Primers were U1(50-CCAGCAGCCGCGGTAATACG-30 , corresponding to nucleotides 518–537 of the Escherichia coli 16S rRNA gene) and U2 (50-ATCGGYTACCTTGTTACGACTTC- 30 , corresponding to nucleotides 1513–1491 of the same gene), as described by Lu et al. (2000).