Microbial Testing Procedures Flashcards
First decision to make during the microbiological testing of foods is:
a. a surface sample
b. a homogenized sample of the food
Types of surface sampling:
Swabbing
contact plates
excision method
Swabbing method
MO collected from a surface with sterile cotton or calcium alginate swabs (alginate swabs are the best since the alginate can be readily dissolved in hexametaphosphate), transferred to broth where they are dislodged, then diluted and used with further tests to determine total numbers. Sponges can be used to swab larger areas then placed in a buffer-filled bag.
Advantages:
a. Easy to perform
b. Inexpensive
c. Well suited to flexible, uneven and heavily contaminated surfaces
Disadvantage:
1. MO recovery may be poor (10% in some studies, but even that is still acceptable for many applications)
Contact Plates (rodac plates) method
raised agar plate that is pressed against a surface and then incubated.
Advantages:
1. Method of choice for smooth, firm and nonporous surfaces (e.g. vat in a cheese plant)
2. Any type of media can be used
Disadvantages:
1. Colony overgrowth makes enumeration difficult on heavily contaminated surfaces
2. Only removes about 0.1% of contact flora - much less than swabs
modified version of Rodac plates:
agar syringe or “sausage.” Tube full of agar, samples are pressed against a surface and then sliced off into a Petri plate for incubation.
excision method
a plug of know surface area is taken from the food and then 1-3 mm is taken from the surface end, homogenized and plated to determine total numbers. Commonly used in whole meat cuts.
Homogenation methods
blender
colwell stomacher
Blender method
Not used extensively now
Stomacher
is the method of choice.
Food is placed in a sterile plastic bag with diluent and then inserted into the machine. The stomacher has two paddles that vigorously disrupt the food and give a nice homogenous fluid for sampling.
Advantages over blenders include:
1. No cleaning since food is in a bag
2. No heat buildup that will injure Mo (better survival in the sample)
3. Homogenates can be frozen in the bags if necessary for further use
4. Decreased noise level
5. Decrease in aerosols
Methods to determine total microbial numbers:
- Standard plate count
- Spiral plate counter
- Dry Petrifilms
- Most Probable Numbers
- Membrane filters
- Hydrophobic Grid Membrane Filters
- Dye reduction
- impedance
Standard Plate Count (SPC)
the most widely used method for viable numbers.
Advantage:
1. Easy to perform
2. AOAC (Assoc. of Official Analytical Chemists) approved for many foods
Disadvantages:
- Results are not available for at least 16-18 h (often much longer)
- Counts in a food are influenced by several factors including:
a. Sampling method and the distribution of MO in the food
b. Nature of the food flora (mostly gets mesophilic aerobes and fac. aerobes)
c. Nature of the food
d. Intrinsic parameters of the growth medium reflect the MO which are detected
e. Incubation time and temp (extrinsic parameters)
f. Microbial antagonism among species on the food
Spiral Plate Counter
An automated version of SPC is the spiral plater, a device that distributes a continuously decreasing volume of liquid over a single rotating agar plate (the dispensing arm moves like a needle on a turntable, only backward). The agar is then incubated and counts are made.
-It can effectively deliver up to a 105 concentration range on one plate, but enumeration requires a special counting grid.
Advantages over SPC:
1. Easy to perform (little training required)
2. Fewer materials are used (agar plates, dilution blanks, pipettes)
3. 3-4X more samples can be run per hour
4. Spiral plating does agree well with SPC values and is an AOAC method.
Disadvantage:
1. Food particles may clog the dispensing arm – more suited to fluids like milk
Dry Petrifilms
Two plastic films held together on one side and coated like a sandwich with
culture media ingredients, tetrazolium dye (a reducing dye) and a water
soluble gelling agent. 1 ml of sample in diluent is placed between the films and gently spread around by pressing the 2 sides of film. After incubation, cell growth reduces the dye and gives red colonies.
Advantages:
1. nonselective and selective media are available in the films. Films are available to perform total plate counts of bacteria or fungi, coliform counts, and specific tests for hemorrhagic E. coli 0157:H7
2. the product has AOAC approval
3. store for long periods, no autoclave required
Disadvantage:
1. expensive
2. difficult to read without training
MPN
A method based upon statistical probability. Food samples are prepared like SPC. Three serial dilutions are prepared and then transferred to 9 or 15 tubes (3- or 5-tube method). The numbers of organisms/g are then estimated using standard MPN tables.
Advantages: 1. easy to perform 2. results from one lab more likely than SPC to agree with those from another lab 3. specific groups of organisms (e.g. coliforms-it is the method of choice) can be enumerated using different selective medias 4. AOAC for coliforms, S. aureus & B. cereus (many others) in various foods Disadvantages: 1. lots of tubes required (clean up intensive) 2. lack of precision, generally higher than SPC results
Membrane filters
- Most useful for microscopic examination of water supplies (e.g. coliform counts) since most foods will clog the filters. 0.45 µm-pore filters allow water to pass but trap bacteria.
- Filters are especially useful for samples with low numbers of bacteria since large volumes of fluid can be passed through.
- Polycarbonate filters are better than the cellulose one because bacteria are caught on the filter surface more efficiently.
- A given volume of fluid is passed through the filter:
a. the membrane is placed on an agar plate and incubated
b. Direct Microscopic Count (DMC) can be made on the filter