A viable but non-culturable (VBNC) bacterial state was originally detected in studies in environmental microbiology. In particular, this state has been demonstrated for a number of human pathogens (Escherichia coli, S...
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A viable but non-culturable (VBNC) bacterial state was originally detected in studies in environmental microbiology. In particular, this state has been demonstrated for a number of human pathogens (Escherichia coli, Salmonella enteritidis, Vibrio cholerae, Legionella pneumophila and Campylobacter jejuni). The presence of VBNC cells poses a major public health problem since they cannot be detected by traditional culturing methods and the cells remain potentially pathogenic under favourable conditions. But, as far as we know, the VBNC state has not been yet described in Listeria monocytogenes. In most studies, this has been assessed by the Kogure procedure based on cellular elongation in the presence of DNA gyrase inhibitors. The antibiotic used was nalidixic acid in order to prevent DNA replication, only efficient in Gram-negative bacteria studies. In this study, we describe a new DVC procedure to detect and count viable of L. monocytogenes suspended in filtered, sterilized distilled water. We used different concentrations of ciprofloxacin, efficient both in Gram-negative and Gram-positive bacteria. Bacteria cells were removed and resuspended in BHI broth, with yeast extract and ciprofloxacin. The mixture was incubated at different incubation times at 37 degrees C. After different incubation times, cells were filtered through an isopore polycarbonate black membrane filter and covered with a DAPI solution or orange acridine. The filters were prepared and examined by epifluorescence microscopy. Elongated cells were counted as viable cells, whereas normal size was regarded as nonactive ones. This method allows determination of ciprofloxacin concentration and incubation time optimal to detect maximum viable cells percentage in L. monocytogenes.
Transmission electron microscopy (TEM) studies revealed that rough cell-forms of L. monocytogenes (designated FR variants), isolated from clinical and food samples (and under conditions of sublethal heat stress), cons...
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Transmission electron microscopy (TEM) studies revealed that rough cell-forms of L. monocytogenes (designated FR variants), isolated from clinical and food samples (and under conditions of sublethal heat stress), consist of either single or paired long-filaments. These FR variants markedly contrast in cell morphology from other previously described avirulent rough-mutants of L. monocytogenes that form long chains consisting of multiple cells of similar size (designated MCR variants). The identity of these Listeria isolates was determined using a commercially available, anti-Listeria polyclonal KPL antibody and by the API Listeria biochemical gallery. This study shows that filamentous rough-forms of L. monocytogenes may occur in clinical and food samples that are of undetermined pathogenicity.
Lag phase durations (t(Lag)) Of individual Listeria monocytogenes cells were analysed using the NightOwl Molecular Imaging System, and results were compared with mean individual cell lag times (t(L)) Obtained from the...
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Lag phase durations (t(Lag)) Of individual Listeria monocytogenes cells were analysed using the NightOwl Molecular Imaging System, and results were compared with mean individual cell lag times (t(L)) Obtained from the detection time (t(d)) method using Bioscreen. With Bioscreen, an average t(L) of 6 . 39 +/- 0 . 89 h was obtained from five separate experiments. With the NightOwl method, an average t(Lag) of 2 . 73 +/- 0 . 06 h was obtained from three experiments consisting of eight total replicates. Lag values from the NightOwl and Bioscreen are related by the equation: t(Lag) = t(L) + DT where DT is the doubling time. The equivalent t(Lag) mean value for the Bioscreen method was 7 . 11 +/- 0 . 84 h. Individual lag times measured by both methods were normally distributed (r(2) for Bioscreen and NightOwl ranged from 0 . 951 to 0 . 999 and from 0 . 884 to 0 . 982, respectively). The results suggest that the NightOwl method can provide accurate estimates of individual cell lag times, which will facilitate the development of combined discrete continuous models for bacterial growth.
The aim of this study was to examine the physicochemical surface properties and the ability to adhere to stainless steel of three strains of Listeria monocytogenes after different cultivation procedures. To this end, ...
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The aim of this study was to examine the physicochemical surface properties and the ability to adhere to stainless steel of three strains of Listeria monocytogenes after different cultivation procedures. To this end, bacteria were cultivated at 37 degrees C after storage at two frequently used temperatures (4 degrees C or -80 degrees C) and were then transferred into the liquid medium (trypticase soy broth supplemented with 6 g liter(-1) of yeast extract, pH 7.3) between one and four times. In addition, the influence of supplementing the growth medium with lactic acid was explored, this organic acid being representative of both the dairy and cured meat industries. The hydrophobic/hydrophilic and electron-acceptor/electron-donor characteristics of the strains were evaluated by the microbial adhesion to solvents method. Using this technique, we recorded an increase in the hydrophobic properties of one strain stored at 4 degrees C, with an increasing number of transfers in the media (P < 0.05), Another plant-isolated strain appeared more hydrophobic and stuck better to stainless steel when cells were stored at 4 degrees C rather than at -80 degrees C. Preculturing L. monocytogenes in a lactic acid-supplemented medium increased the affinity of microbial cells to solvents and the bacterial attachment to stainless steel (P < 0.05).
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