Psychrotrophic strains of Bacillus cereus producing enterotoxin

Assay of Bacillus cereus Emetic toxin produced in orange squash

The contamination of squash by B. cereus, an enterotoxin producer, was found to range between 7.5×104  and 1.8×104 CFU/g in orange squash (during storage), that is hazardous. Orange squash is widely produced and consumed in India, but has a low rating of 3 on the scale of 10 (on feedback), mostly due to high sugars, not preferred these days. It can be preserved for >9 months due to added sugars and preservatives. During processing squash, if juice is not quickly cooled and/or squash is kept for long at temperatures <48 °C after processing, it can be a source of food poisoning. Reason, a large number of toxins can be produced by B. cereus. B. cereus strains, isolated from squash, produce heat stable toxin. Vacuolar assay confirmed them as emetic toxins, produced in squash. The toxin behaved like an ionophore in assay using mitochondria, extracted from liver cells of chicken with potassium ions in buffer. The toxicity of toxin by assay was 3200 IU/ng (BC IV strain) and 800 IU/ng (BC X strain). By the vacuolar expansions of mitochondria in assay, toxins of B. cereus demonstrated a toxic effect, in the range of 20.93 to 60.94 % by BC IV toxin and 43.28 to 45.02 % by BC X toxin, on the 3rd day growth of B. cereus in squash and toxin extraction for assay. It was also possible to produce antibodies against the B. cereus whole cell and toxin of BC IV, as an attempt to detect B. cereus contaminations in foods, by Ouchterlony’s immune-diffusion test

The Bacillus cereus Food Infection as Multifactorial Process

The ubiquitous soil bacterium Bacillus cereus presents major challenges to food safety. It is responsible for two types of food poisoning, the emetic form due to food intoxication and the diarrheal form emerging from food infections with enteropathogenic strains, also known as toxico-infections, which are the subject of this review. The diarrheal type of food poisoning emerges after production of enterotoxins by viable bacteria in the human intestine. Basically, the manifestation of the disease is, however, the result of a multifactorial process, including B. cereus prevalence and survival in different foods, survival of the stomach passage, spore germination, motility, adhesion, and finally enterotoxin production in the intestine. Moreover, all of these processes are influenced by the consumed foodstuffs as well as the intestinal microbiota which have, therefore, to be considered for a reliable prediction of the hazardous potential of contaminated foods. Current knowledge regarding these single aspects is summarized in this review aiming for risk-oriented diagnostics for enteropathogenic B. cereus.

Bacillus cereus Decreases NHE and CLO Exotoxin Synthesis to Maintain Appropriate Proteome Dynamics During Growth at Low Temperature

Cellular proteomes and exoproteomes are dynamic, allowing pathogens to respond to environmental conditions to sustain growth and virulence. Bacillus cereus is an important food-borne pathogen causing intoxication via emetic toxin and/or multiple protein exotoxins. Here, we compared the dynamics of the cellular proteome and exoproteome of emetic B. cereus cells grown at low (16 °C) and high (30 °C) temperature. Tandem mass spectrometry (MS/MS)-based shotgun proteomics analysis identified 2063 cellular proteins and 900 extracellular proteins. Hierarchical clustering following principal component analysis indicated that in B. cereus the abundance of a subset of these proteins—including cold-stress responders, and exotoxins non-hemolytic enterotoxin (NHE) and hemolysin I (cereolysin O (CLO))—decreased at low temperature, and that this subset governs the dynamics of the cellular proteome. NHE, and to a lesser extent CLO, also contributed significantly to exoproteome dynamics; with decreased abundances in the low-temperature exoproteome, especially in late growth stages. Our data therefore indicate that B. cereus may reduce its production of secreted protein toxins to maintain appropriate proteome dynamics, perhaps using catabolite repression to conserve energy for growth in cold-stress conditions, at the expense of virulence.

Enterotoxin Genes, Antibiotic Susceptibility, and Biofilm Formation of Low-Temperature-Tolerant Bacillus cereus Isolated from Green Leaf Lettuce in the Cold Chain

The prevalence and characteristics of low-temperature-tolerant Bacillus cereus (psychrotolerant B. cereus) in green leaf lettuce collected during cold chain were investigated. Among the 101 isolated B. cereus samples, only 18 were capable of growth at 7 °C, and these isolates shared potential health hazard characteristics with mesophilic isolates. Most psychrotolerant B. cereus isolates contained various combinations of nheA, nheB, nheC, hblA, hblA, hblC, hblD, cytK, and entFM. Most isolates of psychrotolerant B. cereus possessed at least two enterotoxin genes and 28% of isolates harbored tested nine enterotoxin genes. Additionally, the psychrotolerant B. cereus isolates showed resistance to tetracycline and rifampin and intermediate levels of resistance to clindamycin. A total of 23% of isolates among psychrotolerant B. cereus displayed a high level of biofilm formation at 7 °C than at 10 °C or 30 °C. The results of this study indicate that cold distribution and storage for green leaf lettuce may fail to maintain food safety due to the presence of enterotoxigenic, antibiotic-resistant, and strong biofilm forming psychrotolerant B. cereus isolates, which therefore poses a potential health risk to the consumer. Our findings provide the first account of the prevalence and characteristics of psychrotolerant B. cereus isolated from green leaf lettuce during cold storage, suggesting a potential hazard of psychrotolerant B. cereus isolates to public health and the food industry.

Risk presented to minimally processed chilled foods by psychrotrophic Bacillus cereus

BACKGROUND

Spores of psychrotrophic Bacillus cereus may survive the mild heat treatments given to minimally processed chilled foods. Subsequent germination and cell multiplication during refrigerated storage may lead to bacterial concentrations that are hazardous to health.

SCOPE AND APPROACH

This review is concerned with the characterisation of factors that prevent psychrotrophic B. cereus reaching hazardous concentrations in minimally processed chilled foods and associated foodborne illness. A risk assessment framework is used to quantify the risk associated with B. cereus and minimally processed chilled foods.

KEY FINDINGS AND CONCLUSIONS

Bacillus cereus is responsible for two types of food poisoning, diarrhoeal (an infection) and emetic (an intoxication); however, no reported outbreaks of food poisoning have been associated with B. cereus and correctly stored commercially-produced minimally processed chilled foods. In the UK alone, more than 1010 packs of these foods have been sold in recent years without reported illness, thus the risk presented is very low. Further quantification of the risk is merited, and this requires additional data. The lack of association between diarrhoeal food poisoning and correctly stored commercially-produced minimally processed chilled foods indicates that an infectious dose has not been reached. This may reflect low pathogenicity of psychrotrophic strains. The lack of reported association of psychrotrophic B. cereus with emetic illness and correctly stored commercially-produced minimally processed chilled foods indicates that a toxic dose of the emetic toxin has not been formed. Laboratory studies show that strains form very small quantities of emetic toxin at chilled temperatures.

Optimum Thermal Processing for Extended Shelf-Life (ESL) Milk

Extended shelf-life (ESL) or ultra-pasteurized milk is produced by thermal processing using conditions between those used for traditional high-temperature, short-time (HTST) pasteurization and those used for ultra-high-temperature (UHT) sterilization. It should have a refrigerated shelf-life of more than 30 days. To achieve this, the thermal processing has to be quite intense. The challenge is to produce a product that has high bacteriological quality and safety but also very good organoleptic characteristics. Hence the two major aims in producing ESL milk are to inactivate all vegetative bacteria and spores of psychrotrophic bacteria, and to cause minimal chemical change that can result in cooked flavor development. The first aim is focused on inactivation of spores of psychrotrophic bacteria, especially Bacillus cereus because some strains of this organism are pathogenic, some can grow at ≤7 °C and cause spoilage of milk, and the spores of some strains are very heat-resistant. The second aim is minimizing denaturation of β-lactoglobulin (β-Lg) as the extent of denaturation is strongly correlated with the production of volatile sulfur compounds that cause cooked flavor. It is proposed that the heating should have a bactericidal effect, B* (inactivation of thermophilic spores), of >0.3 and cause ≤50% denaturation of β-Lg. This can be best achieved by heating at high temperature for a short holding time using direct heating, and aseptically packaging the product.

A Study To Assess the Numbers and Prevalence of Bacillus cereus and Its Toxins in Pasteurized Fluid Milk

Bacillus cereus is a pathogenic adulterant of raw milk and can persist as spores and grow in pasteurized milk. The objective of this study was to determine the prevalence of B. cereus and its enterotoxins in pasteurized milk at its best-before date when stored at 4, 7, and 10°C. More than 5.5% of moderately temperature-abused products (stored at 7°C) were found to contain >10⁵ CFU/mL B. cereus , and about 4% of them contained enterotoxins at a level that may result in foodborne illness; in addition, more than 31% of the products contained >10⁵ CFU/mL B. cereus and associated enterotoxins when stored at 10°C. Results from a growth kinetic study demonstrated that enterotoxin production by B. cereus in pasteurized milk can occur in as short as 7 to 8 days of storage at 7°C. The higher B. cereus counts were associated with products containing higher butterfat content or with those produced using the conventional high-temperature, short-time pasteurization process. Traditional indicators, aerobic colony counts and psychrotrophic counts, were found to have no correlation with level of B. cereus in milk. The characterization of 17 representative B. cereus isolates from pasteurized milk revealed five toxigenic gene patterns, with all the strains carrying genes encoding for diarrheal toxins but not for an emetic toxin, and with one strain containing all four diarrheal enterotoxin genes (nheA, entFM, hblC, and cytK). The results of this study demonstrate the risks associated even with moderately temperature-abused pasteurized milk and the necessity of a controlled cold chain throughout the shelf life of fluid milk to enhance product safety and minimize foodborne illness.

Potential antimicrobial and antiproliferative activities of autochthonous starter cultures and protease EPg222 in dry-fermented sausages

This work studied the presence of nitrogen compounds with bioactive properties in Iberian pork sausages that were manufactured using different autochthonous starter cultures and protease EPg222.

From genome to toxicity: a combinatory approach highlights the complexity of enterotoxin production in Bacillus cereus

In recent years Bacillus cereus has gained increasing importance as a food poisoning pathogen. It is the eponymous member of the B. cereus sensu lato group that consists of eight closely related species showing impressive diversity of their pathogenicity. The high variability of cytotoxicity and the complex regulatory network of enterotoxin expression have complicated efforts to predict the toxic potential of new B. cereus isolates. In this study, comprehensive analyses of enterotoxin gene sequences, transcription, toxin secretion and cytotoxicity were performed. For the first time, these parameters were compared in a whole set of B. cereus strains representing isolates of different origin (food or food poisoning outbreaks) and of different toxic potential (enteropathogenic and apathogenic) to elucidate potential starting points of strain-specific differential toxicity. While toxin gene sequences were highly conserved and did not allow for differentiation between high and low toxicity strains, comparison of nheB and hblD enterotoxin gene transcription and Nhe and Hbl protein titers revealed not only strain-specific differences but also incongruence between toxin gene transcripts and toxin protein levels. With one exception all strains showed comparable capability of protein secretion and so far, no secretion patterns specific for high and low toxicity strains were identified. These results indicate that enterotoxin expression is more complex than expected, possibly involving the orchestrated interplay of different transcriptional regulator proteins, as well as posttranscriptional and posttranslational regulatory mechanisms plus additional influences of environmental conditions.

Growth and enterotoxin production of Bacillus cereus in cow, goat, and sheep milk

The aim of this study was to compare Bacillus cereus growth rates and diarrhoeal enterotoxin production in raw and pasteurized goat, sheep, and cow milk in terms of storage conditions. Milk samples were inoculated with B. cereus (CCM 2010), which produces diarrhoeal enterotoxins. Enterotoxin production was tested by ELISA (Enzyme-Linked Immunosorbent Assay), and the count of B. cereus was determined by the plate method. With raw cow milk, B. cereus growth and enterotoxin production can be completely suppressed; in raw goat and sheep milk, enterotoxin was produced at 22 °C. In pasteurized cow, goat, and sheep milk, the B. cereus count increased under all storage conditions, with more rapid growth being observed at 15 °C (sheep milk) and 22 °C (cow and goat milk). Enterotoxin presence was detected at 15 °C and 22 °C, and with pasteurized cow milk also at 8 °C. Our model experiments have determined that B. cereus multiplication and subsequent enterotoxin production depend on storage temperature and milk type.

Antimicrobial Effect of Nisin against Bacillus cereus in Beef Jerky during Storage

The microbial distribution of raw materials and beef jerky, and the effect of nisin on the growth of Bacillus cereus inoculated in beef jerky during storage, were studied. Five strains of pathogenic B. cereus were detected in beef jerky, and identified with 99.8% agreement using API CHB 50 kit. To evaluate the effect of nisin, beef jerky was inoculated with approximately 3 Log CFU/g of B. cereus mixed culture and nisin (100 IU/g and 500 IU/g). During the storage of beef jerky without nisin, the number of mesophilic bacteria and B. cereus increased unlikely for beef jerky with nisin. B. cereus started to grow after 3 d in 100 IU nisin/g treatment, and after 21 d in 500 IU nisin/g treatment. The results suggest that nisin could be an effective approach to extend the shelf-life, and improve the microbial safety of beef jerky, during storage.

Discrimination between mesophilic and psychrotolerant strains in the Bacillus cereus group based on the PstI digestion of the pycA gene

A simple and rapid assay for the detection of Bacillus weihenstephanensis isolates and other psychrotolerant strains in the Bacillus cereus group was developed. It is based on the presence of a nucleotide substitution at position 795 on the housekeeping pycA gene in all B. weihenstephanensis strains. This mutation creates a PstI recognition site. It is absent in mesophilic strains in the B. cereus group. The pycA gene is amplified by PCR and the amplicons submitted to PstI digestions. In mesophilic strains, a single band of 1,718 bp in length is visualised on an agarose gel. In B. weihenstephanensis strains and in all other psychrotolerant strains from the B. cereus group, the amplicons are cleaved and two bands of 1,175 and 543 bp, respectively, are visualised. This method could be used for the screening of B. cereus collections and for the identification of psychrotolerant and mesophilic isolates from different environments.

A probability model for enterotoxin production of Bacillus cereus as a function of pH and temperature

Bacillus cereus is frequently isolated from a variety of foods, including vegetables, dairy products, meats, and other raw and processed foods. The bacterium is capable of producing an enterotoxin and emetic toxin that can cause severe nausea, vomiting, and diarrhea. The objectives of this study were to assess and model the probability of enterotoxin production of B. cereus in a broth model as affected by the broth pH and storage temperature. A three-strain mixture of B. cereus was inoculated in tryptic soy broth adjusted to pH 5.0, 6.0, 7.2, 8.0, and 8.5, and the samples were stored at 15, 20, 25, 30, and 35°C for 24 h. A total of 25 combinations of pH and temperature, each with 10 samples, were tested. The presence of enterotoxin in broth was assayed using a commercial test kit. The probabilities of positive enterotoxin production in 25 treatments were fitted with a logistic regression to develop a probability model to describe the probability of toxin production as a function of pH and temperature. The resulting model showed that the probabilities of enterotoxin production of B. cereus in broth increased as the temperature increased and/or as the broth pH approached 7.0. The model described the experimental data satisfactorily and identified the boundary of pH and temperature for the production of enterotoxin. The model could provide information for assessing the food poisoning risk associated with enterotoxins of B. cereus and for the selection of product pH and storage temperature for foods to reduce the hazards associated with B. cereus.

Growth of enterotoxin producing Bacillus cereus in meat substrate at 10ºC and 30ºC

The behaviour of enterotoxin-producing Bacillus cereus in meat was investigated by inoculating spore suspensions of five cultures into meat substrate (pH 5.8) and incubating at 10ºC and 30ºC. The bacterial populations were evaluated after different times by plate counts in nutrient agar. All the cultures presented growth at 30ºC with the generation time varying from 28.8 to 36.0 minutes. Three cultures also presented growth at 10ºC with generation times between 10.16 and 28.38 h. Considering the results, it was concluded that meat kept at abusive temperatures would be subject to development of this microorganism.

Growth characteristics and development of a predictive model for Bacillus cereus in fresh wet noodles with added ethanol and thiamine

Response surface methodology was used to determine growth characteristics and to develop a predictive model to describe specific growth rates of Bacillus cereus in wet noodles containing a combination of ethanol (0 to 2% [vol/wt]) and vitamin B(1) (0 to 2 g/liter). B. cereus F4810/72, which produces an emetic toxin, was used in this study. The noodles containing B. cereus were incubated at 10°C. The growth curves were fitted to the modified Gompertz equation using nonlinear regression, and the growth rate values from the curves were used to establish the predictive model using a response surface methodology quadratic polynomial equation as a function of concentrations of ethanol and vitamin B(1). The model was shown to fit the data very well (r(2) = 0.9505 to 0.9991) and could be used to accurately predict growth rates. The quadratic polynomial model was validated, and the predicted growth rate values were in good agreement with the experimental values. The polynomial model was found to be an appropriate secondary model for growth rate (GR) and lag time (LT) based on the correlation of determination (r(2) = 0.9899 for GR, 0.9782 for LT), bias factor (B(f) = 1.006 for GR, 0.992 for LT), and accuracy factor (A(f) = 1.024 for GR, 1.011 for LT). Thus, this model holds great promise for use in predicting the growth of B. cereus in fresh wet noodles using only the bacterial concentration, an important contribution to the manufacturing of safe products.

Regulation of toxin production by Bacillus cereus and its food safety implications

Toxin expression is of utmost importance for the food-borne pathogen B. cereus, both in food poisoning and non-gastrointestinal host infections as well as in interbacterial competition. Therefore it is no surprise that the toxin gene expression is tightly regulated by various internal and environmental signals. An overview of the current knowledge regarding emetic and diarrheal toxin transcription and expression is presented in this review. The food safety aspects and management tools such as temperature control, food preservatives and modified atmosphere packaging are discussed specifically for B. cereus emetic and diarrheal toxin production.

The influence of headspace and dissolved oxygen level on growth and haemolytic BL enterotoxin production of a psychrotolerant Bacillus weihenstephanensis isolate on potato based ready-to-eat food products

The major objective of this study was to determine the influence of the initial headspace and dissolved O(2) level and vacuum packaging on growth and diarrhoeal enterotoxin production by Bacillus weihenstephanensis on potato based ready-to-eat food products. In general, the lower the initial headspace or dissolved O(2) level the slower the maximum growth rate (μ(max), log(10) CFU g(-1) d(-1)), the longer the lag phase duration (λ, d) and the smaller the maximum population density (N(max), log(10) CFU g(-1)) became. The slowest μ(max), the longest λ and the smallest N(max) were generally found for growth under vacuum packaging. This implies shorter shelf-lives will occur at higher initial headspace or dissolved O(2) levels as the growth of B. weihenstephanensis to the infective dose of 10(5) CFU g(-1) in such atmospheres takes a shorter time. Significant consumption of dissolved O(2) only occurred when growth shifted from the lag to the exponential phase and growth generally transitioned from the exponential to the stationary phase when the dissolved O(2) levels fell below ca. 75 ppb. Diarrhoeal enterotoxin production (determined via detection of the L2 component of haemolytic BL) was similar for growth under initial headspace O(2) levels of 1-20.9%, and was only reduced when growth took place under vacuum packaging. The reduction in L2 production when growth took place under vacuum was most probably related to the low final cell densities observed under this condition. Both growth and L2 production were inhibited over a 32-day incubation period at 7 °C by 40% CO(2) irrespective of the headspace or dissolved O(2) levels. The results illustrate the importance of residual O(2) and CO(2) on the shelf-stability and safety of modified atmosphere packaged potato based ready-to-eat food products with regards to B. weihenstephanensis.

Occurrence of psychrotolerant Bacillus cereus group strains in ice creams

The occurrences of Bacillus cereus group strains in 40 ice cream samples were investigated. Among 109 isolated B. cereus group strains confirmed by 16S rDNA sequence analysis only 50 were identified as B. cereus and one as B. thuringiensis by using FDA (U.S. Food and Drug Administration) standard, indicating the two identification standards were highly inconsistent. Furthermore, the psychrotolerant growth properties and the occurrence of specific psychrotolerant genes of the isolates were also studied. Both psychrotolerant 16S rDNA fragments and enterotoxic genes could be detected among mesophilic and psychrotolerant strains. No relationship among psychrotolerance, presence of psychrotolerant 16S rDNA fragments and enterotoxic genes were found and the specific cspA fragment was only detected in a small fraction (9.5%) of the psychrotolerant isolates. One psychrotolerant isolate Bw2-1 was identified as B. weihenstephanensis, but no clear distinguishing characteristics between B. weihenstephanensis and psychrotolerant B. cereus were found. These results might be of importance for gaining further understanding of the growth properties of B. weihenstephanensis and psychrotolerant B. cereus as well as their contribution to food poisoning.

Survival during cooking and growth from spores of diarrheal and emetic types of Bacillus cereus in rice

Bacillus cereus is a gram-positive, spore-forming, facultative anaerobe that is responsible for two types of gastrointestinal diseases: emesis and diarrhea. A significant difference in the D(95 degrees C)-values of spores of the emetic and the diarrheal types was initially determined. A mixture of B. cereus spores of the diarrheal type was inoculated into cooked rice. At inoculation levels of 2.5 x 10(2) spores per g of rice, cell numbers of 6.64 log were detected after 22 h at 20 degrees C and 6.81 log after 34 h at 17 degrees C, whereas at 12 degrees C the counts did not go above 4.0 log even after 48 h. When added to raw rice before cooking at inoculum levels of 10(3)/g, the number of viable spores decreased by 2 log, and a <1-log increase in cell numbers occurred after holding at 20 degrees C for 24 h. In contrast, the emetic spores survived and increased approximately 20-fold. Nonhemolytic enterotoxin was not detected in cooked rice at cell numbers of 8.0 log. Results here provide evidence that the absence of foodborne illness caused by the B. cereus diarrheal biotype with rice as the vehicle is due to the inability of their spores to survive and grow following standard heat processing procedures.

Levels and toxigenicity of Bacillus cereus and Clostridium perfringens from retail seafood

For the period 1990 through 2003, seafood was the most commonly identified food linked to foodborne outbreaks in the United States. Fish as a commodity has rarely been examined for the presence of Bacillus cereus in particular. For the present study, 347 fresh and processed retail seafood samples were examined for the presence of Clostridium botulinum, Clostridium perfringens, and B. cereus. The presence of C. botulinum was not confirmed in any of the isolates, but C. perfringens was confirmed in 17 samples. One of the C. perfringens isolates possessed the enterotoxin gene, as determined by PCR. In contrast, 62 confirmed B. cereus isolates were obtained from separate samples at levels ranging from 3.6 to > 1,100 CFU/g. Thirty (48%) of 62 isolates produced both the hemolysin BL (HBL) and nonhemolytic (NHE) enterotoxins, and 58 (94%) and 31 (50%) produced NHE or HBL toxins, respectively. The presence of at least one of the three genes of the NHE complex was detected in 99% of the isolates; 69% of the isolates possessed all three genes. In contrast, 71% of the isolates possessed at least one of the three genes of the HBL complex, and 37% possessed all three HBL gene components. Fifty of the 62 B. cereus isolates were from imported seafood, and 19 (38%) of these samples were at levels > 100 CFU/g. Twelve of the 14 highest enterotoxin assay results were from isolates from imported food. Only one B. cereus isolate possessed the cereulide synthetase gene, ces; this isolate also possessed the genes for the three-component HBL and NHE complexes. A majority of enterotoxin-producing isolates were resistant to 2 of 10 antibiotics tested, ceftriaxone and clindamycin. Our results demonstrate the potential of seafood as a vehicle for foodborne illness caused by B. cereus, in particular the enterotoxin-producing genotype.

The occurrence of Bacillus cereus, B. thuringiensis and B. mycoides in Chinese pasteurized full fat milk

In 2006, a total of 54 samples of pasteurized full fat milk packaged in cartons were collected in spring and in autumn from chain supermarkets in Wuhan, China. The samples were examined and enumerated by MPN methods strictly according to guidelines laid out in US FDA/CFSAN BAM Chapter 14. Among 102 isolated B. cereus-like bacteria, 92 isolates were identified to be B. cereus, 9 B. thuringiensis and 1 B. mycoides. It was found that the occurrences of B. cereus were 71.4% and 33.3% in spring and in autumn samples respectively and the average count among the positive samples was 11.7 MPN/ml. The PCR detection results revealed that the enterotoxin genes hblA, hblC, hblD, nheA, nheB and nheC occurred in B. cereus isolates with frequencies of 37.0%, 66.3%, 71.7%, 71.7%, 62.0% and 71.7% respectively. Nine B. thuringiensis isolates were also identified from six pasteurized milk samples, and most of them harbored six enterotoxic genes and the insecticidal toxin cry1A gene. The single B. mycoides isolate harbored nheA and nheC genes. The data provides information for further evaluating the effect of B. cereus-like bacteria on food safety of Chinese milk products.