The combined effects of temperature, pH and NaCl on growth of Debaryomyces hansenii analyzed by flow cytometry and predictive microbiology

Flow cytometry was applied to determine growth of Debaryomyces hansenii in a laboratory medium. Viable yeasts were enumerated after staining with the fluorogenic ester fluorescein diacetate (FDA). Initial studies showed that the flow cytometric determinations correlated well with viable yeast populations determined as colony forming units (CFU) whereas the relationship between CFU and optical density was only linear over a narrow range of cell concentrations, 105.5–107.5 cells/ml. The flow cytometric measurements could reliably detect D. hansenii at concentrations as low as 102 cells/ml whereas the lower detection limit using optical density measurements was 105–106 cells/ml. Growth was determined by flow cytometry at different combinations of temperatures (10–30 °C), pH (4.7–6.0) and NaCl concentrations (1–12% w/v). Growth curves were generated by fitting a modified Gompertz equation to the growth data using non-linear regression analysis. Lag phase duration and maximum specific growth rates were derived and quadratic polynomial models were developed describing the effects of environmental conditions on the growth parameters. Model validation based upon repetition of experiments and use of another laboratory medium showed good agreement between observed and predicted maximum specific growth rates whereas predicted lag phases were shorter than the observed lag phases.