No abstract

Changes in the outer membrane subproteome of Escherichia coli along the transition to the viable but nonculturable state (VBNC) were studied. The VBNC state was triggered by exposure of E. coli cells to adverse conditions such as aquatic systems, starvation, suboptimal temperature, visible light irradiation and seawater. The subproteome, obtained according to Molloy et al., was analysed at the beginning of exposure (inoculum, phase 1), after a variable exposure time (95% of population culturable, phase 2) and when populations were mainly in the VBNC state (95% of cells VBNC, phase 3). Proteome changes were dependent on adverse conditions inducing the transition and were detected mainly in phase 2. The permanence of E. coli cells in seawater under illumination conditions entailed a dramatic rearrangement of the outer membrane subproteome involving 106 new spots, some of which could be identified by peptide fingerprinting. However, proteins exclusive to the VBNC state were not detected.

The relative role of components of solar radiation (UV-B, UV-A, and photosynthetically active radiation) as well as the effect of simulated sunlight upon the physiological state of Escherichia coli in fresh water were evaluated. Simulated solar radiation had a sublethal effect on E. coli populations in a short-time exposure by provoking loss of culturability and the formation of viable but nonculturable cells. Prolonged exposure increased the damage to cells but cellular integrity was never affected. However, important differences between the way the sunlight components acted were detected. After photosynthetically active radiation (PAR) exposure, cells remained metabolically active but only 10% of the cells were culturable. When cells were exposed to UV-A, the culturable fraction was similar to the one obtained after PAR irradiation, although formation of viable but nonculturable cells was not observed. For UV-B radiation short-time exposures (6 h) were enough to provoke loss of culturability and a reduction in activity similar to that of simulated sunlight exposed cells. The effect of simulated solar radiation on E. coli cells was mainly attributable to shorter wavelengths, but a synergistic interaction of the UV-B, UV-A and PAR components was detected.

We evaluated the transfer to and from Escherichia coli of endogenously isolated plasmid material from the River Butron during the growth of three donor strains and two recipient strains as well as after the survival of these parental cells in river water. Transfer frequency varied greatly during the growth of donor cells, with minimum values in the exponential phase; frequency remained constant, however, during the growth of recipient strains. After survival in river water, donor cells lost their ability for plasmid transfer before any other physiological variations in the cells caused by environmental stress were detected. Under the same conditions and during equal periods, however, no variation in the ability of recipient cells to receive and express plasmid material was observed.

Discovering the means to control the increasing dissemination of pathogenic vibrios driven by recent climate change is challenged by the limited knowledge of the mechanisms in charge of Vibrio spp. persistence and spread in the time of global warming. To learn about physiological and gene expression patterns associated with the long-term persistence of V. harveyi at elevated temperatures, we studied adaptation of this marine bacterium in seawater microcosms at 30 °C which closely mimicked the upper limit of sea surface temperatures around the globe. We found that nearly 90% of cells lost their culturability and became partly damaged after two weeks, thus suggesting a negative impact of the combined action of elevated temperature and shortage of carbon on V. harveyi survival. Moreover, further gene expression analysis revealed that major adaptive mechanisms were poorly coordinated and apparently could not sustain cell fitness. On the other hand, elevated temperature and starvation promoted expression of many virulence genes, thus potentially reinforcing the pathogenicity of this organism. These findings suggest that the increase in disease outbreaks caused by V. harveyi under rising sea surface temperatures may not reflect higher cell fitness, but rather an increase in virulence enabling V. harveyi to escape from adverse environments to nutrient rich, host-pathogen associations.