Phylogenetic relationship between mer A genes of Gram-negative bacteria. The tree was based on an alignment of 30 published and eight newly determined nucleic acid sequences of merA genes and calculated with the maximum likelihood algorithm. Bootstrap values above 65 % are given at the branching points. The bar indicates one nucleotide exchange per ten nucleotide positions. EMBL accession numbers are given after the names. Inoculum strains are shown in violet, Ps. aeruginosa Bro12 is the invading strain identified on the basis of its band in the merA community profile.