Agriculture Reference
In-Depth Information
is most widely accepted - it is in fact still the standard
against which all other techniques are compared.
depending on the original sample size and the levels of
dilution employed. The colonies on these primary plates
can be subcultured to purify bacterial colonies in order
to help identify the organisms.
The medium used for growth may be selected specifi-
cally to provide information on one particular group/
genus of pathogens or spoilage bacteria (selective media).
Thus, specific agars have been developed for almost
every pathogen based on the resistance of those particu-
lar pathogens to certain antimicrobial substances. For
example,
Listeria monocytogenes
is resistant to nalidixic
acid, cycloheximide, acriflavine and fosfomycin, and the
inclusion of these in the media (Oxoid) restricts the
growth of contaminating organisms, allowing a determi-
nation of the levels of these target organisms.
Despite being both simple and cheap, the
plate count
procedure is not an ideal method for measuring bacterial
numbers. The most significant drawback is the time
interval before a result is obtained, based on the visuali-
sation of colonies, which may range from 2 to 7 days. In
addition, the accuracy of this method, together with the
representativeness of the sample tested, impacts directly
on the variability of results recorded. Thus, manually
completed plate counts are only accurate ±2 log
10
.
The automation of the plate count procedure, remov-
ing operator error as a source of inaccuracy, through the
use of media preparators, automatic dilution and spiral
plate makers and automatic video-based colony coun-
ters, for example, spiral plater (Whitley, Interscience,
SciRobotics), Soleris (Neogene), and VIDAS (bioMer-
ieux), are also available. These are valuable improve-
ments and while expensive to buy are becoming easier
and less expensive to operate; however, there are still
delays in obtaining results associated with the analysis
incubation time.
A number of
rapid methodologies
have been developed
as alternatives to conventional microbiological methods.
These include the use of hydrophobic grid membranes,
bioluminescence (www.ukmeat.org), electrical methods,
radiometry, microcalorimetry, biochemical reactions,
immunological and serological reactions, nucleic acid
(DNA) probes, DNA amplification (polymerase chain
reaction) and flow cytometry. It is outside the scope of
this chapter to consider each in detail. Such methods are
now more commonly being adopted by industry, as on-
site conventional microbiological culture has over recent
years all but disappeared having been being replaced by
low-cost off-site commercial microbiological testing.
Manufacturers of such alternatives have both signifi-
cantly reduced assay costs as well as reduced assay times.
Despite the many problems associated with plate count
procedure, it probably represents the best compromise
between cost and performance and is the method which
Bacteriological standards for meats
Various authorities, for example, the EU, the WHO
Codex Alimentarius Commission and the FSA (United
Kingdom), have laid down acceptance criteria for dif-
ferent types of meats. Although, as of yet, there are no
universally accepted standard for the interpretation of
bacteriological findings, the following are frequently used:
Microbiological
criteria
, which are mandatory criteria
with legal backing
Microbiological specifications
, which are generally
contractual agreements between a manufacturer and a
purchaser to check whether the foods are of the required
quality.
Microbiological guidelines
, which are non-mandatory
criteria usually intended as a guide to good manufactur-
ing practice
The International Commission on Microbiological
Specifications for Foods (ICMSF of the International
Association of Microbiological Societies) has stated that
any microbiological criterion for food should contain
the following information:
1
A statement of the micro-organisms and/or toxins of
concern
2
Laboratory methods for their detection and
quantification
3
The sampling plan
4
The microbiological limits
5
The number of samples required to conform to these
limits
The ICMSF recommended that the
total viable count
at 35°C (or at 20°C in the case of chilled meats) should be
less than 10
7
/g and that
Salmonella
should be detected
The EU Microbiological Criteria Regulation No. 2073/2005
has defined:
Microbiological criterion
is a criterion defining the
acceptability of a product, a batch of foodstuff or a process,
based on the absence, presence or number of micro-
organisms and/or on the quantity of their toxins/metabolites,
per unit(s) of mass, volume, area or batch.
Food safety criterion
means a criterion defining the
acceptability of a product or batch of foodstuff applicable to
products placed on the market.
Process hygiene criterion
is a criterion indicating the
acceptable functioning of the production process. Such a
criterion is not applicable to products placed on the market. It
sets an indicative contamination value above which
corrective actions are required in order to maintain the
hygiene of the process in compliance with food law.
