Cereulide production has until now been restricted to the species Bacillus cereus. Here we report on two psychrotolerant Bacillus weihenstephanensis strains, MC67 and MC118, that produce cereulide. The strains are atypical with regard to pheno-and genotypic characteristics normally used for identification of emetic B. cereus strains. MC67 and MC118 produced cereulide at temperatures of as low as 8°C.

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Among five potentially probiotic lactobacilli investigated, Lactobacillus plantarum MF1298 and Lactobacillus salivarius DC5 showed the highest increase in the transepithelial electrical resistance (TER) of polarized monolayers of Caco-2 cells, and this increase was shown to be dose dependent. Furthermore, preincubation with MF1298 attenuated a decrease in TER induced by Listeria monocytogenes.

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A method to obtain real-time measurements of the interactions between nisin and single cells of Listeria monocytogenes on a solid surface was developed. This method was based on fluorescence ratio-imaging microscopy and measurements of changes in the intracellular pH (pH i ) of carboxyfluorescein succinimidyl esterstained cells during exposure to nisin. Immobilized cells were placed in a chamber mounted on a microscope and attached to a high-precision peristaltic pump which allowed rapid changes in the nisin concentration. In the absence of nisin, the pH i of L. monocytogenes was almost constant (approximately pH 8.0) and independent of the external pH in the pH range from 5.0 to 9.0. In the presence of nisin, dissipation of the pH gradient (⌬pH) was observed, and this dissipation was both time and nisin concentration dependent. The dissipation of ⌬pH resulted in cell death, as determined by the number of CFU. In the model system which we used the immobilized cells were significantly more resistant to nisin than the planktonic cells. The kinetics of ⌬pH dissipation for single cells revealed a variable lag phase depending on the nisin concentration, which was followed by a very rapid decrease in pH i within 1 to 2 min. The differences in nisin sensitivity between single cells in a L. monocytogenes population were insignificant for cells grown to the stationary phase in a liquid laboratory substrate, but differences were observed for cells grown on an agar medium under similar conditions, which resulted in some cells having increased resistance to nisin.Food preservation techniques which include the application of bacteriocin-producing lactic acid bacteria or purified bacteriocins have been studied extensively in order to increase the control of Listeria monocytogenes in particular in foods such as meat products and cheeses (11,13,14,17,19). The best-known and best-studied bacteriocin is the nisin produced by Lactococcus lactis (6, 9). Nisin acts on the cytoplasmic membrane of sensitive cells particularly L. monocytogenes, where it forms pores that lead to dissipation of the membrane potential and the pH gradient (⌬pH) and subsequent collapse of the proton motive force (1,4,27).Most food products of interest in biopreservation are often solid or semisolid, and the probability that bacteriocins will reach the target organism depends on the nature of the food matrix. It has been shown that NaCl concentration, pH, lipid content and agar concentration affect the diffusion of different bacteriocins in an agar matrix (2). It has also been demonstrated that bacteria attached to surfaces are more resistant to disinfectants than free-living bacteria are (7, 10). Williams et al. (25,26) concluded that an increase in the antibiotic resistance of attached Staphylococcus aureus was due to significant physiological adaptation that occurred during the early phases of attached growth.To what extent the solid food matrix influences the probability that bacteriocins will reach the target cells and to what extent the resistance to ...

Aims: To investigate the antibacterial effect of five essential oils (EO) extracted from aromatic plants (Cymbopogon citratus, Ocimum basilicum, Ocimum gratissimum, Thymus vulgaris and Zingiber officinale) of Cameroon against strains of Listeria monocytogenes, L. innocua and Staphylococcus aureus. The ability of selected EO to permeabilize the cytoplasmic membrane of L. innocua was also examined. Methods and Results: The antibacterial activity of the EO determined by the agar diffusion method showed that T. vulgaris had the highest activity followed by O. gratissimum and C. citratus. Lowest activity was recorded from Z. officinale and O. basilicum. Significant differences in sensitivity between strains of Listeria and S. aureus were observed. Flow cytometry of L. innocua stained with carboxy-fluorescein diacetate showed that the fluorescence intensity of cells exposed to EO decreased faster than nonexposed cells, indicating that EO permeabilized the cytoplasmic membrane with the leakage of carboxy-fluorescein. Conclusions: Almost all the EO tested showed antibacterial activity to a different extent. The antibacterial effect was due to permeabilization of the cytoplasmic membrane. Significance and Impact of the Study: This study has identified the preservative potential of the EO examined. The use of sensitive method, such as flow cytometry, is advantageous for quick generation of data on the antibacterial effect of EO.

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Fluorescence ratio imaging microscopy and microelectrode ion flux estimation techniques were combined to study mechanisms of pH homeostasis in Listeria monocytogenes subjected to acid stress at different levels of glucose availability. This novel combination provided a unique opportunity to measure changes in H ؉ at either side of the bacterial membrane in real time and therefore to evaluate the rate of H ؉ flux across the bacterial plasma membrane and its contribution to bacterial pH homeostasis. Responses were assessed at external pHs