Abstracts. Szigeti, E.

Scientific Publications - Work Done by Microbiology Reader Bioscreen C

Szigeti, E., Reichart, O., Farkas, J., Becker, B. and Holzapfel, W.H., Modelling growth of Lactococcus lactis, Listeria monocytogenes and Bacillus cereus as a function of pH and temperature measured by turbidimetry, 13th International Congress of the Hungarian Society for Microbiology, August 30 – September 1, 1999, Eötvös Loránd University, Faculty of Science, Budapest, HUNGARY

ABSTRACT

Combined effects of pH and temperature on growth of Lactococcus lactis BFE 920, Listeria monocytogenes SLCC 2540 serotype 3b and Bacillus cereus CCM 2010 were studied by turbidimetric measurements in two semi-synthetic media using a Bioscreen C equipment (Labsystem Oy, Finland). Detection times (DT) belonging to the first significant changes in optical density were determined. Linear correlation could be established between DT and logarithm of colony forming units (cfu) determined by conventional spread-plate technique.

In the range of 2 - 8 log₁₀ cfu the determination coefficients (R²) of the linear regressions were higher than 0.98 at each strain-medium combination.

In order to describe the relationship between the detection time (DT) and the environmental factors involving the initial cell number, a mathematical model was constructed:

log₁₀DT = a + b₁·log₁₀N_o + b₂·(pH-pH_opt)² + b₃·(T-T_opt)²,

where

N_o : initial cell count (cfu/ml)

a, b₁, b₂, b₃ : constants depending on media and microbes

pH: pH of the medium

pH_opt.: pH optimum of the growth obtained from the model

T: temperature of the medium (°C)

T_opt.: temperature optimum of the growth obtained from the model (°C)

From the results of the mathematical modelling the following conclusions can be drawn:

	The predictive model applied for the experimental data describes the DT values with close correlation as a function of the initial cell number, pH and temperature. The determination coefficients lasted from 0.976 to 0.996.
	There is no significant difference between the determination coefficients of the fitting related to the two media.
	The optimum values of pH obtained from the model for both media were nearly identical in the case of the same micro-organism, while the temperature optimum were differing with 1-4 °C.
	The a, b₁, b₂, coefficients belonging to the two media at the same micro-organism do not differ significantly at 95% level, which means that the effects of the initial cell number and the pH on the growth do not vary significantly in the media.
	The b₃ coefficients proved to be significantly (95% level) differing at every micro-organism, so the temperature effect of the two media on the growth is not identical. This conclusion is in coincidence with the varying temperature-optimum values corresponding to the two media at the same micro-organism.

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