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Food Microbiology > Lab Material > Flashcards

Lab Material Flashcards

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1
Q

BPA

A

Baird-Parker Agar

isolation of Staph aureus

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2
Q

composition of BPA:

A 
sodium pyruvate (protect damaged cells)
egg yolk (diagnostic agent)
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3
Q

NA

A

Nutrient Agar

Nonspecific

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4
Q

SDA

A

Sabouraud Dextrose Agar

dermatophytes/fungi/yeast

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5
Q

Conditions of SDA:

A

acidic

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6
Q

Where does the term “Gram-stain” come from?

A

Hans Christian Gram (microbiologist)

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7
Q

Order of reagents for performing a Gram stain:

A

Crystal violet
Iodine
Decolorizer
Safranin

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8
Q

What does a Gram stain indicate? How?

A

Gram-positive vs Gram-negative (presence of cell wall)

  • positive: cell wall will retain crystal violet (purple)
  • negative: no cell wall, stained with safranin (pink)
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9
Q

A HHD agar showing colonies surrounded by yellow indicates what type of bacteria? Why?

A 
homofermentative LAB (lactobacilli)
produce more acid -> bromocrescol green becomes yellow
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10
Q

What color will streptococci colonies be surrounded by on a HHD agar? Why?

A

blue-green

produce less acid, bromocresol green stays green

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11
Q

How could we safely reduce the salt content of sauerkraut to 1%?

A

Use starter culture (L. mesenteroides) to bring down pH safely while making flavor compounds

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12
Q

How does the bacterial population change in the sauerkraut process?

A

beginning: heterofermentative (make flavors)
2-3 days: homofermentative (acid tolerant) take over
end conc: 10^8 to 10^9

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13
Q

What is the advantage of OGYE vs SDA? Why is this?

A

gives higher yeast counts

uses antibiotic (oxytetracycline) to kill competition instead of low pH (stresses yeast)

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14
Q

The main organic acid in grape wine is: ____. The main organic acid in strawberry wine is: _____.

A

tartaric acid

citric acid

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15
Q

What are the uses for each component in OGYE agar?

A

oxytetracycline: kill competing bacteria
glucose: sugar as food
yeast extract: provide peptides/AA for growth
agar: set gel

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16
Q

What are the purposes of the agents in ADPA?

A

tartic acid: lower pH (inhibit most microbes)
Dextrose: growth stimulant
Potato infusion: nutrients

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17
Q

What are the purposes of the agents in ADPA?

A

tartic acid: lower pH (inhibit most microbes)
Dextrose: growth stimulant
Potato infusion: nutrients

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18
Q

What agar is recommended for isolation of Listeria monocytogenes? What is the appearance of the colonies?

A

PALCAM Agar

Listeria will be grey-green, with black precipitate

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19
Q

What are the agents in PALCAM Agar that make it highly selective?

A

LiCl
ceftazidime,
polymixin B
Acryflavine HCl

20
Q

Mannitol fermenting microbes on a PALCAM agar will appear:

A

with yellow halo/yellow colony

21
Q

What does the abbreviation ‘spp.” mean?

A

referring to subspecies

22
Q

What medium is used to dilute bacterial solutions?

A

0.1% peptone saline

23
Q

What is the catalase test?

A

drop H2O2 onto colony on slide - watch for bubbles

bubbles indicate CATALASE POSITIVE (has catalase enzyme)

24
Q

Pasteurized milk is heated at ___ for ____.

Raw milk is milk that has been kept below ____.

A

63C, 30 min +

40C

25
Q

What agar is used to differentiate between homofermentative and heterofermentative LAB? What is the important ingredient?

A 
HHD agar
Bromocresol green (indicator)
26
Q

What is the appearance of heterofermentative bacteria on HHD agar? Why?

A

colorless colonies surrounded by various colors

produce less acid -> less strong of a color change

27
Q

What is the carb source in HHD media?

A

fructose

28
Q

Why is it important to monitor sugar content in maple syrup?

A

If not sufficiently concentrated, spoilage will occur

29
Q

Can mold grow on maple syrup?

A

Yes; if not sufficiently heat treated, or stored too long after opening (common problem)

30
Q

The agar commonly used for Kirby-Bauer tests: ______.

Why? (3)

A

Mueller Hinton Agar

  1. “loose” agar - allows for antibiotic to easily diffuse, giving clear zone of inhibition
  2. nonselective (can use for many diff. organisms)
  3. starch will absorb toxins from bacteria (prevent from interfering with antibiotic action)
31
Q

What does TSA contain? (2)

A

soy/casein peptones (organic nitrogen)

NaCl (or other salt): maintain osmotic equilibrium or select for halotolerance

32
Q

What is TSA used for?

A

general growth media, cultivation/storage/maintenence/transport of pure cultures

33
Q

According to disc diffusion test inhibition zones, bacteria are classified as:

A

resistant, intermediate, sensitive

34
Q

The greater the zone of inhibition diameter, the more _____ the microbe

A

sensitive

35
Q

Canned foods are classified based on _____.

A

acidity

36
Q

What is TSM and what is it used for? What is done to further acidify the media?

A

tomato serum media
cultivate/enumerate Lactobacilli
Can adjust to pH 5.1 by adding 1 mL/100mL 10% lactic acid

37
Q

What is PCA used for?

A

Plate count agar: nonselective, total viable bacteria growth

38
Q

What is DTB and what is it used for?

A

Dextrose Tryptone Broth

cultivate/enumerate spoilage organisms from low/med acid canned food

39
Q

How much of the sample should be added to 10 mL Dextrose Tryptone broth?

A

2g of food sample

40
Q

What is the recommended procedure/mediums for examining spoilage microbes in low/med acid canned food?

A

aerobic culture in DTB, at 35 and 55C

anaerobic culture in Liver Broth, at 37 and 55C

41
Q

What is liver broth? what growth does it support?

A

liquid with liver particles
saccharolytic/putrefactive; mesophilic/thermophilic;
anaerobic (agar plug)

42
Q

What is the procedure/media for examining spoilage microbes in high acid canned food?

A

aerobic culture in tomato serum broth, transfer to DTA or acidified Sab Agar, at 25, 37, 55C

anaerobic culture in tomato serum broth (with agar plug); incubate at 25, 37, 55C

43
Q

Describe the color change in DTB and why it occurs:

A

purple -> yellow

due to microbes consuming dextrose and making acid (Bacillus and other “flat sour” organisms)

44
Q

What signifies bacterial growth in anaerobic tubes?

A

turbidity

rising of agar plug (gas production)

45
Q

A 0.5 MacFarland standard is equivalent to:

A

10^8 inoculum

Food Microbiology (21 decks)

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