Bacillus cereus | |
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B. cereus colonies on a sheep-blood agar plate | |
Scientific classification | |
Domain: | Bacteria |
Phylum: | Bacillota |
Class: | Bacilli |
Order: | Caryophanales |
Family: | Bacillaceae |
Genus: | Bacillus |
Species: | B. cereus |
Binomial name | |
Bacillus cereus Frankland & Frankland 1887 | |
Biovars | |
Bacillus cereus is a Gram-positive rod-shaped bacterium commonly found in soil, food, and marine sponges. [1] The specific name, cereus, meaning "waxy" in Latin, refers to the appearance of colonies grown on blood agar. Some strains are harmful to humans and cause foodborne illness due to their spore-forming nature, while other strains can be beneficial as probiotics for animals, and even exhibit mutualism with certain plants. [2] [3] [4] B. cereus bacteria may be aerobes or facultative anaerobes, and like other members of the genus Bacillus , can produce protective endospores. They have a wide range of virulence factors, including phospholipase C, cereulide, sphingomyelinase, metalloproteases, and cytotoxin K, many of which are regulated via quorum sensing. [5] [6] B. cereus strains exhibit flagellar motility. [7]
The Bacillus cereus group comprises seven closely related species: B. cereussensu stricto (referred to herein as B. cereus), B. anthracis , B. thuringiensis , B. mycoides , B. pseudomycoides , and B. cytotoxicus ; [8] or as six species in a Bacillus cereus sensu lato: B. weihenstephanensis , B. mycoides, B. pseudomycoides, B. cereus, B. thuringiensis, and B. anthracis. [9] A phylogenomic analysis combined with average nucleotide identity (ANI) analysis revealed that the B. anthracis species also includes strains annotated as B. cereus and B. thuringiensis. [10]
Colonies of B. cereus were originally isolated from a gelatine plate left exposed to the air in a cow shed in 1887. [11] In the 2010s, examination of warning letters issued by the US Food and Drug Administration issued to pharmaceutical manufacturing facilities addressing facility microbial contamination revealed that the most common contaminant was B. cereus. [12]
Several new enzymes have been discovered in B. cereus, such as AlkC and AlkD, both of which are involved in DNA repair. [13]
B. cereus is a rod-shaped bacterium with a Gram-positive cell envelope. Depending on the strain, it may be aerobic or facultatively anaerobic. Most strains are mesophilic, having an optimal temperature between 25 °C and 37 °C, and neutralophilic, preferring neutral pH, but some have been found to grow in environments with much more extreme conditions. [14]
These bacteria are both spore-forming and biofilm-forming, presenting a large challenge to the food industry due to their contamination capability. Biofilms of B. cereus most commonly form on air-liquid interfaces or on hard surfaces such as glass. B. cereus display flagellar motility, which has been shown to aid in biofilm formation via an increased ability to reach surfaces suitable for biofilm formation, to spread the biofilm over a larger surface area, and to recruit planktonic, or single, free-living bacteria. [7] Biofilm formation may also occur while in spore form due to varying adhesion ability of spores. [15]
Their flagella are peritrichous, meaning there are many flagella located all around the cell body that can bundle together at a single location on the cell to propel it. This flagellar property also allows the cell to change directions of movement depending on where on the cell the flagellum filaments come together to generate movement. [15] [16]
Some studies and observations have shown that silica particles the size of a few nanometers have been deposited in a spore coat layer in the extracytoplasmic region of the Bacillus cereus spore. The layer was first discovered by the use of scanning transmission electron microscopy (STEM), however the images taken did not have resolution high enough to determine the precise location of the silica. Some investigators hypothesize that the layer helps different spores from sticking together. It has also been shown to provide some resistance to acidic environments. The silica coat is related to the permeability of the spore's inner membrane. Strong mineral acids are able to break down spore permeability barriers and kill the spore. However, when the spore has a silica coating, it may reduce the permeability of the membrane and provide resistance to many acids. [17]
Bacillus cereus has mechanisms for both aerobic and anaerobic respiration, making it a facultative anaerobe. [18] Its aerobic pathway consists of three terminal oxidases: cytochrome aa3, cytochrome caa3, and cytochrome bd, the use of each dependent on the amount of oxygen present in the environment. [19] The B. cereus genome encodes genes for metabolic enzymes including NADH dehydrogenases, succinate dehydrogenase, complex III, and cytochrome c oxidase, as well as others. Bacillus cereus can metabolize several different compounds to create energy, including carbohydrates, proteins, peptides, and amino acids. [18]
The Embden-Meyerhof pathway is the predominant pathway used by Bacillus cereus to catabolize glucose at every stage of the cell's development, according to estimates of a radiorespirometric method of glucose catabolism. This is true at times of germinative phases, as well as sporogenic phases. At the filamentous, granular, forespore, and transitional stages, the Embden-Meyerhof pathway was responsible for the catabolism of 98% of the cell's glucose. The remainder of the glucose was catabolized by the hexose monophosphate oxidative pathway. [20]
Analysis of the core genome of B. cereus reveals a limited presence of enzymes meant for breakdown of polysaccharides and a prevalence of proteases and amino acid degradation and transport pathways, indicating that their preferred diet consists of proteins and their breakdown products. [21]
An isolate of a bacterium found to produce PHBs was identified as B. cereus through analysis of 16S rRNA sequences as well as similarity of morphological and biochemical characteristics. PHBs may be produced when there is excess carbon or limited essential nutrients present in the environment, and they are later broken down by the microbe as a fuel source under starvation conditions. This indicates the potential role of B. cereus in producing biodegradable plastic substitutes. PHB production was highest when provided with glucose as a carbon source. [22]
The genome of B. cereus has been characterized and shown to contain over 5 million bp of DNA. Out of these, more than 5500 protein-encoding genes have been identified, of which the top categories of genes with known functions include: metabolic processes, processing of proteins, virulence factors, response to stress, and defense mechanisms. Many of the genes categorized as virulence factors, stress responses, and defense mechanisms encode factors in antibiotic resistance. [6] There are approximately 600 genes which are common in 99% of the taxa of B. cereus sensu lato, which constitutes around 1% of all genes in the pan-genome. Due to the prevalence of horizontal gene transfer among bacteria, the pan-genome of B. cereus is continually expanding. [23] The GC content of its DNA across all strains is approximately 35%. [24]
Following exposure to non-lethal acid shock at pH 5.4-5.5, the arginine deiminase gene in B. cereus, arcA, shows substantial up-regulation. This gene is part of the arcABC operon which is induced by low-pH environments in Listeria monocytogenes, and is associated with growth and survival in acidic environments. This suggests that this gene is also important for survival of B. cereus in acidic environments. [25]
The activation of virulence factors has been shown to be transcriptionally regulated via quorum-sensing in B. cereus. The activation of many virulence factors secreted is dependent on the activity of the Phospholipase C regulator (PlcR), a transcriptional regulator which is most active at the beginning of the stationary phase of growth. A small peptide called PapR acts as the effector in the quorum-sensing pathway, and when reimported into the cell, it interacts with PlcR to activate transcription of these virulence genes. [6] When point mutations were introduced into the plcR gene using the CRISPR/Cas9 system, it was observed that the mutated bacteria lost their hemolytic and phospholipase activity. [26]
The flagella of B. cereus are encoded by 2 to 5 fla genes, depending on the strain. [7]
For the isolation and enumeration of B. cereus, there are two standardized methods by International Organization for Standardization (ISO): ISO 7932 and ISO 21871. Because of B. cereus' ability to produce lecithinase and its inability to ferment mannitol, there are some proper selective media for its isolation and identification such as mannitol-egg yolk-polymyxin (MYP) and polymyxin-pyruvate-egg yolk-mannitol-bromothymol blue agar (PEMBA). B. cereus colonies on MYP have a violet-red background and are surrounded by a zone of egg-yolk precipitate. [27]
Below is a list of differential techniques and results that can help to identify B. cereus from other bacteria and Bacillus species. [28]
The Central Public Health Laboratory in the United Kingdom tests for motility, hemolysis, rhizoid growth, susceptibility to γ-phage, and fermentation of ammonium salt-based glucose but no mannitol, arabinose, or xylose. [27]
The optimal growth temperature range for B. cereus is 30-40 °C. [29] At 30 °C (86 °F), a population of B. cereus can double in as little as 20 minutes or as long as 3 hours, depending on the food product. Spores of B. cereus are not metabolically active, but can rapidly become active and begin replicating once they encounter adequate growth conditions. [30] [ better source needed ]
Food | Minutes to double, 30 °C (86 °F) | Hours to multiply by 1,000,000 |
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Milk | 20–36 | 6.6 - 12 |
Cooked rice | 26–31 | 8.6 - 10.3 |
Infant formula | 56 | 18.6 |
Like most Bacilli, the most common ecosystem of Bacillus cereus is the soil. In concert with arbuscular mycorrhiza (and Rhizobium leguminosarum in clover), they can up-regulate plant growth in heavy metal soils by decreasing heavy metal concentrations via bioaccumulation and biotransformation in addition to increasing phosphorus, nitrogen, and potassium uptake in certain plants. [4] B. cereus was also shown to aid in survival of earthworms in heavy metal soils resulting from the use of metal-based fungicides, showing increases in biomass, reproduction and reproductive viability, and a decrease in metal content of tissues in those inoculated with the bacterium. [31] These results suggest strong possibilities for its application in ecological bioremediation. Evidence of bioremediation potential by Bacillus cereus was also found in the aquatic ecosystem, where organic nitrogen and phosphorus wastes polluting a eutrophic lake were broken down in the presence of B. cereus. [29]
In a study measuring the ability of B. cereus to degrade keratin in chicken feathers, bacteria were found to sufficiently biodegrade keratin via hydrolytic mechanisms. These results indicate its potential to degrade keratinous waste from the poultry industry for potential recycling of the byproducts. [32]
B. cereus competes with Gram-negative bacteria species such as Salmonella and Campylobacter in the gut; its presence reduces the number of Gram-negative bacteria, specifically via antibiotic activity via enzymes such as cereins that impede their quorum sensing ability and exhibit bactericidal activity. [33] [34] In food animals such as chickens, [35] rabbits [36] and pigs, [37] some harmless strains of B. cereus are used as a probiotic feed additive to reduce Salmonella in the animals' intestines and cecum. This improves the animals' growth, as well as food safety for humans who eat them. In addition, B. cereus create and release enzymes that aid in the digestion of materials that are typically difficult to digest, such as woody plant matter, in the guts of other organisms. [33]
The strain B. cereus B25 is a biofungicide. [38] A study by Figueroa-López et al. showed that the presence of this strain reduced Fusarium verticillioides growth. [38] B25 shows promise for reduction of mycotoxin concentrations in grains. [38]
B. cereus is responsible for a minority of foodborne illnesses (2–5%), causing severe nausea, vomiting, and diarrhea. [39] Bacillus foodborne illnesses occur due to survival of the bacterial endospores when contaminated food is not, or is inadequately, cooked. [40] Cooking temperatures less than or equal to 100 °C (212 °F) allow some B. cereus spores to survive. [41] This problem is compounded when food is then improperly refrigerated, allowing the endospores to germinate. [42] Cooked foods not meant for either immediate consumption or rapid cooling and refrigeration should be kept at temperatures below 10 °C (50 °F) or above 50 °C (122 °F). [41] Germination and growth generally occur between 10 °C and 50 °C, [41] though some strains can grow at low temperatures, [43] and Bacillus cytotoxicus strains have been shown to grow at temperatures up to 52 °C (126 °F). [44] Bacterial growth results in production of enterotoxins, one of which is highly resistant to heat and acids (pH levels between 2 and 11); [45] ingestion leads to two types of illness: diarrheal and emetic (vomiting) syndrome. [46] The enterotoxins produced by B. cereus have beta-hemolytic activity. [14]
The diarrhetic syndromes observed in patients are thought to stem from the three toxins: hemolysin BL (Hbl), nonhemolytic enterotoxin (Nhe), and cytotoxin K (CytK). [51] The nhe/hbl/cytK genes are located on the chromosome of the bacteria. Transcription of these genes is controlled by PlcR. These genes occur in the taxonomically related B. thuringiensis and B. anthracis, as well. These enterotoxins are all produced in the small intestine of the host, thus thwarting digestion by host endogenous enzymes. The Hbl and Nhe toxins are pore-forming toxins closely related to ClyA of E. coli . The proteins exhibit a conformation known as a "beta-barrel" that can insert into cellular membranes due to a hydrophobic exterior, thus creating pores with hydrophilic interiors. The effect is loss of cellular membrane potential and eventually cell death.[ citation needed ]
Previously, it was thought that the timing of the toxin production was responsible for the two different courses of disease, but it has since been found that the emetic syndrome is caused by the toxin cereulide, which is found only in emetic strains and is not part of the "standard toolbox" of B. cereus. Cereulide is a cyclic polypeptide containing three repeats of four amino acids: D-oxy- Leu —D- Ala —L-oxy- Val —L-Val (similar to valinomycin produced by Streptomyces griseus ) produced by nonribosomal peptide synthesis. Cereulide is believed to bind to 5-hydroxytryptamine 3 (5-HT3) serotonin receptors, activating them and leading to increased afferent vagus nerve stimulation. [52] It was shown independently by two research groups to be encoded on multiple plasmids: pCERE01 [53] or pBCE4810. [54] Plasmid pBCE4810 shares homology with the B. anthracis virulence plasmid pXO1, which encodes the anthrax toxin. Periodontal isolates of B. cereus also possess distinct pXO1-like plasmids. Like most of cyclic peptides containing nonproteogenic amino acids, cereulide is resistant to heat, proteolysis, and acid conditions. [55]
B. cereus is also known to cause difficult-to-eradicate chronic skin infections, though less aggressive than necrotizing fasciitis. B. cereus can also cause keratitis. [56]
While often associated with gastrointestinal illness, B. cereus is also associated with illnesses such as fulminant bacterial infection, central nervous system involvement, respiratory tract infection, and endophthalmitis. Endophthalmitis is the most common form of extra-gastrointestinal pathogenesis, which is an infection of the eye that may cause permanent vision loss. Infections typically cause a corneal ring abscess, followed by other symptoms such as pain, proptosis, and retinal hemorrhage. [57] While different from B. anthracis, B. cereus contains some toxin genes originally found in B. anthracis that are attributed to anthrax-like respiratory tract infections. [58]
A case study was published in 2019 of a catheter-related bloodstream infection of B. cereus in a 91-year-old male previously being treated with hemodialysis via PermCath for end-stage renal disease. He presented with chills, tachypnea, and high-grade fever, his white blood cell count and high-sensitivity C-reactive protein (CRP) were significantly elevated, and CT imaging revealed a thoracic aortic aneurysm. He was successfully treated for the aneurysm with intravenous vancomycin, oral fluoroquinolones, and PermCath removal. [59] Another case study of B. cereus infection was published in 2021 of a 30-year-old woman with lupus who was diagnosed with infective endocarditis after receiving a catheter. The blood samples were positive for B. cereus and the patient was subsequently treated with vancomycin. PCR was also used to verify toxins that the isolate produces. [60]
In case of foodborne illness, the diagnosis of B. cereus can be confirmed by the isolation of more than 100,000 B. cereus organisms per gram from epidemiologically implicated food, but such testing is often not done because the illness is relatively harmless and usually self-limiting. [61]
Most emetic patients recover within 6 to 24 hours, [46] but in some cases, the toxin can be fatal via fulminant hepatic failure. [62] [63] [64] [65] [66] In 2014, 23 newborns in the UK receiving total parenteral nutrition contaminated with B. cereus developed sepsis, with three of the infants later dying as a result of infection. [67] [68]
While B. cereus vegetative cells are killed during normal cooking, spores are more resistant. Viable spores in food can become vegetative cells in the intestines and produce a range of diarrheal enterotoxins, so elimination of spores is desirable. In wet heat (poaching, simmering, boiling, braising, stewing, pot roasting, steaming), spores require more than 5 minutes at 121 °C (250 °F) at the coldest spot to be destroyed. In dry heat (grilling, broiling, baking, roasting, searing, sautéing), 120 °C (248 °F) for 1 hour kills all spores on the exposed surface. [69] This process of eliminating spores is very important, as spores of B. cereus are particularly resistant, even after pasteurization or exposure to gamma rays. [24]
B. cereus and other members of Bacillus are not easily killed by alcohol; they have been known to colonize distilled liquors and alcohol-soaked swabs and pads in numbers sufficient to cause infection. [70] [71]
A study of an isolate of Bacillus cereus that was isolated from the stomach of a sheep was shown to be able to break down β-cypermethrin (β-CY) which has been known to be an antimicrobial agent. This strain, known as GW-01, can break down β-CY at a significant rate when the bacterial cells are in high concentrations relative to the antimicrobial agent. It has also been noted that the ability to break down β-CY is inducible. However, as the concentration of β-CY increases, the rate of β-CY degradation decreases. This suggests that the agent also functions as a toxin against the GW-01 strain. This is significant as it shows that in the right concentrations, β-CY can be used as an antimicrobial agent against Bacillus cereus. [72]
Bacillus cereus group bacteria, notably B. cereus and B. thuringiensis, are also pathogenic to multiple aquatic organisms including Chinese softshell turtle ( Pelodiscus sinensis ), causing infection characterized by gross lesions such as hepatic congestion and enlarged spleen with high mortality. [73]
Bacteria of the B. cereus group are infected by bacteriophages belonging to the family Tectiviridae. This family includes tailless phages that have a lipid membrane or vesicle beneath the icosahedral protein shell and that are formed of approximately equal amounts of virus-encoded proteins and lipids derived from the host cell's plasma membrane. Upon infection, the lipid membrane becomes a tail-like structure used in genome delivery. The genome is composed of about 15-kilobase, linear, double-stranded DNA (dsDNA) with long, inverted terminal-repeat sequences (100 base pairs). GIL01, Bam35, GIL16, AP50, and Wip1 are examples of temperate tectiviruses infecting the B. cereus group. [74]
Bacillus thuringiensis is a gram-positive, soil-dwelling bacterium, the most commonly used biological pesticide worldwide. B. thuringiensis also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well on leaf surfaces, aquatic environments, animal feces, insect-rich environments, flour mills and grain-storage facilities. It has also been observed to parasitize moths such as Cadra calidella—in laboratory experiments working with C. calidella, many of the moths were diseased due to this parasite.
Bacillus is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum Bacillota, with 266 named species. The term is also used to describe the shape (rod) of other so-shaped bacteria; and the plural Bacilli is the name of the class of bacteria to which this genus belongs. Bacillus species can be either obligate aerobes which are dependent on oxygen, or facultative anaerobes which can survive in the absence of oxygen. Cultured Bacillus species test positive for the enzyme catalase if oxygen has been used or is present.
Clostridium botulinum is a gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the ability to produce botulinum toxin, which is a neurotoxin.
Staphylococcus aureus is a gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive for catalase and nitrate reduction and is a facultative anaerobe, meaning that it can grow without oxygen. Although S. aureus usually acts as a commensal of the human microbiota, it can also become an opportunistic pathogen, being a common cause of skin infections including abscesses, respiratory infections such as sinusitis, and food poisoning. Pathogenic strains often promote infections by producing virulence factors such as potent protein toxins, and the expression of a cell-surface protein that binds and inactivates antibodies. S. aureus is one of the leading pathogens for deaths associated with antimicrobial resistance and the emergence of antibiotic-resistant strains, such as methicillin-resistant S. aureus (MRSA). The bacterium is a worldwide problem in clinical medicine. Despite much research and development, no vaccine for S. aureus has been approved.
Clostridium perfringens is a Gram-positive, bacillus (rod-shaped), anaerobic, spore-forming pathogenic bacterium of the genus Clostridium. C. perfringens is ever-present in nature and can be found as a normal component of decaying vegetation, marine sediment, the intestinal tract of humans and other vertebrates, insects, and soil. It has the shortest reported generation time of any organism at 6.3 minutes in thioglycolate medium.
The bacterial capsule is a large structure common to many bacteria. It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily washed off, and it can be the cause of various diseases.
The Ames strain is one of 89 known strains of the anthrax bacterium. It was isolated from a diseased 14-month-old Beefmaster heifer that died in Sarita, Texas in 1981. The strain was isolated at the Texas Veterinary Medical Diagnostic Laboratory and a sample was sent to the United States Army Medical Research Institute of Infectious Diseases (USAMRIID). Researchers at USAMRIID mistakenly believed the strain came from Ames, Iowa because the return address on the package was the USDA's National Veterinary Services Laboratories in Ames and mislabeled the specimen.
Lysogeny, or the lysogenic cycle, is one of two cycles of viral reproduction. Lysogeny is characterized by integration of the bacteriophage nucleic acid into the host bacterium's genome or formation of a circular replicon in the bacterial cytoplasm. In this condition the bacterium continues to live and reproduce normally, while the bacteriophage lies in a dormant state in the host cell. The genetic material of the bacteriophage, called a prophage, can be transmitted to daughter cells at each subsequent cell division, and later events can release it, causing proliferation of new phages via the lytic cycle.
Virulence factors are cellular structures, molecules and regulatory systems that enable microbial pathogens to achieve the following:
A Bacillus phage is a member of a group of bacteriophages known to have bacteria in the genus Bacillus as host species. These bacteriophages have been found to belong to the families Myoviridae, Siphoviridae, Podoviridae, or Tectiviridae. The genus Bacillus includes the model organism, B. subtilis, and two widely known human pathogens, B. anthracis and B. cereus. Other strains of Bacillus bacteria that phage are known to infect include B. megaterium, B. mycoides, B. pseudomycoides, B. thuringiensis, and B. weihenstephanensis. More than 1,455 bacillus phage have been discovered from many different environments and areas around the world. Only 164 of these phages have been completely sequenced as of December 16, 2021.
Aeromonas hydrophila is a heterotrophic, Gram-negative, rod-shaped bacterium mainly found in areas with a warm climate. This bacterium can be found in fresh or brackish water. It can survive in aerobic and anaerobic environments, and can digest materials such as gelatin and hemoglobin. A. hydrophila was isolated from humans and animals in the 1950s. It is the best known of the species of Aeromonas. It is resistant to most common antibiotics and cold temperatures and is oxidase- and indole-positive. Aeromonas hydrophila also has a symbiotic relationship as gut flora inside of certain leeches, such as Hirudo medicinalis.
Lactonase (EC 3.1.1.81, acyl-homoserine lactonase; systematic name N-acyl-L-homoserine-lactone lactonohydrolase) is a metalloenzyme, produced by certain species of bacteria, which targets and inactivates acylated homoserine lactones (AHLs). It catalyzes the reaction
Bacillus anthracis is a gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans. It is the only permanent (obligate) pathogen within the genus Bacillus. Its infection is a type of zoonosis, as it is transmitted from animals to humans. It was discovered by a German physician Robert Koch in 1876, and became the first bacterium to be experimentally shown as a pathogen. The discovery was also the first scientific evidence for the germ theory of diseases.
Microbial toxins are toxins produced by micro-organisms, including bacteria, fungi, protozoa, dinoflagellates, and viruses. Many microbial toxins promote infection and disease by directly damaging host tissues and by disabling the immune system. Endotoxins most commonly refer to the lipopolysaccharide (LPS) or lipooligosaccharide (LOS) that are in the outer plasma membrane of Gram-negative bacteria. The botulinum toxin, which is primarily produced by Clostridium botulinum and less frequently by other Clostridium species, is the most toxic substance known in the world. However, microbial toxins also have important uses in medical science and research. Currently, new methods of detecting bacterial toxins are being developed to better isolate and understand these toxins. Potential applications of toxin research include combating microbial virulence, the development of novel anticancer drugs and other medicines, and the use of toxins as tools in neurobiology and cellular biology.
In bacteriology, a taxon in disguise is a species, genus or higher unit of biological classification whose evolutionary history reveals has evolved from another unit of a similar or lower rank, making the parent unit paraphyletic. That happens when rapid evolution makes a new species appear so radically different from the ancestral group that it is not (initially) recognised as belonging to the parent phylogenetic group, which is left as an evolutionary grade.
Cereulide is a toxin produced by some strains of Bacillus cereus, Bacillus megaterium and related species. It is a potent cytotoxin that destroys mitochondria. It causes nausea and vomiting.
Clostridioides difficile is a bacterium known for causing serious diarrheal infections, and may also cause colon cancer. It is known also as C. difficile, or C. diff, and is a Gram-positive species of spore-forming bacteria. Clostridioides spp. are anaerobic, motile bacteria, ubiquitous in nature and especially prevalent in soil. Its vegetative cells are rod-shaped, pleomorphic, and occur in pairs or short chains. Under the microscope, they appear as long, irregular cells with a bulge at their terminal ends. Under Gram staining, C. difficile cells are Gram-positive and show optimum growth on blood agar at human body temperatures in the absence of oxygen. C. difficile is catalase- and superoxide dismutase-negative, and produces up to three types of toxins: enterotoxin A, cytotoxin B and Clostridioides difficile transferase. Under stress conditions, the bacteria produce spores that are able to tolerate extreme conditions that the active bacteria cannot tolerate.
The exosporium is the outer surface layer of mature spores. In plant spores it is also referred to as the exine. Some bacteria also produce endospores with an exosporium, of which the most commonly studied are Bacillus species, particularly Bacillus cereus and the anthrax-causing bacterium Bacillus anthracis. The exosporium is the portion of the spore that interacts with the environment or host organism, and may contain spore antigens. Exosporium proteins, such as Cot protein, are also discovered related to strains of B. anthracis and B.cereus. This Cot protein share similar sequences with other spore coat proteins, and their putative determinants are believed to include bxpC, lunA, exsA, etc.
Caenorhabditis elegans- microbe interactions are defined as any interaction that encompasses the association with microbes that temporarily or permanently live in or on the nematode C. elegans. The microbes can engage in a commensal, mutualistic or pathogenic interaction with the host. These include bacterial, viral, unicellular eukaryotic, and fungal interactions. In nature C. elegans harbours a diverse set of microbes. In contrast, C. elegans strains that are cultivated in laboratories for research purposes have lost the natural associated microbial communities and are commonly maintained on a single bacterial strain, Escherichia coli OP50. However, E. coli OP50 does not allow for reverse genetic screens because RNAi libraries have only been generated in strain HT115. This limits the ability to study bacterial effects on host phenotypes. The host microbe interactions of C. elegans are closely studied because of their orthologs in humans. Therefore, the better we understand the host interactions of C. elegans the better we can understand the host interactions within the human body.
Cytotoxin-K (CytK) is a protein toxin produced by the gram-positive bacteria Bacillus cereus. It was first discovered in a certain Bacillus cereus strain which was isolated from a food poisoning epidemic that occurred in a French nursing home in 1998. There were six cases of bloody diarrhea, three of which were fatal. None of the known enterotoxins from B. cereus could be detected at this time. Later, this B. cereus strain and its relatives were classified as a brand-new species called Bacillus cytotoxicus, which is the thermo-tolerant member of the B. cereus genus. The cytotoxin-K gene is present in approximately 50% of Bacillus cereus isolates, and its expression is regulated by several factors, including temperature and nutrient availability.