Flow cytometry immunodetection and membrane integrity assessment of Escherichia coli O157:H7 in ready-to-eat pasta salad during refrigerated storage

Rapid and Multiplexed Detection of Single Cells of Salmonella, Escherichia coli O157, and Shigella flexneri in Ground Beef by Flow Cytometry

Salmonella, Escherichia coli O157, and Shigella flexneri are typical foodborne pathogens in ground beef, which can cause severe infection even when present as a single cell. Flow cytometry (FCM) methods are widely applied in the rapid detection of pathogens in food products. In this study, we report an FCM-based method for detecting single cells of Salmonella, E. coli O157, and S. flexneri in 25 g ground beef samples. We fluorescently labeled specific antibodies that could effectively identify bacterial cells, prepared single-cell samples by serial dilution, and optimized the pre-enrichment time. The results showed that 7 h of pre-enrichment is appropriate for sensitive single-cell detection by FCM. Finally, we evaluated this method in artificially contaminated and retail beef samples. This study outlines a novel highly sensitive FCM-based method to detect Salmonella, E. coli O157, and S. flexneri in beef samples within 8 h that can be applied to the rapid and multiplexed detection of foodborne pathogens.

Incidence of common infectious diseases in Japan during the COVID-19 pandemic

Recent reports indicate that respiratory infectious diseases were suppressed during the novel coronavirus disease-2019 (COVID-19) pandemic. COVID-19 led to behavioral changes aimed to control droplet transmission or contact transmission. In this study, we examined the incidence of common infectious diseases in Japan during the COVID-19 pandemic. COVID-19 data were extracted from the national data based on the National Epidemiological Surveillance of Infectious Diseases (NESID). Common infectious diseases were selected from notifiable infectious diseases under the NESID. The epidemic activity of the diseases during 2015-2020 was evaluated based on the Infectious Disease Weekly Reports published by the National Institute of Infectious Diseases. Each disease was then categorized according to the route of transmission. Many Japanese people had adopted hygienic activities, such as wearing masks and hand washing, even before the COVID-19 pandemic. We examined the correlation between the time-series of disease counts of common infectious diseases and COVID-19 over time using cross-correlation analysis. The weekly number of cases of measles, rotavirus, and several infections transmitted by droplet spread, was negatively correlated with the weekly number of cases of COVID-19 for up to 20 weeks in the past. According to the difference-in-differences analysis, the activity of influenza and rubella was significantly lower starting from the second week in 2020 than that in 2015-2019. Only legionellosis was more frequent throughout the year than in 2015-2019. Lower activity was also observed in some contact transmitted, airborne-transmitted, and fecal-oral transmitted diseases. However, carbapenem-resistant Enterobacteriaceae, exanthema subitum, showed the same trend as that over the previous 5 years. In conclusion, our study shows that public health interventions for the COVID-19 pandemic may have effectively prevented the transmission of most droplet-transmitted diseases and those transmitted through other routes.

Inactivation of Escherichia coli O157:H7 in apple juice via induced electric field (IEF) and its bactericidal mechanism

Non-conventional heating technology based on electric fields can be utilized to process liquid foods. In this study, the induced electric field (IEF) was investigated to clarify its inactivation mechanism on E.coli. Staining results show that inactivation of E.coli by IEF can be attributed to the reversible destruction of the cell membrane, followed by the denaturation of intracellular enzymes, and finally the irreversible rupture of the cell membrane. The increased levels of extracellular proteins and nucleic acids were also observed. IEF treatment at 400 Hz and 800 V (or 53 V/cm) results in a reduction of 4.5 log CFU·mL^-1 in the number of E.coli. Storage life analysis shows that IEF treatment can improve the stability of apple juice and the content of bioactive components. Thus, IEF is a potential technique for liquid food processing.

A Diffusion Model to Quantify Membrane Repair Process in Listeria monocytogenes Exposed to High Pressure Processing Based on Fluorescence Microscopy Data

The effects of environmental stresses on microorganisms have been well-studied, and cellular responses to stresses such as heat, cold, acids, and salts have been extensively discussed. Although high pressure processing (HPP) is becoming more popular as a preservation method in the food industry, the characteristics of the cellular damage caused by high pressure are unclear, and the microbial response to this stress has not yet been well-explored. We exposed the pathogen Listeria monocytogenes to HPP (400 MPa, 8 min, 8°C) and found that the high pressure created plasma membrane pores. Using a common staining technique involving propidium iodide (PI) combined with high-frequency fluorescence microscopy, we monitored the rate of diffusion of PI molecules into hundreds of bacterial cells through these pores on days 0, 1, 2, 3, and 4 after pressurization. We also developed a mathematical dynamic model based on mass transfer and passive diffusion laws, calibrated using our microscopy experiments, to evaluate the response of bacteria to HPP. We found that the rate of diffusion of PI into the cells decreased over the 4 consecutive days after exposure to HPP, indicating repair of the pressure-created membrane pores. The model suggested a temporal change in the size of pores until closure. To the best of our knowledge, this is the first time that pressure-created membrane pores have been quantitatively described and shown to diminish with time. In addition, we found that the membrane repair rate in response to HPP was linear, and growth was temporarily arrested at the population level during the repair period. These results support the existence of a progressive repair process in some of the cells that take up PI, which can therefore be considered as being sub-lethally injured rather than dead. Hence, we showed that a subgroup of bacteria survived HPP and actively repaired their membrane pores.

Ohmic heating increases inactivation and morphological changes of Salmonella sp. and the formation of bioactive compounds in infant formula

The effect of ohmic heating (OH) (50, 55, and 60 °C, 6 V/cm) on the inactivation kinetics (Weibull model) and morphological changes (scanning electron microscopy and flow cytometry) of Salmonella spp. in infant formula (IF) was evaluated. In addition, thermal load indicators (hydroxymethylfurfural and whey protein nitrogen index, HMF, and WPNI) and bioactive compounds (DPPH, total phenolics, ACE, α-amylase, and α-glucosidase inhibitory activities) were also studied. OH presented a more intense inactivation rate than conventional heating, resulting in a reduction of about 5 log CFU per mL at 60 °C in only 2.91 min, being also noted a greater cell membrane deformation, higher formation of bioactive compounds, and lower values for the thermal load parameters. Overall, OH contributed to retaining the nutritional value and improve food safety in IF processing.

Protein A-Mediated Binding of Staphylococcus spp. to Antibodies in Flow Cytometric Assays and Reduction of This Binding by Using Fc Receptor Blocking Reagent

Staphylococcus aureus and other coagulase-positive Staphylococcus spp. bind the Fc region of IgG antibodies through expression of protein A (SpA). These species have consequently been a source of false-positive signals in antibody-based assays designed to detect other target bacteria. Here, flow cytometry was used to study the influence of a number of factors on the SpA-mediated binding of single cells to an anti-human IgG antibody, including strain, heat killing, overnight storage, growth phase, cell physiology, surface adhesion, and growth in model food systems. Through the costaining of antibody-stained cells with the permeability dye propidium iodide and calcein violet AM, the cell physiological status was related to SpA-mediated antibody binding. Generally, permeabilized cells lacking esterase activity did not strongly bind antibody. The binding of a number of commercially available polyclonal IgG antibodies to non-Staphylococcus spp. was also characterized. Not all SpA-expressing species showed strong binding of mouse IgG, and one species not known to express SpA showed strong binding. Most SpA-expressing strains bound rabbit IgG antibodies to some extent, whereas only one strain bound goat IgG. To reduce or eliminate SpA-mediated IgG binding, the following products were evaluated as blocking reagents and applied prior to staining with primary or secondary antibody: normal rabbit serum, mouse IgG isotype control, goat IgG, and a commercial FcR blocking reagent. Only the FcR blocking reagent consistently reduced SpA-mediated binding of Staphylococcus spp. to antibodies against other species and could be recommended as a blocking reagent in immunoassays designed to detect non-Staphylococcus species.IMPORTANCE This study characterizes a widespread but little-studied problem associated with the antibody-based detection of microbes-the Staphylococcus protein A (SpA)-mediated binding of IgG antibodies-and offers a solution: the use of commercial FcR blocking reagent. A common source of false-positive signals in the detection of microbes in clinical, food, or environmental samples can be eliminated by applying this study's findings. Using flow cytometry, the authors demonstrate the extent of heterogeneity in a culture's SpA-mediated binding of antibodies and that the degree of SpA-mediated antibody binding is strain, growth phase, and food matrix dependent and influenced by simulated food processing treatments and cell adherence. In addition, our studies of SpA-mediated binding of Staphylococcus spp. to antibodies against other bacterial species produced a very nuanced picture, leading us to recommend testing against multiple strains of S. aureus and S. hyicus of all antibodies to be incorporated into any immunoassay designed to detect a non-Staphylococcus spp.

Membrane modification as a survival mechanism through gastric fluid in non-acid adapted enteropathogenic Escherichia coli (EPEC)

In bacterial cells, the cytoplasmic membrane forms a barrier between the environment and the cell's cytoplasm. This barrier regulates which substances (and the amount) that leave and enter the cell, to maintain homeostasis between the cytoplasm and the external environment. One of the mechanisms employed to maintain structure and functionality during exposure to environmental stress is adaptation of the membrane lipids. The aim of this study was to investigate membrane alteration as a possible survival method of non-acid adapted enteropathogenic Escherichia coli (E. coli) (EPEC) (as could be found in contaminated water or unprocessed food) through simulated gastric fluid (SGF). Enteropathogenic E. coli was grown in nutrient-rich media and then exposed to SGF of various pH (1.5, 2.5, 3.5, or 4.5) for 180 min. Flow cytometry was utilised to examine membrane integrity; and morphological changes were investigated using transmission electron microscopy (TEM). Gas chromatography-mass spectrometry (GC-MS) was used to assess the membrane lipid composition. The results of this study showed that SGF treatment caused membrane damage, as well as cell wall thickening and irregular plasma membranes. The morphological changes were accompanied by membrane lipid changes indicative of decreased membrane fluidity and increased rigidity. The findings suggest that non-acid adapted EPEC can perceive pH change in the environment and adapt accordingly.

Flow Cytometry to Assess the Counts and Physiological State of Cronobacter sakazakii Cells after Heat Exposure

Cronobacter sakazakii is an opportunistic pathogen that is associated with outbreaks of neonatal necrotizing enterocolitis, septicaemia, and meningitis. Reconstituted powdered infant formulae is the most common vehicle of infection. The aim of the present study is to gain insight into the physiological states of C. sakazakii cells using flow cytometry to detect the compromised cells, which are viable but non-culturable using plate-based methods, and to evaluate the impact of milk heat treatments on those populations. Dead-cell suspensions as well as heat-treated and non-heat-treated cell suspensions were used. After 60 or 65 °C treatments, the number of compromised cells increased as a result of cells with compromised membranes shifting from the heat-treated suspension. These temperatures were not effective at killing all bacteria but were effective at compromising their membranes. Thus, mild heat treatments are not enough to guarantee the safety of powered infant formulae. Flow cytometry was capable of detecting C. sakazakii’s compromised cells that cannot be detected with classical plate count methods; thus, it could be used as a screening test to decrease the risk derived from the presence of pathogenic viable but non-culturable cells in this food that is intended for newborns’ nutrition.

Biotin-exposure-based immunomagnetic separation coupled with nucleic acid lateral flow biosensor for visibly detecting viable Listeria monocytogenes

Infectious diseases caused by Listeria monocytogenes pose a great threat to public health worldwide. Therefore, a rapid and efficient method for L. monocytogenes detection is needed. In this study, a biotin-exposure-based immunomagnetic separation (IMS) method was developed. That is, biotinylated antibody was first targeted to L. monocytogenes. Then, streptavidin-functionalized magnetic nanoparticles were added and anchored onto L. monocytogenes cells indirectly through the strong noncovalent interaction between streptavidin and biotin. Biotin-exposure-based IMS exhibited an excellent capability to enrich L. monocytogenes. Specifically, more than 90% of L. monocytogenes was captured when the bacterial concentration was lower than 10⁴ colony-forming units (CFU)/mL. Importantly, the antibody dosage was reduced by 10 times of that in our previous study, which used antibody direct-conjugated magnetic nanoparticles. Propidium monoazide (PMA) treatment prior to PCR amplification could eliminate the false-positive results from dead bacteria and detected viable L. monocytogenes sensitively and specifically. For viable L.monocytogenes detection, enriched L. monocytogenes was treated with PMA prior to asymmetric PCR amplification. The detection limits of the combined IMS with nucleic acid lateral flow (NALF) biosensor for viable L. monocytogenes detection were 3.5 × 10³ CFU/mL in phosphate buffer solution and 3.5 × 10⁴ CFU/g in lettuce samples. The whole assay process of recognizing viable L. monocytogenes was completed within 6 h. The proposed biotin-exposure-mediated IMS combined with a disposable NALF biosensor platform posed no health risk to the end user, and possessed potential applications in the rapid screening and identification of foodborne pathogens.

Identification of microbes from the surfaces of food-processing lines based on the flow cytometric evaluation of cellular metabolic activity combined with cell sorting

In this study the design of a flow cytometry-based procedure to facilitate the detection of adherent bacteria from food-processing surfaces was evaluated. The measurement of the cellular redox potential (CRP) of microbial cells was combined with cell sorting for the identification of microorganisms. The procedure enhanced live/dead cell discrimination owing to the measurement of the cell physiology. The microbial contamination of the surface of a stainless steel conveyor used to process button mushrooms was evaluated in three independent experiments. The flow cytometry procedure provided a step towards monitoring of contamination and enabled the assessment of microbial food safety hazards by the discrimination of active, mid-active and non-active bacterial sub-populations based on determination of their cellular vitality and subsequently single cell sorting to isolate microbial strains from discriminated sub-populations. There was a significant correlation (r = 0.97; p < 0.05) between the bacterial cell count estimated by the pour plate method and flow cytometry, despite there being differences in the absolute number of cells detected. The combined approach of flow cytometric CRP measurement and cell sorting allowed an in situ analysis of microbial cell vitality and the identification of species from defined sub-populations, although the identified microbes were limited to culturable cells.