what is serial dilution?
where a percise volume of a sample to be tested is progressivly diluted through a series of known diluent volumes resulting in an agar plate with a number of colonies that can be easily counted
how can one determine the kind of bacterium?
by using the appropriate media (e.g. EMB agar) with the food product to be tested.
a precise volume of a sample to be tested
a substance used to dilute an aliquot
define "too numerous to count":
An agar plate with more than 250 colonies of bacteria
describe the process of serial dilution
- obtain 4 petri dishes and label 3 of them 10-2,10-3,10-4 using sample streak one dish for isolation
- the other three dishes will be diluted 10-2,10-3,10-4
- obtain 99ml sterile water blank
- obtain melted agar tube
- using a sterile pipette and pipetor withdraw 1.1 ml of the 10-1 sample from blender
- dispense 0.1 ml of sample into agar tube and the remaining 1.0 ml into the 99 ml blank
- secure caps to water blank and agar tube
- mix the sample in agar tube by slowly inverting 20 times DO NOT SHAKE
- pour the contents of the agar tube with 0.1 ml of sample into the BOTTOM of a petri dish labled 10-2
- obtain 2 more melted agar tubes and another pipette
- withdraw 1.1 ml from the blank containing 1.0 ml of sample and 99 ml water
- dispense 0.1 ml into one agar tube labled 10-4 invert it and place it in the BOTTOM the the 10-4 dish.
- dispense the remaining 1.0 ml into the agar tube labled 10-3, invert it and place it in the bottom of the 10-3 dish
- replace caps
- replace dish covers and let cool for 5 mins
- once dishes are cooled tape them and place them in the 37 deg incubator.
what is disk diffusion?
a teqnique used to evaluate the relative effectivness of chemicals against bacteria
describe the process of creating a bactria lawn
saturate a sterile swap in a bacteria broth. starting from the outer edge of the agar surface move the swab side to side, coming half way down the plate rotate the plate 90 degrees and do it again and rotate 90 degrees do this twice more to cover entire surface
what 2 types of bacteria commonly survive the pasteuration process?
thermoduric and thermophilic
what are the two methods used to determine the number of bacteria in milk?
- direct microscopic count
- plate count
what are the advantages and disadvantages of microscopic count?
- could detect infection
- counts all bacteria living and dead which would not show in plate count
what is pasteurization
milk is mildly heated to 72 degrees C for 15 seconds
what bacteria levels must pasteurized milk be within? and raw milk?
fewer than 30,000 bacteria per milliliter for pasteurized and less than 50,000 for raw
what is the difference between quantitative and qualitative
- quantitative - data that can be measured (numbers, etc.) deals with quantity
- qualitative - deals with descriptions and data that can be observed but not measured
describe pasteurized milk inoculation protocol
- label 2 empty petri dishes pasteurized 10-1 and 10-2 along with usual data
- obtain 2 sterile 9 ml water blanks label them 10-1 and 10-2
- using sterile pipette withdraw 1.0 ml of pasteurized milk and dispense it into the 10-1 blank
- invert 20 times
- using another pipette withdraw 1.0 ml of the 10-1 dilution and dispense it into the 10-2 blank
- invert the 10-2 blank 20 times
- obtain 2 tubes of melted agar
- using 2 sterile pipettes dispense 1.0 ml from the 10-1 and place in one agar tube and do the same for the 10-2
- pour contents of tubes into respective dish bottoms cover and let cool then place in 37 deg incubator
describe the raw milk inoculation protocol
- label 3 dishes raw 10-2 raw 10-3 and raw 10-4 along with usual data
- obtain to 99 ml sterile water blanks label them 10-2 and 10-4
- using a 1.0 ml pipette with draw 1.0 ml of raw milk and dispense it into the 10-2 blank
- swirl it to mix for 15 secs
- using another pipette with draw 1.0 ml from the 10-2 dilution and place it in the 10-4 blank
- mix blank
- with another sterile pipette remove 1.1 ml from the 10-2
- obtain 2 melted agar tubes
- dispense 0.1 ml into one tube for the 10-3 plate and the remaining 1.0 ml into the tube for the 10-2 plate
- replace caps and invert 20 times
- pour tubes into respective dish bottoms and replace tops and let cool
- using another pipette withdraw 1.0 ml from the 10-4 bottle and obtain another agar tube
- place the 1.0 ml of 10-4 into tube replace cap and invert
- pour tube into 10-4 dish, cover and cool
- tape all plates together and place in 37 degree incubator