Dear Sharleen,
Yr question is philosophical and statistical. I think you should appreciate in the following that bacteriological assessment is quantitatively not a very exact science compared to, say, weighing objects.
There is the usual statistical requirement of achieving a representative sample and also which can be “reliably” counted. I hv hardly ever seen the question of statistical number of locations addressed, this may possibly reflect the known large variances involved
. Frequently IMEX, the locations are grouped by risk assessment such as via probability of direct food contact / ease of cleaning etc, eg tables, hands and an average of 2 or 3 duplicate samples taken for comparison to appropriate levels. Statistically, bacteriological theory requires a minimum number of colonies to grow on the standard plate used for counting so as to provide a desired nominal accuracy. The logic is validated in most standard textbooks. Practically, the ease of provision of a suitable sample concentration will obviously vary between a slaughterhouse reception and a fully cooked product packaging room. The typical choice of quantity of wiped surface area as referred has been found to provide a suitable practical compromise for many food situations. It can presumably be enlarged if required to increase sensitivity although in practice, the use of “less than (a specified maximum level of the cleanliness parameter /unit area” tends to be a more common solution. Hence the type of procedure referred in this thread.
Concerning the dilution liquid, this also has to possess certain specified characteristics. Over time, the choices hv narrowed. The details / validation are, I think, less common in text books. Maybe someone knows a literature ref? (I cannot remember if given in BAM, probably not since this aspect is a recognised standard setup already). Suppliers product manuals, eg Difco, Oxoid etc probably cover it but these are not available on-line i think.
Rgds / Charles.C
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