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Exercise 6-1 Micro Lab: Standard Plate Count

front 1

_______ are the primary vehicle responsible for the transmission of microbial diseases of the gastrointestinal system

back 1

food products

front 2

heterotrophic plate count

back 2

used to determine the number of viable bacteria in a food sample. the larger the count, the greater the likelihood that specific pathogens capable of causing disease will be present and also that the food will spoil.

front 3

Normally, raw hamburger should not contain over _____ bacteria per gram

back 3

10^6

front 4

One of the limitations of the heterotrophic plate count

back 4

only bacteria capable of growing in the culture medium under the environmental conditions provided will be counted

front 5

heterotrophic

back 5

requiring organic carbon

front 6

standard plate count

back 6

a procedure that allows microbiologists to estimate the population density in a liquid sample by plating a very dilute portion of that sample and counting the number of colonies it produces

front 7

serial dilution

back 7

a series of controlled transfers down a line of dilution blanks. the series begins with a sample containing an unknown concentration of cells (density) and ends with a very dilute mixture containing only a few cells.

front 8

dilution blanks

back 8

tubes containing a known volume of sterile diluent - water, saline, or buffer

front 9

If a dilution is 10^-4, the proportion of the original sample inside the tube would be what?

back 9

1/10000th of the total volume inside.

front 10

When 0.1 mL of a 10^-4 solution is transferred to a plate, what is the volume of the sample in the plate?

back 10

0.1 mL x 10^-4 = 10^-5 mL

front 11

countable plate

back 11

contains between 30-300 colonies

front 12

Plates with fewer than 30 colonies are ___

back 12

TFTC (too few to count)

front 13

Plates with more than 300 colonies are ____

back 13

TMTC (too many to count)

front 14

Dilutions can be calculated using the following formula

back 14

V1D1 = V2D2

front 15

V1 and D1 are what?

back 15

The volume and dilution of the concentrated broth

front 16

V2 and D2 are what?

back 16

the volume and dilution of the completed dilution

front 17

Undiluted samples are always expressed as ____

back 17

1

front 18

To calculate the dilution of a 1 mL sample transferred to 9 mL of dilutent, the permuted formula would be:

back 18

D2 = (V1D1/V2) = [(1.0 mL x 1) / 10 mL] = (1/10) = 10^-1

front 19

Formula for calculating original cell density (OCD)

back 19

OCD = CFU / (D x V)

D is the dilution as written on the dilution tube from which the inoculum comes.

V is the volume transferred to the plate.

front 20

Colony Forming Unit (CFU)

back 20

the number of colonies that develop on the plate

front 21

Why is CFU the preferred term?

back 21

Because colonies could develop from single cells or from groups of cells, depending on the typical cellular arrangement of the organism.

front 22

Original sample volume

back 22

density and volume from the OCD = CFU / (D x V)

it becomes OCD = CFU / original sample volume

front 23

When is the sample volume written on the plate?

back 23

at the time of inoculation

front 24

viable count

back 24

provides an estimate of actual living cells in the sample

front 25

What bacteria will we be using in this lab?

back 25

Escherichia coli (E. coli)

front 26

In this lab we will inoculate the plates using what technique?

back 26

spread plate technique

front 27

We will count bacterial colonies using what?

back 27

colony counter