Highly chlorinatedEscherichia colicannot be stained by propidium iodide

Label-Free Flow Cytometry: A Powerful Tool to Rapidly and Accurately Assess the Efficacy of Chemical Disinfectants

A rapid and accurate evaluation of a chemical disinfectant's bactericidal efficacy is crucial for ensuring effective infection control, preventing the spread of pathogens, and supporting the development of new disinfectant formulations. In this study, we report a rapid, label-free flow cytometry (FCM) protocol for evaluating the bactericidal efficacy of disinfectants. Five commercial disinfectants (alcohols, oxidizing agents, and alkylating agents) were evaluated against type strains recommended by EN 13727+A2 and ten clinical strains. The label-free FCM method allowed the determination of disinfectant efficacy through assessment of scatter light profiles (FSC-H/SSC-H) and count differences. The label-free FCM provided the results in approximately 4 h and showed strong correlation with standard tests (91.4%, sensitivity 0.94 and specificity 0.98) that can take up to 48 h. Our results represent a proof-of-principle that label-free FCM can reliably assess the efficacy of chemical disinfectants, the same day, and substantially faster than the current growth-based methods. Additionally, the study highlights the potential of the FCM method for detecting the occurrence of viable but non-culturable bacteria following treatment with chlorine-based disinfectants. With its speed, accuracy, and capability to identify bacterial injuries at a single-cell level, the FCM method is a powerful tool for assessing the efficacy of new disinfectant formulations.

A Label-Free Optical Flow Cytometry Based-Method for Rapid Assay of Disinfectants' Bactericidal Activity

Selecting the appropriate disinfectant to control and prevent healthcare-associated infections (HAIs) is a challenging task for environmental health experts due to the large number of available disinfectant products. This study aimed to develop a label-free flow cytometry (FCM) method for the rapid evaluation of bactericidal activity and to compare its efficacy with that of standard qualitative/quantitative suspension tests. The bactericidal efficiency of eight commercial disinfectants containing quaternary ammonium compounds (QACs) was evaluated against four strains recommended by EN 13727 (Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Enterococcus hirae) and four multidrug-resistant pathogens. The proposed FCM protocol measures changes in scattered light and counts following disinfectant exposure, neutralization, and culture steps. Unlike other available FCM-based methods, this approach does not rely on autofluorescence measurements, impedance cytometry, or fluorescent dyes. The FCM scattered light signals revealed both decreased count rates and morphological changes after treatment with minimum inhibitory concentrations (MICs) and higher concentrations for all tested bacteria. The results from the FCM measurements showed excellent correlation with those from standard assays, providing a rapid tool for monitoring the susceptibility profile of clinical, multidrug-resistant pathogens to chemical disinfectants, which could support infection prevention and control procedures for healthcare environments. This label-free FCM protocol offers a novel and rapid tool for environmental health experts, aiding in the optimization of disinfectant selection for the prevention and control of HAIs.

Kinetics and mechanisms of bacteria disinfection by performic acid in wastewater: In comparison with peracetic acid and sodium hypochlorite

Performic acid (PFA) has been increasingly used in wastewater disinfection due to its strong oxidizing ability and few disinfection byproducts. However, its disinfection pathways and mechanisms towards pathogenic bacteria disinfection are poorly understood. In this study, E. coli, S. aureus, and B. subtilis were inactivated using sodium hypochlorite (NaClO), PFA, and peracetic acid (PAA) in simulated turbid water and municipal secondary effluent. Cell culture-based plate counting showed that E. coli and S. aureus were extremely susceptible to NaClO and PFA and achieved a 4-log inactivation at CTs ≤ 1 mg/L·min with an initial disinfectant concentration of 0.3 mg/L. B. subtilis was much more resistant. At the initial disinfectant dose of 7.5 mg/L, PFA required CTs of 3-13 mg/L·min to achieve a 4-log inactivation. Turbidity negatively affected the disinfection. In the secondary effluent, the CTs required for PFA to achieve a 4-log inactivation of E. coli and B. subtilis were 6-12 times higher than those required in simulated turbid water, and a 4-log inactivation of S. aureus could not be achieved. PAA showed a much weaker disinfection ability than the other two disinfectants. The reaction pathways of E. coli inactivation by PFA included both direct and indirect reactions, in which the PFA molecule accounted for 73 %, and ·OH and peroxide radicals accounted for 20 % and 6 %, respectively. During PFA disinfection, E. coli cells were severely disintegrated, while the S. aureus cell exteriors remained mostly intact. B. subtilis was the least affected. Compared with cell culture-based analysis, the inactivation detected by flow cytometry was significantly lower. Viable but non-culturable bacteria after disinfection were believed to be primarily responsible for this inconsistency. This study suggested that PFA was able to control regular bacteria in wastewater, but it should be used with caution when treating recalcitrant pathogens.

Combining microscopy assays of bacteria-surface interactions to better evaluate antimicrobial polymer coatings

Validation of the antimicrobial performance of contact-killing polymer surfaces through experimental determination of bacterial adhesion or viability is essential for their targeted development and application. However, there is not yet a consensus on a single most appropriate evaluation method or procedure. Combining and benchmarking previously reported assays could reduce the significant variation and misinterpretation of efficacy data obtained from different methods. In this work, we systematically investigated the response of bacteria cells to anti-adhesive and antiseptic polymer coatings by combining (i) bulk solution-based, (ii) thin-film spacer-based and (iii) direct contact assays. In addition, we evaluated the studied assays using a five-point scoring framework that highlights key areas for improvement. Our data suggest that combined microscopy assays provide a more comprehensive representation of antimicrobial performance, thereby helping to identify effective types of antibacterial polymer coatings. Importance We present and evaluate a combination of methods for validating the efficacy of antimicrobial surfaces. Antimicrobial surfaces/coatings based on contact-killing components can be instrumental to functionalise a wide range of products. However, there is not yet a consensus on a single, most appropriate method to evaluate their performance. By combining three microscopy methods, we were able to discern contact killing effects at the single cell level that were not detectable by conventional bulk microbiological analyses. The developed approach is considered advantageous for the future targeted development of robust and sustainable antimicrobial surfaces.

Evaluation by Flow Cytometry of Escherichia coli Viability in Lettuce after Disinfection

Foodborne outbreaks due to the consumption of ready-to-eat vegetables have increased worldwide, with Escherichia coli (E. coli) being one of the main sources responsible. Viable but nonculturable bacteria (VBNC) retain virulence even after some disinfection procedures and constitute a huge problem to public health due to their non-detectability through conventional microbiological techniques. Flow cytometry (FCM) is a promising tool in food microbiology as it enables the distinction of the different physiological states of bacteria after disinfection procedures within a short time. In this study, samples of lettuce inoculated with E. coli were subject to disinfection with sodium hypochlorite at free chlorine concentrations of 5, 10, 25, 50, and 100 mg·L⁻¹ or with 35% peracetic acid at concentrations of 5, 10, 25, and 50 mg·L⁻¹. The efficiency of these disinfectants on the viability of E. coli in lettuce was evaluated by flow cytometry with LIVE/DEAD stains. Results from this study suggest that FCM can effectively monitor cell viability. However, peracetic acid is more effective than sodium hypochlorite as, at half the concentration, it is enough to kill 100% of bacteria and always induces a lower percentage of VBNC. Finally, we can conclude that the recommended levels of chemical disinfectants for fresh fruit and vegetables are adequate when applied in lettuce. More importantly, it is possible to ensure that all cells of E. coli are dead and that there are no VBNC cells even with lower concentrations of those chemicals. These results can serve as guidance for lettuce disinfection, improving quality and the safety of consumption.

Flow cytometry applications in water treatment, distribution, and reuse: A review

Ensuring safe and effective water treatment, distribution, and reuse requires robust methods for characterizing and monitoring waterborne microbes. Methods widely used today can be limited by low sensitivity, high labor and time requirements, susceptibility to interference from inhibitory compounds, and difficulties in distinguishing between viable and non-viable cells. Flow cytometry (FCM) has recently gained attention as an alternative approach that can overcome many of these challenges. This article critically and systematically reviews for the first time recent literature on applications of FCM in water treatment, distribution, and reuse. In the review, we identify and examine nearly 300 studies published from 2000 to 2018 that illustrate the benefits and challenges of using FCM for assessing source-water quality and impacts of treatment-plant discharge on receiving waters, wastewater treatment, drinking water treatment, and drinking water distribution. We then discuss options for combining FCM with other indicators of water quality and address several topics that cut across nearly all applications reviewed. Finally, we identify priority areas in which more work is needed to realize the full potential of this approach. These include optimizing protocols for FCM-based analysis of waterborne viruses, optimizing protocols for specifically detecting target pathogens, automating sample handling and preparation to enable real-time FCM, developing computational tools to assist data analysis, and improving standards for instrumentation, methods, and reporting requirements. We conclude that while more work is needed to realize the full potential of FCM in water treatment, distribution, and reuse, substantial progress has been made over the past two decades. There is now a sufficiently large body of research documenting successful applications of FCM that the approach could reasonably and realistically see widespread adoption as a routine method for water quality assessment.

Assessment of flow cytometry for microbial water quality monitoring in cooling tower water and oxidizing biocide treatment efficiency

The control of Legionella proliferation in cooling tower water circuits requires regular monitoring of water contamination and effective disinfection procedures. In this study, flow cytometry was assessed to monitor water contamination and disinfection treatment efficiency on bacterial cells regarding nucleic acid injury (SYBR® Green II), cell integrity (SYBR® Green II and propidium iodide) and metabolism activity (ChemChrome V6). A total of 27 cooling tower water samples were analyzed in order to assess water contamination levels regarding viable populations: standard culture, ATP measurement and flow cytometry methods were compared. Flow cytometry and plate counts methods showed a significant correlation for changes in concentrations despite a 1 to 2-log difference regarding absolute quantification. Concerning intracellular activity, the use of two different flow cytometers (FACSCanto™ II and Accuri™ C6) showed no statistical difference while a difference was observed between flow cytometry and usual methods (culture and ATP measurement). The standard culture and flow cytometry methods were also compared for in vitro bacteria inactivation measurements in the presence of 3 different types of oxidizing biocides commonly used for cooling tower disinfection. Reductions observed ranged between 1 and 2 log depending on (1) the detection method, (2) the bacterial population origin and/or (3) the active biocide molecule used. In conclusion, flow cytometry represents an efficient, accurate and fast approach to monitor water contamination and biocide treatment efficiency in cooling towers.

Comparison between Flow Cytometry and Traditional Culture Methods for Efficacy Assessment of Six Disinfectant Agents against Nosocomial Bacterial Species

The present study was undertaken to compare the use of flow cytometry (FCM) and traditional culture methods for efficacy assessment of six disinfectants used in Quebec hospitals including: two quaternary ammonium-based, two activated hydrogen peroxide-based, one phenol-based, and one sodium hypochlorite-based. Four nosocomial bacterial species, Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Vancomycin-resistant Enterococci faecalis, were exposed to minimum lethal concentrations (MLCs) and sublethal concentrations (1/2 MLCs) of disinfectants under study. The results showed a strong correlation between the two techniques for the presence of dead and live cell populations, as well as, evidence of injured populations with the FCM. The only exception was observed with sodium hypochlorite at higher concentrations where fluorescence was diminished and underestimating dead cell population. The results also showed that FCM can replace traditional microbiological methods to study disinfectant efficacy on bacteria. Furthermore, FCM profiles for E. coli and E. faecalis cells exposed to sublethal concentrations exhibited distinct populations of injured cells, opening a new aspect for future research and investigation to elucidate the role of injured, cultural/noncuturable/resuscitable cell populations in infection control.

Microbial cell budgets of an Arctic glacier surface quantified using flow cytometry

Uncertainty surrounds estimates of microbial cell and organic detritus fluxes from glacier surfaces. Here, we present the first enumeration of biological particles draining from a supraglacial catchment, on Midtre Lovénbreen (Svalbard) over 36 days. A stream cell flux of 1.08 × 10(7)  cells m(-2)  h(-1) was found, with strong inverse, non-linear associations between water discharge and biological particle concentrations. Over the study period, a significant decrease in cell-like particles exhibiting 530 nm autofluorescence was noted. The observed total fluvial export of ~7.5 × 10(14) cells equates to 15.1-72.7 g C, and a large proportion of these cells were small (< 0.5 μm in diameter). Differences between the observed fluvial export and inputs from ice-melt and aeolian deposition were marked: results indicate an apparent storage rate of 8.83 × 10(7)  cells m(-2)  h(-1). Analysis of surface ice cores revealed cell concentrations comparable to previous studies (6 × 10(4)  cells ml(-1)) but, critically, showed no variation with depth in the uppermost 1 m. The physical retention and growth of particulates at glacier surfaces has two implications: to contribute to ice mass thinning through feedbacks altering surface albedo, and to potentially seed recently deglaciated terrain with cells, genes and labile organic matter. This highlights the merit of further study into glacier surface hydraulics and biological processes.

Escherichia coli, enterococci, and Bacteroides thetaiotaomicron qPCR signals through wastewater and septage treatment

Fecal indicators such as Escherichia coli and enterococci are used as regulatory tools to monitor water with 24 h cultivation techniques for possible input of sewage or feces and presence of potential enteric pathogens yet their source (human or animal) cannot be determined with routine methods. This critical uncertainty has furthered water pollution science toward new molecular approaches. Members of Bacteroides genus, such as Bacteroides thetaiotaomicron are found to have features that allow their use as alternative fecal indicators and for Microbial Source Tracking (MST). The overall aim of this study was to evaluate the concentration and fate of B. thetaiotaomicron, throughout a wastewater treatment facility and septage treatment facility. A large number of samples were collected and tested for E. coli and enterococci by both cultivation and qPCR assays. B. thetaiotaomicron qPCR equivalent cells (mean: 1.8 × 10(7)/100 mL) were present in significantly higher concentrations than E. coli or enterococci in raw sewage and at the same levels in raw septage. The removal of B. thetaiotaomicron target qPCR signals was similar to E. coli and enterococci DNA during the treatment of these wastes and ranged from 3 to 5 log(10) for wastewater and was 7 log(10) for the septage. A significant correlation was found between B. thetaiotaomicron marker and each of the conventional indicators throughout the waste treatment process for both raw sewage and septage. A greater variability was found with enterococci when compared to E. coli, and CFU and equivalent cells could be contrasted by various treatment processes to examine removal and inactivation via septage and wastewater treatment. These results are compared and contrasted with other qPCR studies and other targets in wastewater samples providing a view of DNA targets in such environments.

Agreement, precision, and accuracy of epifluorescence microscopy methods for enumeration of total bacterial numbers

To assess interchangeability of estimates of bacterial abundance by different epifluorescence microscopy methods, total bacterial numbers (TBNs) determined by most widely accepted protocols were statistically compared. Bacteria in a set of distinctive samples were stained with acridine orange (AO), 4'-6-diamidino-2-phenylindole (DAPI), and BacLight and enumerated by visual counting (VC) and supervised image analysis (IA). Model II regression and Bland-Altman analysis proved general agreements between IA and VC methods, although IA counts tended to be lower than VC counts by 7% on a logarithmic scale. Distributions of cells and latex beads on polycarbonate filters were best fitted to negative binomial models rather than to Poisson or log-normal models. The fitted models revealed higher precisions of TBNs by the IA method than those by the VC method. In pairwise comparisons of the staining methods, TBNs by AO and BacLight staining showed good agreement with each other, but DAPI staining had tendencies of underestimation. Although precisions of the three staining methods were comparable to one another (intraclass correlation coefficients, 0.97 to 0.98), accuracy of the DAPI staining method was rebutted by disproportionateness of TBNs between pairs of samples that carried 2-fold different volumes of identical cell suspensions. It was concluded that the TBN values estimated by AO and BacLight staining are relatively accurate and interchangeable for quantitative interpretation and that IA provides better precision than does VC. As a prudent measure, it is suggested to avoid use of DAPI staining for comparative studies investigating accuracy of novel cell-counting methods.

Assessment of Damage to Nucleic Acids and Repair Machinery in Salmonella typhimurium Exposed to Chlorine

Water disinfection is usually evaluated using mandatory methods based on cell culturability. However, such methods do not consider the potential of cells to recover, which should also be kept as low as possible. In this paper, we hypothesized that a successful disinfection is achieved only when the applied chlorine leads to both intracellular nucleic acid damage and strong alterations of the DNA repair machinery. Monitoring the SOS system responsiveness with a umuC'-'lacZ reporter fusion, we found that the expression of this important cellular machinery was altered after the beginning of membrane permeabilization but prior to the total decline of both the cell culturability and the nucleic acid integrity as revealed by Sybr-II staining. Rapid measurement of such nucleic acid alterations by fluorochrome-based staining could be used as an alternative method for assessing the effectiveness of disinfection with chlorine.

Viability PCR, a culture-independent method for rapid and selective quantification of viable Legionella pneumophila cells in environmental water samples

PCR-based methods have been developed to rapidly screen for Legionella pneumophila in water as an alternative to time-consuming culture techniques. However, these methods fail to discriminate between live and dead bacteria. Here, we report a viability assay (viability PCR [v-PCR]) for L. pneumophila that combines ethidium monoazide bromide with quantitative real-time PCR (qPCR). The ability of v-PCR to differentiate viable from nonviable L. pneumophila cells was confirmed with permeabilizing agents, toluene, or isopropanol. v-PCR suppressed more than 99.9% of the L. pneumophila PCR signal in nonviable cultures and was able to discriminate viable cells in mixed samples. A wide range of physiological states, from culturable to dead cells, was observed with 64 domestic hot-water samples after simultaneous quantification of L. pneumophila cells by v-PCR, conventional qPCR, and culture methods. v-PCR counts were equal to or higher than those obtained by culture and lower than or equal to conventional qPCR counts. v-PCR was used to successfully monitor in vitro the disinfection efficacy of heating to 70 degrees C and glutaraldehyde and chlorine curative treatments. The v-PCR method appears to be a promising and rapid technique for enumerating L. pneumophila bacteria in water and, in comparison with conventional qPCR techniques used to monitor Legionella, has the advantage of selectively amplifying only viable cells.

Single-cell vs. bulk activity properties of coastal bacterioplankton over an annual cycle in a temperate ecosystem

The connections between single-cell activity properties of heterotrophic planktonic bacteria and whole community metabolism are still poorly understood. Here, we show flow cytometry single-cell analysis of membrane-intact (live), high nucleic acid (HNA) content and actively respiring (CTC+) bacteria with samples collected monthly during 2006 in northern Spain coastal waters. Bulk activity was assessed by measuring 3H-Leucine incorporation and specific growth rates. Consistently, different single-cell relative abundances were found, with 60-100% for live, 30-84% for HNA and 0.2-12% for CTC+ cells. Leucine incorporation rates (2-153 pmol L(-1) h(-1)), specific growth rates (0.01-0.29 day(-1)) and the total and relative abundances of the three single-cell groups showed marked seasonal patterns. Distinct depth distributions during summer stratification and different relations with temperature, chlorophyll and bacterial biovolume suggest the existence of different controlling factors on each single-cell property. Pooled leucine incorporation rates were similarly correlated with the abundance of all physiological groups, while specific growth rates were only substantially explained by the percentage of CTC+ cells. However, the ability to reduce CTC proved notably better than the other two single-cell properties at predicting bacterial bulk rates within seasons, suggesting a tight linkage between bacterial individual respiration and biomass production at the community level.