Clostridium botulinum

          

           Clostridium botulinum is a rod-shaped microorganism. It is an obligate anaerobe, meaning that oxygen is poisonous to the cells. However, C. botulinum tolerates traces of oxygen due to the enzyme called superoxide dismutase (SOD) which is an important antioxidant defense in nearly all cells exposed to oxygen.C. botulinum is only able to produce the neurotoxin during sporulation, which can only happen in an anaerobic environment. Other bacterial species produce spores in an unfavorable growth environment to preserve the organism's viability and permit survival in a dormant state until the spores are exposed to favorable conditions.

             In the laboratory Clostridium botulinum is usually isolated in tryptose sulfite cycloserine (TSC) growth media in an anaerobic environment with less than 2% of oxygen. This can be achieved by several commercial kits that use a chemical reaction to replace O₂ with CO₂ (E.J. GasPak System). C. botulinum is a lipase negative microorganism that grows between pH of 4.8 and 7 and it can't use lactose as a primary carbon source, characteristics important during a biochemical identification.

           Clostridium botulinum was first recognized and isolated in 1895 by Emile van Ermengem from home cured ham implicated in a botulism outbreak. The isolate was originally named Bacillus botulinus. However, isolates from subsequent outbreaks were always found to be anaerobic spore formers, so Bengston proposed that the organism be placed into the genus Clostridium as the Bacillus genus was restricted to aerobic spore-forming rods.

             Since 1959 all species producing the botulinum neurotoxins (types A-G) have been designated C. botulinum. Substantial phenotypic and genotypic evidence exists to demonstrate heterogeneity within the species. This has led to the reclassification of C. botulinum type G strains as a new species Clostridium argentinense.

             Clostridium botulinum strains that do not produce a botulin toxin are referred to as Clostridium sporogenes. The complete genome of C. botulinum has been sequenced Sanger.

Bacillus cereus

           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 food is improperly cooked. Cooking temperatures less than or equal to 100 °C (212 °F) allows 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 above 60 °C (140 °F).Germination and growth generally occurs between 10–50 °C (50–122 °F) though some strains are psychrotrophic. Bacterial growth results in production of enterotoxins, one of which is highly resistant to heat and to pH between 2 and 11; ingestion leads to two types of illness, diarrheal and emetic (vomiting) syndrome.

               The diarrheal type is associated with a wide-range of foods, has an 8- to 16.5-hour incubation time and is associated with diarrhea and gastrointestinal pain. Also known as the long-incubation form of B. cereus food poisoning, it might be difficult to differentiate from poisoning caused by Clostridium perfringens. 

             The emetic form is commonly caused by rice that is not cooked for a time and temperature sufficient to kill any spores present, then improperly refrigerated. It can produce a toxin, cereulide, which is not inactivated by later reheating. This form leads to nausea and vomiting 1–5 hours after consumption. It can be difficult to distinguish from other short-term bacterial foodborne pathogens such as Staphylococcus aureus.

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 as well in the toxonomically related B. thuringensis and B. anthracis. These enterotoxins are all produced in the small intestine of the host, thus thwarting the issue of 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 "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.

It was previously thought that the timing of the toxin production might be responsible for the two different courses of disease, but in fact the emetic syndrome is caused by a toxin called cereulide that is found only in emetic strains and is not part of the "standard toolbox" of B. cereus. Cereulide contains 3 repeats of 4 amino acids (similar to Valinomycin produced by Streptomyces griseus) produced by nonribosomal peptide synthesis (NRPS). Cereulide is believed to activate 5-HT3 (serotonin) receptors leading to increased afferent vagus nerve stimulation. It was shown independently by two research groups to be encoded on multiple plasmids: pCERE01^[14] or pBCE4810. Plasmid pBCE4810 shares homology with the Bacillus anthracis virulence plasmid pXO1, which encodes the anthrax toxin. Periodontal isolates of B. cereus also possess distinct pXO1-like plasmids.

B. cereus is also known to cause chronic skin infections that are difficult to eradicate though less aggressive than necrotizing fasciitis. B. cereus can also cause keratitis. It recommended as pathogenic microflora in pharmaceutical oral products in Brazilian Phamacopaeia.