Bacillus cereus
Bacillus cereus is a Gram-positive, rod-shaped, facultatively anaerobic, motile, beta-hemolytic, spore-forming bacterium commonly found in soil, food and marine sponges. 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, while other strains can be beneficial as probiotics for animals. The bacteria is classically contracted from fried rice dishes that have been sitting at room temperature for hours. "B. cereus" bacteria are facultative anaerobes, and like other members of the genus "Bacillus", can produce protective endospores . Its virulence factors include phospholipase C, cereulide, sphingomyelinase, metalloproteases, and cytotoxin K. The "Bacillus cereus" group comprises seven closely related species: "B. cereus" "sensu stricto" (referred to herein as "B. cereus"), "B. anthracis", "B. thuringiensis", "B. mycoides", "B. pseudomycoides", and "B. cytotoxicus"; or as six species in a "Bacillus cereus" sensu lato: "B. weihenstephanensis", "B. mycoides", "B. pseudomycoides", "B. cereus", "B. thuringiensis", and "B. anthracis". Ecology. Like most "Bacilli," the most common ecosystem of "Bacillus cereus" is land. In concert with Arbuscular mycorrhiza (and Rhizobium leguminosarum in clover), they can regenerate heavy metal soil by increasing phosphorus, nitrogen, and potassium  content in certain plants. "B. cereus" competes with other microorganisms such as "Salmonella" and "Campylobacter" in the gut; its presence reduces the numbers of those microorganisms. In food animals such as chickens, rabbits and pigs, 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. "B. cereus" can parasitize codling moth larvae. "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. Some strains of "B. cereus" produce cereins, bacteriocins active against different "B. cereus" strains or other Gram-positive bacteria. Reproduction. At , a population of "B. cereus" can double in as little as 20 minutes or as long as 3 hours, depending on the food product. Pathogenesis. "B. cereus" is responsible for a minority of foodborne illnesses (2–5%), causing severe nausea, vomiting, and diarrhea. "Bacillus" foodborne illnesses occur due to survival of the bacterial endospores when infected food is not, or is inadequately, cooked. Cooking temperatures less than or  equal to allow some "B. cereus" spores to survive. This problem is compounded when food is then improperly refrigerated, allowing the endospores to germinate. Cooked foods not meant for either immediate consumption or rapid cooling and refrigeration should be kept at temperatures below or above . Germination and growth generally occur between 10 °C and 50 °C, though some strains can grow at low temperatures. Bacterial growth results in production of enterotoxins, one of which is highly resistant to heat and acids (pH levels between 2 and 11); ingestion leads to two types of illness: diarrheal and emetic (vomiting ) syndrome. The diarrhetic syndromes observed in patients are thought to stem from the three toxins: hemolysin BL (Hbl), nonhemolytic enterotoxin (Nhe), and cytotoxin K (CytK). 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. CytK is a pore-forming protein more related to other hemolysins. The timing of the toxin production was previously thought to be possibly responsible for the two different courses of disease, but in fact the emetic syndrome is caused by a toxin, cereulide, 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: -oxy-—-—-oxy-—- (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. It was shown independently by two research groups to be encoded on multiple plasmids: pCERE01 or pBCE4810. 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. "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. 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. Spore elimination. 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 at the coldest spot to be destroyed. In dry heat (grilling, broiling, baking, roasting, searing, sautéing), for 1 hour kills all spores on the exposed surface. Diagnosis. 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. Identification. 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. Below is a list of differential techniques and results that can help to identify "B. cereus" from other bacteria and "Bacillus" species. 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. Characteristics of "B. cereus". Colony, morphological, physiological, and biochemical characteristics of marine "B. cereus" are shown in the Table below. Note : + = Positive, – =Negative, V= Variable (+/–) Prognosis. Most emetic patients recover within 6 to 24 hours, but in some cases, the toxin can be fatal via fulminant hepatic failure. In 2014, 23 newborns in the UK receiving total parenteral nutrition contaminated with "B. cereus" developed septicaemia, with three of the infants later dying as a result of infection. Bacteriophage. 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. History. Colonies of "B. cereus" were originally isolated from an agar plate left exposed to the air in a cow shed. 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". Several new enzymes have been discovered in "B. cereus", such as AlkC and AlkD, both of which are involved in DNA repair.