Bacterial Regrowth Research Articles

Antimicrobial efficacy of cordless sonic or ultrasonic devices on Enterococcus faecalis-infected root canals.

The aim of the present study was to evaluate the immediate and residual antimicrobial activity of cordless sonic or ultrasonic devices on Enterococcus faecalis (Efaecalis)-infected teeth. A total of 140 single-rooted extracted teeth with Efaecalis were grouped as follows (N=15): conventional syringe irrigation with 3% sodium hypochlorite activated by ultrasonic device (group 1) or cordless ultrasonic device (group 2), EndoActivator (group 3) or without activation (group 4), conventional syringe irrigation with sterile bi-distilled water and ultrasonic device (group 5), EndoUltra (group 6), EndoActivator (group 7), or without activation (group 8). The remaining 20 teeth were used for positive and negative controls. Colony-forming units (CFU) and turbidity were recorded from bacteriological samples taken before and after irrigation and after 24hours of re-incubation. Data were statistically analyzed using Student's t test, Mann-Whitney test, Kruskal-Wallis test, and Dunn's multiple comparison tests (P<.05). Groups 1 and 2 showed higher antimicrobial efficacy than groups 3 and 4 (P<.05). No bacteria reduction was found in groups 5-8 (P>.05). After 24hours, regrowth of bacteria was not significantly different for all groups (P>.05). Ultrasound produced lower CFU and turbidity after treatment and after re-incubation of 24hours than sonic or no activation.

Assessing pretreatment and seawater reverse osmosis performance using an ATP-based bacterial growth potential method

Various bacterial growth potential (BGP) methods have been developed recently to monitor biofouling in seawater reverse osmosis (SWRO) systems such as assimilable organic carbon and bacterial regrowth potential. However, the relationship between these methods and biofouling in SWRO desalination plants has not yet been demonstrated. In this research, an attempt is made to investigate if a correlation exists between BGP of SWRO feed water and the chemical cleaning frequency in SWRO plants using an ATP-based BGP method employing an indigenous microbial consortium. Using ATP-based BGP method at 5 different seawater locations showed low variations of bacterial yield.The BGP method was applied to assess the pretreatment performance of three full-scale SWRO plants with different pretreatment processes. Dual media filtration (DMF) showed the highest BGP removal (>50%) in two SWRO plants. Removal of BGP and hydrophilic organic carbon in dissolved air floatation combined with ultrafiltration was similar to the removal achieved with DMF in combination with inline coagulation. For the three SWRO plants investigated, a higher BGP in SWRO feed water corresponded to a higher chemical cleaning frequency. However, more data is required to confirm if a real correlation exists between BGP and biofouling in SWRO plants.

Analyzing the performance of biological versus conventional drinking water filtration under warm and cold water conditions: A pilot scale study

Conventional filtration practices typically operate with a pre-chlorination step for the disinfection of harmful pathogens. Without pre-chlorination, biofilters will develop, which are capable of reducing the formation of disinfection by-products (DBPs) and can decrease bacterial regrowth in the distribution system. However, concerns, particularly in North America, still exist with the implementation of biofiltration and there is a lack of side-by-side comparisons with pre-chlorinated filters in literature. A pilot scale study comparing conventional and biological dual media anthracite/sand filters was conducted to assess their performance under warm (15–25 °C) and cold (0–5 °C) water conditions. The filters were operated under various backwash conditions, including the addition of air scour and extended terminal subfluidization wash (ETSW). The biofilter effluent exhibited significantly lower (p < 0.05) DBP formations under both temperature conditions, with trihalomethanes (TTHM) and haloacetic acids (HAA9) concentrations 33–35% and 36–46% lower than the conventional filters, respectively. There was no significant difference when it came to particle passage or turbidity during ripening for both filter types, however the conventional filter proved to be more vulnerable to changes in terms of particles between backwashes. The biofilter exhibited greater average headloss development by 8.8 cm and 4.5 cm under warm and cold water conditions, respectively. Biofilter headloss was found to be minimized by 18% when applying air scour and ETSW during backwash compared to water only, under warm temperature conditions.

A novel method: using an adenosine triphosphate (ATP) luminescence-based assay to rapidly assess the biological stability of drinking water.

The rapid and credible evaluations of the microbial stability of a drinking water distribution system (DWDS) are of great significance for ensuring the safety of drinking water and predicting microbial pollution. Conventional biostability assessment methods mainly focus on bacterial regrowth or evaluation of the level of nutrients that support bacterial regrowth. However, such methods are time-consuming and have many limitations. An adenosine triphosphate (ATP) assay can rapidly measure all active microorganisms and is known to be a useful method to assess the microbial activity of drinking water. The measurement of ATP has been used for more than a decade in the field of drinking water research. This article reviews the application of an ATP luminescence-based method to assess the biostability of drinking water and discusses the feasibility of ATP measurement as a parameter for quickly evaluating this criterion. ATP measurement will help researchers and water managers better monitor the biological stability of drinking water from the source to the consumer's tap. This review covers the: (1) principle and application of the ATP measurement in drinking water quality assessment; (2) comparison of the merits and demerits of several methods for evaluating the biostability of drinking water; (3) discussions on using ATP measurement in evaluating biostability; and (4) improvements in the use of ATP measurement in evaluating biostability. At the end of this review, recommendations were given for better application of the ATP measurement as a parameter for monitoring the microbial quality of drinking water.

Influence of industrial contamination in municipal secondary effluent disinfection by UV/H2O2.

Advanced oxidation processes, including UV/H2O2, are methods able to remove diverse classes of organic contaminants and disinfect water and wastewater. However, the variation in the matrix composition can influence the inactivation of microorganisms due to the presence of competing reactive material, which consumes the available oxidants. This problem can lead to the use of inadequate oxidant/radiation dose and disturb a correct treatment. The aim of this study was to assess the efficiency of UV/H2O2 to inactivate microbiological indicators in secondary effluents in the presence of high concentration of carbonate, nitrate, metals, and industrial organic contaminants. Metals had a positive influence on inactivation acting as catalysts. Zn, Fe, and all metals simultaneously presented toxic effects to the indicator organisms in the higher concentrations before the treatment. Even in metals presence, the negative effect of carbonate and the industrial organic contaminants on indicators inactivation was very important. Bacteria regrowth after 72h was also affected by the same inhibiting substances, but the metals acted positively inhibiting it. The disinfection indicators had different sensibilities to the spiked substances. Escherichia coli inactivation was more affected than total coliforms by the presence of the industrial contamination, which can lead to different interpretation of inhibition degree depending of the used disinfection indicator.

Pharmacodynamics of ciprofloxacin against Pseudomonas aeruginosa planktonic and biofilm-derived cells.

The influence of growth phase and state on the survival and recovery of Pseudomonas aeruginosa exposed to ciprofloxacin was investigated using batch culture grown planktonic cells and disaggregated biofilm populations. Biofilms were either nonantibiotic exposed or previously exposed to ciprofloxacin before disaggregation and subsequent challenge with ciprofloxacin. Viable counts showed that late stationary phase cells were tolerant to ciprofloxacin over 24h exposure, while all other populations presented a biphasic killing pattern. In contrast, the metabolic activity of planktonic and biofilm-derived cells remained similar to controls during the initial 6h of ciprofloxacin exposure, despite a significant reduction in viable cell numbers. A similar effect was observed when assessing the postantibiotic effect of 1h ciprofloxacin exposure. Thus, although cell reduction occurred, the metabolic status of the cells remained unchanged. The recovery of disaggregated biofilm cells previously exposed to ciprofloxacin was significantly quicker than naïve biofilm cells, and this latter population's recovery was significantly slower than all planktonic populations. Results from this work have implications for our understanding of biofilm-related infections and their resilience to antimicrobial treatment. SIGNIFICANCE AND IMPACT OF THE STUDY: Removal of biofilms from surfaces and infection sites via disaggregation and induction of dispersion may reverse their antibiotic tolerant state. However, little is known of the recovery of the cells upon disaggregation from biofilms. Driven by this gap in knowledge we quantified the effect of ciprofloxacin on disaggregated biofilms of Pseudomonas aeruginosa, including those previously exposed to ciprofloxacin. Our results provide further insight into bacterial resilience, regrowth, and antimicrobial efficacy, as reduction in cell viability does not directly correlate with the metabolic activity of bacteria at the time of the exposure to antimicrobials. Thus, despite a perceived reduction in viability, the potential for cell persistence and regrowth remains and recovery is quicker upon subsequent exposure to antimicrobial, supporting the increase in resilience and recurrence of infections.

Heterogeneous photocatalysis using UVA-LEDs for the removal of antibiotics and antibiotic resistant bacteria from urban wastewater treatment plant effluents

Secondary urban wastewater samples were spiked with azithromycin (AZT), trimethoprim (TMP), ofloxacin (OFL) and sulfamethoxazole (SMX) at 100 µg L−1 to investigate the efficiency of a TiO2-photocatalytic treatment using UVA-LEDs. Different operating parameters were studied, such as the irradiation conditions, catalyst load and the use of methanol as carrier solvent and radical scavenger. The most efficient conditions to treat spiked urban wastewater (4 LEDs symmetrically distributed and 1.00 g L−1 of catalyst) were also assessed on the removal of the antibiotics at real concentrations, as well as on the inactivation and regrowth of bacteria after 3-day storage (total and resistant heterotrophs, Escherichia coli and enterococci). Clindamycin (CLI) was targeted when SMX was not detected. One-hour treatment was enough to reduce the analysed antibiotics to values below the detection limits and to decrease the bacterial load by 2 log-units. Bacterial regrowth was observed for total heterotrophs, after the storage of photocatalytic treated wastewater, to values close to pre-treatment. However, the antibiotic resistance percentage of such stored wastewater was always similar or lower than that of secondary urban wastewater. Thus, the potential of this process as part of the tertiary treatment is demonstrated, but conditions must be adjusted to minimize microbial regrowth.

Does plastic incise drape prevent recolonization of endogenous skin flora during lumbar spine procedures?

Background:The aim of this study was to compare the PID with bare skin (without PID) regarding bacterial recolonization and bacterial regrowth of the adjacent skin of surgical incision in lumbar spine surgery patients.Methods:This quasi-experimental study was conducted from February to May 2018 on 88 patients who were candidate for lumbar spine surgery. Patients were assigned to one of two groups, treatment (with PID) and control (without PID). Skin sampling (adjacent of surgical incision) for bacterial culture was done in two steps, immediately after surgical skin prep (IASSP) and immediately after surgical wound closure (IASWC) by researcher. Finally, samples were sent to the laboratory.Results:The mean total bacterial count of patient's skin in stage IASSP was not significantly different between treatment and control groups (0.34 vs 0.27, P = 0.68). However, mean total bacterial count in stage IASWC in treatment group was significantly higher than control group (2.2 vs 0.93, P = 0.03). The frequency distribution of S. aureus (P = 0.04) and S. epidermidis (P = 0.02) was significantly higher in treatment group compared with control group in stage IASWC.Conclusions:The results showed that using PID is unable to reduce recolonization and regrowth of bacteria on patients’ skin adjacent to surgical wound in clean lumbar spine surgeries. However, making a definite decision about using or not using of PID requires further studies.

Brown and Red Seaweeds Serve as Potential Efflux Pump Inhibitors for Drug-Resistant Escherichia coli.

Multidrug-resistant pathogens are a significant clinical problem. Efflux pump inhibitors (EPIs) can restore the activities of existing antibiotics by interfering with drug efflux pumps located in bacterial cell membranes. Seaweeds are important sources of biologically active metabolites of natural origin; however, their potential as EPIs remains uninvestigated. Here, functional extracts from the brown seaweeds Laminaria japonica and Sargassum horneri and the red seaweeds Gracilaria sp. and Porphyra dentata were evaluated as potential EPIs against drug-resistant Escherichia coli. All these extracts were found to potentiate the activities of drugs in modulation tests, although not to the same extent. Synergistic effects of the extracts and the drug clarithromycin were observed from the onset of Time-kill assays, with no evidence of bacterial regrowth. Ethidium bromide accumulation studies revealed that the efflux decreased in the presence of each extract, as indicated by the presence of EPIs. Most identified EPIs that have been discovered to date have aromatic structures, and the seaweed extracts were found to contain various terpenes, terpenoids, phenolic compounds, indoles, pyrrole derivatives, alkaloids, and halogenated aromatic compounds. Our study highlights the potential of these compounds of the seaweeds as drug EPIs.

Intelligent Management of Secondary Water Supply Systems in Downtown Shanghai

Secondary water supply, as the final component of the urban water supply system, is critical for ensuring the safety of urban water supply, and is directly related to customers’ water use. However, water stored in secondary water supply facilities will more likely to lose disinfectants (chlorine residual) which can lead to bacterial regrowth and water quality deterioration. Therefore, it is important to solve problems associated with secondary water supply and manage the system more effectively. In the context of the Internet of Things, the intelligent management model based on information technology has become the growing trend. This paper introduces the application of the secondary water supply intelligent management model in Shanghai.

Lack of evidence for the necessity of root canal obturation.

Root canal obturation still is a relevant research topic and patients spend substantial amounts of financial resources for this step of endodontic treatment. Three experiments were conducted challenging the necessity of root canal obturation. Applying micro computed tomography, the volume of dentin tubules that cannot be instrumented during root canal therapy was determined. Using a simple biofilm model of human tooth segments, the effect of root canal obturation on the persistency of bacteria was evaluated and freshly extracted root canal treated teeth were examined for bacteria remaining in dentin. The volume of dentinal canals was found to be at least three times greater than the volume of the root canal itself. Bacterial growth was observed both in specimens with and without root canal obturation implying that the treatment rendered was ineffective in removing bacterial biofilm and the obturation material was incapable of hindering bacterial regrowth. Despite showing adequate root canal obturation radiographically, persistent bacteria could be identified in all teeth extracted. While perfect disinfection of root canals is mandatory, root canal obturation seems questionable as current materials have no antibacterial activity, do not stabilize the tooth, and cannot seal the canal system if a coronal restoration is missing.

Impact of blending for direct potable reuse on premise plumbing microbial ecology and regrowth of opportunistic pathogens and antibiotic resistant bacteria

Little is known about how introducing recycled water intended for direct potable reuse (DPR) into distribution systems and premise plumbing will affect water quality at the point of use, particularly with respect to effects on microbial communities and regrowth. The examination of potential growth of opportunistic pathogens (OPs) and spread of antibiotic resistance genes (ARGs), each representing serious and growing public health concerns, by introducing DPR water has not previously been evaluated. In this study, the impact of blending purified DPR water with traditional drinking water sources was investigated with respect to treatment techniques, blending location, and blending ratio. Water from four U.S. utility partners was treated in bench- and pilot-scale treatment trains to simulate DPR with blending. Water was incubated in simulated premise plumbing rigs made of PVC pipe containing brass coupons to measure regrowth of total bacteria (16S rRNA genes, heterotrophic plate count), OPs (Legionella spp., Mycobacterium spp., Pseudomonas aeruginosa), ARGs (qnrA, vanA), and an indicator of horizontal gene transfer and multi-drug resistance (intI1). The microbial community composition was profiled and the resistome (i.e., all ARGs present) was characterized in select samples using next generation sequencing. While regrowth of total bacteria (16S rRNA genes) from the start of the incubation through week eight consistently occurred across tested scenarios (Wilcoxon, p ≤ 0.0001), total bacteria were not more abundant in the water or biofilm of any DPR scenario than in the corresponding conventional potable condition (p ≥ 0.0748). Regrowth of OP marker genes, qnrA, vanA, and intI1 were not significantly greater in water or biofilm for any DPR blends treated with advanced oxidation compared to corresponding potable water (p ≥ 0.1047). This study of initial bacteria colonizing pipes after introduction of blended DPR water revealed little evidence (i.e., one target in one water type) of exacerbated regrowth of total bacteria, OPs, or ARGs in premise plumbing.

Solar photo-Fenton oxidation followed by adsorption on activated carbon for the minimisation of antibiotic resistance determinants and toxicity present in urban wastewater

This work evaluated the removal of a mixture of antibiotics from urban wastewater, by a combined process consisting of solar photo-Fenton (SPF) followed by granular activated carbon (GAC). The effects of the SPF process were investigated at a toxicological, microbiological and genomic level, using species of plants and aquatic organisms, bacteria, antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). The results demonstrated that SPF could completely degrade antibiotics present in two types of effluent deriving from a conventional activated sludge system (CAS) and a membrane bioreactor (MBR), operated at the optimum oxidant dose ([H2O2]CAS = 100 mg/L; [H2O2]MBR = 50 mg/L) and illumination time (tCAS = 115 min; tMBR = 111 min) at acidic pH (2.8–2.9). Moreover, total disinfection was achieved by the SPF process, as cultivable bacteria, including ARB, were inactivated after 60 min of treatment, disabling also the bacterial regrowth after 24 h of storage of the treated effluent. Furthermore, SPF was shown to be effective in degrading the cellular DNA of the effluent, which was reduced to the detection limit after 60 min of treatment in both effluents. The abundance of 16S rRNA was found to be preferentially decreased by the SPF in the MBR-treated effluent (8 folds) compared to the CAS-treated effluent (7.4 folds). The abundance of blaOXA, blaCTX-M, qnrS, sul1, and tetM genes was decreased to the limit of quantification after 60 min of SPF treatment in both effluents. However, the SPF treated flow resulted in increased toxicity, probably due to the oxidation of the dissolved effluent organic matter of the wastewater leading to the formation of toxic products. Therefore, SPF-treated samples collected at different time intervals (30, 60, 90, 120, and 180 min) were subjected to adsorption onto GAC (500 mg/L), and the removal of both the toxicity and the residual antibiotics remaining after SPF, was explored. The combined processes (30 min SPF; 15 min GAC) provided almost complete removal of toxicity and elimination of antibiotics, ensuring wastewater decontamination.

Sequential Combined Effect of Phages and Antibiotics on the Inactivation of Escherichia coli.

The emergence of antibiotic resistance in bacteria is a global concern. The use of bacteriophages (or phages) alone or combined with antibiotics is consolidating itself as an alternative approach to inactivate antibiotic-resistant bacteria. However, phage-resistant mutants have been considered as a major threat when phage treatment is employed. Escherichia coli is one of the main responsible pathogens for moderate and serious infections in hospital and community environments, being involved in the rapid evolution of fluoroquinolones and third-generation cephalosporin resistance. The aim of this study was to evaluate the effect of combined treatments of phages and antibiotics in the inactivation of E. coli. For this, ciprofloxacin at lethal and sublethal concentrations was added at different times (0, 6, 12 and 18 h) and was tested in combination with the phage ELY-1 to inactivate E. coli. The efficacy of the combined treatment varied with the antibiotic concentration and with the time of antibiotic addition. The combined treatment prevented bacterial regrowth when the antibiotic was used at minimum inhibitory concentration (MIC) and added after 6 h of phage addition, causing less bacterial resistance than phage and antibiotic applied alone (4.0 × 10−7 for the combined treatment, 3.9 × 10−6 and 3.4 × 10−5 for the antibiotics and the phages alone, respectively). Combined treatment with phage and antibiotic can be effective in reducing the bacterial density and it can also prevent the emergence of resistant variants. However, the antibiotic concentration and the time of antibiotic application are essential factors that need to be considered in the combined treatment.

1409. Evaluation of Alternative Piperacillin–tazobactam Dosing Strategies Against ESBL-Producing Enterobacteriaceae Using a Hollowfiber Infection Model

BackgroundExtended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae exhibit variable response to treatment with piperacillin–tazobactam. Current clinical practice with piperacillin–tazobactam involves dosing the components simultaneously at a fixed ratio of 8:1 piperacillin to tazobactam. However, it remains unclear whether this ratio is optimal for enzyme inhibition and bactericidal activity. Using a hollowfiber infection model (HFIM), we evaluated the efficacy of various exposures of piperacillin–tazobactam against ESBL-producing Enterobacteriaceae.MethodsA clinical strain of K. pneumoniae expressing CTX-M-15 was used as a reference isolate. Piperacillin minimum inhibitory concentrations (MIC) were determined using a range of tazobactam concentrations and fitted to an inhibitory Emax model. An HFIM was used to simulate and evaluate the impact of escalating tazobactam dosing in the context of a fixed piperacillin exposure (equivalent to 4 g every 8 hours). Serial samples were collected to verify the pharmacokinetic simulations (by LC–MS/MS) and determine bacterial density for up to 120 hours. Measured drug concentrations were incorporated in the Emax model to determine the free-time above instantaneous MIC (fT>MICi) associated with each experimental exposure. The target fT>MICi associated with growth suppression was subsequently validated using a clinical strain of E. coli (producing SHV-12) and a second K. pneumoniae (producing CTX-M-15).ResultsFor the reference strain, a clinical regimen of 4 g piperacillin and 0.5 g tazobactam administered every 8 hours resulted in a fT > MICi of 39.6% and bacterial regrowth. An exposure equivalent to 1.5 g tazobactam (fT > MICi of 55.1%) was needed to suppress growth. These regrowth findings were validated with the two other ESBL-producers with tazobactam exposures characterized by fT > MICi of 36.8 and 43.8%.ConclusionImproved bacterial killing was observed with increasing tazobactam exposures. As a novel PK/PD index, fT > MICi may be used to characterize response to a β-lactamase inhibitor and provide efficacy targets to guide the development and clinical dosing of these inhibitors.Disclosures A. M. Sánchez-Díaz, European Union’s Seventh Framework Programme: Grant Investigator, Research grant. R. Cantón, European Union’s Seventh Framework Programme: Grant Investigator, Research grant. V. Tam, European Union’s Seventh Framework Programme: Grant Investigator, Research grant.

ATP-Dependent Dynamic Protein Aggregation Regulates Bacterial Dormancy Depth Critical for Antibiotic Tolerance

Cell dormancy is a widespread mechanism used by bacteria to evade environmental threats, including antibiotics. Here we monitored bacterial antibiotic tolerance and regrowth at the single-cell level and found that each individual survival cell shows different "dormancy depth," which in return regulates the lag time for cell resuscitation after removal of antibiotic. We further established that protein aggresome-a collection of endogenous protein aggregates-is an important indicator of bacterial dormancy depth, whose formation is promoted by decreased cellular ATP level. For cells to leave the dormant state and resuscitate, clearance of protein aggresome and recovery of proteostasis are required. We revealed that the ability to recruit functional DnaK-ClpB machineries, which facilitate protein disaggregation in an ATP-dependent manner, determines the lag time for bacterial regrowth. Better understanding of the key factors regulating bacterial regrowth after surviving antibiotic attack could lead to new therapeutic strategies for combating bacterial antibiotic tolerance.

Antimicrobial Photodynamic Inactivation Mediated by Tetracyclines in Vitro and in Vivo: Photochemical Mechanisms and Potentiation by Potassium Iodide

Tetracyclines (including demeclocycline, DMCT, or doxycycline, DOTC) represent a class of dual-action antibacterial compounds, which can act as antibiotics in the dark, and also as photosensitizers under illumination with blue or UVA light. It is known that tetracyclines are taken up inside bacterial cells where they bind to ribosomes. In the present study, we investigated the photochemical mechanism: Type 1 (hydroxyl radicals); Type 2 (singlet oxygen); or Type 3 (oxygen independent). Moreover, we asked whether addition of potassium iodide (KI) could potentiate the aPDI activity of tetracyclines. High concentrations of KI (200–400 mM) strongly potentiated (up to 5 logs of extra killing) light-mediated killing of Gram-negative Escherichia coli or Gram-positive MRSA (although the latter was somewhat less susceptible). KI potentiation was still apparent after a washing step showing that the iodide could penetrate the E. coli cells where the tetracycline had bound. When cells were added to the tetracycline + KI mixture after light, killing was observed in the case of E. coli showing formation of free molecular iodine. Addition of azide quenched the formation of iodine but not hydrogen peroxide. DMCT but not DOTC iodinated tyrosine. Both E. coli and MRSA could be killed by tetracyclines plus light in the absence of oxygen and this killing was not quenched by azide. A mouse model of a superficial wound infection caused by bioluminescent E. coli could be treated by topical application of DMCT and blue light and bacterial regrowth did not occur owing to the continued anti biotic activity of the tetracycline.

E. coli – MS2 bacteriophage interactions during solar disinfection of wastewater and the subsequent post-irradiation period

In this study, the effect of MS2 bacteriophage on the inactivation of its Escherichia coli host was evaluated in an effort to recreate some the main factors governing solar disinfection of wastewater (namely microbial growth, infection, photonic flux). As such, the kinetics of sole MS2 bacteriophage or Escherichia coli and their simultaneous inactivation were investigated in synthetic secondary effluent wastewater and accurately modeled by log-linear and shoulder-log linear models, respectively. The observed inactivation kinetics revealed a minor influence between the microorganisms during the solar treatment phase. However, the simultaneous presence of both strains affected bacterial regrowth during the post-irradiation period. Specifically, our results show minor MS2 increase (1-log) in presence of E. coli, but bacterial survival restriction in presence of the bacteriophage (24- vs. 48-h decay). Experiments conducted at three different temperatures (4, 20 and 37 °C) and two discharge matrices (lake water, seawater) indicate that both healthy cells and non-cultivable bacteria influence population dynamics, with (re)growth under mesophilic temperatures and support of the viral life cycle. Noteworthy cases were observed in lake water (high regrowth levels) and in seawater (no growth). In overall, this study contributes to the deeper understanding of population dynamics (i.e. coliphage-bacteria interactions). In extension, it unveils the capacity of solar exposure as disinfection method and indicates possible health risks associated with enteric viral and bacterial-related diseases due to incomplete solar wastewater disinfection or post-irradiation mishandling.

Inhibition of regrowth of planktonic and biofilm bacteria after peracetic acid disinfection

Peracetic acid (PAA) is a promising alternative to chlorine for disinfection; however, bacterial regrowth after PAA disinfection is poorly understood. This study compared the regrowth of bacteria (Gram-negative Pseudomonas aeruginosa PAO1 and Gram-positive Bacillus sp.) after disinfection with PAA or free chlorine. In the absence of organic matter, PAA and free chlorine prevented the regrowth of planktonic cells of P. aeruginosa PAO1 at C·t (= disinfectant concentration × contact time) doses of (28.5 ± 9.8) mg PAA·min·L−1 and (22.5 ± 10.6) mg Cl2·min·L−1, respectively, suggesting that they had comparable efficiencies in preventing the regrowth of planktonic bacteria. For comparison, the minimum C·t doses of PAA and free chlorine to prevent the regrowth of P. aeruginosa PAO1 biofilm cells in the absence of organic matter were (14,000 ± 1,732) mg PAA·min·L−1 and (6,500 ± 2,291) mg Cl2·min·L−1, respectively. PAA was less effective than free chlorine in killing bacteria within biofilms in the absence of organic matter most likely because PAA reacts with biofilm matrix constituents slower than free chlorine. In the presence of organic matter, although the bactericidal efficiencies of both disinfectants significantly decreased, PAA was less affected due to its slower reaction with organic matter and/or slower self-decomposition. For instance, in a dilute Lysogeny broth-Miller, the minimum concentrations of PAA and free chlorine to prevent the regrowth of planktonic P. aeruginosa PAO1 were 20 mg PAA·L−1 and 300 mg Cl2·L−1, respectively. While both disinfectants are strong oxidants disrupting cell membrane, environmental scanning electron microscopy (ESEM) revealed that PAA made holes in the center of the cells, whereas free chlorine desiccated the cells. Overall, this study shows that PAA is a powerful disinfectant to prevent bacterial regrowth even in the presence of organic matter.

Reduced Chlorine in Drinking Water Distribution Systems Impacts Bacterial Biodiversity in Biofilms.

In drinking water distribution systems (DWDS), a disinfectant residual is usually applied to limit bacterial regrowth. However, delivering water with no or reduced chlorine residual could potentially decrease the selection for antimicrobial resistant microorganisms, favor bacterial regrowth and result in changes in bacterial populations. To evaluate the feasibility of water reduction in local DWDS while ensuring water safety, water quality was measured over 2 months in two different networks, each of them harboring sub-areas with normal and reduced chlorine. Water quality remained good in chlorine reduced samples, with limited development of total flora and absence of coliforms. Furthermore, 16S rRNA amplicon-based metagenomics was used to investigate the diversity and the composition of microbial communities in the sub-networks. Taxonomic classification of sequence reads showed a reduced bacterial diversity in sampling points with higher chlorine residuals. Chlorine disinfection created more homogeneous bacterial population, dominated by Pseudomonas, a genus that contains some major opportunistic pathogens such as P. aeruginosa. In the absence of chlorine, a larger and unknown biodiversity was unveiled, also highlighted by a decreased rate of taxonomic classification to the genus and species level. Overall, this experiment in a functional DWDS will facilitate the move toward potable water delivery systems without residual disinfectants and will improve water taste for consumers.