EFFECT OF THE PRETREATMENTS FOR MILK SAMPLES FILTRATION ON DIRECT VIABLE CELL COUNTS

Escherichia coli O25:H-42 was selected to study the effect of pre-trea tments on the enumeration of direct viable cells from milk samples. Be fore and after inducing cell elongation by cellular division inhibitor s, three pre-treatments for milk-filtration were used. One involved a pre-treatment with trypsin (1.5 min at 50 degrees C), addition of hot Triton X-100 after heating and filter rinses with phosphate saline buf fer. The other two involved pre-treatment with trypsin and Triton X-10 0 (10 min at 50 degrees C), filter rinses with hot Triton X-100 and or ganic solvents. Pre-treatments applied after inducing cell elongation had an effect on cell recovery from milk samples depending on the pre- treatment used. The most suitable, on the basis of the number and perc entage of enlarged cells obtained was the first described. The others selectively affected recovery of elongated cells. Pretreatments applie d before inducing the cell elongation, negatively affected viability w ith enumerations in milk samples being significantly (P < 0.001) lower than those found in controls. However, the negative effects of first pre-treatment on viability was lower than that produced by the pre-tre atments involving organic solvents.