Examination of Foods for Staphylococcus aureus - DB Server Test ...

stored for extended periods frequently develop less black coloration than typical colonies
and may have rough appearance and dry texture.
2. For each dilution to be plated, place 1 ml of sample suspension onto center bottom film
of 3M Petrifilm Rapid S. aureus Count Plates. Refer to the “Reminders of Use for
3M Petrifilm Rapid S. aureus Count Plates” that is attached to your handout.
Incubate plates with clear side up in stacks of up to 10. Incubate at 35 o C or 37 o C for 24
hours.
3. Refer to the “Reminders of Use for 3M Petrifilm Rapid S. aureus Count
Plates” that is attached to your handout. Transfer petrifilm plates to a 62 o C incubator.
Incubate for 1-4 hours. With sterile forceps, remove the round reactive disk from the
outer square frame. Lift the top film of the Petrifilm plate and place the Petrifilm reactive
disk in the well of the plate. Lower the top film. Apply gentle pressure across the reactive
disk. Incubate plates with inserted Petrifilm reactive disks for 1-3 hours at 35 o C or 37 o C.
4. Read and record results. Refer to the “Interpretation Guide Section” that is
attached to your handout.
Dilution 10 -1
Dilution 10- 2
Dilution 10 -3
Dilution 10- 4
Dilution 10 -5
S. aureus count
5. Count and record colonies on Baird-Parker Agar plates. If several types of colonies are
observed which appear to be S. aureus on selected plates, count number of colonies of
each type and record counts separately. When plates of the lowest dilution contain 200 colonies have colonies with the
typical appearance of S. aureus and typical colonies do not appear at higher dilutions, use
these plates for the enumeration of S. aureus, but do not count nontypical colonies. Select
> 1 colony of each type counted and test for coagulase production. Add number of
colonies on triplicate plates represented by colonies giving positive coagulase test and
multiply by the sample dilution factor. Report this number as number of S. aureus/g of
food tested.
cfu [Plate 1 (0.4
ml)]
cfu [Plate 2 (0.3
ml)]
cfu [Plate 3 (0.3
ml)]
cfu/g food
Dilution 10 -2
4