The detrimental influence of bacteria (E. coli, Shigella and Salmonella) on the degradation of organic compounds (and vice versa) in TiO2 photocatalysis and near-neutral photo-Fenton processes under simulated solar light.

TiO2 photocatalytic and near-neutral photo-Fenton processes were tested under simulated solar light to degrade two models of natural organic matter - resorcinol (R) (which should interact strongly with TiO2 surfaces) and hydroquinone (H) - separately or in the presence of bacteria. Under similar oxidative conditions, inactivation of Escherichia coli, Shigella sonnei and Salmonella typhimurium was carried out in the absence and in the presence of 10 mg L(-1) of R and H. The 100% abatement of R and H by using a TiO2 photocatalytic process in the absence of bacteria was observed in 90 min for R and in 120 min for H, while in the presence of microorganisms abatement was only of 55% and 35% for R and H, respectively. Photo-Fenton reagent at pH 5.0 completely removed R and H in 40 min, whereas in the presence of microorganisms their degradation was of 60% to 80%. On the other hand, 2 h of TiO2 photocatalytic process inactivated S. typhimurium and E. coli cells in three and six orders of magnitude, respectively, while S. sonnei was completely inactivated in 10 min. In the presence of R or H, the bacterial inactivation via TiO2 photocatalysis was significantly decreased. With photo-Fenton reagent at pH 5 all the microorganisms tested were completely inactivated in 40 min of simulated solar light irradiation in the absence of organics. When R and H were present, bacterial photo-Fenton inactivation was less affected. The obtained results suggest that in both TiO2 and iron photo-assisted processes, there is competition between organic substances and bacteria simultaneously present for generated reactive oxygen species (ROS). This competition is most important in heterogeneous systems, mainly when there are strong organic-TiO2 surface interactions, as in the resorcinol case, suggesting that bacteria-TiO2 interactions could play a key role in photocatalytic cell inactivation processes.

Bactericidal effects of 405 nm light exposure demonstrated by inactivation of Escherichia, Salmonella, Shigella, Listeria, and Mycobacterium species in liquid suspensions and on exposed surfaces.

The bactericidal effect of 405 nm light was investigated on taxonomically diverse bacterial pathogens from the genera Salmonella, Shigella, Escherichia, Listeria, and Mycobacterium. High-intensity 405 nm light, generated from an array of 405-nm light-emitting diodes (LEDs), was used to inactivate bacteria in liquid suspension and on exposed surfaces. L. monocytogenes was most readily inactivated in suspension, whereas S. enterica was most resistant. In surface exposure tests, L. monocytogenes was more susceptible than Gram-negative enteric bacteria to 405 nm light when exposed on an agar surface but interestingly less susceptible than S. enterica after drying onto PVC and acrylic surfaces. The study findings, that 405 nm light inactivates diverse types of bacteria in liquids and on surfaces, in addition to the safety advantages of this visible (non-UV wavelength) light, indicate the potential of this technology for a range of decontamination applications.

Mutants of Shigella sonnei deficient in DNA polymerase I.

Mutants of Shigella sonnei (S. sonnei) deficient in DNA polymerase I were isolated after mutagenesis with nitrosoguanidine. The isolation of the mutants was facilitated by the use of a strain harboring plasmid pBR313 which required DNA polymerase I for its muliplication. The mutants isolated could not maintain the plasmid and became sensitive to methyl methanesulfonate (MMS) and to ultraviolet light (UV) irradiation. Assays performed on crude extracts established that the mutants were deficient in an enzyme with DNA polymerase activity. All of these properties are the same as those of E. coli polA. Several MMS-resistant revertants isolated from one of the S. sonnei polA mutants regained 3-120% of the DNA polymerase activity found in the extracts of the wild-type parent strain. Most though not all of the revertants could support the multiplication of plasmid pBR313.

Post-UV survival of Escherichia coli strains carrying more than one UV-sensitising plasmid.

The post-UV phenotypes conferred by wild-type plasmids R391 and pYD1, which increase UV-induced mutagenesis but sensitise Escherichia coli AB1157 umuC+ uvrB+ to UV, were compared, alone and in combination with that of plasmid pGW16, which sensitises AB1157 to low, but protects against high UV doses. All three plasmids increased UV resistance when present in Shigella sonnei. No plasmid significantly affected the UV sensitivity of E. coli TK501 umuC uvrB, in which pKM101, the parent of pGW16 increases UV resistance up to 1000-fold. Both pYD1 and R391 reduced the UV protective effect of pKM101, and increased UV-sensitisation conferred by pGW16. UV-sensitisation conferred by pYD1 and R391 was additive when the plasmids were together in strain AB1157, and both pKM101 and pGW16 reduced this additive sensitisation.

High-intensity 405 nm light inactivation of Listeria monocytogenes.

The antimicrobial properties of light is an area of increasing interest. This study investigates the sensitivity of the significant foodborne pathogen Listeria monocytogenes to selected wavelengths of visible light. Results demonstrate that exposure to wavelength region 400-450 nm, at sufficiently high dose levels (750 J cm(-2)), induced complete inactivation of a 5 log(10) population. Exposure to wavelengths longer than 450 nm did not cause significant inactivation. Analysis of 10 nm bandwidths between 400 and 450 nm confirmed 405(± 5) nm light to be most effective for the inactivation of L. monocytogenes, with a lesser bactericidal effect also evident at other wavelengths between 400 and 440 nm. Identification of the optimum bactericidal wavelength enabled the comparison of inactivation using 405(± 5) nm filtered light and a 405 nm light-emitting diode (LED) array (14 nm FWHM). Results demonstrate similar inactivation kinetics, indicating that the applied dose of 405 nm light is the important factor. Use of the 405 nm LED array for the inactivation of L. monocytogenes and other Listeria species resulted in similar kinetics, with up to 5 log(10) reductions with a dose of 185 J cm(-2). Comparative data for the 405 nm light inactivation of L. monocytogenes and other important foodborne pathogens, Escherichia coli, Salmonella enteritidis and Shigella sonnei, are also presented, with L. monocytogenes showing higher susceptibility to inactivation through 405 nm light exposure.

Induction of colicin 1a at high temperature.

Induction of colicin la at a high temperature (42 C) was demonstrated in a wild strain of Shigella sonnei. After transfer of the Col factor, a similar effect was observed in those recipient cells acquiring colicinogeny. The induction is suggested to be due to the presence of a thermosensitive colicin repressor. The wild-type strain segregated sensitive cells and showed heterogeneity in colicin production.