Stable foam formation is a major problem in activated sludge wastewater treatment plants worldwide increasing maintenance costs, safety risks and generating poor quality effluent. Foaming requires the presence of air bubbles, surfactants, and hydrophobic bacteria from several mycolic acid producing hydrophobic genera including Nocardia, Tsukamurella, Gordonia, Rhodococcus, and Skermania. Many foam control strategies have been tried, but no method is universally successful.
Over 120 bacteriophages infective for these foaming Mycolata have been isolated from Australian wastewater, natural water, and soil samples. The 454 pyrosequencing of two of these, GDE1 and GDE2, revealed they each possess circularly permuted dsDNA genomes of 76.96 kb and 76.26 kb respectively, with both phages having G+C contents of 59% mol. GDE1 and GDE2 genomes share 78.9% pairwise nucleotide sequence identity, and 51.2-51.3% identity to phage GTE7 (Petrovski et al., 2011). All genomes display a modular architecture with evidence of recombination events suggesting a common ancestry. These Siphoviridae phages are morphologically similar, displaying isometric capsids of 60 (±8) nm and long non-contractile tails of 440 (±10) nm in length. Phages GDE1 and GDE2 have identical polyvalent host ranges targeting specific members of the genera Gordonia, Nocardia and Tsukamurella, and quite different to that of GTE7, with which they share a single common host N. asteroides (Nast23T). It has been suggested that phages may be used as biological control agents against foam stabilizing Mycolata and laboratory scale foaming assays have supported this (Petrovski et al., 2011; Thomas et al., 2002). Studies are currently underway to develop a delivery system for trialling these phages at pilot plant scale.