Evaluation of the Bactoscan 8000 method for the determination of total bacterial count in raw milk
1998
Suhren, G. (Federal Dairy Research Centre, Kiel (Germany). Inst. for Hygiene)
The fluorescent automatic microscopic counting of single bacteria in milk (Bactoscan 8000) is applied as a routine method for the determination of the bacteriological quality of ex-farm milk in an increasing number of countries. One major aim of methodological modifications in the past was to decrease the lower limit of detection due to the increasing bacteriological quality of raw milk, in view of the pressure under legal requirements for decreasing class limits. The following descriptive parameters were derived: lower and upper limit of detection 10-20000 and more than 3 mill colony forming units (cfu)/ml, respectively; precision data are dependent on the counting level; repeatability (r) and reproducibility (R) on a level of 100 Bactoscan counts (BC) were 0.06 and 0.12 log cfu/ml, respectively; the accuracy of the estimate was indicated by 0.2-0.35 log cfu/ml. Calibration and standardization are, as for all quantitative microbiological methods very critical and difficult to solve. Changes in sensitive reagents, as for example, technical standard and dye, can cause systematic deviation of BCs. The ratio between BCs and cfu is influenced by numerous factors, as for example, type, number, physiological status of the microorganisms, milk production and time/temperature conditions of the sample. Pure culture trials with mastitis pathogens revealed for some types (Staphylococcus aureus, Streptococcus uberis) an underestimation of the cfu using the Bactoscan method.
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