Survival strategy of Escherichia coli and Enterococcus faecalis in illuminated fresh and marine systems

Abstract

Some effects of visible light on Escherichia coli and Enterococcus faecalis in natural freshwater and seawater were studied by plate counts, colony area measurements, and direct counts. A large number of somnicells (non-culturable cells) were noted in illuminated systems as compared with non-illuminated ones. Colony areas were significantly smaller in illuminated systems. Indirect activity measurements were used to test the effects of visible light on the ability of E. coli and Ent. faecalis to metabolize substrates ([14C]glucose) in natural waters. In illuminated systems, a decrease of glucose uptake was observed. When percentages of assimilation and respiration with respect to the total glucose uptake were analysed a decrease of assimilation percentages and an increase of respiration percentages were observed. In addition, differences in glucose uptake, assimilation and respiration by enteric bacteria were detected for E. coli at the beginning of the experiments between fresh- and seawater and these were interpreted as a toxic effect exerted by seawater on E. coli cells. Differences between species, natural waters and parameters studied (excepting glucose assimilation) were detected in the illuminated systems. We concluded, however, that enteric bacteria under visible light illumination show a general survival strategy characterized by reaching progressively a somnicell stage which can be defined in terms of their (1) inability to form colonies on standard bacteriological media, (2) inability to incorporate substrates, and (3) inactivation of biosynthetic processes.

Avirulence of Viable But Non-Culturable Listeria monocytogenes cells demonstrated by in vitro and in vivo models

“…However, some studies have demonstrated that VBNC cells retain some pathogenic effects [23][24][25]. Some authors have shown that E. coli VBNC cells retain their pathogenicity by keeping the ability to produce enterotoxin [3,23], or by preserving plasmids involved in pathogenicity [9]. Oliver and Bockian [22] showed that injecting Vibrio vulnificus VBNC cells in mice killed the animals.…”

Avirulent Viable But Non Culturable cells ofListeria monocytogenesneed the presence of an embryo to be recovered in egg yolk and regain virulence after recovery

“…Oliver and Bockian [36] observed a lethal effect in mice inoculated with VBNC cells of Vibrio vulnificus. Other works established that Escherichia coli VBNC cells kept their pathogenicity by keeping the ability to produce little amounts of enterotoxin [2,37] or by preserving plasmids involved in pathogenicity [9]. Chaiyanan et al showed conservation of cholera toxin and toxinassociated genes in VBNC cells of Vibrio cholerae [13].…”

Salmonellae in the Environment

“…In freshwaters however, the survival of salmonellae may be extended for periods longer than in seawater, as a result of the presence of humic materials that may absrobe U. V. radiation, therefore, protecting the cells from possible damage to the DNA by radiation (Davies & Evison, 1991). Generally, natural light affects the uptake of nutrients and inhibits active transport and biosynthesis in Escherichia coli (Barcina et al, 1990); its effect on salmonellae serovars may require further investigation (Sinton et al, 2007).…”