result).
Left tube is result for a non-motile bacterium. Right tube is the result for a