Viable but non-culturable E. coli induced by low level chlorination have higher persistence to antibiotics than their culturable counterparts

Regrowth of bacteria after light-based disinfection - What we know and where we go from here

Regrowth of bacteria after water/wastewater disinfection is a serious risk to public health, particularly when such pathogens carry antibiotic resistance genes. Despite increasing interest in light-based disinfection using ultraviolet or solar radiation, the mechanism of bacterial regrowth and their concentration upon light exposure (i.e., during storage, or after discharge into rivers or lakes) remain poorly understood. Therefore, we present a focused critical review to 1) elucidate regrowth mechanisms, 2) summarize the pros and cons of available experimental designs and detection techniques for regrowth evaluation, and 3) provide an outlook of key research directions for further investigations of post-disinfection bacterial regrowth. Bacterial regrowth can occur through reactivation from a viable but non-culturable state, repair of photo-induced DNA damage, and reproduction of bacteria surviving disinfection. Many studies have underestimated the degree of actual regrowth because of the use of simple experimental designs and plate count methods, which cannot quantify actual abundance of viable bacteria. Further research should investigate the effects of various factors on bacterial regrowth in realistic conditions in regrowth tests and adopt multiplex detection methods that combine culture-based and culture-independent approaches. An accurate understanding of the mechanisms involved in bacterial regrowth following disinfection is critical for safeguarding public health and aquatic environments.

Detection and distribution of vbnc/viable pathogenic bacteria in full-scale drinking water treatment plants

Viable but non-culturable (VBNC) bacteria have attracted widespread attention since they are inherently undetected by traditional culture-dependent methods. Importantly, VBNC bacteria could resuscitate under favorable conditions leading to significant public health concerns. Although the total number of viable bacteria has been theorized to be far greater than those that can be cultured, there have been no reports quantifying VBNC pathogenic bacteria in full-scale drinking water treatment plants (DWTPs). In this work, we used both culture-dependent and quantitative PCR combination with propidium monoazide (PMA) dye approaches to characterize cellular viability. Further, we established a method to quantify viable pathogens by relating specific gene copies to viable cell numbers. Ratios of culturable bacteria to viable 16S rRNA gene copies in water and biological activated carbon (BAC) biofilms were 0-4.75% and 0.04-56.24%, respectively. The VBNC E. coli, E. faecalis, P. aeruginosa, Salmonella sp., and Shigella sp. were detected at levels of 0-10³ cells/100 mL in source water, 0-10² cells/100 mL in chlorinated water, and 0-10³ cells/g in BAC biofilms. In addition, differences between the total and viable community structures after ozonation and chlorination were investigated. The relative abundance of opportunistic pathogens such as Mycobacterium, Sphingomonas, etc. increased in final water, likely due to their chlorine resistance. In summary, we detected significant quantities of viable/VBNC opportunistic pathogens in full-scale DWTPs, confirming that traditional, culture-dependent methods are inadequate for detecting VBNC bacteria. These findings suggest a need to develop and implement rapid, accurate methods for the detection of VBNC pathogenic bacteria in DWTPs to ensure the safety of drinking water.

Tolerance of Listeria monocytogenes to biocides used in food processing environments

Listeria monocytogenes is a foodborne pathogen that causes a life-threatening disease in humans known as listeriosis. Contamination of food during processing is the main route of transmission of Listeria monocytogenes. Therefore, biocides play a crucial role in food processing environments as they act as the first line of defense in the prevention and control of L. monocytogenes. Residues of biocides may be present at sublethal concentrations after disinfection. This, unfortunately, subjects L. monocytogenes to selection pressure, giving rise to tolerant strains, which pose a threat to food safety and public health. This review will give a brief description of L. monocytogenes, the clinical manifestation, treatment of listeriosis as well as recently recorded outbreaks. The article will then discuss the current literature on the ability of L. monocytogenes strains to tolerate biocides especially quaternary ammonium compounds as well as the mechanisms of tolerance towards biocides including the activation of efflux pump systems.

Effect of ferrate and monochloramine disinfection on the physiological and transcriptomic response of Escherichia coli at late stationary phase

Biological mechanisms of disinfection not only vary by disinfectant but also remain not well understood. We investigated the physiological and transcriptomic response of Escherichia coli at late stationary phase to ferrate and monochloramine in amended lake water. Although ferrate and monochloramine treatments similarly reduced culturable cell concentrations by 3-log₁₀, 64% and 11% of treated cells were viable following monochloramine and ferrate treatment, respectively. This observed induction of viable but non-culturable (VBNC) state following monochloramine treatment but not ferrate is attributed to slower monochloramine disinfection kinetics (by 2.8 times) compared to ferrate. Transcriptomic analysis of E. coli at 15 min of exposure revealed that 3 times as many genes related to translation and transcription were downregulated by monochloramine compared to ferrate, suggesting that monochloramine treatment may be inducing VBNC through reduced protein synthesis and metabolism. Downregulation of universal stress response genes (rpoS, uspA) was attributed to growth-related physiological stressors during late stationary phase which may have contributed to the elevated expression levels of general stress responses pre-disinfection and, subsequently, their significant downregulation post-disinfection. Both disinfectants upregulated oxidative stress response genes (trxC, grxA, soxS), although levels of upregulation were time sensitive. This work shows that bacterial inactivation responses to disinfectants is mediated by complex molecular and growth-related responses.

How to Evaluate Non-Growing Cells-Current Strategies for Determining Antimicrobial Resistance of VBNC Bacteria

Thanks to the achievements in sanitation, hygiene practices, and antibiotics, we have considerably improved in our ongoing battle against pathogenic bacteria. However, with our increasing knowledge about the complex bacterial lifestyles and cycles and their plethora of defense mechanisms, it is clear that the fight is far from over. One of these resistance mechanisms that has received increasing attention is the ability to enter a dormancy state termed viable but non-culturable (VBNC). Bacteria that enter the VBNC state, either through unfavorable environmental conditions or through potentially lethal stress, lose their ability to grow on standard enrichment media, but show a drastically increased tolerance against antimicrobials including antibiotics. The inability to utilize traditional culture-based methods represents a considerable experimental hurdle to investigate their increased antimicrobial resistance and impedes the development and evaluation of effective treatments or interventions against bacteria in the VBNC state. Although experimental approaches were developed to detect and quantify VBNCs, only a few have been utilized for antimicrobial resistance screening and this review aims to provide an overview of possible methodological approaches.

The diagnostic tools for viable but nonculturable pathogens in the food industry: Current status and future prospects

Viable but nonculturable (VBNC) microorganisms have been recognized as pathogenic contaminants in foods and environments. The failure of VBNC cells to form the visible colonies hinders the ability to use conventional media for their detection. Efficient and rapid detection of pathogens in the VBNC state is a prerequisite to ensure the food safety and public health. Despite their nonculturability, VBNC cells have distinct characteristics, such as morphology, metabolism, chemical composition, and gene and protein expression, that have been used as the basis for the development of abundant diagnostic tools. This review covers the current status and advances in various approaches for examining microorganisms in the VBNC state, including but not limited to the methodological aspects, advantages, and drawbacks of each technique. Existing methods, such as direct viable count, SYTO/PI dual staining, and propidium monoazide quantitative polymerase chain reaction (PCR), as well as some techniques with potential to be applied in the future, such as digital PCR, enhanced-surface Raman spectroscopy, and impedance-based techniques, are summarized in depth. Finally, future prospects for the one-step detection of VBNC bacteria are proposed and discussed. We believe that this review can provide more optional methods for researchers and promote the development of rapid, accurate detecting methods, and for inspectors, the diagnostic tools can provide data to undertake risk analysis of VBNC cells.

Stress Resistance and Pathogenicity of Nonthermal-Plasma-Induced Viable-but-Nonculturable Staphylococcus aureus through Energy Suppression, Oxidative Stress Defense, and Immune-Escape Mechanisms

The occurrence of viable-but-nonculturable (VBNC) bacteria poses a potential risk to food safety due to failure in conventional colony detection. In this study, induction of VBNC Staphylococcus aureus was conducted by exposure to an atmospheric-pressure air dielectric barrier discharge-nonthermal-plasma (DBD-NTP) treatment with an applied energy of 8.1 kJ. The stress resistance profiles and pathogenicity of VBNC S. aureus were further evaluated. We found that VBNC S. aureus showed levels of tolerance of heat, acid, and osmosis challenges comparable to those shown by culturable S. aureus, while VBNC S. aureus exhibited enhanced resistance to oxidative and antibiotic stress, relating to the mechanisms of cellular energy depletion, antioxidant response initiation, and multidrug efflux pump upregulation. Regarding pathogenicity, NTP-induced VBNC S. aureus retained the capacity to infect the HeLa host cells. Compared with the culturable counterparts, VBNC S. aureus caused reduced immune responses (Toll-like receptor [TLR], nucleotide-binding oligomerization domain [NOD]) in HeLa cells, which was attributed to suppression of biosynthesis of the recognized surface ligands (e.g., peptidoglycan). Additionally, the proteomic analysis revealed that upregulation of several virulence factors (ClfB, SdrD, SCIN, SasH, etc.) could ensure that VBNC S. aureus would adhere to and internalize into host cells and avoid the host attack. The camouflaged mechanisms described above led to VBNC S. aureus causing less damage to the host cells, and their activity might result in longer intracellular persistence, posing potential risks during NTP processing.IMPORTANCE The consumer demand for freshness and nutrition has accelerated the development of mild decontamination technologies. The incomplete killing of nonthermal (NT) treatments might induce pathogens to enter into a viable-but-nonculturable (VBNC) status as a survival strategy. The use of nonthermal plasma (NTP) as a novel food decontamination technology received increased attention in food industry during recent decades. Our previous work confirmed that the foodborne pathogen S. aureus was induced into VBNC status in response to NTP exposure. This work further revealed the development of stress resistance and virulence retention of NTP-induced VBNC S. aureus through the mechanisms of energy suppression, oxidative stress defense, and immune escape. The data provide fundamental knowledge of the potential risks posed by NTP-induced VBNC S. aureus, which require further parameter optimization of the NTP process or combination with other techniques to avoid the occurrence of VBNC bacteria.

Remediation of textile effluents for water reuse: Decolorization and desalination using Escherichia fergusonii followed by detoxification with activated charcoal

Textile effluents contain high levels of pollutants of different categories like dyes, metal salts, acids, bases and microorganisms. Remediation of textile effluents is often challenging because of its composition, which also varies between dyeing units. In this study, we demonstrate the novel use of a waste-water bacterium, Escherichia fergusonii, in the effective remediation of textile effluents. The bacteria application efficiently caused a reduction of color (98.4%), total dissolved solids (75%), sulphates (87%), bicarbonates (83%), chlorides (64%), calcium (84%), and chemical oxygen demand (81%) of the textile effluents. The bacteria-treated effluents were further disinfected and detoxified by treating with rice husk activated charcoal. After the charcoal treatment, the chemical oxygen demand decreased further by 11.5% and biochemical oxygen demand decreased by 85%. The effluents remediated using the two-step process were subjected to toxicity assays using zebrafish (Danio rerio) model. The textile effluents treated using Escherichia fergusonii, followed by activated charcoal were found to be non-toxic and suitable for reuse for domestic applications. Thus, we present here, a simple, less energy-intensive, economic, two-step process as a complete solution for textile effluent treatment. The results of this investigation can be used to simplify the remediation process of textile effluents in common treatment plants as well as the individual dyeing units.

Modelling the effect of chlorination/chloramination on induction of viable but non-culturable (VBNC) Escherichia coli

Many bacteria, including Escherichia coli, are known to enter into a viable but non-culturable (VBNC) state when exposed to harsh environmental stresses. The VBNC cells introduced by chlorination/chloramination have raised increasing concern about biological safety of drinking water. A quantitative relationship between chlorination/chloramination and number of VBNC cells has not been found. In this study, a mathematical model was developed to quantify the effect of chlorination/chloramination on induction of viable but non-culturable (VBNC) Escherichia coli. the model was generated based on a first order kinetics of chlorination/chloramination using the data collected from laboratory disinfection experiments. The disinfection rates of culturable cells (k_c ) and viable cells (k_v ) were dose-dependent, and they were also modelled in different initial concentrations by regression analysis to overcome the shortcoming of dose-dependent. In general, the k_c and k_v values for chlorination (k_c , 2.59-29.89 h^-1; k_v , 19.52-26.74 h^-1) was 2-58 times greater than that for chloramination (k_c , 0.5446-10.81 h^-1; k_v , 0.3398-14.57 h^-1), suggesting that chlorine was more effective than chloramine in reducing the number of culturable and VBNC cells at same dose of disinfectant. Ultimately, the generated models, which could describe the dynamics of VBNC cells formation in chlorination/chloramination, can provide practical guidance in drinking water treatment and it can also be applied to risk assessment of drinking water management systems.

Induction of Escherichia coli O157:H7 into a viable but non-culturable state by high temperature and its resuscitation

Escherichia coli O157:H7, a causative agent of haemolytic uremic syndrome, can enter into a viable but non-culturable (VBNC) state in response to harsh stress. Bacteria in this state can retain membrane integrity, metabolic activity and virulence expression, which may present health risks. However, virulence expression and resuscitation ability of the VBNC state are not well understood. Here, we induced E. coli O157:H7 into a VBNC state by high temperature, which is commonly used to prevent the proliferation of pathogens in process of soil solarization, composting and anaerobic digestion of organic wastes. The virulence genes were highly expressed in the VBNC state and resuscitated daughter cells. The resuscitation of VBNC cells occurred after the removal of heat stress in Luria-Bertani medium. In addition, E. coli O157: H7 cells can leave the VBNC state and resuscitate with the clearance of protein aggregates. Notably, with the accumulation of protein aggregation and increased levels of reactive oxygen species, cells lost their ability to resuscitate. The results of this study not only can facilitate a better understanding of the health risks associated with the VBNC state but also have the potential to provide a theoretical basis for thermal disinfection processing.

Microbial contamination in distributed drinking water purifiers induced by water stagnation

Small-scale distributed water purifiers (SSDWPs), providing better quality drinking water, are popularly used both in homes and in the public domain. Non-continuous operation leads to water stagnation and ultimately induces microbial contamination. However, information related to such contamination in these purifiers is reported scarcely. In the present study, an SSDWP, consisting of sand filtration (SF), granular activated carbon (GAC), and ultrafiltration (UF) processes, was established to explore microbial changes induced by water stagnation, based on the aspects of bacterial count, microbial size, microbiome and pathogenic communities. Our results primary showed that: first, compared with drinking water distribution system (DWDS), bacterial counts increased more rapidly in SSDWPs, growing to > 500 cfu/mL after 2.5 h stagnation. The proportion of intact cells also increased with stagnation time. Conversely, microbial size decreased with stagnation time according to changes in forward scatter detected using flow cytometry. Second, microbiome evolution followed the isolated island model, while in stagnated DWDS, microbiome evolved according to the continent island model, and the former had higher abundance of biodiversity. Furthermore, stagnation evidently caused microbiome changes in each unit, and spatial differences contributed to microbiome dissimilarity more significantly than temporal differences. Third, Mycobacterium was the dominant pathogenic genus in the SF and GAC units while Acinetobacter was the most abundant in the UF unit. Pathogenic risks increased with water stagnation time and lower nutrients level contributed to pathogenic community richness. Therefore, terminal disinfection of SSDWPs is strongly advised.

Resuscitation, isolation and immobilization of bacterial species for efficient textile wastewater treatment: A critical review and update

Given highly complex and recalcitrant nature of synthetic dyes, textile wastewater poses a serious challenge on surrounding environments. Until now, biological treatment of textile wastewater using efficient bacterial species is still considered as an environmentally friendly and cost-effective approach. The advances in resuscitating viable but non-culturable (VBNC) bacteria via signaling compounds such as resuscitation-promoting factors (Rpfs) and quorum sensing (QS) autoinducers, provide a vast majority of potent microbial resources for biological wastewater treatment. So far, textile wastewater treatment from resuscitating and isolating VBNC state bacteria has not been critically reviewed. Thus, this review aims to provide a comprehensive picture of resuscitation, isolation and application of bacterial species with this new strategy, while the recent advances in synthetic dye decolorization were also elaborated together with the mechanisms involved. Discussion was further extended to immobilization methods to tackle its application. We concluded that the resuscitation of VBNC bacteria via signaling compounds, together with biochar-based immobilization technologies, may lead to an appealing biological treatment of textile wastewater. However, further development and optimization of the integrated process are still required for their wide applications.

Surviving Reactive Chlorine Stress: Responses of Gram-Negative Bacteria to Hypochlorous Acid

Sodium hypochlorite (NaOCl) and its active ingredient, hypochlorous acid (HOCl), are the most commonly used chlorine-based disinfectants. HOCl is a fast-acting and potent antimicrobial agent that interacts with several biomolecules, such as sulfur-containing amino acids, lipids, nucleic acids, and membrane components, causing severe cellular damage. It is also produced by the immune system as a first-line of defense against invading pathogens. In this review, we summarize the adaptive responses of Gram-negative bacteria to HOCl-induced stress and highlight the role of chaperone holdases (Hsp33, RidA, Cnox, and polyP) as an immediate response to HOCl stress. We also describe the three identified transcriptional regulators (HypT, RclR, and NemR) that specifically respond to HOCl. Besides the activation of chaperones and transcriptional regulators, the formation of biofilms has been described as an important adaptive response to several stressors, including HOCl. Although the knowledge on the molecular mechanisms involved in HOCl biofilm stimulation is limited, studies have shown that HOCl induces the formation of biofilms by causing conformational changes in membrane properties, overproducing the extracellular polymeric substance (EPS) matrix, and increasing the intracellular concentration of cyclic-di-GMP. In addition, acquisition and expression of antibiotic resistance genes, secretion of virulence factors and induction of the viable but nonculturable (VBNC) state has also been described as an adaptive response to HOCl. In general, the knowledge of how bacteria respond to HOCl stress has increased over time; however, the molecular mechanisms involved in this stress response is still in its infancy. A better understanding of these mechanisms could help understand host-pathogen interactions and target specific genes and molecules to control bacterial spread and colonization.

Coupling changes of disinfectant and bacteria induced by the water stagnation and disinfection strategy

This paper studied stagnation-induced changes of disinfectant and bacteria using an orthogonal test and kinetic analysis, and then proposed a disinfection strategy. Tap water from a drinking water distribution system and ultrafiltrated water were collected and disinfected with four disinfectants (concentrations were set 0.2-1 mg/L as Cl₂. The study had several findings. First, disinfectants expanded lag phases and shortened generation times of the microbiome. Reduction in culturability, substrate responsiveness, respiratory activity, membrane potential and integrity subsequently occurred with increasing disinfection concentration. Second, the disinfectant decay rate decreased with initial disinfection concentration, and the effective disinfection phase (heterotrophic plate count (HPC) was less than 100 cfu/mL) was longer in water samples with lower organic matter. Moreover, the disinfection process was divided into an effective phase and an invalid phase (HPC>100 cfu/mL). Then a disinfection efficiency model was built and the regulation of disinfection by-products (DBPs) production was studied in chlorinated water samples, which provides a general method for other disinfectant studies. The average trihalomethanes (THMs) production during the effective phase (marked as THM/t_h) and THMs production during the invalid phase (marked as ΔTHM) were proposed to evaluate the DBPs production. The level of THM/t_h and ΔTHM were lower in ultrafiltrated water than those in tap water. THM/t_h were negatively correlated with initial chlorine concentration while ΔTHM were positively correlated with initial chlorine concentration. Finally, for the purpose of raising disinfection efficiency and decreasing DBPs, we propose periodic pulse disinfection.

Characterization and potential mechanisms of highly antibiotic tolerant VBNC Escherichia coli induced by low level chlorination

Escherichia coli is an important pathogenic indicator in drinking water. Viable but non-culturable (VBNC) E. coli induced by low level chlorination was found to have higher antibiotic tolerance. The emerging of VBNC bacteria in drinking water systems is posing challenges to the control of bio-safety. It is necessary to study the underlying mechanisms of VBNC state E. coli under actual residual chlorine condition of drinking water pipe network. In this study, we investigated the changes of morphology and gene expressions that might present such state. The results indicated that the size of VBNC E. coli was not remarkably changed or recovered culturability under favorable environmental conditions. Results from transcriptomic analysis revealed that the regulated genes related to fimbrial-like adhesin protein, putative periplasmic pilin chaperone, regulators of the transcriptional regulation, antibiotic resistance genes and stress-induced genes, rendering VBNC cells more tolerant to adverse environmental conditions. In total of 16 genes were significantly up-regulated under the VBNC state, including three genes encoding toxic protein (ygeG, ibsD, shoB), indicating that VBNC E. coil was still a threat to human. The work is of great relevance in the context of better understanding this poorly understood physiological state.

Benzalkonium Chloride Induces a VBNC State in Listeria monocytogenes

The objective of our study was to investigate the effects of benzalkonium chloride (BC) adaptation of L. monocytogenes on the susceptibility to antimicrobial agents and on the viable but non culturable (VBNC) state of the bacterial cells. We adapted L. monocytogenes SLCC2540 to BC by applying BC below minimum inhibitory concentration (MIC) to above minimum bactericidal concentration (MBC). The culturable fractions and the susceptibility of adapted and parental cells to BC were assessed. In addition, cell membrane permeability and glucose uptake were analyzed by multi parametric flow cytometry using the fluorescent agents SYTO9, propidium iodide, and 2-deoxy-2-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]-D-glucose (2-NBDG). Adapted cells displayed a two-fold MIC increase of BC and reduced antibiotic susceptibility. At high BC concentrations, the decrease in the number of colony forming units was significantly lower in the population of adapted cells compared to parental cells. At the same time, the number of metabolically active cells with intact membranes was significantly higher than the number of culturable cells. Growth-independent viability assays revealed an adapted subpopulation after BC application that was not culturable, indicating increased abundance of viable but nonculturable (VBNC) cells. Moreover, adapted cells can outcompete non-adapted cells under sublethal concentrations of disinfectants, which may lead to novel public health risks.

The Association of Cell Division Regulated by DicC With the Formation of Viable but Non-culturable Escherichia coli O157:H7

The viable but non-culturable (VBNC) state, in which bacteria fail to grow on routine culture media but are actually alive, has been widely recognized as a strategy adopted by bacteria to cope with stressful environments. However, little is known regarding the molecular mechanism of VBNC formation. Here, we aimed to elucidate the specific roles of cell division regulatory proteins and the cell growth rate during VBNC Escherichia coli O157:H7 formation. We have previously found that expression of dicC is reduced by 20.08-fold in VBNC E. coli O157:H7 compared to non-VBNC cells. Little is known about DicC except that it, along with DicA, appears to act as a regulator of cell division by regulating expression of the cell division inhibitor DicB. First, our results showed that the VBNC cell number increased in the ΔdicC mutant as well as the DicA-overexpressing strain but decreased in the DicC-overexpressing strain induced by high-pressure carbon dioxide, acid, and H₂O₂. Furthermore, the growth rates of both the DicA-overexpressing strain and the ΔdicC mutant were higher than that of the control strain, while DicC-overexpressing strain grew significantly more slowly than the vector strain. The level of the dicB gene, regulated by dicA and dicC and inhibiting cell division, was increased in the DicC-overexpressing strain and decreased in the ΔdicC mutant and DicA-overexpressing strain, which was consistent with the growth phenotypes. In addition, the dwarfing cell morphology of the ΔdicC mutant and DicA-overexpressing strain were observed by SEM and TEM. Taken together, our study demonstrates that DicC negatively regulates the formation of the VBNC state, and DicA enhances the ability of cells to enter the VBNC state. Besides, the cell growth rate and dwarfing cell morphology may be correlated with the formation of the VBNC state.

Effect of hydraulic conditions on the prevalence of antibiotic resistance in water supply systems

The incidence of antibiotic resistance genes (ARGs) in tap water leads to potential risks to human health and draws more and more attention from the public. However, ARGs harbored in drinking water remain largely unexplored. In this study, a simulated water supply system was designed to study the effects of different pipe flow rates on the transmission of antibiotic resistance in water supply systems. We observed that the biofilm in low flow rate pipeline (0.1 m/s, 0.3 m/s) had higher concentration of both antibiotic resistant bacteria (ARB) and ARGs, while high flow rate (0.5 m/s and 0.7 m/s) resulted in low relative abundance of ARB and high relative abundance of ARGs in biofilms. The results showed that the high flow rate led to an abundance in non-culturable bacteria and a scarcity of nutrients in the biofilm, giving rise to its antibiotic resistance. High-throughput sequencing pointed out that the high content of Caulobacteraceae and Paenibacillus were determined in biofilms of high flow rate pipelines. Similarity analysis of microbial community composition of inlet water (IW), biofilms and outlet water (OW) showed that the composition of microbial community in OW was more similar to that in biofilms than in IW. Genera of bacteria in biofilms and OW (Brevundimonas, Brevibacillus and Pseudomonas) which had relationship with sulⅠ, sulⅡ in biofilms (P < 0.05) had higher relative abundance than that in IW. Different flow rate conditions had an impact on the biomass, microbial community, ARB and ARGs composition of biofilms. Thus, the detachment of biofilms can increased the antibiotic resistance of the water.

Population and single cell metabolic activity of UV-induced VBNC bacteria determined by CTC-FCM and D2O-labeled Raman spectroscopy

The occurrence of viable but non-culturable (VBNC) bacteria will result in significant underestimation of viable bacterial counts in drinking water. Whereas, much is unknown in characterizing their viability. In this study, two environmental isolates (Aeromonas sp. and Pseudomonas sp.) and two model strains (E. coli and S. aureus) were induced into VBNC state by UV irradiation. Then, their metabolic activity was determined by 5-cyano-2,3-ditolyl tetrazolium chloride combination flow cytometry (CTC-FCM) and D₂O-labeled Raman spectroscopy, respectively, at both population and single cell levels. The results showed that almost all strains could enter VBNC state irradiated by ≥ 5 mJ/cm² UV. When determined by CTC-FCM, the population metabolic activity for each strain did not vary significantly (p > 0.05) unless the UV dose reached 200 mJ/cm². Their single cell activity spectrum narrowed slightly, as indicated by changes in the standard deviation of the logarithmic normal distribution (σ) of 0.015-0.033. This minute difference suggested the CTC-FCM method was suitable for assessing the essential viability of VBNC bacteria. With respect to Raman method, an obvious dose-response effect was recorded. With the UV dosages increased from 10 to 200 mJ/cm², the CD/(CD + CH) for the four strains were reduced to between 95.7% and 47.9% of unirradiated controls, depending on strain and UV dose. Meanwhile, the single cellular Raman spectrum showed much more heterogeneously metabolic activity distribution, with some cells even entering metabolic "silence". Considering the ubiquitous participation of water in biochemical processes, the Raman method was more appropriate in assessing the overall metabolic activity. The above findings can not only be a reference for VBNC mechanism studies, but also have the potential in optimizing disinfection and other bacterial removal processes.

Citric acid can force Staphylococcus aureus into viable but nonculturable state and its characteristics

Pathogens in viable but nonculturable (VBNC) state can escape traditional detection methods based on culturable ability, thus bringing risks to food safety and human health. Considering Staphylococcus aureus as a kind of primary foodborne pathogen, this study attempted to investigate whether citric acid, a food additive commonly used, can force S. aureus into VBNC state along with low temperature. Treated with citric acid solution (pH 4.0) at 4 °C, S. aureus was confirmed to enter into VBNC state after induction for 18 days. Meanwhile, resuscitation was achieved in culture medium rather than in nutrition-free saline solution. In VBNC cells, ATP concentration still maintained at a high level, as about two-thirds of exponential-phase cells. For survival, intracellular structure of VBNC cells changed remarkably, including irregular cell shape, denser cytoplasm, space between cell wall and cell membrane, and decreased density of nuclear region. Notably, resistance of VBNC cells to simulated gastric fluid improved when compared with exponential-phase cells. What are noted above suggests that VBNC state adopted by S. aureus might be a survival strategy to the adverse environment (acidity stress and low temperature). In conclusion, our study sounds an alarm for the safety of citric acid-containing foods.

A Fast and Easy ATP-Based Approach Enables MIC Testing for Non-resuscitating VBNC Pathogens

Many bacteria enter the viable but non-culturable (VBNC) state to maximize resources and increase their tolerance to harmful conditions to cope with environmental stress, which has been described for a plethora of important human and foodborne pathogens. VBNC pathogens can potentially present a serious risk to human health as they are invisible to routine microbiological culture-based methods. Of high importance is the increased tolerance to antibiotics or disinfectant measures while in the VBNC state. The greatest remaining challenge for such investigations is the lack of an appropriate, cost-effective multi-species screening method due to experimental constraints. In this study, we investigated if de novo ATP production of cells in the VBNC state is a suitable indicator for overall cell viability that can be utilized to determine the minimum ATP inhibitory concentration (MAIC) of antibiotics and other antimicrobials. To validate this approach, heat-stress time-kill experiments were performed with both culturable and VBNC cells. We developed a comprehensive experimental setup and demonstrated the applicability of this VBNC–MIC assay for testing the tolerance of 12 strains of 4 important bacterial species (Escherichia coli, Bacillus cereus, Pseudomonas aeruginosa, and Listeria monocytogenes) in the VBNC state to eight important antimicrobials including four different antibiotics. We confirmed that bacteria in the VBNC state were resistant to all tested antibiotics (ampicillin, imipenem, ciprofloxacin, and gentamicin) and additionally insensitive to disinfectants (benzalkonium chloride and trioctylmethylammonium chloride) and preservatives (bronopol and sodium azide). These data emphasize the need for further research regarding the characteristics of bacterial pathogens in the VBNC state and present the advantages and high-throughput capabilities of ATP determinations to investigate tolerance of VBNC pathogens to antimicrobials. The presented method should be helpful in order to identify appropriate countermeasures, treatments, or disinfectants when confronted with bacterial pathogens in the VBNC state.

Chlorine injury enhances antibiotic resistance in Pseudomonas aeruginosa through over expression of drug efflux pumps

Adaption to adverse environments plays an important role in bacterial survival and is receiving increasing globe attention now. Here, cultivable chlorine-injured Pseudomonas aeruginosa, produced on the chlorination process, was investigated about their resistance to antibiotics. Then, global transcriptional analyses, quantitative PCR (qPCR) validation and antioxidant enzymes measurement were performed to explore the underlying mechanisms. The results showed that chlorine injury enhanced antibiotic resistance in P. aeruginosa and cultivable chlorine-injured P. aeruginosa exposed to 4 mg/L sodium hypochlorite (half of the lethal dose) improved antibiotic resistance against ceftazidime, chloramphenicol and ampicillin by 1.4-5.6 fold. This increase in antibiotic resistance was not hereditable and over expression of the MexEF-OprN efflux pump resulting from oxidative stress contributed to it. These results demonstrate temporal physiological persistence to antibiotics in cultivable chlorine-injured pathogens, suggesting their survival from adverse environments with antibiotic exposure and thereby posing lasting hazards to human health.

Biocidal Agents Used for Disinfection Can Enhance Antibiotic Resistance in Gram-Negative Species

Biocidal agents used for disinfection are usually not suspected to enhance cross-resistance to antibiotics. The aim of this review was therefore to evaluate the effect of 13 biocidal agents at sublethal concentrations on antibiotic resistance in Gram-negative species. A medline search was performed for each biocidal agent on antibiotic tolerance, antibiotic resistance, horizontal gene transfer, and efflux pump. In cells adapted to benzalkonium chloride a new resistance was most frequently found to ampicillin (eight species), cefotaxime (six species), and sulfamethoxazole (three species), some of them with relevance for healthcare-associated infections such as Enterobacter cloacae or Escherichia coli. With chlorhexidine a new resistance was often found to ceftazidime, sulfamethoxazole and imipenem (eight species each) as well as cefotaxime and tetracycline (seven species each). Cross-resistance to antibiotics was also found with triclosan, octenidine, sodium hypochlorite, and didecyldimethylammonium chloride. No cross-resistance to antibiotics has been described after low level exposure to ethanol, propanol, peracetic acid, polyhexanide, povidone iodine, glutaraldehyde, and hydrogen peroxide. Taking into account that some biocidal agents used in disinfectants have no health benefit (e.g., in alcohol-based hand rubs) but may cause antibiotic resistance it is obvious to prefer products without them.

Photodynamic inactivation as an emergent strategy against foodborne pathogenic bacteria in planktonic and sessile states

Foodborne microbial diseases are still considered a growing public health problem worldwide despite the global continuous efforts to ensure food safety. The traditional chemical and thermal-based procedures applied for microbial growth control in the food industry can change the food matrix and lead to antimicrobial resistance. Moreover, currently applied disinfectants have limited efficiency against biofilms. Therefore, antimicrobial photodynamic therapy (aPDT) has become a novel alternative for controlling foodborne pathogenic bacteria in both planktonic and sessile states. The use of aPDT in the food sector is attractive as it is less likely to cause antimicrobial resistance and it does not promote undesirable nutritional and sensory changes in the food matrix. In this review, aspects on the antimicrobial photodynamic technology applied against foodborne pathogenic bacteria and studied in recent years are presented. The application of photodynamic inactivation as an antibiofilm strategy is also reviewed.

Induction of Escherichia coli into a VBNC state through chlorination/chloramination and differences in characteristics of the bacterium between states

Many pathogens can enter into a viable but nonculturable (VBNC) state in response to harsh environmental stresses. Bacteria in this state can retain certain features of viable cells, such as cellular integrity, metabolic activity, or virulence and may present health risks associated with drinking water. In this study, we investigated the ability of chlorination and chloramination, which are widely used methods to disinfect drinking water, to induce Escherichia coli into a VBNC state. After treatment with chlorine and chloramine at concentrations of 1, 2, 3, and 4 mg/L, the counts of culturable E. coli cells decreased from 10⁶ CFU/mL to 0 CFU/mL at 5-60 min post treatment. Meanwhile, viable cell counts were still approximately 10³-10⁵ cells/mL. These viable E. coli cells may be induced into a VBNC state by chlorination and chloramination. Scanning electron microscopy and laser confocal microscopy showed that some bacteria maintained cellular integrity, but the average length of VBNC cells was less than that of culturable cells. Respiratory activity of VBNC cells decreased approximately 50% relative to that of culturable cells. We also used heavy water (D₂O) combined with Raman microspectroscopy to show that E. coli in a VBNC state retained metabolic activity involving water (e.g. condensation reactions) at the single-cell level. Furthermore, soxR, gadA, and katG genes remained highly expressed, suggesting that VBNC cells were physiologically active. Finally, resuscitation of VBNC cells induced by chlorine in Luria-Bertani (LB) broth was identified by calculating the generation time. Results of this study will facilitate a better understanding of the health risks associated with VBNC bacteria and the development of more effective strategies for drinking water disinfection.

Induction of Escherichia coli Into a VBNC State by Continuous-Flow UVC and Subsequent Changes in Metabolic Activity at the Single-Cell Level

A viable but non-culturable (VBNC) state of bacteria induced by disinfection in water treatment poses serious health risks because of possible resuscitation of VBNC cells during transportation. In this study, a setup using continuous-flow ultraviolet (UVC) irradiation ranging from 0 to 172.2 mJ cm^-2 was designed to simulate real-world disinfection in both drinking water (SDW) and reclaimed water (SRW) treatment plants. A systematic investigation of UVC-induced VBNC bacteria, including occurrence, resuscitation, and time-dependent recovery of metabolic activity during post-incubation, was conducted. Different techniques including two new ones of “single cell culture” and D₂O-labeled single-cell Raman spectroscopy were employed to gain comprehensive insights into VBNC cells. Heterotrophic plate counts (HPC) and 5-cyano-2,3-ditoyl tetrazolium chloride flow cytometry (CTC-FCM) assay demonstrated that exposure to continuous-flow UVC can induce E. coli into a VBNC state. Membranes integrity and 16S rRNA transcription level of VBNC bacteria were demonstrated to be unaffected by UVC exposure even at a high dose of 172.2 mJ cm^-2. Resuscitation of VBNC bacteria was identified in a more accurate way based on “single cell culture.” Finally, time-dependent evolution of metabolic activity of UVC-treated cells during post-incubation was examined by D₂O-labeled Raman spectroscopy at a high-resolution of single-cell level. C-D Raman bands resulting from incorporation of D₂O-derived D into bacterial biomass were used as a sensitive and quantitative indicator of bacterial metabolic activity. A lower UVC dose, longer post-incubation time, and higher initial number of bacteria were demonstrated to result in a faster recovery of metabolic activity. Heterogeneous metabolic activity and subpopulation with higher metabolic activity were also revealed by single-cell Raman, even for UVC-treated cells losing cultivability. The comprehensive assessment of VBNC bacteria in UVC-disinfected drinking and reclaimed water points out treatment deficiencies of UVC and the necessity to develop more effective strategies to eliminate VBNC cells.

Relationship between the Viable but Nonculturable State and Antibiotic Persister Cells

Bacteria have evolved numerous means of survival in adverse environments with dormancy, as represented by "persistence" and the "viable but nonculturable" (VBNC) state, now recognized to be common modes for such survival. VBNC cells have been defined as cells which, induced by some stress, become nonculturable on media that would normally support their growth but which can be demonstrated by various methods to be alive and capable of returning to a metabolically active and culturable state. Persister cells have been described as a population of cells which, while not being antibiotic resistant, are antibiotic tolerant. This drug-tolerant phenotype is thought to be a result of stress-induced and stochastic physiological changes as opposed to mutational events leading to true resistance. In this review, we describe these two dormancy strategies, characterize the molecular underpinnings of each state, and highlight the similarities and differences between them. We believe these survival modes represent a continuum between actively growing and dead cells, with VBNC cells being in a deeper state of dormancy than persister cells.

Formation of Sublethally Injured Yersinia enterocolitica, Escherichia coli O157:H7, and Salmonella enterica Serovar Enteritidis Cells after Neutral Electrolyzed Oxidizing Water Treatments

The impact of neutral electrolyzed oxidizing (NEO) water treatments on the formation of sublethally injured Yersinia enterocolitica, Escherichia coli O157:H7, and Salmonella enterica serovar Enteritidis cells was evaluated. When pathogens were treated with 6% NEO water, approximately 38% of the treated Yersinia population and 25% of the treated Salmonella population became sublethally injured. The highest sublethally injured population was found when Salmonella cultures were treated with 3% NEO water. Regardless of the NEO water concentration used, no sublethally injured E. coli O157:H7 cells were found. To evaluate the sensitivity of NEO water-treated cells, four additional stresses (heat treatment, pH, NaCl, and bile salt) were tested. NEO water treatments did not generate any cross protection of treated cells against the other stresses. The diluted NEO water treatments in combination with heat treatment at 51°C for 10 min led to the best synergistic antimicrobial effects with a combined reduction of 7 logs. The gene expression results showed that NEO water treatments led to the upregulation of ompR, ail, and ycfR These genes are known for their involvement in cells' environmental stress responses. In summary, this study investigated the sublethal injury in pathogenic cells caused by NEO water treatments. Although sublethal injury was discovered, when combined with other mild stresses, the synergistic antimicrobial effects were able to further reduce the numbers of viable pathogenic cells. These results demonstrate the great application potential of NEO water as a nonthermal and less corrosive antimicrobial treatment.IMPORTANCE Neutral electrolyzed oxidizing (NEO) water is a nonthermal and less corrosive antimicrobial treatment that has been demonstrated to have efficacy in reducing microbial contamination in food, including meat, fresh fruit, and vegetables. However, NEO water treatments can cause sublethal injury to pathogenic cells, resulting in cells that retain their viability. Consequently, these sublethally injured pathogenic cells become a serious food safety concern. This study evaluated the formation of sublethally injured Yersinia enterocolitica, Escherichia coli O157:H7, and Salmonella enterica serovar Enteritidis cells by NEO water treatments and the potential cross protection against heat, pH, NaCl, or bile salt stresses that it may generate. No cross protection was observed. By combining NEO water treatments with sublethal levels of additional stresses, significant synergistic antimicrobial outcomes were achieved. These results indicate that mild processing treatments, when combined, can effectively reduce pathogen populations while minimizing the negative impacts on food quality.

EmPis-1L, an Effective Antimicrobial Peptide Against the Antibiotic-Resistant VBNC State Cells of Pathogenic Bacteria

The antibiotic-resistant viable but non-culturable (VBNC) pathogenic bacteria are considered as a new threat to public health. Antimicrobial peptides (AMPs), possessing bactericidal effects in a rapid membrane attacking mode, are supposed to be effective against bacteria entering the VBNC state. In the current study, the activity of grouper AMP piscidin killing the VBNC state cells of pathogenic bacteria Escherichia coli O157, Staphylococcus aureus, and Vibrio parahaemolyticus OS4 was studied. After entering the VBNC state, cells of E. coli O157, S. aureus, and V. parahaemolyticus OS4 developed resistance to the antibiotics Ampicillin and Kanamycin. Rather than truncated form of Malabar grouper piscidin 1 (EmPis-1S), full-length Malabar grouper piscidin 1 (EmPis-1L) showed strong activity to kill the above VBNC bacteria. The VBNC state cells (1 × 10⁵ CFU/mL) of the three species of bacteria could be totally lysed by 10 μmol/L of EmPis-1L in 1 h. The VBNC state cells of S. aureus were most susceptible to EmPis-1L, which killed the cells by 100% in 30 min at the low concentration of 2.0 μmol/L. In addition, EmPis-1L at the concentration of no more than 10 μmol/L showed no observed toxicity to human lung carcinoma epithelial cells (A549) and mouse neuroblastoma cells (N2a). Accordingly, EmPis-1L could be a promisingly safe and efficient agent for eliminating the traditional antibiotic-resistant VBNC state cells of pathogenic bacteria, E. coli, S. aureus, and V. parahaemolyticus.

Viable-but-Nonculturable Listeria monocytogenes and Salmonella enterica Serovar Thompson Induced by Chlorine Stress Remain Infectious

The microbiological safety of fresh produce is monitored almost exclusively by culture-based detection methods. However, bacterial food-borne pathogens are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses such as chlorine, which is commonly used for fresh produce decontamination. Here, complete VBNC induction of green fluorescent protein-tagged Listeria monocytogenes and Salmonella enterica serovar Thompson was achieved by exposure to 12 and 3 ppm chlorine, respectively. The pathogens were subjected to chlorine washing following incubation on spinach leaves. Culture data revealed that total viable L. monocytogenes and Salmonella Thompson populations became VBNC by 50 and 100 ppm chlorine, respectively, while enumeration by direct viable counting found that chlorine caused a <1-log reduction in viability. The pathogenicity of chlorine-induced VBNC L. monocytogenes and Salmonella Thompson was assessed by using Caenorhabditis elegans Ingestion of VBNC pathogens by C. elegans resulted in a significant life span reduction (P = 0.0064 and P < 0.0001), and no significant difference between the life span reductions caused by the VBNC and culturable L. monocytogenes treatments was observed. L. monocytogenes was visualized beyond the nematode intestinal lumen, indicating resuscitation and cell invasion. These data emphasize the risk that VBNC food-borne pathogens could pose to public health should they continue to go undetected.IMPORTANCE Many bacteria are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses. VBNC cells cannot be detected by standard laboratory culture techniques, presenting a problem for the food industry, which uses these techniques to detect pathogen contaminants. This study found that chlorine, a sanitizer commonly used for fresh produce, induces a VBNC state in the food-borne pathogens Listeria monocytogenes and Salmonella enterica It was also found that chlorine is ineffective at killing total populations of the pathogens. A life span reduction was observed in Caenorhabditis elegans that ingested these VBNC pathogens, with VBNC L. monocytogenes as infectious as its culturable counterpart. These data show that VBNC food-borne pathogens can both be generated and avoid detection by industrial practices while potentially retaining the ability to cause disease.