Lessons from the organization of a proficiency testing program in food microbiology by interlaboratory comparison: Analytical methods in use, impact of methods on bacterial counts and measurement uncertainty of bacterial counts

The repeatability uncertainty is only indirectly affected by the method in use but depends essentially on the number of counted colonies. On the other hand, the between-laboratory uncertainty varies with the enumeration method in use, this variability is relatively small for the enumerations calling for methods without colony confirmation, i.e. for the enumeration of aerobic micro-organisms, Enterobacteriaceae, 'total' and thermotolerant coliforms, β-glucuronidase-positive E. coli and coagulase-positive staphylococci with the technique using the rabbit-plasma fibrinogen agar. For these methods, the average between-laboratory standard deviation is 0.17 log10 cfu g−1. The between-laboratory uncertainty is, on the contrary, larger for more complex techniques. For the enumeration of coagulase-positive staphylococci with the Baird-Parker agar, the between-laboratory standard deviation is equal to 0.23 log10 cfu g−1, it is equal to 0.28 log10 cfu g−1 for the enumeration of L. monocytogenes, to 0.34 log10 cfu g−1 for the enumeration of C. perfringens, and to 0.47 log10 cfu g−1 for the enumeration of anaerobic sulfito-reducing bacteria.