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BIO 220
MICROBIAL GROWTH MEDIA
A culture medium is a nutrient material prepared for the growth of microorganisms. Culture
media must provide a source of carbon and energy as well as other mineral and growth factors
required by the microbe for growth. Oxygen levels and pH are other considerations when
selecting a growth medium. Some microbes can grow on a wide variety of media. Fastidious
microorganisms, on the other hand, require specific media for growth. These microbes have
relatively simple enzyme systems and thus have complex nutritional requirements. They only
grow if specific nutrients are present in the culture medium. Fastidious microbes require a
complex medium for growth.
Complex media
Complex media has an undefined chemical composition. Peptones, yeast and meat extracts are
common components of this media, and are responsible for the variability that exists between
prepared batches. Most heterotrophic bacteria and fungi are routinely grown on this type of
medium.
Examples of complex media include nutrient agar (NA) and nutrient broth (NB).
Enriched media
Medium supplied with nutrient-rich materials such as animal tissue, plant or yeast extracts, or
blood to support the growth of fastidious heterotrophs.
Selective media
Selective media is designed to suppress the growth of unwanted bacteria and encourage the
growth of the desired microbes.
Differential media
Differential media allows us to distinguish between different microbes that grow on the same
medium.
Some media are both selective and differential.
Sabouraud Dextrose Agar (SAB)
Selective for: fungal growth
• Low pH (5.6) supports the growth of fungi, but largely inhibits bacterial
growth
Phenylethyl alcohol medium
Selective for: growth of gram-positive bacteria such as streptococci and other micrococci
• Phenylethyl alcohol significantly impacts DNA synthesis and the
membrane integrity of gram-negative microbes, resulting in the inhibition
or significant reduction in growth of these organisms.
Deoxycholate medium
Selective for: growth of enteric bacilli from fecal samples
• Deoxycholate disrupts the integrity of gram-positive cell membranes.
Citrate potentiates the action of deoxycholate.
Differential for: lactose fermentation
• pH indicator is neutral red
• Bacteria that ferment lactose produce acid and, in the presence of neutral
red, form pink to red colonies. (i.e. E. coli)
• Bacteria that do not ferment lactose form colorless colonies.
Mannitol Salt Agar (MSA)
Selective for: growth of salt-tolerant Staphylococci bacteria
• 7.5 % NaCl inhibits the growth of most other bacteria
Differential for: mannitol fermentation
• pH indicator is phenol red
o if pH <6.8, turns yellow
o if pH>7, stays red
• if microbe can ferment mannitol, more free H+ will be produced, which
decreases pH (medium becomes yellow)
• if microbe cannot ferment mannitol, there should be no decrease in pH
(medium remains red)
This can help us differentiate between bacteria like S. epidermidis and S. aureus. Both of
these microbes are found on the skin, but only one typically causes serious infections.
• Both can grow on MSA
• S. epidermidis cannot ferment mannitol, but instead uses amino acids as an
energy source
• S. aureus can ferment mannitol, so what color will the medium be?
Eosin Methylene Blue Agar (EMB)
Selective for: gram-negative bacteria (Enterobacteriaceae and related enterics)
• Eosin and methylene blue dyes inhibit the growth of gram-positive
bacteria
Differential for: lactose fermentation
• If fermentation occurs, the dyes precipitate and form dark colonies
• If fermentation does not occur, the colonies are colorless
This can help us differentiate between bacteria like Escherichia coli and Enterobacter
aerogenes. Both of these microbes grow on the EMB medium and both ferment lactose,
but the resulting colonies look different.
• E.coli produces colonies with a metallic green sheen (high acid producers)
• E. aerogenes produces pink colonies with a dark dot in the center (“fish
eye” colonies) (low acid producers)
Blood Agar
Enriched with: 5% sheep blood
Differential for: hemolysis
• How well does the microbe break down hemoglobin/red blood cells?
o α-hemolysis – color of agar surrounding colonies is green to light
brown because bacteria can partially break down hemoglobin into
biliverdin (green pigment)
Microbe produces α-hemolysins
o β-hemolysis – complete lysis of hemoglobin/RBCs, see clearing of
agar around the colonies
Microbe produces β-hemolysins
o γ-hemolysis – no lysis (no change)
Sources
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http://www.austincc.edu/microbugz/blood_agar_test.php
http://www.austincc.edu/microbugz/mannitol_salt_agar.php
http://www.austincc.edu/microbugz/phenyethyl_alcohol_agar.php
http://foodsafety.neogen.com/pdf/Acumedia_PI/7130_PI.pdf
Tortora et al., Microbiology:An Introduction.12th ed. (2016). Pearson Education, Inc.
Chan et al., Laboratory Exercises in Microbiology. 6th ed. (1993). McGraw-Hill, Inc.
Meyer et al., Microbiology (BIO 440) Laboratory Manual. (2012). Sacramento City
College.