Modification of the aggregation behaviour of the environmental Ralstonia eutropha-like strain AE815 is reflected by both surface hydrophobicity and amplified fragment length polymorphism (AFLP) patterns

After inoculation of the plasmid-free non-aggregative Ralstonia eutropha-li ke strain AE815 in activated sludge, followed by reisolation on a selective medium, a mutant strain A3 was obtained, which was characterized by an aut oaggregative behaviour. Strain A3 had also acquired an IncP1 plasmid, pLME1 , co-aggregated with yeast cells when co-cultured, and stained better with Congo red than did the AE815 strain. Contact angle measurements showed that the mutant strain was considerably more hydrophobic than the parent strain AE815, and scanning electron microscopy (SEM) revealed the production of a n extracellular substance. A similar hydrophobic mutant (AE176R) could be i solated from the AE815-isogenic R, eutropha-like strain AE176, With the DNA fingerprinting technique repetitive extragenic palindromic-polymerase chai n reaction (REP-PCR), no differences between these four strains, AE815, A3, AE176 and AE176R, could be revealed. However, using the amplified fragment length polymorphism (AFLP) DNA fingerprinting technique with three differe nt primer combinations, small but clear reproducible differences between th e banding patterns of the autoaggregative mutants and their non-autoaggrega tive parent strains were observed for each primer set. These studies demons trate that, upon introduction of a strain in an activated sludge microbial community, minor genetic changes readily occur, which can nevertheless have major consequences for the phenotype of the strain and its aggregation beh aviour.