BIO203 Laboratory Media
and Biochemical Tests
Table of Contents
I. Media
1
TSA – Tryptic Soy Agar
1
Blood Agar
2
EMB – Eosin Methylene Blue Agar
3
MSA – Mannitol Salt Agar
4
MacConkey Agar
5
II. Colony Morphology of 12 bacteria in our laboratory
6
III. Biochemical Tests
15
Motility Test
15
Oxidase Test
16
Glucose Fermentation Test
17
Nitrate Test
18
FT – Fluid Thioglycollate Test
19
Urea Test
20
IMViC: Tryptone broth/Indole test (I)
21
IMViC: MR-Methyl Red (“M”)
22
IMViC: VP-Voges-Proskauer Test (V)
23
IMViC: Simmons citrate slant (C)
24
IMViC test results for specific bacteria
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MEDIA
Isolation of bacteria is accomplished by growing ("culturing") them on the surface of
solid nutrient media. Such a medium normally consists of a mixture of protein
digests (e.g., peptone, tryptone) and inorganic salts, hardened by the addition of
1.5% agar. Standard or general purpose media will support the growth of a wide
variety of bacteria – for example: TSA. A medium may be enriched by the addition
of blood or serum – we use sheep’s blood agar.
Selective media contain ingredients that inhibit the growth of some organisms but
allow others to grow. For example, mannitol salt agar contains a high concentration
of sodium chloride that inhibits the growth of most organisms but permits
staphylococci to grow.
Differential media contain compounds that allow groups of microorganisms to be
visually distinguished by the appearance of the colony or the surrounding media,
usually on the basis of some biochemical difference between the two groups. Blood
agar is one type of differential medium, allowing bacteria to be distinguished by the
type of hemolysis produced. Some differential media are also selective, for example,
eosin-methylene blue agar, which is selective for gram-negative coliforms and can
differentiate lactose-fermenting and non-lactose-fermenting bacteria.
Examples of the agars and some of the bacteria that you will see in this class:
TSA – Tryptic Soy Agar: used for the isolation and cultivation of nonfastidious and fastidious
microorganisms – not the medium of choice for anaerobes.
TSA Streak Plate: pigmented Serratia
marcescens
TSA Streak Plate: Stapholococcus
aureus
1
Blood Agar – used for the detection of hemolytic activity of microorganisms.
alpha (α) hemolysis: green halo around colony.
Blood agar – Streptococcus
pneumoniae
beta (β) hemolysis: a clear, colorless zone appears around the colonies, indicating
the red blood cells have undergone complete lysis.
Blood agar – Staphylococcus aureus
gamma (γ) hemolysis: normal-looking colony.
Blood agar Micrococcus luteus
2
EMB – Eosin Methylene Blue Agar: selective and differential medium. Eosin differentiates
between two major coliforms: E. coli (smaller, green-metallic sheen) and Enterobacter
aerogenes (larger, rose color). Methylene blue selectively inhibits the growth of Gram+
bacteria. With this media we can also determine which bacteria are Gram-negative and which
are Gram-positive, because only Gram-negative bacteria grow on this special media. The
enhanced cell walls of Gram-negative bacteria protect these bacteria from the dye in the EMB
plates. The dye is able to enter the cells of Gram positive bacteria and kill them.
EMB: Escherichia coli
EMB:
A=Escherichia coli
B=Pseudomonas
C=Klebsiella
D=Enterobacter aerogenes
3
MSA – Mannitol Salt Agar: Demonstrates the ability of a bacterium to grow in a 7.5%
salt environment (growth indicates tolerance for high salt environment – no growth
shows intolerance/no determination of ability to ferment mannitol w/ no growth).
Demonstrates the ability of a bacterium to ferment mannitol (“+” mannitol
fermentation = yellow due to lowered pH from acids in waste. “-“ mannitol
fermentation = no color change/no acid).
MSA – Staphylococcus epidermidis
MSA – Staphylococcus aureus
4
MacConkey Agar: Demonstrates the ability of a gram negative bacterium to
metabolize Lactose. MacConkey agar is both a selective and differential medium
frequently used in culture testing. It contains crystal violet dye and bile salts, both
of which inhibit the growth of most gram-positive bacteria. It contains lactose (a
sugar) and neutral red indicator (a pH indicator which is yellow in a neutral solution,
but turns pink to red in an acidic environment), which allow for differentiation. On
MacConkey agar, Escherichia coli and Enterobacter aerogenes would ferment the
lactose producing acid and would form colonies pink to red in color. On the same
medium, Salmonella, Shigella, and Pseudomonas species would not ferment the
lactose and would form off-white colonies. The red colored colonies show that acid
was produced from lactose, meaning the bacteria could utilize lactose as a carbon
source.
Quadrant 1: Growth on the plate indicates the organism, Enterobacter aerogenes,
is not inhibited by bile salts and crystal violet and is a gram-negative bacterium.
The pink color of the bacterial growth indicates E. aerogenes is able to ferment
lactose.
Quadrant 2: Growth on the plate indicates the organism, Escherichia coli, is not
inhibited by bile salts and crystal violet and is a gram-negative bacterium. The
pink color of the bacterial growth indicates E. coli is able to ferment lactose.
Quadrant 3: Absence of growth indicates the organism, Staphylococcus epidermidis,
is inhibited by bile salts and crystal violet and is a gram-positive bacterium.
Quadrant 4: Growth on the plate indicates the organism, Salmonella typhimurium,
is not inhibited by bile salts and crystal violet and is a gram-negative bacterium.
The absence of color in the bacterial growth indicates S. typhimurium is unable to
ferment lactose.
5
COLONY MORPHOLOGY
Bacillus subtilis
Colonies which are dry, flat, and irregular, with lobate margins.
Proteus vulgaris
6
Pseudomonas
This gram negative rod forms mucoid colonies with umbonate elevation.
Staphylococcus aureus
Circular, pinhead colonies which are convex with entire margins. This gram positive coccus often
produces colonies which have a golden-brown color.
7
Mycobacterium smegmatis. Note the waxy appearance of this Acid-fast
bacterium.
8
Escherichia coli
This gram negative rod (coccobacillus) forms shiny, mucoid colonies which have entire margins
and are slightly raised. Older colonies often have a darker center.
9
Serratia marcescens. These gram negative rods produce mucoid colonies which have entire
margins and umbonate elevation. Note that there are both red and white colonies present on
this plate. Some strains of S. marcescens produce the red pigment prodigiosin in response to
incubation at 30o C, but do not do so at 37o C. This is an example of temperature-regulated
phenotypic expression.
10
Corynebacterium xerosis
Lactobacillus
11
Staphylococcus epidermidis
Circular, pinhead colonies which are convex with entire margins. The colonies of this grampositive coccus appear either the color of the agar, or whitish.
12
Micrococcus luteus. Circular, pinhead colonies which are convex with entire margins. This gram
positive coccus produces a bright yellow, non-diffusable pigment.
13
Enterobacter aerogenes. This gram negative rod is a common contaminant of vegetable matter
which forms shiny colonies with entire margins and convex elevation.
14
Biochemical Tests
Motility Agar: identifies the ability bacteria to move (i.e. flagellated cells). We use plates
instead of tubes.
Non-motile – no
movement from
stab
Motile – growth
far away from
stab
15
Oxidase Test: Used to demonstrate the ability of a bacterium to produce the enzyme
cytochrome- c oxidase, capable of reducing oxygen. Only Aerobic bacteria have this enzyme.
This test will distinguish Aerobic vs. Anaerobic metabolism. A positive test will show a color
change to blue, then to dark purple or black, within 10 to 30 seconds.
16
Glucose Fermentation tubes: determines the ability of a bacterium to ferment the sugar glucose
as well as its ability to convert end products (pyruvic acid) into gaseous byproducts. Phenol red
indicator is used to show acid fermentation (yellow below pH 6.8) or alkaline fermentation (red
above pH 8.4). Durham tubes collect CO2 gas produced from fermentation process.
Phenol Red + Sugar:
Left = nonfermenter of glucose, no
gas produced.
Middle = glucose fermentation, but
no gas produced.
Right = glucose fermentation and gas
production.
17
N -Nitrate broth: demonstrates the ability of a bacterium to produce the enzyme nitratase,
capable of converting nitrate to nitrite.
1. Red color after addition of Nitrate A + Nitrate B = positive
test for nitrate reduction (end)
2. No color change after addition of Nitrate A + Nitrate B =
possibly negative for nitrate reduction (continue to next
step to confirm)
a) Red color after adding Zn = confirmed negative
for nitrate reduction
b) No color change after adding Zn = positive test
for nitrate reduction (organism reduced nitrate
to nitrite then further to ammonia or N2). May
see gas bubble in Durham tube.
18
FT – Fluid Thioglycollate tubes: show oxygen usage of bacteria by where they grow in the tube.
Resazurin is an indicator that is pink in the presence of oxygen (notice upper portion of tube
where media touches air – if it is pink deeper in the tube, the tube has too much oxygen
diffused and should not be used). Thioglycollate binds w/ oxygen that diffuses into the media,
making it unavailable deeper in the tube.
Tube 1 = obligate anaerobe. Growth in the bottom but none in the top.
Tube 2 and 5(mislabeled!!) = obligate aerobe. Growth only at the top.
Tube 3 = aerotolerant facultative anaerobe. Dense growth throughout tube.
Tube 4 = facultative anaerobe. Growth throughout, but more dense at top.
19
U -Urea broth: demonstrates the ability of a bacterium to produce the enzyme urease, capable
of hydrolyzing urea. Phenol red indicator is added (fuchsia above pH 8.4) to show rise in pH due
to accumulation of ammonia.
20
IMViC: A battery of biochemical tests known as IMViC are used in the clinical lab to distinguish
between enteric microorganisms. The acronym IMViC stands for indole, methyl red, VogesProskauer and citrate. The "i" in the acronym is added for pronunciation purposes.
Tryptone broth/Indole test (“I”): Used to demonstrate the ability of a bacterium to produce the
enzyme tryptophanase. This enzyme acts on the amino acid to produce “indole”.
Tryptone
broth/Indole –
positive result
Tryptone
broth/Indole –
negative result
21
Methyl Red (“M”) – an indicator of low pH (red below pH of 4.4) – used to show the mixed acid
fermentation ability of bacteria.
22
VP -Voges-Proskauer Test (“Vi”) – used to show bacterial production of acetoin, also known as
2,3-butanediol.
23
Simmons citrate slant (“C”) – Simmons citrate agar tests for the ability of a gram-negative
organism to import citrate for use as the sole carbon and energy source. Only bacteria that can
utilize citrate as the sole carbon and energy source will be able to grow on the Simmons citrate
medium, thus a citrate-negative test culture will be virtually indistinguishable from an
uninoculated slant.
Simmons citrate – blue is a positive
citrate test, while green is
negative/no growth
24
IMViC test results for specific bacteria:
Escherichia coli: Tube A: + Idole
Tube B: + methyl red
Tube C: - VP
Tube D: - citrate
Proteus vulgaris: Tube A: + Idole
Tube B: + methyl red
Tube C: - VP
Tube D: - citrate
Citrobacter freundii: Tube A: - Idole
Tube B: + methyl red
Enterobacter aerogenes: Tube A: - Idole
Tube B: - methyl red
Tube C: - VP
Tube C: + VP
Tube D: + citrate
Tube D: + citrate
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