-Investigations of bacterial survival in natural environments have indicated that some organisms lose culturability on appropriate media under certain conditions and yet still exhibit signs of metabolic activity and thus viability. This reproducible loss of culturability in many bacterial species led to the description of a "Viable But Non Culturable" (VBNC) state. The purpose of this article is to determine environmental and physico-chemical factors which induce the VBNC state in a food-borne pathogen that has become a public concern: Listeria monocytogenes. The factors, i.e. inoculum size, natural sunlight, temperature (4 o C or 20 o C), NaCl concentration (0% or 7%) and pH (5 or 6) were studied on 4 strains (LO28, ATCC 19115, Scott A, CNL 895807). The culturability of the starved cell suspension was determined in each condition tested by the spread plate count, and the cell activity was determined by the Direct Viable Count technique and CTC-DAPI double staining. A strain effect was found in different test conditions. For the LO 28 and ATCC 19115 strains, the VBNC state was very transient in certain conditions. For the other strains tested (Scott A, CNL 895807), the VBNC state was maintained throughout the observation period. In the dark, the incubation temperature was the main factor in the production of VBNC forms in L. monocytogenes. However, natural sunlight rapidly produced the VBNC state in L. monocytogenes cells in microcosm water. We conclude that because of its ubiquity and the factors studied which are met in the food industry, the presence of VBNC L. monocytogenes cells could pose a major public health problem since they cannot be detected by traditional culturing methods. Further investigations are needed to establish virulence before and after resuscitation of VBNC L. monocytogenes cells. bactéries perdent leur capacité à former des colonies sur des milieux de culture, tout en conservant une activité métabolique. L'objectif de ce travail est d'étudier l'influence de facteurs physico-chimiques et environnementaux, intervenant dans l'entrée à l'état VNC de Listeria monocytogenes. Les facteurs : taille de l'inoculum, exposition à la lumière naturelle, température (4-20 o C), concentration en NaCl (0-7%), pH (5-6) ont été étudiés chez 4 souches de L. monocytogenes : LO28, ATCC19115, Scott A, CNL 895807. Les cellules ont été placées dans des conditions de privation nutritionnelle (eau distillée filtrée). La capacité à former des colonies a été déterminée par étalement sur boîte de gélose, alors que l'activité métabolique des bactéries a été établie par 2 techniques : le Direct Viable Count et la double coloration CTC-DAPI. Un effet souche a été constaté: pour les souches LO28 et ATCC 19115, l'état VNC semblait très transitoire, alors que pour les souches Scott A et CNL 895807, l'état VNC s'est maintenu pendant toute la durée de l'expérimentation. A l'obscurité, la température d'incubation est apparue comme le facteur primordial d'entrée à l'état VNC, mais l'exposition à la lumière naturelle a...

Campylobacter jejuni is a microaerophilic pathogen but is able to survive oxidative stress conditions during its transmission to the human host. Strains of different origins (reference, poultry, or human clinical) were tested for survival under oxidative stress conditions. C. jejuni strains were grown in Mueller Hinton broth to obtain late exponential-phase cultures. Then they were exposed to 2 different stresses: (1) cultures were either plated on Columbia agar plates and exposed to atmospheric oxygen or (2) paraquat (a chemical oxidizing agent) was added to liquid cultures to reach a 500-microM concentration. Both of these experimental conditions were realized at 3 different temperatures: 4 degrees C, 25 degrees C, and 42 degrees C. Results obtained with paraquat and atmospheric oxygen were similar. Surprisingly, C. jejuni was found to be very sensitive to oxidative stress at 42 degrees C, which is its optimal growth temperature, whereas it was more resistant at 4 degrees C. A strain effect was observed, but no relationship was found between the origin of the strains and level of resistance. High temperature (42 degrees C) combined with oxidative stress allowed a rapid decrease in the C. jejuni population, whereas low temperature considerably decreased the effect of oxidative stress.

Aims:  To investigate potential resuscitation of Listeria monocytogenes and Salmonella Typhimurium after high hydrostatic pressure treatments. Methods and Results:  Pressure treatments were applied at room temperature for 10 min on bacterial suspensions in buffers at pH 7 and 5·6. Total bacterial inactivation (8 log10 CFU ml−1 of bacterial reduction) obtained by conventional plating was achieved regarding both micro‐organisms. Treatments at 400 MPa in pH 5·6 and 600 MPa in pH 7 for L. monocytogenes and at 350 MPa in pH 5·6 and 400 MPa in pH 7 for S. Typhimurium were required respectively. A ‘direct viable count’ method detected some viable cells in the apparently totally inactivated population. Resuscitation was observed for the two micro‐organisms during storage (at 4 and 20°C) after almost all treatments. In the S. Typhimurium population, 600 MPa, 10 min, was considered as the treatment achieving total destruction because no resuscitation was observed under these storage conditions. Conclusions:  We suggest a delay before performing counts in treated samples in order to avoid the under‐evaluation of surviving cells. Significance and Impact of the Study:  The resuscitation of pathogen bacteria after physical treatments like high hydrostatic pressure has to be considered from the food safety point of view. Further studies should be performed in food products to study this resuscitation phenomenon.

The effect of a high hydrostatic pressure treatment on the Gram-positive Listeria monocytogenes strain Scott A and the Gram-negative Salmonella typhimurium strain Mutton (ATCC13 311) has been determined in stationary phase cell suspensions. Pressure treatments were done at room temperature for 10 min in sodium citrate (pH 5·6) and sodium phosphate (pH 7·0) suspension buffers. Increasing pressure treatments resulted in an exponential decrease of cell counts. Salmonella typhimurium suspended at low pH was more sensitive to pressure treatments. Progressive morphological changes were evident with the pressure increase. Cell lysis only appeared with the highest pressure treatments. Cell volume was not affected by pressure treatment. A progressive decrease of dpH (pH in -pH out ), intracellular potassium and ATP contents was demonstrated with the pressure increase. A parallel lowering of membrane potentials was measured.