Exercise 6-1 Micro Lab: Standard Plate Count
_______ are the primary vehicle responsible for the transmission of microbial diseases of the gastrointestinal system
heterotrophic plate count
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.
Normally, raw hamburger should not contain over _____ bacteria per gram
One of the limitations of the heterotrophic plate count
only bacteria capable of growing in the culture medium under the environmental conditions provided will be counted
requiring organic carbon
standard plate count
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
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.
tubes containing a known volume of sterile diluent - water, saline, or buffer
If a dilution is 10^-4, the proportion of the original sample inside the tube would be what?
1/10000th of the total volume inside.
When 0.1 mL of a 10^-4 solution is transferred to a plate, what is the volume of the sample in the plate?
0.1 mL x 10^-4 = 10^-5 mL
contains between 30-300 colonies
Plates with fewer than 30 colonies are ___
TFTC (too few to count)
Plates with more than 300 colonies are ____
TMTC (too many to count)
Dilutions can be calculated using the following formula
V1D1 = V2D2
V1 and D1 are what?
The volume and dilution of the concentrated broth
V2 and D2 are what?
the volume and dilution of the completed dilution
Undiluted samples are always expressed as ____
To calculate the dilution of a 1 mL sample transferred to 9 mL of dilutent, the permuted formula would be:
D2 = (V1D1/V2) = [(1.0 mL x 1) / 10 mL] = (1/10) = 10^-1
Formula for calculating original cell density (OCD)
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.
Colony Forming Unit (CFU)
the number of colonies that develop on the plate
Why is CFU the preferred term?
Because colonies could develop from single cells or from groups of cells, depending on the typical cellular arrangement of the organism.
Original sample volume
density and volume from the OCD = CFU / (D x V)
it becomes OCD = CFU / original sample volume
When is the sample volume written on the plate?
at the time of inoculation
provides an estimate of actual living cells in the sample
What bacteria will we be using in this lab?
Escherichia coli (E. coli)
In this lab we will inoculate the plates using what technique?
spread plate technique
We will count bacterial colonies using what?