Leon G. M. Gorris

Bactericidal activity of carvacrol towards the food‐borne pathogen Bacillus cereus

Carvacrol, a natural plant constituent occurring in oregano and thyme, was investigated for its bactericidal effect towards the food‐borne pathogen Bacillus cereus. Carvacrol showed a dose‐related growth inhibition of B. cereus. At concentrations of 0·75 mmol l−1 and above, total inhibition of the growth was observed. Below this concentration, carvacrol extended the lag‐phase, reduced the specific growth rate and reduced the maximum population density. Incubation for 40 min in the presence of 0·75–3 mmol l−1 carvacrol decreased the number of viable cells of B. cereus exponentially. Spores were found to be approximately 2·3‐fold less sensitive to carvacrol than vegetative cells. Bacillus cereus cells showed reduced susceptibility towards carvacrol at pH 7·0 compared with different values between pH 4·5 and 8·5. The culture and exposure temperatures had a significant influence on the survival of vegetative cells. The highest death rate of cells was observed at an exposure temperature of 30 °C. Membrane fluidity was found to be an important factor influencing the bactericidal activity of carvacrol.

Effect of elevated oxygen and carbon dioxide on the surface growth of vegetable‐associated micro‐organisms

The impact of a novel type of Modified Atmosphere (MA), referred to as high O2‐MA, on micro‐organisms associated with the spoilage of minimally‐processed vegetables was studied. Pure cultures of Pseudomonas fluorescens, Enterobacter agglomerans, Aureobacterium strain 27, Candida guilliermondii, C. sake, Salmonella typhimurium, Salm. enteritidis, Escherichia coli, Listeria monocytogenes, Leuconostoc mesenteroides var. mesenteroides, Lactobacillus plantarum and Lactococcus lactis were cultured on an agar‐surface model system and incubated at 8 °C under an atmosphere composed of O2 (80 or 90%, balanced with N2), CO2 (10 or 20%, balanced with N2), or a combination of both gases. In general, exposure to high O2 alone did not inhibit microbial growth strongly, while CO2 alone reduced growth to some extent in most cases. Consistently strong inhibition was observed only when the two gases were used in combination. With minimally‐processed vegetables, where CO2 levels of around 20% or above cannot be used because of physiological damage to the produce, the combined treatment of high O2 and 10–20% CO2 may provide adequate suppression of microbial growth, allowing a safe, prolonged shelf‐life.

Comparison of two optical-density-based methods and a plate count method for estimation of growth parameters of Bacillus cereus.

ABSTRACT Quantitative microbiological models predicting proliferation of microorganisms relevant for food safety and/or food stability are useful tools to limit the need for generation of biological data through challenge testing and shelf-life testing. The use of these models requires quick and reliable methods for the generation of growth data and estimation of growth parameters. Growth parameter estimation can be achieved using methods based on plate counting and methods based on measuring the optical density. This research compares the plate count method with two optical density methods, namely, the 2-fold dilution (2FD) method and the relative rate to detection (RRD) method. For model organism Bacillus cereus F4810/72, the plate count method and both optical density methods gave comparable estimates for key growth parameters. Values for the maximum specific growth rate (μmax) derived by the 2FD method and by the RRD method were of the same order of magnitude, but some marked differences between the two approaches were apparent. Whereas the 2FD method allowed the derivation of values for lag time (λ) from the data, this was not possible with the RRD method. However, the RRD method gave many more data points per experiment and also gave more data points close to the growth boundary. This research shows that all three proposed methods can be used for parameter estimation but that the choice of method depends on the objectives of the research.

Risk-based Estimate of Effect of Foodborne Diseases on Public Health, Greece

TOC summary: These infections may account for 896 disability-adjusted life years per 1 million inhabitants annually.

Secondary plant metabolites as control agents of postharvest Penicillium rot on tulip bulbs

Abstract Fifteen essential oil components were screened for their antifungal activity towards Penicillium hirsutum. From these, carvone, cuminaldehyde, perillaldehyde, cinnamaldehyde, salicylaldehyde and benzaldehyde were selected as the most potent inhibitors of in vitro growth. Growth inhibition by carvone was found to be reversible. Exposure to cuminaldehyde, perillaldehyde, cinnamaldehyde and salicylaldehyde caused irreversible inhibition of fungal growth. Storage of tulip bulbs in atmospheres containing cuminaldehyde, perillaldehyde, salicylaldehyde or carvone resulted in a significant reduction of the natural Penicillium infection. Dipping tulip bulbs in an aqueous solution of 3.9 mM cinnamaldehyde gave a 40-fold reduction of the fungal population but did not affect the bacterial population. Treatment of tulip bulbs with carvone, cuminaldehyde, perillaldehyde, cinnamaldehyde or salicylaldehyde had no effect on the total stalk length or the flowering capacity of tulip bulbs.

Inhibition of propionate degradation by acetate in methanogenic fluidized bed reactors

SummaryThe degradation of acetate, propionate and butyrate was monitored during start-up of five lab-scale methanogenic fluidized bed reactors on an artificially prepared waste water. The acetate concentration in the reactor content was found to influence the degradation of propionate but not of butyrate. In general, at acetate levels over 200 mg/l the degradation of propionate was below 60%, whereas the degradation was complete at acetate levels under 100 mg/l. The rationale of the inhibition of propionate degradation by acetate is discussed.

Early stages in biofilm development in methanogenic fluidized-bed reactors.

SummaryBiofilm development in methanogenic fluidized-bed reactors with sand as the carrier was studied on a laboratory scale. The microorganisms present in consecutive layers of the biofilm of mature sludge granules were preliminarily characterized on the basis of their morphology, element composition and adhesion capacity and were compared to bacteria which take part in the initial colonization of sand. The early phase of biofilm development was monitored with reactors receiving waste-waters containing different mixtures of volatile fatty acids and inoculated with fluidized-bed reactor effluent for different lengths of time. The results obtained indicate that facultative anaerobic bacteria abundantly present in the outermost biofilm layers of mature sludge granules are probably the main primary colonizers of the sand. Methanothrix spp. or other methanogens were rarely observed among the primary colonizers. The course of biofilm formation was comparable under the various start-up conditions employed including variations in waste-water composition, inoculation and anaerobicity. However, omission of waste-water and thus of substrate resulted in rapid wash-out of the attached biomass.

Separation and quantification of cofactors from methanogenic bacteria by high-performance liquid chromatography: optimum and routine analyses

Abstract Four assays were developed, employing high-performance liquid chromatography, which gave optimal detection and separation of derivatives of 7-methylpterin, coenzyme F 420 , factor F 430 or vitamin B 12 . In addition an assay was developed in which thirteen of these cofactors can be separated and quantified simultaneously and which can be used in routine analysis of methanogenic populations in anaerobic digesters. The application of the different assays is demonstrated by analyses of extracts of pure cultures of Methanobacterium thermoautotrophicum and Methanosarcina barkeri and of sludge from a methanogenic fluidized bed reactor. Mixtures of authentic methanogenic cofactors were used in reference analyses and a relative peak area method was employed to identity the various cofactors in the extracts.

Influence of waste water composition on biofilm development in laboratory methanogenic fluidized bed reactors

SummaryThe influence of the volatile fatty acid composition of waste waters on biofilm development and on the time course of reactor start-up was investigated in laboratory scale fluidized bed reactors. It was found that biofilm development proceeded in a similar way with either acetate, butyrate, propionate or a mixture of these compounds as carbon source in the waste water. Startup was retarded, however, with propionate as sole carbon source. Scanning electron microscopic examination revealed that immobilization of bacteria on the sand used as adhesive support initially occurred in crevices and that thereupon the surface of the sand particles became colonized. The composition of the newly developed biomass was determined when reactors reached steady state. Significant differences in the relative substrate spectra and in the amounts of hydrogenotrophic and acetotrophic methanogenic bacteria were measured. The differences reflected the differences in the composition of the waste waters. The results obtained emphasize the role of the structure of the carrier surface in start-up of methanogenic fluidized bed reactors.

Comparing Nonsynergistic Gamma Models with Interaction Models To Predict Growth of Emetic Bacillus cereus when Using Combinations of pH and Individual Undissociated Acids as Growth-Limiting Factors

ABSTRACT A combination of multiple hurdles to limit microbial growth is frequently applied in foods to achieve an overall level of protection. Quantification of hurdle technology aims at identifying synergistic or multiplicative effects and is still being developed. The gamma hypothesis states that inhibitory environmental factors aiming at limiting microbial growth rates combine in a multiplicative manner rather than synergistically. Its validity was tested here with respect to the use of pH and various concentrations of undissociated acids, i.e., acetic, lactic, propionic, and formic acids, to control growth of Bacillus cereus in brain heart infusion broth. The key growth parameter considered was the maximum specific growth rate, μmax, as observed by determination of optical density. A variety of models from the literature describing the effects of various pH values and undissociated acid concentrations on μmax were fitted to experimental data sets and compared based on a predefined set of selection criteria, and the best models were selected. The cardinal model developed by Rosso (for pH dependency) and the model developed by Luong (for undissociated acid) were found to provide the best fit and were combined in a gamma model with good predictive performance. The introduction of synergy factors into the models was not able to improve the quality of the prediction. On the contrary, inclusion of synergy factors led to an overestimation of the growth boundary, with the inherent possibility of leading to underestimation of the risk under the conditions tested in this research.