Agriculture Reference
In-Depth Information
members of the family Enterobacteriaceae,
Moraxella
,
Brochothrix and Acinetobacter
. Destructive surface
sampling is used commonly in poultry where, for
example, a 10 g sample of neck skin is hygienically col-
lected for laboratory analysis.
usda.gov/wps/wcm/connect/3efc7f8e-e6a2-4997-91ba-
9c579c2a1f14/Guideline_for_Ecoli_Testing_Slaughter_
Estab.pdf ?MOD=AJPERES) where microbial counts of
over 1000 cfu/ml are considered to be unsatisfactory
(cfu - colony-forming units).
The UK Food Standards Agency (FSA) supports a web
page (www.ukmeat.org) dedicated to providing the UK
meat industry with information on microbiological cri-
teria regulations, how to comply with the regulation,
how to undertake the testing, a place to record results
and advice on how to undertake corrective action when
the criteria are not met. Detailed sampling methodolo-
gies across several species are provided (Fig. 12.3).
Non-destructive surface sponge or swab method
Using a sterile technique, a sterile sponge or swab, usually
10 × 10 cm, is moistened with 10 ml of diluent. The swab is
then applied to a given site on the carcase, over a prescribed
area. This may be achieved using a template applied to the
surface with the standard area of, for example, 100 cm
2
wiped
10 times or wiped over a standard length of carcase - for
example, 50 or 100 cm. The swab is then hygienically
enclosed in a plastic bag and sent for laboratory processing.
The nature of the swab or the sponge, the pressure applied
during sampling and the detail of the process all affect the
outcome, making comparison of results difficult.
Microbiological laboratory analysis
The meat sample or the rinses from swabs or sponges form
the material for the microbiological analysis. The majority
of such samples are commonly sent by refrigerated trans-
port to commercial testing laboratories for microbiological
analysis. In brief, the solid samples are homogenised in a
suitable quantity of diluent to suspend the organisms. A
dilution series is then prepared by decimally diluting this
original bacterial suspension. This dilution series ensures
the growth plates are not subsequently overgrown by a
large inoculum of bacteria. All suspensions are then plated
out on a suitable growth medium.
A general growth medium such as a total count agar
(TCA) will allow the growth of almost all organisms
present that are capable of growth under the selected
incubation conditions (non-selective media). TCA is
normally incubated aerobically at 20 and 35°C to assess the
levels of psychrotrophic and mesophilic Gram-positive
and Gram-negative organisms present. The plated
samples can also be incubated anaerobically to assess
anaerobic and facultative anaerobic populations. On
completion of incubation, the number of organisms per
gram of sample or per cm
2
may be back calculated
Deep samples
are used to determine the levels of sys-
temic contamination/infection within a carcase. Such
contamination is not normally of environmental origin,
having resulted from pre-existing disease or infection.
Bacteria isolated from deep samples tend to be patho-
genic in nature. Deep samples of meat must be taken
with care in order to avoid contamination by superficial
organisms. Such samples can be obtained using sterile
scalpels and forceps or, in the case of frozen meat, a cork
borer or an electric drill fitted with a bore-extracting bit.
The surface should first be prepared by flaming, fol-
lowed by an aseptic dissection of about 10 g of meat.
Destructive sampling
may use surface slices where a
known weight (usually 10 or 25 g) is removed with sterile
scalpels and forceps and then homogenised in a suitable
diluent, for example, Ringer's solution, using a stom-
acher or other means, to provide a 1:10 dilution before
plating on appropriate culture media.
Alternatively, excision of surface material a few mm
thick of a fixed surface area (5 cm
2
from each of 4 carcass
locations, rump, flank, brisket and neck) using a similar
methodology may be used.
Non-destructive sampling method using rinses and
washes prepared by washing or rinsing one part by
weight of the meat in 10 parts by weight of the sterile
diluents is now more frequently used in the EU. Samples
of 1:10 dilutions may also be prepared from comminuted
forms of the meat since surface washing and rinsing do
not give a true picture of the degree of contamination.
The US Department of Agriculture has mandated a
rinse method for sampling poultry carcases for generic
E. coli
as a tool to verify process control (http://www.fsis.
Figure 12.3
Non-destructive surface sampling swab.
