The survival of Salmonella enterica serovar Typhimurium rpoS+ and rpoS exposed to sunlight or incubated in the dark was studied in a seawater microcosm and a diffusion chamber. The total number of bacteria and physiologically active ones were estimated by direct counting using epifluorescence microscopy, while cultivable bacteria were enumerated by plating onto an agar culture medium. The results obtained showed that the bacteria exposed to sunlight, either in the microcosm or in the diffusion chamber, exhibited a fast decline of cultivable cells, whereas the epifluorescence enumeration revealed a conservation of physiological activity after a total loss of cultivability. In a transparent diffusion chamber, the evolution of cultivable and physiological active bacteria was characterized by a diurnal decrease and a nocturnal stabilization in the count levels, reflecting the effect of phototoxicity on the survival of Salmonella. The transformation and persistence of a physiological active state and cellular integrity in the diffusion chamber or the microcosm showed a clear dependence on the presence of the rpoS gene. Survival in the dark was also characterized by a decrease of cultivability; however, this was significantly inferior to that observed in bacteria exposed to sunlight. This loss of cultivability was accompanied by the preservation of a physiologically active state. The study of seasonal survival carried out in the transparent diffusion chamber was characterized by an enhanced survival of the rpoS+ strain in comparison with the rpoS mutant. The highest survival rates were obtained in the autumn, thus highlighting the relevant effect of temperature and exposure to sunlight on the persistence of bacteria in seawater.

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