Disinfection kinetics: a new hypothesis and model for the tailing of log-survivor/time curves

A lattice model based on percolation theory for cold atmospheric DBD plasma decontamination kinetics

The tailing phenomenon, where the survival curve of bacteria shows a slow tailing period after a rapid decline, is a ubiquitous inactivation kinetics process in the advanced plasma sterilization field. While classical models suggest that bacterial resistance dispersion causes the tailing phenomenon, experiments suggest that the non-uniform spatial distribution of spores (clustered structure) is the cause. However, no existing inactivation kinetics model can accurately describe spatial heterogeneity. In this paper, we propose a lattice model based on percolation theory to explain the inactivation kinetics by considering the non-uniform spatial distribution of spores and plasma. Our model divides spores into non-clustered and clustered types and distinguishes between short-tailing and long-tailing compositions and their formation mechanisms. By systematically studying the effects of different spore and plasma parameters on the tailing phenomenon, we provide a reasonable explanation for the kinetic law of the plasma sterilization survival curve and the mechanism of the tailing phenomenon in various cases. As an example, our model accurately explains the 80-second kinetics of atmospheric pressure plasma inactivation of spores, a process that previous models struggled to understand due to its multi-stage and long-tail phenomena. Our model predicts that increasing the spatial distribution probability of plasma can shorten the complete killing time under the same total energy, and we validate this prediction through experiments. Our model successfully explains the seemingly irregular plasma sterilization survival curve and deepens our understanding of the tailing phenomenon in plasma sterilization. This study offers valuable insights for the sterilization of food surfaces using plasma technology, and could serve as a guide for practical applications.

A Novel Platform for Root Protection Applies New Root-Coating Technologies to Mitigate Soil-Borne Tomato Brown Rugose Fruit Virus Disease

Tomato brown rugose fruit virus (ToBRFV) is a soil-borne virus showing a low percentage of ca. 3% soil-mediated infection when the soil contains root debris from a previous 30–50 day growth cycle of ToBRFV-infected tomato plants. We designed stringent conditions of soil-mediated ToBRFV infection by increasing the length of the pre-growth cycle to 90–120 days, adding a ToBRFV inoculum as well as truncating seedling roots, which increased seedling susceptibility to ToBRFV infection. These rigorous conditions were employed to challenge the efficiency of four innovative root-coating technologies in mitigating soil-mediated ToBRFV infection while avoiding any phytotoxic effect. We tested four different formulations, which were prepared with or without the addition of various virus disinfectants. We found that under conditions of 100% soil-mediated ToBRFV infection of uncoated positive control plants, root-coating with formulations based on methylcellulose (MC), polyvinyl alcohol (PVA), silica Pickering emulsion and super-absorbent polymer (SAP) that were prepared with the disinfectant chlorinated-trisodium phosphate (Cl-TSP) showed low percentages of soil-mediated ToBRFV infection of 0%, 4.3%, 5.5% and 0%, respectively. These formulations had no adverse effect on plant growth parameters when compared to negative control plants grown under non ToBRFV inoculation conditions.

Study on antibacterial properties of heated oyster shell particle against Bacillus subtilis spores in rainwater by response surface methodology based on central composite design

Taiwan's oyster industry produces shell waste in abundant quantities every year. This study explored the feasibility of applying this resource as a simple and low-cost disinfectant to improve the microbial quality of harvested rainwater. Critical parameters affecting the disinfection efficacy of calcined oyster shell particles, i.e., heating temperature and duration, dosage, and contact time of the calcined shell material against Bacillus subtilis endospores in rainwater, were investigated. A central composite design of response surface methodology was employed to study the relative effects. As estimated from R² coefficients, a quadratic model was identified to predict the response variable satisfactorily. Results indicated that the heating temperature, dosage, and contact time of the calcined material in the rainwater significantly influenced (p < 0.05) the sporicidal effect, consistent with the prior literature on calcined shells of similar nature. However, heating time had a relatively low influence on the sporicidal impact, suggesting that the rate of shell activation, i.e., conversion of the carbonate compound in the shell material to oxide, occurs rapidly at high calcination temperatures. In addition, the sterilization kinetics for heated oyster shell particles in aqueous media under stagnant storage conditions were investigated and found to be in good agreement with Hom's model.

Modeling carbonate/bicarbonate and nitrate disturbance during secondary effluent disinfection by UV/H2O2 and UV/ozone

The disinfection of effluents has been considered the main step to inactivate pathogenic organisms to prevent the spread of waterborne diseases. The variation in the matrix composition can lead to the use of inadequate oxidant dose and disturb a correct treatment. The objective of this study was to develop a simple and practical mathematical model to simulate the disturbance of inorganic anions (CO₃^2-/HCO₃^- and NO₃^-) during secondary effluent disinfection by UV/H₂O₂ and UV/O₃. The pathogenic agents chosen for this study were total coliforms and E. coli. To build the mathematical model, a modification of the Chick model (referred to as 'Modified Chick Model') was proposed by employing a weighted average in the calculation of the kinetic constant. Both treatments were affected by the presence of the anions. However, with the highest NO₃^- concentration, less inhibition of disinfection was observed in the UV/H₂O₂. The use of the arithmetic means to calculate the value of k, as indicated by the Chick model, demonstrates a lesser precision in the prediction of the microorganisms' concentrations. On the other hand, using the Modified Chick Model, a better prediction of the inactivation of the microorganisms was obtained, which can be confirmed by the validation performed.

Surfaces with instant and persistent antimicrobial efficacy against bacteria and SARS-CoV-2

Surfaces contaminated with bacteria and viruses contribute to the transmission of infectious diseases and pose a significant threat to global public health. Modern day disinfection either relies on fast-acting (>3-log reduction within a few minutes), yet impermanent, liquid-, vapor-, or radiation-based disinfection techniques, or long-lasting, but slower-acting, passive antimicrobial surfaces based on heavy metal surfaces, or metallic nanoparticles. There is currently no surface that provides instant and persistent antimicrobial efficacy against a broad spectrum of bacteria and viruses. In this work, we describe a class of extremely durable antimicrobial surfaces incorporating different plant secondary metabolites that are capable of rapid disinfection (>4-log reduction) of current and emerging pathogens within minutes, while maintaining persistent efficacy over several months and under significant environmental duress. We also show that these surfaces can be readily applied onto a variety of desired substrates or devices via simple application techniques such as spray, flow, or brush coating.

A Plasma-Based Decontamination Process Reveals Potential for an in-Process Surface-Sanitation Method

Methods, which use an indirect plasma treatment for the inactivation of microorganisms in foods, claim a vastly growing field of research. This paper presents a method that uses plasma-processed air (PPA) as a sanitizer. In addition to a sanitation concept for the decontamination of produce in the value chain, the presented method offers a possible application as an “in-process” surface sanitation. PPA provides antimicrobial-potent species, which are predominantly reactive nitrogen species (RNS); this has an outstanding groove penetration property. In an experimental approach, surfaces, made from materials, which are frequently used for the construction of food-processing plants, were inoculated with different microorganisms. Listeria monocytogenes (ATCC 15313), Staphylococcus aureus (ATCC 6538), Escherichia coli (ATCC 10538), Salmonella enterica subsp. enterica serovar Typhimurium (ATCC 43971), and Salmonella enterica subsp. enterica serovar Enteritidis (ATCC 13076) are all microorganisms that frequently appear in foods and possess the risk for cross-contamination from the plant to the produce or vice versa. The contaminated samples were treated for various treatment times (1–5 min) with PPA of different antimicrobial potencies. Subsequently, the microbial load on the specimens was determined and compared with the load of untreated samples. As a result, reduction factors (RF) up to several log10-steps were obtained. Although surface and the bacterial strain showed an influence on the RF, the major influence was seen by a prolongation of the treatment time and an increase in the potency of the PPA.

Regrowth of Escherichia coli in environmental waters after chlorine disinfection: shifts in viability and culturability

Bacterial regrowth after water/wastewater disinfection poses severe risks to public health. However, regrowth studies under realistic water conditions that might critically affect bacterial regrowth are scarce. This study aimed to assess for the first time the regrowth of Escherichia coli (E. coli) in terms of its viability and culturability in environmental waters after chlorine disinfection, which is the most widely used disinfection method. Post-chlorination regrowth tests were conducted in 1) standard 0.85% NaCl solution, 2) river water receiving domestic wastewater effluents, and 3) river water that is fully recharged by domestic wastewater effluents. The multiplex detection of plate count and fluorescence-based viability test was adopted to quantify the culturable and viable E. coli to monitor the regrowth process. The results confirmed that chlorine treatment (0.2, 0.5 and 1.0 mg L-1 initial free chlorine) induced more than 99.95% of E. coli to enter a viable but non-culturable (VBNC) state and the reactivation of VBNC E. coli is presumably the major process of the regrowth. A second-order regrowth model well described the temporal shift of the survival ratio of culturable E. coli after the chlorination (R2: 0.73-1.00). The model application also revealed that the increase in initial chlorine concentration and chlorine dose limited the maximum regrowth rate and the maximum survival ratio, and the regrowth rate and percentage also changed with the water type. This study gives a better understanding of the potential regrowth after chlorine disinfection and highlights the need for investigating the detailed relation of the regrowth to environmental conditions such as major components of water matrices.

Ozone for SARS-CoV-2 inactivation on surfaces and in liquid cell culture media

This study evaluated the inactivation of SARS-CoV-2, the virus responsible for COVID-19, by ozone using virus grown in cell culture media either dried on surfaces (plastic, glass, stainless steel, copper, and coupons of ambulance seat and floor) or suspended in liquid. Treatment in liquid reduced SARS-CoV-2 at a rate of 0.92 ± 0.11 log₁₀-reduction per ozone CT dose(mg min/L); where CT is ozone concentration times exposure time. On surface, the synergistic effect of CT and relative humidity (RH) was key to virus inactivation; the rate varied from 0.01 to 0.27 log₁₀-reduction per ozone CT value(g min/m³) as RH varied from 17% to 70%. Depletion of ozone by competitive reactions with the medium constituents, mass transfer limiting the penetration of ozone to the bulk of the medium, and occlusion of the virus in dried matrix were postulated as potential mechanisms that reduce ozone efficacy. RH70% was found plausible since it provided the highest disinfection rate while being below the critical RH that promotes mould growth in buildings. In conclusion, through careful choice of (CT, RH), gaseous ozone is effective against SARS-CoV-2 and our results are of significance to a growing field where ozone is applied to control the spread of COVID-19.

Use of bacteriophage to inactivate pathogenic bacteria from wastewater

The aim of this study was to enhance the rhizobacterium potential in horizontal subsurface flow constructed wetland (CW) system planted by Phragmites australis using specific and lytic phages. The bioinoculation of specific bacteriophage for target bacteria; Salmonella typhi, and the monitoring of bacterial inactivation under different conditions showed the effectiveness of this methodology to enhance bacteria reduction and consequentially ameliorate purification performance of this studied biological treatment system. The injection of the phage at a concentration equal to 10³ UFP/mL within the rhizosphere of the inoculated filter (F) was allowed 1 U-Log₁₀ of improvement of bacterial inactivation compared to the control filter (T) nearly 1 logarithmic unit thus, a 90% improvement of bacteria reduction. When we increased the phage titer (10⁵ UFP/mL), the bacterial reduction equal to 2.75 U-Log₁₀ (N/N₀) was registered that corresponds to a decrease of nearly 99.9%. According to the first-order model, the inactivation coefficient is equal to 2.29 min^-1 (0.88 min^-1 for the first experiment) and the bacterial reduction rate is 5 times higher than that determined for the control filter. This results show the positive impact of the phage in the bacterial inactivation and the improvement of water treatment of the biofilter C.

Persistence against benzalkonium chloride promotes rapid evolution of tolerance during periodic disinfection

Biocides used as disinfectants are important to prevent the transmission of pathogens, especially during the current antibiotic resistance crisis. This crisis is exacerbated by phenotypically tolerant persister subpopulations that can survive transient antibiotic treatment and facilitate resistance evolution. Here, we show that E. coli displays persistence against a widely used disinfectant, benzalkonium chloride (BAC). Periodic, persister-mediated failure of disinfection rapidly selects for BAC tolerance, which is associated with reduced cell surface charge and mutations in the lpxM locus, encoding an enzyme for lipid A biosynthesis. Moreover, the fitness cost incurred by BAC tolerance turns into a fitness benefit in the presence of antibiotics, suggesting a selective advantage of BAC-tolerant mutants in antibiotic environments. Our findings highlight the links between persistence to disinfectants and resistance evolution to antimicrobials.

UVC disinfection robot

The aim of the present work is to contribute in the fight against the spread of Covid-19, a novel human coronavirus, in hospitals, public transport, airlines, and any enclosed areas. In this study, we have adopted the physical disinfection method by using UVC light as agent. The UVC devices are studied and classified according their disinfectant units, complementary devices, combined disinfection agents, mobilities, and order types. Our finding shows that a mobile robot is the most efficient device to inactivate microorganisms, so we have developed a robot called i-Robot UVC. The robot is equipped with eight UVC lamps around a central column and two lamps on the top. The column is fixed on a mobile base where several sensors are integrated to measure temperature and humidity on the one hand, and on the other, to detect motion plus position and to avoid obstacles. The robot can estimate automatically the disinfection time while monitored by Wi-Fi connection from a phone or a tablet. I-Robot UVC disinfects rooms and equipment with ultraviolet light, and shuts down when humans are around to keep them safe. The robot can kill 99,999% bacteria and various through UVC lamps led. The innovative robot UVC was patented under the number TN2020/0063.

Mechanistic Study of the Kinetic Phenomena Influencing the Bacteriostatic Action of Silver Ions in Agar Bioassays

Bacteriostatic action of a biocidal agent results from the cumulative impact of different kinetics, including those of bacterial growth, mass transfer of the agent and its antibacterial action against the targeted bacteria. Current studies on bacteriostatic effects always directly consider the combination of these kinetics at given times, without discrimination between each other. This work introduces a novel approach, consisting of first studying independently, by the experiment and the model, the different kinetics involved, and then in coupling these kinetics to obtain a model that will be confronted with experimental data. An agar diffusion test with silver ions against Escherichia coli bacteria was implemented herein to assess the relevance of this approach. This work achieved to characterize the different kinetics and to propose a dynamic model combining them, which fits the experimental data with a silver diffusivity in the biofilm fixed to 7.0 ± 0.1 × 10⁻¹² m² s⁻¹. This study also proves that the diffusive phenomenon was limiting the bacteriostatic action of silver ions over the test duration.

Modeling the dynamic kinetics of microbial disinfection with dissipating chemical agents-a theoretical investigation

The most notable microbial survival models of disinfection kinetics are the original and modified versions of the static Chick-Watson-Hom's (CWH) initially developed for water chlorination. They can all be viewed as special cases of the Weibull survival model, where the observed static curve is the cumulative form (CDF) of the times at which the individual targeted microbes succumb to the treatment. The CWH model time's exponent is the distribution's shape factor, and its concentration-dependent rate parameter represents the distribution's scale factor's reciprocal. Theoretically, the concentration- dependence of the Weibull model's rate parameter need not to be always in a form of a power-law relationship as the CWH model requires, and two possible alternatives are presented. Apart from being chemically reactive, most chemical disinfectants are also volatile, and their effective concentration rarely remains constant. However, the published dynamic versions of the original CWH model are mathematically incongruent with their static versions. The issue is nonexistent in the dynamic version of the Weibull or other distribution-based models, provided that the momentary inactivation rate is expressed as the static rate at the momentary concentration, at the time that corresponds to the momentary survival ratio. The resulting model is an ordinary differential equation (ODE) whose numerical solution can describe survival curves under realistic regular and irregular disinfectant dissipation patterns, as well as during the disinfectant dispersion and/or its replenishment. KEY POINTS: • The Chick-Watson-Home models are treated as special cases of the Weibull distribution. • Dynamic microbial survival curve described as ordinary differential equation solution. • Survival rate models of disinfectant dissipation and replenishment patterns presented.

Improving the microalgae inactivating efficacy of ultraviolet ballast water treatment in combination with hydrogen peroxide or peroxymonosulfate salt

Due to the increasing number of ecosystem invasions with the introduction of exogenous species via ballast water, the International Maritime Organization adopted the Ballast Water Convention (BWMC). The BWMC establishes standards for the concentration of viable organisms in a ballast water discharge. Ultraviolet (UV) irradiation is commonly used for treating ballast water; however, regrowth after UV irradiation and other drawbacks have been reported. In this study, improvement in UV treatment with the addition of hydrogen peroxide or peroxymonosulfate salt was investigated using the microalgae Tetraselmis suecica as the target organism. Results reported that each of these reagents added in a concentration of 10 ppm reduced the concentration of initial cells by more than 96%, increased the UV inactivation rate, and enabled reaching greater level of inactivation with the treatment. These improvements imply a reduction of the UV doses required for a consistent compliance with the BWMC standards.

Microbial Dose-Response Curves and Disinfection Efficacy Models Revisited

The same term “dose-response curve” describes the relationship between the number of ingested microbes or their logarithm, and the probability of acute illness or death (type I), and between a disinfectant’s dose and the targeted microbe’s survival ratio (type II), akin to survival curves in thermal and non-thermal inactivation kinetics. The most common model of type I curves is the cumulative form of the beta-Poisson distribution which is sometimes indistinguishable from the lognormal or Weibull distribution. The most notable survival kinetics models in static disinfection are of the Chick-Watson-Hom’s kind. Their published dynamic versions, however, should be viewed with caution. A microbe population’s type II dose-response curve, static and dynamic, can be viewed as expressing an underlying spectrum of individual vulnerabilities (or resistances) to the particular disinfectant. Therefore, such a curve can be described mathematically by the flexible Weibull distribution, whose scale parameter is a function of the disinfectant’s intensity, temperature, and other factors. But where the survival ratio’s drop is so steep that the static dose-response curve resembles a step function, the Fermi distribution function becomes a suitable substitute. The utility of the CT (or Ct) concept primarily used in water disinfection is challenged on theoretical grounds and its limitations highlighted. It is suggested that stochastic models of microbial inactivation could be used to link the fates of individual viruses or bacteria to their manifestation in the survival curve’s shape. Although the emphasis is on viruses and bacteria, most of the discussion is relevant to fungi, protozoa, and perhaps worms too.

Chlorination of Schistosoma mansoni cercariae

BACKGROUND

Schistosomiasis is a water-based disease acquired through contact with cercaria-infested water. Communities living in endemic regions often rely on parasite-contaminated freshwater bodies for their daily water contact activities, resulting in recurring schistosomiasis infection. In such instances, water treatment can provide safe water on a household or community scale. However, to-date there are no water treatment guidelines that provide information on how to treat water containing schistosome cercariae. Here, we rigorously test the effectiveness of chlorine against Schistosoma mansoni cercariae.

METHOD

S. mansoni cercariae were chlorinated using sodium hypochlorite under lab and field condition. The water pH was controlled at 6.5, 7.0 or 7.5, the water temperature at 20°C or 27°C, and the chlorine dose at 1, 2 or 3 mg/l. Experiments were conducted up to contact times of 45 minutes. 100 cercariae were used per experiment, thereby achieving up to 2-log10 inactivations of cercariae. Experiments were replicated under field conditions at Lake Victoria, Tanzania.

CONCLUSION

A CT (residual chlorine concentration x chlorine contact time) value of 26±4 mg·min/l is required to achieve a 2-log10 inactivation of S. mansoni cercariae under the most conservative condition tested (pH 7.5, 20°C). Field and lab-cultivated cercariae show similar chlorine sensitivities. A CT value of 30 mg·min/l is therefore recommended to disinfect cercaria-infested water, though safety factors may be required, depending on water quality and operating conditions. This CT value can be achieved with a chlorine residual of 1 mg/l after a contact time of 30 minutes, for example. This recommendation can be used to provide safe water for household and recreational water activities in communities that lack safe alternative water sources.

Rapid assessment and prediction of the efficiency of two preservatives against S. aureus in cosmetic products using High Content Screening-Confocal Laser Scanning Microscopy

Most cosmetic products are susceptible to microbiological spoilage due to contaminations that could happen during fabrication or by consumer’s repetitive manipulation. The composition of cosmetic products must guarantee efficient bacterial inactivation all along with the product shelf life, which is usually assessed by challenge-tests. A challenge-test consists in inoculating specific bacteria, i.e. Staphylococcus aureus, in the formula and then investigating the bacterial log reduction over time. The main limitation of this method is relative to the time-consuming protocol, where 30 days are needed to obtain results. In this study, we have proposed a rapid alternative method coupling High Content Screening—Confocal Laser Scanning Microscopy (HCS-CLSM), image analysis and modeling. It consists in acquiring real-time S. aureus inactivation kinetics on short-time periods (typically 4h) and in predicting the efficiency of preservatives on longer scale periods (up to 7 days). The action of two preservatives, chlorphenesin and benzyl alcohol, was evaluated against S. aureus at several concentrations in a cosmetic matrix. From these datasets, we compared two secondary models to determine the logarithm reduction time (Dc) for each preservative concentration. Afterwards, we used two primary inactivation models to predict log reductions for up to 7 days and we compared them to observed log reductions. The IQ model better fits datasets and the Q value gives information about the matrix level of interference.

On the adsorption kinetics and mechanism of enhanced photocatalytic activity of Fe3 O4 -SiO2 -TiO2 core-multishell nanoparticles against E. coli

In the present study, a Fe₃ O₄ -TiO₂ (FT) core-shell and a core-multishell structure of Fe₃ O₄ -SiO₂ -TiO₂ (FST) were synthesized, and their bactericidal capability was investigated on Escherichia coli (E. coli). Scanning electron microscopy (SEM), ultraviolet-visible spectroscopy (UV-vis), X-ray diffraction, Brunauer-Emmett-Teller, zeta potential, and fluorimetry were carried out to characterize properties of synthesized nanoparticles. An efficiency of 98% adsorption and harsh bacterial damage was observed when E. coli was put in contact with FST. Weaker adsorption of bacteria in contact with FT demonstrated that heterojunction has destructive effects on nanostructure. Further investigation proved that more OH were produced on the surface of FST, which is a sign of its longer lifetime. Moreover, results revealed that the presence of SiO₂ in the structure caused enhanced coverage, surface area, and porosity in TiO₂ outer layer, all of which have positive effects on adsorption. However, UV-vis showed smaller band gap for FT. It suggests that although photoactivity of FST is less influenced by light absorption, it possesses more e/h lifetime for generation of reactive oxygen species. Results point to the importance of SiO₂ as an obstacle of heterojunction on both adsorption and photoactivity. It was also proposed that increasing band gap in FST can be attributed to the porosity of SiO₂ that causes suppression of TiO₂ nanocrystallite growth.

Differential Susceptibility of Xylella fastidiosa Strains to Synthetic Bactericidal Peptides

The kinetics of cell inactivation and the susceptibility of Xylella fastidiosa subspecies fastidiosa, multiplex, and pauca to synthetic antimicrobial peptides from two libraries (CECMEL11 and CYCLO10) were studied. The bactericidal effect was dependent on the relative concentrations of peptide and bacterial cells, and was influenced by the diluent, either buffer or sap. The most bactericidal and lytic peptide was BP178, an enlarged derivative of the amphipathic cationic linear undecapeptide BP100. The maximum reduction in survivors after BP178 treatment occurred within the first 10 to 20 min of contact and at micromolar concentrations (<10 μM), resulting in pore formation in cell membranes, abundant production of outer membrane vesicles, and lysis. A threshold ratio of 10⁹ molecules of peptide per bacterial cell was estimated to be necessary to initiate cell inactivation. There was a differential susceptibility to BP178 among strains, with DD1 being the most resistant and CFBP 8173 the most susceptible. Moreover, strains showed a proportion of cells under the viable but nonculturable state, which was highly variable among strains. These findings may have implications for managing the diseases caused by X. fastidiosa.

Higher functionality of bacterial plasmid DNA in water after peracetic acid disinfection compared with chlorination

Peracetic acid (PAA) is an emerging disinfectant with a low disinfection by-product formation potential, but how PAA destroys gene function after killing bacteria remains to be studied. Bacterial plasmid DNA is a mobile genetic element that often harbors undesirable genes encoding antibiotic resistance and virulence factors. Even though PAA efficiently kills bacteria, bacterial plasmids and other mobile genetic elements might still be intact and functional after PAA disinfection, posing potential public health and environmental risks. This study evaluated the impact of PAA disinfection on the functionality of plasmid DNA in vivo and compared the results with those from chlorination. We delivered a plasmid DNA harboring two antibiotic resistance genes to Escherichia coli TOP10 to form an antibiotic-resistant bacterium (ARB). The planktonic ARB was treated with PAA and chlorine to find the minimum doses inhibiting the regrowth of the strain. PAA and chlorine stopped the regrowth at 8 ± 1 mg PAA·L^-1 and 20 ± 9 mg Cl₂·L^-1, respectively. The functionality of the plasmid DNA after PAA and chlorine disinfection was then determined at higher doses in vivo. Neither PAA nor chlorine completely destroyed the plasmid DNA. However, chlorine was more efficient than PAA in eliminating the plasmid DNA. PAA at 25 mg PAA·L^-1 reduced the transforming activity of the plasmid DNA by less than 0.3 log₁₀ units, whereas chlorine at 25 mg Cl₂·L^-1 reduced the transforming activity by approximately 1.7 log₁₀ units. Chlorine had a more pronounced impact on the functionality of the plasmid DNA because it oxidizes or destroys bacterial components including plasmid DNA faster than PAA. In addition, environmental scanning electron microscopy shows that chlorination desiccated the cells resulting in the flat cellular structure and possibly more complete loss of plasmid DNA, whereas PAA disinfection had a less impact on cell structure and morphology. This study demonstrates that more plasmid DNA remains functional in water after PAA disinfection than after chlorination. These functional genetic elements could be acquired by other microorganisms via horizontal gene transfer to pose potential public health and environmental risks.

Optimization of E. coli Inactivation by Benzalkonium Chloride Reveals the Importance of Quantifying the Inoculum Effect on Chemical Disinfection

Optimal disinfection protocols are fundamental to minimize bacterial resistance to the compound applied, or cross-resistance to other antimicrobials such as antibiotics. The objective is twofold: guarantee safe levels of pathogens and minimize the excess of disinfectant after a treatment. In this work, the disinfectant dose is optimized based on a mathematical model. The model explains and predicts the interplay between disinfectant and pathogen at different initial microbial densities (inocula) and dose concentrations. The study focuses on the disinfection of Escherichia coli with benzalkonium chloride, the most common quaternary ammonium compound. Interestingly, the specific benzalkonium chloride uptake (mean uptake per cell) decreases exponentially when the inoculum concentration increases. As a consequence, the optimal disinfectant dose increases exponentially with the initial bacterial concentration.

Action of Monomeric/Gemini Surfactants on Free Cells and Biofilm of Asaia lannensis

We investigated the biological activity of surfactants based on quaternary ammonium compounds: gemini surfactant hexamethylene-1,6-bis-(N,N-dimethyl-N-dodecylammonium bromide) (C6), synthesized by the reaction of N,N-dimethyl-N-dodecylamine with 1,6-dibromohexane, and its monomeric analogue dodecyltrimethylammonium bromide (DTAB). The experiments were performed with bacteria Asaia lannensis, a common spoilage in the beverage industry. The minimal inhibitory concentration (MIC) values were determined using the tube standard two-fold dilution method. The growth and adhesive properties of bacterial cells were studied in different culture media, and the cell viability was evaluated using plate count method. Both of the surfactants were effective against the bacterial strain, but the MIC of gemini compound was significantly lower. Both C6 and DTAB exhibited anti-adhesive abilities. Treatment with surfactants at or below MIC value decreased the number of bacterial cells that were able to form biofilm, however, the gemini surfactant was more effective. The used surfactants were also found to be able to eradicate mature biofilms. After 4 h of treatment with C6 surfactant at concentration 10 MIC, the number of bacterial cells was reduced by 91.8%. The results of this study suggest that the antibacterial activity of the gemini compound could make it an effective microbiocide against the spoilage bacteria Asaia sp. in both planktonic and biofilm stages.

A novel combined solar pasteurizer/TiO2 continuous-flow reactor for decontamination and disinfection of drinking water

A new combined solar plant including an annular continuous-flow compound parabolic collector (CPC) reactor and a pasteurization system was designed, built, and tested for simultaneous drinking water disinfection and chemical decontamination. The plant did not use pumps and had no electricity costs. First, water continuously flowed through the CPC reactor and then entered the pasteurizer. The temperature and water flow from the plant effluent were controlled by a thermostatic valve located at the pasteurizer outlet that opened at 80 °C. The pasteurization process was simulated by studying the effect of heat treatment on the death kinetic parameters (D and z values) of Escherichia coli K12 (CECT 4624). 99.1% bacteria photo-inactivation was reached in the TiO₂-CPC system (0.60 mg cm^-2 TiO₂), and chemical decontamination in terms of antipyrine degradation increased with increasing residence time in the TiO₂-CPC system, reaching 70% degradation. The generation of hydroxyl radicals (between 100 and 400 nmol L^-1) was a key factor in the CPC system efficiency. Total thermal bacteria inactivation was attained after pasteurization in all cases. Chemical degradation and bacterial photo-inactivation in the TiO₂-CPC system were improved with the addition of 150 mg L^-1 of H₂O₂, which generated approximately 2000-2300 nmol L^-1 of HO^● radicals. Finally, chemical degradation and bacterial photo-inactivation kinetic modelling in the annular CPC photoreactor were evaluated. The effect of the superficial liquid velocity on the overall rate constant was also studied. Both antipyrine degradation and E. coli photo-inactivation were found to be controlled by the catalyst surface reaction rate.

Visible light photocatalytic water disinfection and its kinetics using Ag-doped titania nanoparticles

The UN estimated about five million deaths every year due to water-borne diseases, accounting from four billion patients. Keeping in view, the ever increasing health issues and to undermine this statistics, a reliable and sustainable water-treatment method has been developed using visible light for water treatment. titania nanoparticles (NPs) have been synthesized successfully by a more applicable method Viz: liquid impregnation (LI) method. The bacterial death rate by photocatalysis under visible light was studied by employing a typical fluorescent source and was found to follow pseudo first-order reaction kinetics. The nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy, and energy-dispersive X-ray spectroscopy to deduce their size range, surface morphology, and elemental compositions, respectively. Among all the prepared grades, 1% Ag-TiO2 was found to be a very effective photocatalytic agent against Escherichia coli. The resulted photoinactivated data were also evaluated by different empirical kinetic models for bacterial inactivation. Hom, Hom-power, Rational, and Selleck models were not able to explain the disinfection kinetics but modified-Hom model fitted best with the experimentally obtained data by producing a shoulder, log-linear, and a tail region.

Disinfection action of electrostatic versus steric-stabilized silver nanoparticles on E. coli under different water chemistries

The capping layer stabilizing silver nanoparticles (AgNPs) affects its aggregation, dissolution, and net disinfection action, especially under conditions of varying water composition, such as, pH, ionic strength and organic matter content. Herein, we correlate the silver ion (Ag(+)) release and reactive oxygen species (ROS) generation rates for AgNPs of varying functionalization to their net disinfection coefficient on Escherichia coli, under conditions of differing water chemistries. For electrostatically stabilized citrate-capped AgNPs, the rate of ROS generation, as measured using a fluorescent dye, is found to dominate over that of Ag(+) release, especially for smaller sized AgNP suspensions (~10nm) at low pH (~6.2). For these AgNPs, the ROS disinfection mechanism is confirmed to dominate net disinfection action, as measured by the live/dead assay, especially at low levels of organic matter. Steric stabilization of AgNPs by protein or starch-capped layers enables disinfection through reducing AgNP aggregation and promoting silver dissolution over ROS generation. We suggest the involvement of protons and dissolved oxygen in causing the independent formation of Ag(+) and ROS, regardless of the AgNP capping layer. While protein-capping layers effectively stabilize AgNPs, the generated ROS is likely dissipated by interference with the bulky capping layer, whereas the interference is lower with citrate-capping layers. Steric stabilization of AgNPs enables disinfection within a wide range of water chemistries, whereas effective disinfection can occur under electrostatic stabilization, only at low NaCl (<1 mmol/L) and organic matter (<5 mg/L) levels.

Evaluation of bactericidal efficacy of silver ions on Escherichia coli for drinking water disinfection

PURPOSE

The purpose of this study is the development of a suitable process for the disinfection of drinking water by evaluating bactericidal efficacy of silver ions from silver electrodes.

METHODS

A prototype of a silver ioniser with silver electrodes and control unit has been fabricated. Silver ions from silver electrodes in water samples were estimated with an atomic absorption spectrophotometer. A fresh culture of Escherichia coli (1.75 × 10(3) c.f.u./ml) was exposed to 1, 2, 5, 10 and 20 ppb of silver ions in 100 ml of autoclaved tap water for 60 min. The effect of different pH and temperatures on bactericidal efficacy was observed at constant silver ion concentration (5 ppb) and contact time of 30 min.

RESULTS

The maximum bactericidal activity (100%) was observed at 20 ppb of silver ion concentration indicating total disinfection after 20 min while minimum bactericidal activity (25%) was observed after 10 min at 01 ppb of silver ions. Likewise, 100% bactericidal activity was noticed with 2, 5 and 10 ppb of silver ions after 60, 50 and 40 min, respectively. Bactericidal activity at pH 5, 6, 7, 8 and 9 was observed at 79.9%, 79.8%, 80.5%, 100% and 100%, respectively, whereas it was 80.4%, 88.3%, 100%, 100% and 100% at 10°C, 20°C, 30°C, 40°C and 50°C, respectively.

CONCLUSION

The findings of this study revealed that very low concentrations of silver ions at pH 8-9 and temperature >20°C have bactericidal efficacy for total disinfection of drinking water. Silver ionisation is suitable for water disinfection and an appropriate alternative to chlorination which forms carcinogenic disinfection by-products.

A matter of life or death: modeling DNA damage and repair in bacteria

DNA damage is a hazard all cells must face, and evolution has created a number of mechanisms to repair damaged bases in the chromosome. Paradoxically, many of these repair mechanisms can create double-strand breaks in the DNA molecule which are fatal to the cell. This indicates that the connection between DNA repair and death is far from straightforward, and suggests that the repair mechanisms can be a double-edged sword. In this report, we formulate a mathematical model of the dynamics of DNA damage and repair, and we obtain analytical expressions for the death rate. We predict a counterintuitive relationship between survival and repair. We can discriminate between two phases: below a critical threshold in the number of repair enzymes, the half-life decreases with the number of repair enzymes, but becomes independent of the number of repair enzymes above the threshold. We are able to predict quantitatively the dependence of the death rate on the damage rate and other relevant parameters. We verify our analytical results by simulating the stochastic dynamics of DNA damage and repair. Finally, we also perform an experiment with Escherichia coli cells to test one of the predictions of our model.

Hydrogen peroxide vapor: a novel method for the environmental control of lactococcal bacteriophages

Bacteriophage contamination can be problematic, especially in industrial settings. We examined the in vitro efficacy of hydrogen peroxide vapor (HPV) for the inactivation of two lactococcal bacteriophages dried onto stainless steel discs. A more than 6-log reduction was achieved on both bacteriophages compared with unexposed controls by 50 min of HPV exposure in an isolator. HPV might be useful for the environmental control of bacteriophages.

A model for the thermal inactivation of micro-organisms

AIMS

To mathematically model published thermal inactivation data sets using an empirical model based on a double Arrhenius function.

METHODS AND RESULTS

A mathematical model, the log R-fat, provided an excellent description of the data sets available: the thermal inactivation of Salmonella anatum at 55 degrees C, Pseudomonas viscosa at 48 degrees C and Streptococcus faecalis at 60 degrees C; Clostridium botulinum spores at various temperatures in the range of 101-121 degrees C (two data sets); thermal inactivation of Salmonella Bedford over the temperature range 50-58 degrees C, water activity range of 0.94-0.99 and a pH range of 4-7; Bacillus stearothermophilus spores from 105 to 121 degrees C and the dry heat sterilization of an indigenous mesophilic soil population over the temperature range of 120-160 degrees C.

CONCLUSIONS

The log R-fat model, derived from previously published chemical inactivation studies provides as good, if not better, description of thermal inactivation kinetics as other published models. The model does not invoke either of the two hypotheses of inactivation: the mechanistic or vitalistic, although it is closely linked to descriptions of the former.

SIGNIFICANCE AND IMPACT OF THE STUDY

The log R-fat double Arrhenius function provides the investigator with a relatively simple and easy mathematical model to apply to data of thermal inactivation. This model may allow a more accurate description of thermal food processing, especially when the safety of marginal heat processes are concerned.

Theory of antimicrobial combinations: biocide mixtures - synergy or addition?

AIMS

To demonstrate the effect that non-linear dose responses have on the appearance of synergy in mixtures of antimicrobials.

METHODS AND RESULTS

A mathematical model, which allows the prediction of the efficacy of mixtures of antimicrobials with non-linear dose responses, was produced. The efficacy of antimicrobial mixtures that would be classified as synergistic by time-kill methodology was shown to be a natural consequence of combining antimicrobials with non-linear dose responses.

CONCLUSIONS

The effectiveness of admixtures of biocides and other antimicrobials with non-linear dose responses can be predicted. If the dose response (or dilution coefficient) of any biocidal component, in a mixture, is other than one, then the time-kill methodology used to ascertain the existence of synergy in antimicrobial combinations is flawed.

SIGNIFICANCE AND IMPACT OF THE STUDY

The kinetic model developed allows the prediction of the efficacy of antimicrobial combinations. Combinations of known antimicrobials, which reduce the time taken to achieve a specified level of microbial inactivation, can be easily assessed once the kinetic profile of each component has been obtained. Most patented cases of antimicrobial synergy have not taken into account the possible effect of non-linear dose responses of the component materials. That much of the earlier literature can now be predicted, suggests that future cases will require more thorough proof of the alleged synergy.

A rapid method for assessing the suitability of quenching agents for individual biocides as well as combinations

AIMS

To develop a novel, rapid method for testing the ability of quenching agents to neutralize disinfectants.

METHODS AND RESULTS

Tests were performed to determine the suitability of different neutralizers for a range of disinfectants, using a new method based on the Bioscreen optical density analyser. Results showed that during disinfection tests, efficacy could be over-estimated due to poor, or no, neutralization of the disinfectant after a specified time of exposure to the bacteria. The failure to distinguish adequately between bacteriostatic and bactericidal effects can lead to false results during disinfectant testing. Experiments also showed that dilution of the disinfectant, following exposure to the bacteria, was not always sufficient to stop the activity of the disinfectant for chemicals with low dilution coefficients.

CONCLUSIONS

The quench test proved to be very quick and easy to perform, with results being available within 18 h. Using the Bioscreen, the test is automated and determines whether dilution into a particular neutralizer is able to inactivate a disinfectant within 30 s.

SIGNIFICANCE AND IMPACT OF THE STUDY

This new approach allows the efficacy of quenching agents to be determined, prior to undertaking each disinfection study, and can help in the development of more suitable quenching solutions. The test has also been used to find suitable neutralizers for mixtures of disinfectants which might be used during studies on synergistic biocide combinations.

Escherichia coli disinfection by electrohydraulic discharges

We study the survival of single-strain Escherichia coli colonies in aqueous media exposed to 5.5 kV, 90 kA electrohydraulic discharges (EHD). The probability of survival (Pn) of a 4 x 10(7) cfu mL(-1) E. coli population after n consecutive EHDs follows a logit distribution: In(Pn/ 100 - Pn) = 1.329 - 1.579 ln n with r2 = 0.993 that corresponds to lethal doses of LD50 = 2.2 and LD90 = 10.5 EHDs. Considering that the reactor is thoroughly mixed during each discharge and that LD50 = 0.9 values are nearly independent of E. coli concentrations in the range of 2 x 10(3) < or = E coli/cfu mL(-1) < or = 3 x 10(6), we ascribe the nonexponential Pn decay of single-strain E. coli colonies to a shielding phenomenon where inactive cells protect the successively smaller numbers of viable cells in the EHD. The qualitatively similar concentration dependence observed for survival under 254 nm of radiation, in contrast with the lower resistance of denser colonies to 20 kHz power ultrasound and the delayed onset of extracellular beta-D-galactosidase activity in bacterial populations already decimated by EHDs, support the view that UV radiation is the dominant disinfection agent generated by electrohydraulic discharges.

The effect of interfering substances on the disinfection process: a mathematical model

AIMS

To gain a greater understanding of the effect of interfering substances on the efficacy of disinfection.

METHODS AND RESULTS

Current kinetic disinfection models were augmented by a term designed to quantify the deleterious effect of soils such as milk on the disinfection process of suspended organisms. The model was based on the assumption that inactivation by added soil occurred at a much faster rate than microbial inactivation. The new model, the fat-soil model, was also able to quantify the effect of changing the initial inoculum size (1 x 10(7)-5 x 10(7) ml(-1) of Staphylococcus aureus) on the outcome of the suspension tests. Addition of catalase to the disinfection of Escherichia coli by hydrogen peroxide, resulted in changes to the shape of the log survivor/time plots. These changes were modelled on the basis of changing biocide concentration commensurate with microbial inactivation.

CONCLUSIONS

The reduction in efficacy of a disinfectant in the presence of an interfering substance can be quantified through the use of adaptations to current disinfection models.

SIGNIFICANCE AND IMPACT OF THE STUDY

Understanding the effect of soil on disinfection efficacy allows us to understand the limitations of disinfectants and disinfection procedures. It also gives us a mechanism with which to investigate the soil tolerance of new biocides and formulations.