Abstract
Culturability and viability techniques such as plate count on solid agar (PC), Most Probable Number (MPN) and Direct Viable Count-Fluorescence in Situ Hybridation (DVC FISH) were used to study the inactivation of Salmonella typhimurium by photo-Fenton process at pH 5.5. In the presence of only simulated solar irradiation (500 W·m-2), S. typhimurim showed that both culturability measured by MPN and viability (measured by DVC FISH) underwent just a slight decreasing of 2 and 1 log respectively after 240 min of light exposition while culturability measured by PC did not show any change. Results after 48 h of dark conditions did not reveal re-growth. However, when experiment was carried out in the presence of 2 mg L-1 of Fe3+ and 20 mg L-1 of H2O2 and pH 5.5, culturability was strongly affected after 240 min of simulated solar irradiation; nevertheless, viability was only slightly altered (~1 log). During dark period of 48 h changes on culturability and viability were not observed. On the other hand, it was also found that sugar metabolism was affected rather the amino-acids in S. typhimurium cells irradiated at different times upon photo-Fenton conditions. These findings might suggest for the first time that photo-Fenton process at pH 5.5 could induce viable but nonculturable state (VBNC) on waterborne S. typhimurium and that probably sugar metabolism damage could activate the VBNC state.
Highlights
Culturability and viability techniques such as plate count on solid agar (PC), Most Probable Number (MPN) and Direct Viable Count-Fluorescence in Situ Hybridation (DVC Fluorescence in situ Hybridation (FISH)) were used to study the inactivation of Salmonella typhimurium by photo-Fenton process at pH 5.5
This later fact is a key point since some authors have suggested Salmonella typhimurium cells could maintain their virulence during viable but nonculturable state (VBNC) state, and this could imply a serious threat to human health in disinfection processes [11,12,13]
The experiments were followed by 48 h in dark conditions and the results showed that Direct Viable count (DVC) FISH and MPN did not undergo neither decreasing nor re-growth on the bacterial population
Summary
When the oxidative stress ceases, microbial cells can undergo resuscitation where they recover their culturability under specific conditions [10] This later fact is a key point since some authors have suggested Salmonella typhimurium cells could maintain their virulence during VBNC state, and this could imply a serious threat to human health in disinfection processes [11,12,13]. It is suggested that this photochemical process could induce on the bacterial cells VBNC state avoiding their growing on solid agar and liquid media after 240 min of simulated solar irradiation. This fact is extremely important since it might allow evaluate precisely the ability and limitations of photo-Fenton process to inactive pathogen waterborne bacteria
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