Exotoxins are extremely potent compounds that cause major damage to the animal either locally (in the intestines, for example) or through systemic infection. They are secreted by bacteria while the bacteria are still alive or released upon cellular disruption, adding to the overall exotoxin levels in their environment.
Most exotoxins are proteins, and many of them are enzymes that are highly recycled. This is why even small amounts of exotoxins can be highly toxic. Diseases caused by certain pathogenic bacteria that produce potent exotoxins are due to these small amounts of toxins and not to the presence of the bacteria themselves. Most exotoxins are heat labile, and as such, feed thermal processing can destroy exotoxins already in the feed. However, feed thermal processing does not stop bacteria from producing new quantities after feed processing, during storage or in the animal’s gastrointestinal tract. Plus, not all bacteria enter the animal’s organism through feed.
Although exotoxins are susceptible to antibodies produced by the immune system, many are so toxic that they can cause death before the animal’s immune system has been fully activated to build defenses against them. Such cases include diseases as botulism, tetanus, gas gangrene, diphtheria, cholera, plague and several types of food intoxication. In most non-lethal cases, enough critical damage occurs before the immune system is able to fully respond.
One very well known exotoxin is that produced by Clostridium botulinum; popularly known under the trade name of Botox.
In domestic animals, the most important exotoxins are those that cause damage to the enteric epithelium of the gastrointestinal tract. These exotoxins are known as enterotoxins. As a general rule, enterotoxins are produced by Gram-positive bacteria rather than by Gram-negative bacteria; although there are exceptions to this rule. A well-known example of an enterotoxin is toxin B, produced by Staphylococcus aureus, which is associated with food-borne illness. This kind of toxin causes fever, chills, headache, chest pain and persistent coughing. To distinguish this disease from bacterial endotoxin illness (food poisoning) it is known as food intoxication.
Multiple modes to consider
Enterotoxins have three different modes of actions:
1. One type (invasins) destroys the host cell to which it is bound. In essence, these enterotoxins make a hole in the host cell membrane. An example of such an enterotoxin is hemolysin, produced by Escherichia coli, causing hemolytic diarrhea.
2. Another type, known as superantigen toxins, cause overstimulation of the immune system, leading to systemic shock. This is the exotoxin secreted by Staphylococcus aureus.
3. Finally, a third type is known as A-B toxins because it has two subunits. The A subunit binds to the host cell and perforates the membrane, which allows the B subunit to enter the cell and cause damage. This is the enterotoxin produced by Vibrio cholerae.
Some exotoxins only attack specific types of cells, but most (such as those produced by staphylococci, streptococci and clostridia) have very broad cytotoxic activity causing damage to various types of cells and tissues, resulting in tissue necrosis (death). Thus, similar to the very persistent problem of hemolytic diarrhea in pigs, the also-quite-significant disease of necrotic enteritis in poultry is ascribed to exotoxins.
Theory and practice
In order to better understand exotoxins, one can draw an analogy to mycotoxins; the former are toxins produced by bacteria, the latter are toxins produced by molds. Today, the importance of controlling mycotoxins is well appreciated, but that of controlling exotoxins remains in the sphere of therapeutic treatment rather than strategic intervention. Nevertheless, drawing again from the example of mycotoxins, it could be possible to control enterotoxins by similar mechanisms, but up to now, there has been a lack of information concerning the efficacy of infeed supplements on enterotoxins.
In a very recent trial, the in-vitro efficacy of a binder (containing selected clay minerals and yeast cell wall fragments) on Clostridium perfringens alpha-toxin was investigated. The trial followed a very simple procedure:
1. Prepare toxin and binder solutions with increasing concentration of binder.
2. Incubate at 37 degrees Celsius for one hour.
3. Centrifuge the mixture and remove supernatant for assay.
4. Estimate the toxin concentration using an ELISA kit.
The results are shown in Figure 1, where it is apparent that by adding increasing amounts of this binder, it was able to bind more and more of the exotoxin present. At the highest dosage, at 0.1 percent binder in the mixture, virtually all exotoxin was bound.
As a follow up, a preliminary animal study with broilers was designed to compare the in-vivo efficacy of this new binder. Treatments included the following:
1. Challenged Control
2. Binder at 0.1 percent
The broilers were challenged orally with Clostridium perfringens type A culture broth at three and four days of age with 108 colony-forming units per chick, using a blunted needle. Results, shown in Table 1, indicated that the product was effective in controlling the effects of exotoxins in the culture broth. It appears that the binder was quite effective from early on, something that needs further investigation as it gives evidence of a stronger response that could be used to reduce medication (the feeds were indeed medicated) and reduce overall mortality. As this was an initial small-scale trial, it merits further investigation.
In conclusion, we are already in an era where nutrition and disease prevention are strongly interlinked, and new functions are being discovered for existing products. Such is the case of traditional binders when it comes to binding bacterial enterotoxins. Preliminary results are very promising, and it appears further work is warranted to explore this new frontier in disease prevention.