Effect of the pre-treatments for milk samples filtration on direct viable cell counts

Escherichia coli O25:H-42 was selected to study the effect of pre-treatments on the enumeration of direct viable cells from milk samples. Before and after inducing cell elongation by cellular division inhibitors, 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 buffer. The other two involved pre-treatment with trypsin and Triton X-100 (10 min at 50 degrees C), filter rinses with hot Triton X-100 and organic 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 percentage of enlarged cells obtained was the first described. The others selectively affected recovery of elongated cells. Pre-treatments applied before inducing the cell elongation, negatively affected viability with enumeration 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-treatments involving organic solvents.