Several MPN models for serial dilutions with suppressed growth at low dilutions

Most probable number (MPN) estimates of microbial concentrations from serial dilutions employ a usual model of probabilities of the various possible outcomes. Outcomes challenge the usual model when they occur at markedly different rates from what the usual model predicts. Too many outcomes that suggest suppression of growth at low dilutions have occurred in some foods. These challenge the usual model and suggest new models. This paper provides four new models: growth suppression by a toxicant released from the food product by sample preparation; interference by a non-target species of microbe; disaggregation of clumped microbes at a single dilution step; disaggregation spread evenly over several dilution steps. The assumptions and discussion of how to find the maximum likelihood estimate for the parameters are given. (C) 1998 Academic Press Limited.