September 2006
The perfect Contact plate
Over the last 25
years Cherwell Laboratories have developed a range of contact
plate products to overcome both general problems of handling
and specific problems brought to us by individual customers.
We currently manufacture
62 varieties of contact plate products all of which now
use the latest grip-lid plastic dish. The grip-lid design
ensures that the plates are secure if stacked on the bench
or in the incubator and the lid remains in place until it
is removed by the user. When the lid is replaced after sampling,
inspection or subculture it is again secure, yet it is no
more difficult to manipulate with one hand than the traditional
design.
Contact plates, sometimes
known as RODAC (Replicate Organism Detection and Counting)
plates, take significant skill to fill perfectly but if
you can buy them pre-filled they have unrivalled convenience.
The contact plate is a plastic dish filled with agar to
give a convex surface with an area of 25cm². The two
principal applications are surface sampling, so that the
count of colonies after incubation can be directly related
to the contamination as cfu/unit area, and air sampling,
where the contact plate is used in a SAS active air sampler
to produce cfu/unit volume.
The choice of media
and pack option is dictated by the environment and the microbiological
risk at the test site combined with the regulatory requirements
or the preference of the microbiologist. Total viable counts
in the pharmaceutical industry use mostly TSA, with SDA
for yeasts and moulds in air, while others prefer Nutrient
Agar or Blood Agar with Malt Extract Agar or Rose Bengal.
When specific contaminants are suspected selective and differential
media can be helpful. MacConkey for Coliforms, Pseudomonas
selective agars, Baird Parker for Staphylococcus are common
examples.
If the surfaces to
be sampled have been cleaned with disinfectants it is necessary
to incorporate an appropriate neutraliser for each biocide
into the medium. Lecithin and Tween (Polysorbate) are common
with Thiosulphate, L-Histidine, Thioglycollate and bisulphate
in various combinations.
Where the environment
might contain other growth inhibitors these also need to
be considered. For example we incorporate b -Lactamase enzyme
in plates for use in Cephalosporin production areas.
Back
to top
For use in isolators
or clean rooms it is required that not only the agar surface
but the container and the outer packaging are all sterile.
This is achieved by gamma irradiation of the plates after
packing. Most plates in this application are expected to
show no growth so it is essential that the plates are free
from visible contaminants.
The secret is in
the manufacturing process. At Cherwell plates are filled
automatically under laminar flow in an ISO Class 7 clean
room and inkjet labelled. They are then stored for several
days at room temperature before being inspected. Any plate
with visible imperfections such as cracks or bubbles is
rejected (this would also remove any contaminated plate)
before the perfect plates are stacked in 10s, bagged, heat-sealed
and labelled, all inside the clean room. Plates for irradiation
are then wrapped in additional layers of clear heat sealed
bag if required and packed into plastic lined cardboard
boxes. Boxes of 100 plates are sealed, labelled and sent
for irradiation the same day. The box should be stored at
room temperature and opened close to the point of use. The
final reassurance for the user is that the plates are visible
through the clear wrapper and could be rejected before opening
in an aseptic environment.
Cherwell Laboratories
would be delighted to help you select the right product
and if you need something different from our other users,
we will offer to make variety 63 for you.
For further information,
please complete our contact form here
Back to top
