Sub-emetic toxicity of Bacillus cereus toxin cereulide on cultured human enterocyte-like Caco-2 cells

Development of a simple screening method for analyzing cereulide toxin in fried rice using liquid chromatography-tandem mass spectrometry

PURPOSE

The presence of cereulide, an emetic toxin produced by Bacillus cereus, in fried rice samples is critical evidence of food poisoning even in situations where B. cereus could not be detected. This study aims to develop a screening method for analyzing cereulide in fried rice using the QuEChERS procedure and liquid chromatography-tandem mass spectrometry (LC-MS/MS).

METHODS

Cereulide was identified and quantified in fried rice samples using the QuEChERS extraction method and LC-MS/MS. The accuracies of the methods were determined by analyzing fortified blank samples at two concentrations (10 and 50 µg/kg) conducted on three samples daily for five days.

RESULTS

The QuEChERS procedure removed matrix compounds from fried rice. Characteristic MS/MS spectra enabled the identification of cereulide. As the matrix effects in seven fried rice samples were within ± 6%, an external solvent calibration curve could be used for quantification. This method exhibited good accuracy ranging from 88 to 89%. The relative standard deviations for both repeatability and intra-laboratory reproducibility were < 4%. These standard deviations satisfied the criteria of the Japanese validation guidelines for residues (MHLW 2010, Director Notice, Syoku-An No. 1224-1). The limit of quantification was 2 μg/kg. The applicability of this method was confirmed using the analysis of cereulide in fried rice samples incubated with emetic Bacillus cereus.

CONCLUSIONS

The QuEChERS extraction procedure described herein showed substantial promise as a reliable screening tool for cereulide in fried rice sample.

Cereulide and Emetic Bacillus cereus: Characterizations, Impacts and Public Precautions

Cereulide, which can be produced by Bacillus cereus, is strongly associated with emetic-type food poisoning outbreaks. It is an extremely stable emetic toxin, which is unlikely to be inactivated by food processing. Considering the high toxicity of cereulide, its related hazards raise public concerns. A better understanding of the impact of B. cereus and cereulide is urgently needed to prevent contamination and toxin production, thereby protecting public health. Over the last decade, a wide range of research has been conducted regarding B. cereus and cereulide. Despite this, summarized information highlighting precautions at the public level involving the food industry, consumers and regulators is lacking. Therefore, the aim of the current review is to summarize the available data describing the characterizations and impacts of emetic B. cereus and cereulide; based on this information, precautions at the public level are proposed.

Chronic cereulide exposure causes intestinal inflammation and gut microbiota dysbiosis in mice

Known as a cause of food poisoning, Bacillus cereus (B. cereus) is widespread in nature. Cereulide, the heat-stable and acid-resistant emetic toxin which is produced by some B. cereus strains, is often associated with foodborne outbreaks, and causes acute emetic toxicity at high dosage exposure. However, the toxicological effect and underlying mechanism caused by chronic low-dose cereulide exposure require to be further addressed. In the study, based on mouse model, cereulide exposure (50 μg/kg body weight) for 28 days induced intestinal inflammation, gut microbiota dysbiosis and food intake reduction. According to the cell models, low dose cereulide exposure disrupted the intestinal barrier function and caused intestinal inflammation, which were resulted from endoplasmic reticulum (ER) stress IRE1/XBP1/CHOP pathway activation to induce cell apoptosis and inflammatory cytokines production. For gut microbiota, cereulide decreased the abundances of Lactobacillus and Oscillospira. Furthermore, cereulide disordered the metabolisms of gut microbiota, which exhibited the inhibitions of butyrate and tryptophan. Interestingly, cereulide exposure also inhibited the tryptophan hydroxylase to produce the serotonin in the gut and brain, which might lead to depression-like food intake reduction. Butyrate supplementation (100 mg/kg body weight) significantly reduced intestinal inflammation and serotonin biosynthesis suppression caused by cereulide in mice. In conclusion, chronic cereulide exposure induced ER stress to cause intestinal inflammation, gut microbiota dysbiosis and serotonin biosynthesis suppression. IRE1 could be the therapeutic target and butyrate supplementation is the potential prevention strategy.

New Insights into the Potential Cytotoxic Role of Bacillus cytotoxicus Cytotoxin K-1

The thermotolerant representative of the Bacillus cereus group, Bacillus cytotoxicus, reliably harbors the coding gene of cytotoxin K-1 (CytK-1). This protein is a highly cytotoxic variant of CytK toxin, initially recovered from a diarrheal foodborne outbreak that caused the death of three people. In recent years, the cytotoxicity of B. cytotoxicus has become controversial, with some strains displaying a high cytotoxicity while others show no cytotoxicity towards cell lines. In order to better circumscribe the potential pathogenic role of CytK-1, knockout (KO) mutants were constructed in two B. cytotoxicus strains, E8.1 and E28.3. The complementation of the cytK-1 KO mutation was implemented in a mutant strain lacking in the cytK-1 gene. Using the tetrazolium salt (MTT) method, cytotoxicity tests of the cytK-1 KO and complemented mutants, as well as those of their wild-type strains, were carried out on Caco-2 cells. The results showed that cytK-1 KO mutants were significantly less cytotoxic than the parental wild-type strains. However, the complemented mutant was as cytotoxic as the wild-type, suggesting that CytK-1 is the major cytotoxicity factor in B. cytotoxicus.

Phenotypic homogeneity of emetic Bacillus cereus isolates in China

Emetic Bacillus cereus strains produce a potent cereulide cytotoxin, which can cause acute and fatal cases of food poisoning. We isolated 18 emetic B. cereus strains from a food poisoning event, and from clinical and non-random food surveillance in China and phenotypic characteristics of haemolysis, starch hydrolysis, salicin fermentation, gelatin liquefaction, cytotoxicity, and susceptibility to antibiotics were assessed. All isolates were positive for haemolysis and gelatin liquefaction, and negative for starch hydrolysis and salicin fermentation. Their haemolytic potentials were intermediate to Bacillus anthracis and B. cereus ATCC 14579 (a non-emetic strain). All isolates were cytotoxic to CHO, Hep-2, and Vero cells, and were sensitive to ampicillin. The homogeneous phenotypes of emetic isolates from China are similar to the corresponding traits of European and Japanese isolates that have been characterized, suggesting highly similar phenotypes of emetic B. cereus worldwide.

Bacillus cereus food intoxication and toxicoinfection

Bacillus cereus is one of the leading etiological agents of toxin-induced foodborne diseases. Its omnipresence in different environments, spore formation, and its ability to adapt to varying conditions and produce harmful toxins make this pathogen a health hazard that should not be underestimated. Food poisoning by B. cereus can manifest itself as an emetic or diarrheal syndrome. The former is caused by the release of the potent peptide toxin cereulide, whereas the latter is the result of proteinaceous enterotoxins (e.g., hemolysin BL, nonhemolytic enterotoxin, and cytotoxin K). The final harmful effect is not only toxin and strain dependent, but is also affected by the stress responses, accessory virulence factors, and phenotypic properties under extrinsic, intrinsic, and explicit food conditions and host-related environment. Infamous portrait of B. cereus as a foodborne pathogen, as well as a causative agent of nongastrointestinal infections and even nosocomial complications, has inspired vast volumes of multidisciplinary research in food and clinical domains. As a result, extensive original data became available asking for a new, both broad and deep, multifaceted look into the current state-of-the art regarding the role of B. cereus in food safety. In this review, we first provide an overview of the latest knowledge on B. cereus toxins and accessory virulence factors. Second, we describe the novel taxonomy and some of the most pertinent phenotypic characteristics of B. cereus related to food safety. We link these aspects to toxin production, overall pathogenesis, and interactions with its human host. Then we reflect on the prevalence of different toxinotypes in foods opening the scene for epidemiological aspects of B. cereus foodborne diseases and methods available to prevent food poisoning including overview of the different available methods to detect B. cereus and its toxins.

Assessing Mixture Effects of Cereulide and Deoxynivalenol on Intestinal Barrier Integrity and Uptake in Differentiated Human Caco-2 Cells

The human intestine is regularly exposed to ingested food contaminants, such as fungal and bacterial toxins, which have been described to co-occur in a mixed diet. Thus, it is of utmost importance to understand possible interactions between contaminants of different origin. Hence, we investigated the single and combined effects of one of the most abundant mycotoxins, deoxynivalenol (DON; 0.1 to 10 µg/mL), and the bacterial toxin cereulide (CER; 1 to 100 ng/mL) on differentiated human Caco-2 (C2BBe1) cells cultured in a transwell system. We tested the capacity of the two toxins to alter the intestinal integrity and further investigated the uptake of both compounds and the formation of selected DON metabolites. CER alone (10 and 100 ng/mL) and in combination with DON (10 ng/mL CER with 1 µg/mL DON) was found to alter the barrier function by increasing the transepithelial electrical resistance and the expression of the tight junction protein claudin-4. For the first time, DON-3-sulfate was identified as a metabolite of human intestinal cells in vitro. Moreover, co-incubation of CER and DON led to an altered ratio between DON and DON-3-sulfate. Hence, we conclude that co-exposure to CER and DON may alter the intestinal barrier function and biotransformation of intestinal cells.

Combinatory effects of cereulide and deoxynivalenol on in vitro cell viability and inflammation of human Caco-2 cells

Deoxynivalenol (DON), one of the most abundant mycotoxins in cereal products, was recently detected with other mycotoxins and the emetic bacterial toxin cereulide (CER) in maize porridge. Within a cereal-based diet, co-exposure to these toxins is likely, hence raising the question of combinatory toxicological effects. While the toxicological evaluation of DON has quite progressed, consequences of chronic, low-dose CER exposure are still insufficiently explored. Information about the combinatory toxicological effects of these toxins is lacking. In the present study, we investigated how CER (0.1-100 ng/mL) and DON (0.01-10 µg/mL) alone and in a constant ratio of 1:100 (CER:DON) affect the cytotoxicity and immune response of differentiated human intestinal Caco-2 cells. While DON alone reduced cell viability only in the highest concentration (10 µg/mL), CER caused severe cytotoxicity upon prolonged incubation (starting from 10 ng/mL after 24 h and 48 h, 2.5 ng/mL and higher after 72 h). After 72 h, synergistic effects were observed at 2.5 ng/mL CER and 0.25 µg/mL DON. Different endpoints of inflammation were investigated in interleukin-1β-stimulated Caco-2 cells. Notably, DON-induced interleukin-8 transcription and secretion were diminished by the presence of 10 and 25 ng/mL CER after short-term (5 h) incubation, indicating immunosuppressive properties. We hypothesise that habitual consumption of cereal-based foods co-contaminated with CER and DON may cause synergistic cytotoxic effects and an altered immune response in the human intestine. Therefore, further research concerning effects of co-occurring bacterial toxins and mycotoxins on the impairment of intestinal barrier integrity, intestinal inflammation and the promotion of malnutrition is needed.

Oxygen Consumption Rate Analysis of Mitochondrial Dysfunction Caused by Bacillus cereus Cereulide in Caco-2 and HepG2 Cells

The emetic syndrome of Bacillus cereus is a food intoxication caused by cereulide (CER) and manifested by emesis, nausea and in most severe cases with liver failure. While acute effects have been studied in the aftermath of food intoxication, an exposure to low doses of cereulide might cause unnoticed damages to the intestines and liver. The toxicity which relies on the mitochondrial dysfunction was assessed on Caco-2 and HepG2 cells after exposure of one, three and ten days to a range of low doses of cereulide. Oxygen consumption rate analyses were used to study the impact of low doses of CER on the bioenergetics functions of undifferentiated Caco-2 and HepG2 cells using Seahorse XF extracellular flux analyzer. Both Caco-2 and HepG2 cells experienced measurable mitochondrial impairment after prolonged exposure of 10 days to 0.25 nM of cereulide. Observed mitochondrial dysfunction was greatly reflected in reduction of maximal cell respiration. At 0.50 nM CER, mitochondrial respiration was almost completely shut down, especially in HepG2 cells. These results corresponded with a severe reduction in the amount of cells and an altered morphology, observed by microscopic examination of the cells. Accurate and robust quantification of basal respiration, ATP production, proton leak, maximal respiration, spare respiratory capacity, and non-mitochondrial respiration allowed better understanding of the effects of cereulide in underlying respiratory malfunctions in low-dose exposure.

Molecular discrimination of Bacillus cereus group species in foods (lettuce, spinach, and kimbap) using quantitative real-time PCR targeting groEL and gyrB

The aim of the study was to identify and evaluate specific biomarkers to differentiate within Bacillus cereus group species from contaminated food samples with the use of real-time PCR. A total of 120 strains, comprising of 28 reference, 2 type, 78 wild strains of B. cereus and B. thuringiensis along with 12 strains representing 2 bacterial groups - B. mycoides, B. pseudomycoides, B. weihenstephanensis (B. cereus group); B. amyloliquefaciens, B. subtilis, Enterococcus faecalis, Escherichia coli, Listeria monocytogenes, Micrococcus luteus, Salmonella enterica, Staphylococcus aureus, Streptococcus pyogenes (non-Bacillus sp.) were identified by applying valid biomarkers (groEL and gyrB). In addition, the presence of B. cereus group was determined in three different artificially contaminated vegetable samples (lettuce, spinach, and kimbap), using prominent biomarkers targeting on chaperonin protein (GroEL) and topoisomerase enzyme protein (gyrB). Direct analysis of samples revealed the specificity towards identification and characterization of the B. cereus group among wild, reference and type strains and the type strain inoculated in vegetables. Our results demonstrated two existing biomarkers groEL and gyrB with a high specificity of 98% and 96% respectively to analyze the total B. cereus group. Further, we also reported the detection limit of groEL and gyrB in food samples was 3.5 and 3.7 log CFU/g respectively. Thus, the developed real-time PCR approach can be a reliable and effective tool for the identification of B. cereus group strains present in environment and food samples. This does not require band isolation, re-amplification, sequencing or sequence identification, thus reducing the time and cost of analysis.

Screening of Cytotoxic B. cereus on Differentiated Caco-2 Cells and in Co-Culture with Mucus-Secreting (HT29-MTX) Cells

B. cereus is an opportunistic foodborne pathogen able to cause diarrhoea. However, the diarrhoeal potential of a B. cereus strain remains difficult to predict, because no simple correlation has yet been identified between the symptoms and a unique or a specific combination of virulence factors. In this study, 70 B. cereus strains with different origins (food poisonings, foods and environment) have been selected to assess their enterotoxicity. The B. cereus cell-free supernatants have been tested for their toxicity in vitro, on differentiated (21 day-old) Caco-2 cells, using their ATP content, LDH release and NR accumulation. The genetic determinants of the main potential enterotoxins and virulence factors (ces, cytK, entFM, entS, hbl, nhe, nprA, piplC and sph) have also been screened by PCR. This analysis showed that none of these genes was able to fully explain the enterotoxicity of B. cereus strains. Additionally, in order to assess a possible effect of the mucus layer in vitro, a cytotoxicity comparison between a monoculture (Caco-2 cells) and a co-culture (Caco-2 and HT29-MTX mucus-secreting cells) model has been performed with selected B. cereus supernatants. It appeared that, in these conditions, the mucus layer had no notable influence on the cytotoxicity of B. cereus supernatants.

Development and validation of ultra-high-performance liquid chromatography-tandem mass spectrometry methods for the simultaneous determination of beauvericin, enniatins (A, A1, B, B1) and cereulide in maize, wheat, pasta and rice

Rapid and accurate UPLC-MS/MS methods for the simultaneous determination of beauvericin and the related enniatins (A, A1, B, B1), together with cereulide were successfully developed and validated in cereal and cereal-based food matrices such as wheat, maize, rice and pasta. Although these emerging foodborne toxins are of different microbial origin, the similar structural, toxicological and food safety features provided rationale for their concurrent detection in relevant food matrices. A Waters Acquity UPLC system coupled to a Waters Quattro Premier XE™ Mass Spectrometer operating in ESI+ mode was employed. Sample pretreatment involved a fast and simple liquid extraction of the target toxins without any further clean-up step. For all toxins the sample preparation resulted in acceptable extraction recoveries with values of 85-105% for wheat, 87-106% for maize, 84-106% for rice and 85-105% for pasta. The efficient extraction protocol, together with a fast chromatographic separation of 7min allowed substantial saving costs and time showing its robustness and performance. The validation of the developed method was performed based on Commission Decision 2002/657/EC. The obtained limits of detection ranged from 0.1 to 1.0μgkg^-1 and the limits of quantification from 0.3 to 2.9μgkg^-1 for the targeted toxins in the selected matrices. The obtained sensitivities allow detection of relevant toxicological concentrations. All relative standard deviations for repeatability (intra-day) and intermediate precision (inter-day) were lower than 20%. Trueness, expressed as the apparent recovery varied from 80 to 107%. The highly sensitive and repeatable validated method was applied to 57 naturally contaminated samples allowing detection of sub-clinical doses of the toxins.

Cereulide food toxin, beta cell function and diabetes: Facts and hypotheses

The incidence of both type 1 and type 2 diabetes is increasing and although environmental pollutants are believed to be potential culprits, the extent to which they can be held responsible remains uncertain. Some bacterial strains of the Bacillus cereus produce a toxin, cereulide, which is frequently found in starchy meals and which is difficult to eradicate from the food chain as it is highly resistant to heat, acidity and proteolysis. While cereulide is well known to cause acute emetic toxicity when ingested at high doses, several in vitro studies have shown that also extremely low doses of cereulide can be toxic, with beta cells being particularly sensitive. Mechanistically, such low doses impair the mitochondrial activity of the beta cells thereby leading to hampered insulin secretion and cell death, both key traits in the pathophysiology of diabetes. In vivo studies of chronic or repeated low dose exposure to cereulide are currently lacking, but should be performed to further clarify the true relevance of cereulide as a potential environmental contributor to the ongoing diabetes epidemic.

Effect of apple, baobab, red-chicory, and pear extracts on cellular energy expenditure and morphology of a Caco-2 cells using transepithelial electrical resistance (TEER) and scanning electron microscopy (SEM)

The present study investigated the effects of four food extracts on the Caco-2 intestinal cell line using a new transepithelial electrical resistance method (TEER) concurrent with electron microscopy (SEM).