Gut microbiota a key organ for human health
The human gastrointestinal tract harbors trillions of microorganisms (≈ 1011 per g feces) that play an important role in host physiology, metabolism and nutrition. This microbiota “organ” is the central bioreactor of the gastrointestinal tract, populated by a complex mixture of microbial organisms, which represents more than 100 times the human genome. The gut microbiota co-evolves with its human host from birth to adulthood. During early life, several external factors, such as mode of birth, diet, antibiotic exposure or other environmental stimuli can affect microbiota composition. Alterations in the composition of gut microbial communities, also known as dysbiosis, have been associated with a number of intestinal and extra-intestinal disorders. Intestinal disorders include inflammatory bowel disease, irritable bowel syndrome, and coeliac disease, while extra-intestinal disorders include asthma, metabolic syndrome, obesity, and diabetes (type 1 and 2).
Gastrointestinal microbiota as partner for human defense
The microbiota acts as a barrier against the proliferation of exogenous pathogens by preventing their invasion with a highly dynamic modality, exerting metabolic functions and stimulating the maturation of the immunitary system. Microbiota commensals inhibit pathogen growth by the producing and releasing organic acids, volatile fatty acids, antibiotic compounds, bacteriocins and/or host immune response stimulating factors.
Strategies to modulate the gut microbiota
Several approaches have been investigated including supplementation with prebiotics, use of probiotics, reconstitution of bacterial populations by faecal transplantation or by employing antimicrobials to eliminate pathogens or manipulate the gut microbiota in a way that will benefit host health.
Probiotics are defined as “living microbial organisms, which upon ingestion in sufficient numbers exert health benefits beyond inherent basic nutrition” (FAO). Many mechanisms of action have been proposed to explain the stabilizing effects of probiotics on the intestinal microbiota. These include:
- Interfering with the attachment of pathogens to adhesion sites
- Out-competing pathogens for nutrients
- Degradation or other alterations of toxin receptors
- Production of inhibitory substances
- Stimulation of immunity/immunomodulation.
Schematic representation of potential or known mechanisms by which probiotic bacteria might have an impact on the stability of intestinal microbiota (O'Toole and Cooney, 2008)
Recent reports have revealed that some intestinal lactobacilli and bifidobacteria produce bacteriocinss that are active against enterovirulent microorganisms. Bacteriocins are bactericidal proteinaceous molecules that have a relatively narrow killing spectrum, being toxic only to bacteria closely related to the producing strain. Bacteriocins make up a highly diverse family of proteins in terms of size, microbial target, mode of action and release and mechanism of immunity. The laboratory hosts BACTIBASE, a database dedicated to bacteriocins produced by both Gram-positive and Gram-negative bacteria (http://bactibase.hammamilab.org). This database provides physicochemical, structural, microbiological, and taxonomic information about bacteriocins, which would allow better and more comprehensive structural and functional analysis of this special group of antimicrobial peptides.
The efficacy of bacteriocin as well as their producing strains for inhibiting several bacterial pathogens has been shown in different food matrices including cheese, meat and vegetable. However, implication of bacteriocins as a mechanism of action in the inhibitory activity of probiotics remains unclear and need to more investigations.