In a numerical taxonomic survey of foam-forming actinomycetes, representatives of four groups were selected for further taxonomic study because they either did not cluster with known reference strains of nocardioforms or their relationship to them was uncertain. Phylogenetic studies, using 16S rDNA sequencing, showed that one group probably represented a previously described species, Rhodococcus erythropolis, but two possibly represented new species of Gordona and Rhodococcus. The fourth group represented an independent line of descent, possibly a new genus. These, together with strains of foam-formers Gordona amarae and Skermania piniformis, were evaluated for their growth on hydrophobic substrates likely to be found in activated sludge plants. All grew well on vegetable oils (olive, safflower and coconut) and on mixtures of hydrocarbons like paraffin oil, but at varying rates. However, growth on kerosene and hexadecane was less consistent. One feature of growth on the hydrophobic substrates was the attachment of most cells to the hydrophobic substrate rather than growth in the aqueous phase. This suggests a possible method for competing for nutrients with faster-growing (but not hydrophobic) organisms in activated sludge.

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