FEMS Microbiology Ecology 85 (1991) 335-344 © 1991 Federation of European Microbiological Societies 0168-6496/91/$03.50 Published by Elsevier ADONIS 016864969100087C 335 FEMSEC 00342 The effect of the incubation period on the result of MPN enumerations of nitrite-oxidizing bacteria: theoretical considerations G.J. Both and H.J. Laanbroek Institute for Ecological Research, Heteren, The Netherlands Received 19 July 1990 Revision received 26 March 1991 Accepted 27 March 1991 Key words: Nitrobacter; MPN enumeration; Model formulation; Kinetic parameters; Substrate inhibition 1. SUMMARY A computer model based on Monod- and Hal- dane-kinetics was used to estimate the minimum incubatio~a period required for MPN enumera- tions of nitrite-oxidizing bacteria. The minimum incubation period was defined as the time needed for one cell, present in the tubes inoculated with the highest dilutions, to grow into a population that oxidized all the nitrite present at the start of the incubation. Kinetic parameters used in the model were derived from literature data and ap- plied in different combinations. The results show that the minimum incubation period may increase with decreasing initial nitrite concentrations in the incubation medium. They also show that the opposite trend, i.e. increasing minimum incuba- tion periods with increasing nitrite concentra- Correspondence to: G.J. Both. Present address: Department of Microbiology, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands. tions, can be explained by introducing a term for substrate inhibition in the model. A MPN enu- meration result obtained with samples from a waterlogged peat bog soil could only partly be explained by the model if only one set of parame- ters was used. This indicates that the community of nitrite-oxidizing bacteria in this soil is com- posed of at least two types of nitrite-oxidizing bacteria, with different kinetic parameters of ni- trite oxidation and growth. 2. INTRODUCTION Nitrifying bacteria from various environments have been enumerated by numerous authors us- ing the Most Probable Number (MPN) technique. This technique has been employed using a large number of different nitrite concentrations [1], as well as different incubation periods ranging from 3 weeks to 3 months [2]. If the length of the incubation was considered at all, an empirical approach was chosen to determine its length by Downloaded from https://academic.oup.com/femsle/article/85/4/335/488417 by guest on 15 December 2023