Further Insights into the Toxicity of Bacillus cytotoxicus Based on Toxin Gene Profiling and Vero Cell Cytotoxicity Assays

New insights into Bacillus cytotoxicus sources, screening, toxicity, and persistence in food production facilities

Bacillus cytotoxicus is a thermotolerant member of the Bacillus cereus group. It has been linked to rare, but at times fatal cases of diarrheal disease and might be missed at routine diagnostic screening temperatures commonly used for the B. cereus group. The pathogen is mostly found on dehydrated foods containing potato starch or insects. How it enters the food chain or whether it persists in food producing environments is largely unknown. Increased consumption of insects and convenience foods in Europe and the lack of information on the persistence of B. cytotoxicus in food environments and its virulence demand for further characterization. In this study, we aimed to obtain a better understanding of i) the food sources of B. cytotoxicus, ii) screening temperatures needed for its isolation from food matrices, iii) cytotoxicity of the organism, and iv) its ecological niche and potential epidemiological links. To this end, 112 food samples were collected, with a focus on foods exhibiting low water activity. The samples were screened for B. cytotoxicus at 42 °C and at 50 °C. Presumptive isolates were characterized by cytK-1 toxin gene PCR for differentiation of B. cytotoxicus from other B. cereus group members. Vero cell cytotoxicity assays were performed, and selected isolates were sequenced. Our results show that screening at 42 °C might be insufficient for detecting B. cytotoxicus in foods that harbor other less thermophilic Bacillus species. When screening at 50 °C, B. cytotoxicus was detected in 23% of the food samples (n = 26 isolates). The highest prevalence was detected in mashed potato products (82%) and potato flakes (67%). In contrast, a wide range of products not containing any potato ingredients did not yield B. cytotoxicus isolates. All B. cytotoxicus isolates exhibited either low or no detectable cytotoxicity. WGS analysis revealed that a highly toxic isolate is closely related to the French outbreak strain NVH 391-98. In addition, we could show that two isolates sampled 5 years apart from the same production facility only differed by seven SNPs, making it likely that B. cytotoxicus is able to persist in production facilities over a long time. Interestingly, the reoccurring strain possessed an additional plasmid and did not show cytotoxic potential when re-isolated after 5 years.

Detection of Enterotoxigenic Psychrotrophic Presumptive Bacillus cereus and Cereulide Producers in Food Products and Ingredients

In the last decade, foodborne outbreaks and individual cases caused by bacterial toxins showed an increasing trend. The major contributors are enterotoxins and cereulide produced by Bacillus cereus, which can cause a diarrheal and emetic form of the disease, respectively. These diseases usually induce relatively mild symptoms; however, fatal cases have been reported. With the aim to detected potential toxin producers that are able to grow at refrigerator temperatures and subsequently produce cereulide, we screened the prevalence of enterotoxin and cereulide toxin gene carriers and the psychrotrophic capacity of presumptive B. cereus obtained from 250 food products (cereal products, including rice and seeds/pulses, dairy-based products, dried vegetables, mixed food, herbs, and spices). Of tested food products, 226/250 (90.4%) contained presumptive B. cereus, which communities were further tested for the presence of nheA, hblA, cytK-1, and ces genes. Food products were mainly contaminated with the nheA B. cereus carriers (77.9%), followed by hblA (64.8%), ces (23.2%), and cytK-1 (4.4%). Toxigenic B. cereus communities were further subjected to refrigerated (4 and 7 °C) and mild abuse temperatures (10 °C). Overall, 77% (94/121), 86% (104/121), and 100% (121/121) were able to grow at 4, 7, and 10 °C, respectively. Enterotoxin and cereulide potential producers were detected in 81% of psychrotrophic presumptive B. cereus. Toxin encoding genes nheA, hblA, and ces gene were found in 77.2, 55, and 11.7% of tested samples, respectively. None of the psychrotrophic presumptive B. cereus were carriers of the cytotoxin K-1 encoding gene (cytK-1). Nearly half of emetic psychrotrophic B. cereus were able to produce cereulide in optimal conditions. At 4 °C none of the examined psychrotrophs produced cereulide. The results of this research highlight the high prevalence of B. cereus and the omnipresence of toxin gene harboring presumptive B. cereus that can grow at refrigerator temperatures, with a focus on cereulide producers.

Bacillus cytotoxicus Genomics: Chromosomal Diversity and Plasmidome Versatility

Bacillus cytotoxicus is the thermotolerant representative of the Bacillus cereus group. This group, also known as B. cereus sensu lato, comprises both beneficial and pathogenic members and includes psychrotolerant and thermotolerant species. Bacillus cytotoxicus was originally recovered from a fatal outbreak in France in 1998. This species forms a remote cluster from the B. cereus group members and reliably contains the cytk-1 gene, coding for a cytotoxic variant of cytotoxin K. Although this species was originally thought to be homogenous, intra-species diversity has been recently described with four clades, six random amplified polymorphic DNA (RAPD) patterns, and 11 plasmids profiles. This study aimed to get new insights into the genomic diversity of B. cytotoxicus and to decipher the underlying chromosomal and plasmidial variations among six representative isolates through whole genome sequencing (WGS). Among the six sequenced strains, four fitted the previously described genomic clades A and D, while the remaining two constituted new distinct branches. As for the plasmid content of these strains, three large plasmids were putatively conjugative and three small ones potentially mobilizable, harboring coding genes for putative leaderless bacteriocins. Mobile genetic elements, such as prophages, Insertion Sequences (IS), and Bacillus cereus repeats (bcr) greatly contributed to the B. cytotoxicus diversity. As for IS elements and bcr, IS3 and bcr1 were the most abundant elements and, along with the group II intron B.c.I8, were found in all analyzed B. cytotoxicus strains. When compared to other B. cytotoxicus strains, the type-strain NVH 391-98 displayed a relatively low number of IS. Our results shed new light on the contribution of mobile genetic elements to the genome plasticity of B. cytotoxicus and their potential role in horizontal gene transfer.

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.

Bacillus cytotoxicus-A potentially virulent food-associated microbe

Bacillus cytotoxicus is a member of the Bacillus cereus group with the ability to grow at high temperatures (up to 52℃) and to synthesize cytotoxin K-1, a diarrhoeagenic cytotoxin, which appears to be unique to this species and more cytotoxic than the cytotoxin K-2 produced by other members of this group. Only a few isolates of this species have been characterized with regard to their cytotoxic effects, and the role of cytotoxin K-1 as a causative agent of food poisoning remains largely unclear. Bacillus cytotoxicus was initially isolated from a food-borne outbreak, which led to three deaths, and the organism has since been linked to other outbreaks all involving plant-based food matrices. Other studies, as well as food-borne incidents reported to the UK Food Standards Agency, detected B. cytotoxicus in insect-related products and in dried food products. With insect-related food becoming increasingly popular, the association with this pathogen is concerning, requiring further investigation and evidence to protect public health. This review summarizes the current knowledge around B. cytotoxicus and highlights gaps in the literature from a food safety perspective.