Food Microbiology exam quizzes and
answers well graded A+
Liquid Medium - ANS✅✅0% agar
broths, infusions
used for growing large numbers of bacteria & biochemical testing
Semisolid media - ANS✅✅0.4% agar
used to determine motility
promotes anaerobic growth
Solid media - ANS✅✅>1.5% agar
used to observe colonies, pure culture isolation, storage, biochemical tests
Complex media - ANS✅✅usually has unknown attributes
usually beef extract, yeast extract, or peptic digests of proteins
used when exact needs of microbe is unknown
used commonly for heterotrophs
Chemically defined (minimal) media - ANS✅✅used to culture autotrophs
known amounts of pure chemicals
Selective media - ANS✅✅allows certain kinds of microbes to grow while inhibiting growth of
others
provides only specific carbon sources
selective inhibition with dyes, salt, antibiotics
examples: mannitol salt agar, bile esculin agar
Differential media - ANS✅✅used to differentiate between closely related microbes or groups of
microbes
certain dyes or chemicals shows characteristic changes or growth patterns in microbes used for
identification
, examples: Eosin methylene blue agar, MacConkey's agar, Blood agar, bile esculin agar
Enriched media - ANS✅✅contains growth promoting nutrients, support a wide variety of
microorganisms as well as fastidious ones
examples: blood agar, Mueller hinton agar
Gram staining - ANS✅✅peptidoglycan layer is 20x thicker in gram positive cells, which allows them
to retain the crystal violet dye
problems with gram stains:
over decolorize - gram positive looks like gram negative
under color - gram negative looks like gram positive
gram variable - within a species due to old culture
Structure of lipopolysaccharide - ANS✅✅O-polysaccharide repeat (40 units) -> core polysaccharide
-> glucosamine phosphate dimer -> fatty acids
Decolorizer (gram stain) - ANS✅✅95% ethanol + acetone
in gram positive cells, ethanol shrinks pores and increases dye retention, removes excess stain
in gram negative cells, ethanol increases porosity by removing lipids from cell envelope
Food sampling - ANS✅✅small sample is used to test large batch of food
aim for ~100g for each sample unit
Steps:
-weigh out food sample
-mix with fluid to homogenize (1:10 ratio)
-homogenize food sample
-plate sample for bacterial counts
Fluid for homogenizing (food samples) - ANS✅✅suspension only:
-sterile water, sterile saline, or PBS (phosphate buffered saline) is used
growth & suspension:
-buffered peptone water
answers well graded A+
Liquid Medium - ANS✅✅0% agar
broths, infusions
used for growing large numbers of bacteria & biochemical testing
Semisolid media - ANS✅✅0.4% agar
used to determine motility
promotes anaerobic growth
Solid media - ANS✅✅>1.5% agar
used to observe colonies, pure culture isolation, storage, biochemical tests
Complex media - ANS✅✅usually has unknown attributes
usually beef extract, yeast extract, or peptic digests of proteins
used when exact needs of microbe is unknown
used commonly for heterotrophs
Chemically defined (minimal) media - ANS✅✅used to culture autotrophs
known amounts of pure chemicals
Selective media - ANS✅✅allows certain kinds of microbes to grow while inhibiting growth of
others
provides only specific carbon sources
selective inhibition with dyes, salt, antibiotics
examples: mannitol salt agar, bile esculin agar
Differential media - ANS✅✅used to differentiate between closely related microbes or groups of
microbes
certain dyes or chemicals shows characteristic changes or growth patterns in microbes used for
identification
, examples: Eosin methylene blue agar, MacConkey's agar, Blood agar, bile esculin agar
Enriched media - ANS✅✅contains growth promoting nutrients, support a wide variety of
microorganisms as well as fastidious ones
examples: blood agar, Mueller hinton agar
Gram staining - ANS✅✅peptidoglycan layer is 20x thicker in gram positive cells, which allows them
to retain the crystal violet dye
problems with gram stains:
over decolorize - gram positive looks like gram negative
under color - gram negative looks like gram positive
gram variable - within a species due to old culture
Structure of lipopolysaccharide - ANS✅✅O-polysaccharide repeat (40 units) -> core polysaccharide
-> glucosamine phosphate dimer -> fatty acids
Decolorizer (gram stain) - ANS✅✅95% ethanol + acetone
in gram positive cells, ethanol shrinks pores and increases dye retention, removes excess stain
in gram negative cells, ethanol increases porosity by removing lipids from cell envelope
Food sampling - ANS✅✅small sample is used to test large batch of food
aim for ~100g for each sample unit
Steps:
-weigh out food sample
-mix with fluid to homogenize (1:10 ratio)
-homogenize food sample
-plate sample for bacterial counts
Fluid for homogenizing (food samples) - ANS✅✅suspension only:
-sterile water, sterile saline, or PBS (phosphate buffered saline) is used
growth & suspension:
-buffered peptone water
