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Aflatoxin Binding by Probiotic Bacteria

Aspergillus aflatoxin

The term ‘probiotic’ comes from the Greek words ‘προ’ and ‘βιοτος’, which mean ‘for life’. In 1953, the ‘probiotic’ term was introduced by Kollath as organic and inorganic supplements necessary to restore health to patients suffering a form of malnutrition resulting from eating too much highly refined food (Hamilton-Miller et al., 2003).

The World Health Organisation (WHO) defines ‘probiotic’ as live microorganisms which, when administered in adequate amounts, confer health benefits to the host (WHO, 2001). Furthermore, microbial cell preparations or components of microbial cells that have beneficial effects on the health and well-being of the host (Salminen et al., 1999), including viable and non-viable bacteria (Owuhand & Salminen, 1998) are also considered to be ‘probiotic’. Therefore, probiotics can generally be defined as a “treatment” that gives beneficial health effects to humans and animals.

Binding of probiotics to food mutagens and carcinogens

Some of the health benefits of probiotics include the improvement of intestinal functions and integrity (De Preter et al., 2004; Koebnick et al., 2003; De Preter et al., 2006; De Preter et al., 2007; Matsumoto et al., 2010; Krammer et al., 2011) and modulation of immune function (Matsuzaki et al., 1997; Shida et al., 2002; Baken et al., 2006; de Jonge et al., 2008; Chiba et al., 2009). Several studies have also suggested that probiotic bacteria and fermented dairy products possess anti-carcinogenic activity.

A study conducted by Hosono et al. (1990) showed that probiotics had the ability to bind mutagens. In addition, Zhang and Ohta (1991) indicated that probiotic bacteria were able to bind to Trp-P-1 (3-amino-1,4-dimethyl-[5H]pyrido[4,3-b]indole) and Trp-P-2 (3-amino-1-methyl-[5H]-pyrido[4,3-b]indole).

Probiotic bacteria can be used as agents of dietary detoxification (Turbic et al., 2002) and it is evident that probiotic bacteria can inactivate toxic compounds and thus prevent the activation of carcinogens and mutagens (Peltonen et al., 2000). For example, probiotic Lactobacillus casei Shirota decreased DNA damage in the rat colon after exposure to N-methyl-N-nitro-N-nitrosoguanidyne (MNNG) and ameliorated genotoxicity induced by dimethylhydrazine (DMH) (Commane et al., 2005).

Nevertheless, the mechanism that explains the ability of probiotic bacteria to bind or metabolise different colon carcinogens is not completely known.

Physical binding of aflatoxin to the bacterial cell wall

Aflatoxins are highly substituted coumarins containing a fused dihydrofurofuran (Kensler et al., 2011). Four metabolites of aflatoxin occur naturally, namely aflatoxin B1 (AFB1), aflatoxin B2 (AFB2), aflatoxin G1 (AFG1) and aflatoxin G2 (AFG2). Of these, AFB1 is the most potent and carcinogenic food contaminant; the International Agency for Research on Cancer (IARC) has classified AFB1 as a Group 1 carcinogen that is linked to the development of hepatocellular carcinoma (HCC) (IARC 1993; IARC 2002).

It is said that probiotics remove aflatoxin by binding to the bacterial cell wall. For example, Shetty and Jespersen (2006) indicated that mycotoxin removal is by adhesion to cell wall components, rather than by covalent bindings or metabolism, as non-viable and dead bacteria do not lose their binding ability. Moreover, Hernandez-Mendoza et al. (2010) showed that AFB1 binding to the bacterial cell wall involved a physical interaction. However, the mechanism by which aflatoxin binds to the bacterial cell wall is unclear.

Bueno et al. (2007) proposed theoretical model of aflatoxin binding by probiotic bacteria and involved two processes namely adsorption and desorption during the binding of AFB1 to the bacteria cell wall. Bueno et al. (2007) used probiotic lactic acid bacteria and Saccharomyces cerevisiae to determine the efficiency of microorganism (M x Keq) as a binder of AFB1 based on two parameters; the number of binding sites per microorganism (M) and reaction equilibrium constant (Keq). The authors concluded that different microorganism have different binding sites for AFB1.

The binding and interaction of AFB1 molecule to the bacterial cell wall using a computer-generated simulation model was assessed in a study by Yiannikouris et al. (2006). The authors examined the interaction between β-D-glucan structures of Saccharomyces cerevisiae and AFB1 molecules and found that the binding involved a 2-step mechanism process.

Firstly, AFB1 molecule is trapped inside the single helix of the (1➔3)-β-D-glucan chain. Then, the AFB1 molecule is covered by the branched (1➔6)-β-D-glucan chain, where it is maintained inside the helix. Hydrogen bonds only account for a small portion (-3.8 kcal/mol) of the docking energy for the binding, therefore the authors stated that Van der Waals interaction plays a major role in the binding of AFB1 (Yiannikouris et al., 2006).

On the other hand, Haskard et al. (2001) showed some involvement of both electrostatic interactions and hydrogen bonds between AFB1 molecules and the bacterial cell wall of Lactobacillus rhamnosus strain GG (LGG) during the removal of AFB1. Furthermore, AFB1 can be bound to the bacterial cell wall through weak non-covalent interactions such as associating with hydrophobic pockets on the bacterial surface (Haskard et al., 2001).

Lastly, it has been postulated that binding involves other components of bacterial cell walls. Zhang and Ohta (1991) showed the role of peptidoglycan and polysaccharides in overall binding of toxin bacteria. The AFB1 binding involved peptidoglycan or compounds tightly associated with the peptidoglycan (Lahtinen et al., 2004).

Conclusion

Probiotic bacteria has many beneficial health effects. One of them is their ability to bind aflatoxin. As functional foods, probiotics are of great interest as a dietary approach to prevent human exposure to aflatoxins. Therefore, it is important to study the chemical interactions between the cell wall of probiotic bacteria and its relation to aflatoxin molecules; such data can provide further justification of the use of probiotics as adsorbents of aflatoxin.

References
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Mohd Redzwan Sabran


Mohd Redzwan Sabran
 is a Senior Lecturer in the Department of Nutrition and Dietetics, Faculty of Medicine and Health at UPM. Mohd Redzwan’s main research interest is in food safety and contaminants. His work was recently presented at the 2nd Probiotics Congress: Asia.


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