Image analysis as a mean to model growth of Escherichia coli O157:H7 in gel cassettes.

Aims: The potential of image analysis for rapid and quantitative determination of the effect of environmental parameters such as temperature and pH on the growth of colonies of Escherichia coli O157:H7 derived from immobilized cells in gel cassettes was investigated.

Methods And Results: The organism was grown in brain heart infusion agar contained within a cassette formed between sheets of PVC film. The medium was adjusted to pH 5, 6 or 7 and incubated at 10, 20, 30 or 40 degrees C. The primary model of Baranyi was used to fit the growth data obtained by conventional plate counting and changes in colony area (2-dimensional spread of colonies) by light microscopy to derive estimates of maximum specific growth rates (micromax and Area micromax) in both cases. Growth rate values from both measurements were correlated and a secondary quadratic model was developed to predict micromax obtained via image analysis in response to environmental factors (temperature and pH). A progressive decrease of micromax and Area micromax was observed at lower temperatures and pH values. Immobilized cells failed to initiate growth at a pH of 5.0 and 10 degrees C. There was high correlation between micromax values estimated by conventional plate counting and Area micromax values from microscopic observations in gel cassettes, regardless of temperature and pH. The values of micromax derived indirectly from the correlation with Area micromax values fitted well to the secondary model and gave realistic predictions of maximum specific growth rate values estimated by standard plate counting.

Conclusions: The micromax of E. coli O157:H7 determined by plate counting was linearly correlated with Area micromax estimated by light microscopy, enabling indirect determination of micromax via the Area micromax. The estimates of micromax via the image analysis technique may be further modelled in response to environmental factors such as temperature and pH to predict the response of the organism in intermediate conditions.

Significance And Impact Of The Study: Image analysis in combination with gel cassettes could be a potential tool for rapid and convenient data collection and construction of accurate mathematical models as an alternative to conventional plate counting methods.