Microcolony formation as a viability index for ammonia-oxidizing bacteria: Nitrosomonas europaea and Nitrosospira sp.

Abstract

A microcolony technique was developed to estimate viability of autotrophic ammonia-oxidizing bacteria. The technique was based on the ability of cells to divide on membrane filters. The numbers of microcolony-forming units were followed in liquid batch cultures of two different type strains of Nitrosomonas europaea (ATCC 19718 and NCIMB 11850) and two isolates of Nitrosospira sp. In the Nitrosomonas europaea ATCC 19718 it was observed that a relatively small (<0.5%) fraction of the cells was able to form microcolonies when harvested during exponential phase, while this fraction became larger (10–25%) when the culture entered stationary phase. The same trend was observed for the Nitrosomonas europaea NCIMB 11850 and Nitrosospira isolates. By comparison, a larger and apparently constant fraction of cells showed viability as judged by conventional most probable number counts. The results indicate that a specific fraction of the cell population in batch culture is capable of microcolony formation. This fraction is strongly growth phase-dependent and may represent a sessile cell type which is capable of surface growth on membrane filters.