Mini Review
Mechanisms of probiotic actions – A review

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Abstract

Probiotics are gaining more and more interest as alternatives for antibiotics or anti-inflammatory drugs. However, their mode of action is poorly understood. This review will present examples of probiotic actions from three general modes of actions into which probiotic effects can be classified. Probiotics might modulate the host's immune system, affect other microorganisms directly or act on microbial products, host products or food components. What kind of effect(s) a certain probiotic executes depends on its metabolic properties, the molecules presented at its surface or on the components secreted. Even integral parts of the bacterial cell such as its DNA or peptidoglycan might be of importance for its probiotic effectiveness. The individual combination of such properties in a certain probiotic strain determines its specific probiotic action and as a consequence its effective application for the prevention and/or treatment of a certain disease.

Introduction

In this review I will give an overview on the effects of probiotics. It has to be taken into account, that many reported mechanisms of probiotic actions are the results of in vitro experiments. Therefore, these results must be confirmed by in vivo studies.

The effects of probiotics may be classified in three modes of action. (i) Probiotics might be able to modulate the host's defences including the innate as well as the acquired immune system. This mode of action is most likely important for the prevention and therapy of infectious diseases but also for the treatment of (chronic) inflammation of the digestive tract or parts thereof. In addition, this probiotic action could be important for the eradication of neoplastic host cells. (ii) Probiotics can also have a direct effect on other microorganisms, commensal and/or pathogenic ones. This principle is in many cases of importance for the prevention and therapy of infections and restoration of the microbial equilibrium in the gut. (iii) Finally, probiotic effects may be based on actions affecting microbial products like toxins, host products e.g. bile salts and food ingredients. Such actions may result in inactivation of toxins and detoxification of host and food components in the gut.

All three modes of probiotic action are in all likelihood involved in infection defence, prevention of cancer and in stabilising or reconstituting the physiological balance between the intestinal microbiota and its host. However, it has to be stressed that there seems not to be one probiotic exhibiting all three principles, at least not to that extent that it could be a remedy for prevention or therapy of all mentioned kinds of disease. It depends on the metabolic properties, the kind of surface molecules expressed and components to be secreted which probiotic actions a certain probiotic strain might show. The era of “omics” we are living in at this time, will make crucial contributions because it enables the analysis and comparison of whole genomes, transcriptomes, proteomes and secretomes (see also Wohlgemuth et al., 2010). This all will lead to the identification and characterisation of probiotic properties, distinguishing real probiotics from those, which are not.

Section snippets

Immune modulation

Probiotics can influence the immune system by products like metabolites, cell wall components and DNA. Obviously, immune modulatory effects might be even achieved with dead probiotic bacteria or just probiotics-derived components like peptidoglycan fragments or DNA. Probiotic products are recognized by host cells sensitive for these because they are e.g. equipped with recognition receptors. The main target cells in that context are therefore gut epithelial and gut-associated immune cells. The

Antimicrobial substances produced by probiotics

Bacteriocins. In vitro-studies showed inhibition of pathogen replication mediated by low-molecular-weight substances. Top of this list are short chain fatty acids e.g. lactic acid. A similar effect was observed for hydrogen peroxide. Also low-molecular-weight bacteriocins (LMWB) and high-molecular-weight bacteriocins (class III) are produced by lactobacilli. The LMBW are antimicrobial peptides. LMWB can be grouped into three classes: (class I) lantibiotics, posttranslationally modified peptides

Designer probiotics

The identification of toxin receptors paved the way for recombinant E. coli expressing receptor structures at their surface identical with the receptor for a certain toxin. Such designer probiotics bind very efficiently e.g. shiga toxin 1 (Stx1) and 2 (Stx2). One mg bacterial dry weight is able to bind more than 100 μg Stx1 or Stx2. This was achieved by replacing the O-side chains of the original LPS by Gb3, the receptor for these toxins. The resulting recombinant strain protected all mice after

Inhibitory activity against genotoxins

There are claims for anti-cancer activity of probiotics. If such a protection occurs in humans who consume the appropriate probiotic is unknown. However, the repression of putrefactive bacteria such as Clostridium, coliforms or Bacteroides species and an increase in numbers of lactobacilli and bifidobacteria might at least reduce the probability of incidence for colorectal cancer. The incidence of adenocarcinoma in the colon of IL-10 knockout mice was factually reduced in mice treated with

Final remarks

Taken into account the low number of probiotic bacteria administered with a daily dose of 109 or 1010 bacteria compared to up to 1014 bacteria in the colon it is to some extent surprising that there are probiotic effects observable at all. Still this minority is able to kill pathogens, inhibit probably adhesion and invasion of pathogens, inactivate toxins and compete for limited resources successfully. This might be explained by colonization of the same gut compartment by probiotics and

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