**4. Probiotics and inhibition of bacterial toxins**

*Clostridium botulinum* is an ubiquitous, gram-positive, anaerobic spore-forming organism that is the causative agent of botulinum. The botulinum neurotoxins are one of the most lethal toxins known to mankind with a parenteral lethal dosage of 0.1–1 ng/kg and an oral dose of 1 μg/kg. Due to this high toxicity and potential for bioterrorism, botulinum neurotoxins (BoNTs) are considered Tier 1 category Select Agents by the Centers for Disease Control and Prevention (CDC). BoNTs are a public health and safety threat in the form of foodborne, wound, and infant botulism.

In order to cause disease for foodborne botulism, BoNTs must first be able to survive in the intestinal lumen, bind to and translocate through the intestinal epithelium to reach the bloodstream [95]. Once in the bloodstream, BoNTs bind to peripheral cholinergic neurons to cleave SNAREs and block exocytosis of neurotransmitters hence leading to flaccid muscle paralysis. Similar to other classic A-B chain toxins, the heavy chain (B chain) of BoNTs bind to carbohydrate and protein receptors on their target cell while the light chain (A chain) has the enzymatic function. Therefore, there are two potential therapeutic pathways to block BoNT intoxication: (1) blocking binding/translocation at the intestinal epithelium/ target cells and (2) degradation or inactivation of the toxin It has been shown that pre-treatment with probiotics (*Saccharomyces cerevisiae* var. *boulardii*, *Lactobacillus acidophilus*, *Lactobacillus rhamnosus* LGG, and *Lactobacillus reuteri*) was able to block toxin binding to cells in an *in vitro* Caco-2 cell culture model and that the mechanism most likely used by the probiotics was steric hindrance of binding to host receptors [96].

Another mechanism to inactivate bacterial toxins would to be to subject them to proteolysis thus rendering them inactive. *S. boulardii* has been shown to produce a 54-KDa protease that is able to cleave and inactivate the two main *C. difficile* toxins, TcdA and TcdB [97] in a HT-29 colonic cell model. Whether or not this occurs in any significant degree in human infection is still unclear.
