Bacterial strain inoculum was prepared from 48 h slants (Nutrient Agar (Fluka Biochemika)) washed with physiological salt solution. Suspension density was estimated to be 5 in the McFarland scale. Filter-sterilized colourants were added to tubes containing 10 ml of 48-h-old cultures in stationary growth phase (∼15 × 108 cfu ml−1).