1.5. Effect on intestinal microflora

A great amount of alive microorganisms (up to 1014) inhabit the human being gastrointestinal tract during its life. The constitution of the microflora numbers about 400 species. It is not well-known as the big part of it can not be analyzed due to lack of adequate techniques. It is considered that the available classical and molecular approaches are not powerful enough to describe all this diversity of microorganisms. The type of microbial population changes subsequently in respect to the composition and amount from oral cavity (where a dominant microbial species are lactic acid bacteria, streptococci and some anaerobic species), to the stomach (transient acid tolerant microbes), to the small intestine (populated by colon - like microflora), to the colon, where microbial entities reach up to 1012 grams/ dry weight.

The population colonizing the colon is dominated by anaerobes like Bifidobacterium, Eubacterium, Bacteroides and Clostridium. The microaerophilic and facultative anaerobes such as lactobacilli, enterococci, and coliforms are usually less - 3 to 4 orders of magnitude. This endogenous microflora living in gastrointestinal tract represents a main barrier against exogenous microflora tending to be invading GI and assure the so-called colonization resistance.

The composition of the gastrointestinal (GI) microflora in quantitative and qualitative aspects strongly depends on a number of factors including:  

  • environmental factors - type of diet, the antimicrobial therapy applied, disinfectants, food additives, occupation, climate.
  • host associated factors - age, gender, intestinal motility, transit time, pH, bile acids defenses etc.
  • interrelation of   species in GI microflora - rate of nutrient uptake, oxygen, H+, H2S, production of antimicrobial agents, organic acids, NH3

All these factors have a concerted action on the overall status of the GI microflora.

There are some interactions between GI microfolora and different mammalian host - associated structures and functions. These interactions could be found at different levels: organs, cell and molecular, and could be summarized as: prokaryotic-eukaryotic cross-links at cellular level; production of organic acids, nucleotides etc.; interaction with enterohepatic circulation; development of gut associated immune system (GAIS); influence on intestinal motility and enterocystic mitosis. They depend on the age and the health status of the individual. Thus, the survival of the exogenous microflora (including GMMs) is subordinated to their ability to overwhelm the influence of the indigenous GI microflora and above mentioned host - associated factors. Also the colonization resistance takes part in evaluation of survival, but these mechanisms are not well understood. Sometimes microbes can leave the gut lumen and appear elsewhere. The term describing this behavior is translocation. To determine the intestinal survival of microbes in vitro is a difficult task. For this reason a selection of suitable animal models, simulating human GI system and experiments with patients are required. These investigations should be supported by reliable strain identification methodology.

In case the introduced GMMs into GI system survive digestion, they could appear as transient one or may establish itself for different time in the gut. This phenomenon is described by term colonization measured with a constant level of microorganisms detected for a relevant period of time.

Long life (permanent) colonization of the GI tract of adults by exogenous microorganisms is very rare. But if it happens, an application of certain probiotic strains indicates that the normal microflora could be recovered in the facies and colonic mucosa for weeks after oral administration. To describe the survival of microorganisms in GI tract for period longer than two intestinal transit times term “persistence” has been involved.

In case of introduction of GMMs in the GI tract, independently of its establishment (detected or not), the possibility exists for interaction with microflora of the mammalian host. The anticipated effect on gut flora could partly depend on the functions expressed by GMM (phenotypic expression) and this way on horizontal gene transfer.

The effect of GMMs on mammalian host could be defined as direct and indirect. The direct one is characterized by total influence on all structures and functions listed above and indirect could be mediated by interaction with endogenous microflora and particularly with their active elements. Both types of interactions (direct/indirect) could be provoked also by non-viable microorganisms as they preserve functional properties (i.e. immuno modulation, chemical binding, cell adhesion). Some secretion of biologically active compounds like toxins, enzymes could also be anticipated.

The possibility for gene transfer has been already discussed. So it is reasonable to consider conjugate transfer between microorganisms in the gut which depends both on the relatedness of GMM to the intestinal microflora and its residence time in the GI tract. This effect could be expected with persistent or colonizing strains. The transient strains are of low influence in this respect. At present there is no doubt that a measurable persistence of DNA in the intestinal tract exists. It has been found that plant and recombinant DNA could enter the blood system, tissue cells and even nuclei of mammalian host.