Activated sludge plants suffer frequently from the operational problem of stable foam formation on aerobic reactor surfaces, which can be difficult to prevent. Many foams are stabilized by mycolic acid-containing
Actinobacteria, the mycolata. The in situbiocontrol of foaming using phages is an attractive strategy. We describe two polyvalent phages, GTE5 and GRU1, targeting Gordonia terraeand Gordonia rubrupertincta, respectively, isolated from activated sludge. Phage GRU1 also propagates on Nocardia nova. Both phages belong to the family Siphoviridaeand have similar-size icosahedral heads that encapsulate double-stranded DNA genomes (∼65 kb). Their genome sequences are similar to each other but markedly different from those of other sequenced phages. Both are arranged in a modular fashion. These phages can reduce or eliminate foam formation by their host cells under laboratory conditions.