Ammonia oxidizers were enumerated in 34 activated sludges, including sludges which were from sewage treatment plants (S-sludge), nightsoil treatment plants (N-sludge) and activated sludges cultivated with organic (O-sludge) or inorganic (I-sludge) artificial wastewaters in the laboratory. Two media were used for enumeration, one containing 0.76 mM (AL medium) and the other 37.9 mM (AH medium) of (NH4)2SO4. The MPN estimated with AL medium were higher than those with AH medium in S-sludges, N-sludges and O-sludges, while both media gave almost the same MPN for I-sludges. Ten ammonia oxidizers, all identified as Nitrosomonas spp, were isolated from sludge samples. Isolates obtained as predominants in S- and O-sludges were sensitive to (NH4)2SO4
they grew in medium containing 0.71 mM of (NH4)2SO4 but not in medium containing 35.7 mM of (NH4)2SO4. On the other hand, those obtained as predominants in an I-sludge, as well as ATCC strains grew in both media. The Monod equation described the relationship between (NH4)2SO4 concentration and nitrite production rates for (NH4)2SO4-insensitive strains but not for sensitive strains, which exhibited a relationship closer to the Haldane equation describing substrate inhibition kinetics. Among isolates obtained from S-sludge sample, Km and Vmax values of an (NH4)2SO4-sensitive strain were much lower than those of an insensitive strain. Similarly, among laboratory sludge isolates, Km and Km values of an (NH4)2SO4-sensitive strain from an O-sludge were lower than those of an insensitive strain from an I-sludge. These results suggest that (NH4)2SO4-sensitive strains had a growth advantage in lower (NH4)2SO4 concentrations, while insensitive strains had an advantage in higher (NH4)2SO4 concentrations. © 1994.