Scientific Publications - Work Done by Microbiology Reader Bioscreen C

W. Holzapfel, W. Lack and B. Becker,
Validation of microbial growth data  on vegetable products, WWW 2003 Poster - Institute of Hygiene and Toxicology, BFE, Haid-und-Neu-Str. 9, D-76131 Karlsruhe

ABSTRACT

INTRODUCTION

Modelling growth behaviour of food-associated microorganisms under simulated conditions should take account of multiple influencing factors. Providing optimal conditions in terms of pH, temperature and complex growth factors may not always suffice practical conditions. The approach of „worst case situation“ or „best possible conditions“ for studying growth of food bacteria should enable a safe range for prediction of microbial risks in foods. Comparing the suitability of different semisynthetic media (thus far considered to provide most beneficial conditions) with typical food substrates, indicates that this assumption may not always be applicable.

METHODS

Growth behaviour of the food-borne pathogens and/or toxinogens Listeria monocytogenes WS 2250, Staphylococcus aureus ATCC 14458, Salmonella enteritidis 5271, Escherichia coli DSM 30083 and Bacillus cereus DSM 2301 was studied in six different media (pH 7) by turbidimetry (at 580 nm), using the Bioscreen C system at 30°C. Five semisynthetic media (St-I broth, MRS-broth, two modified St-I broths and modified MRS broth) were compared with salad juice as natural substrate. In a separate study, growth behaviour of three isolates from prepacked salads (Aeromonas salmonicida 408/1, Enterobacter cloacae 003/2 and Erwinia carotovora 065/2) was studied in freshly prepared iceberg salad juice and St-I broth at different combinations of temperatures (10 °C, 20 °C, 30 °C, 37 °C and 42 °C) and pH values (6.0, 6.5, 7.0 and 7.5). The strains were incubated in a BIOSCREEN C turbidimetric analyser (Labsystems, Finland) for at least 24 hrs. Turbidimetric detection times (DT) were chosen as growth indicator for all investigations calculated by the BioLink software.

Fig. 1:
Comparison of detection times (DT) of Enterobacter cloacae 003/2 in salad juice and St-I broth at different combinations of pH and temperature

Fig. 2:
Comparison of detection times (DT) of Erwinia carotovora 065/2 in salad juice and St-I broth at different combinations of pH and temperature

Fig. 3:
Comparison of detection times (DT) of Aeromonas salmonicida 408/1 in salad juice and St-I broth at different combinations of pH and temperature

Table 1:
Software-Parameters for the growth of Staphylococcus aureus ATCC 14458 in different media

Table 2:
Detection Times (DT) of Enterobacter cloacae 003/2, Erwinia carotovora 065/2 and Aeromonas salmonicida 408/1 in salad juice at different combinations of temperature and pH

RESULTS

For all strains from the first study, highest growth rates and shortest detection times (DT ranging from 6 to 16 h) were found ins alad juice, and the slowest growth rate in MRS broth (DT ranging from 22 to 54 h). For Staph. aureus (Table 1), only minor differences in DT were determined for the respective media studied (DT ranging from 9 to 11:20 h). It is however of interest that especially Staph. aureus ATCC 14458 showed clearly improved growth characteristics in salad juice during 36 h, with respect to slope of the curve and growth rate. At temperatures between 20° and 42°, only minor differences between media were found in DT (Figures 1, 2 and 3) for all strains from the second study. However, drastically shorter DT‘s were determined in salad juice (Table 2) at 10°C for all 3strains studied, as compared to St-I broth. This study underlines the importance of an in situ approach, that more closely resembles natural conditions prevailing in food substrates, as basis for reliable prediction of food hygienic risks.

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Last modified: May 25, 2005