Bacteroides fragilis metabolises exopolysaccharides produced by bifidobacteria

The use of bacterial EPS by Bacteroides thetaiotaomicron has been described before with HoPS produced by some lactic acid bacteria and Streptococcus spp. [6, 16, 25]. Although the use of HePS from bifidobacteria had not been definitely demonstrated yet, we have previously identified changes in the metabolism of B. fragilis in the presence of these polymers in an undefined medium [18]. Results from the current study demonstrated an effective growth of this specie in the presence of HePS. As compared to glucose, Bacteroides population levels attained with our EPS at early stages of growth were lower, but probably the slow utilization of these polymers and hence their availability as carbon source along incubation contributed to maintain the microbial levels at late states of growth, this phenomenon being more pronounced with EPS E44 than with EPS R1. Besides, a differential metabolic activity of B. fragilis in the presence of EPS as compared to glucose was evidenced. B. fragilis remained metabolically active in cultures with EPS for a longer period of time, with the highest activity corresponding to the cultures with the polymer E44. Variations obtained in metabolic profiles of Bacteroides cultures as depending on the carbon sources were similar to those indicated previously by us using an undefined medium [18]. Additionally, higher propionic to succinic acid ratios with complex carbon sources relative to glucose, similar to that found in the current study, have been previously reported [26].

In the present study we observed a clear shift in the metabolite production by B. fragilis during the time course of fermentation in the presence of EPS with respect to glucose, which was more pronounced with the polymer E44 than with R1. Coinciding with the depletion in the amount of EPS polymers from 48 h of incubation, a gradual increase in the concentrations of acetic, propionic and succinic acids and a decrease in the levels of lactic acid occurred until the end of incubation. It has been previously suggested that, in Bacteroides, the production of propionate through the succinate/propionate pathway could be a cell response to optimize cell energy production while keeping the intracellular redox balance [18, 19]. Although B. fragilis has the capacity to metabolize moderate amounts of lactic acid [27] as well as amino acids [28], our work was performed in a minimal medium, so that no amino acid sources were available and the scarce consumption of lactic acid that may occur does not explain the growth and metabolic activity of the bacterium in such conditions.

The results from SEC-MALS chromatography indicated that B. fragilis was able to use the EPS E44 produced by Bif. longum as a fermentable substrate. The chemical structure of EPS E44 has not been elucidated yet but it is known that EPS E44 contains glucose and galactose in its composition [22]. It is then possible that the saccharolytic enzymatic machinery of B. fragilis could include enzymes able to participate in the degradation of this complex substrate. On the other hand, changes in growth and metabolic patterns occurring at late stages of growth in EPS E44 could be related with the cessation of consumption of the smaller peak beyond 72 h of incubation, and hence with a scarcity of carbon source available from this time. Our results are not conclusive about the possible degradation of the EPS R1 fraction by B. fragilis. Even though there were no significant changes attributable to the activity of B. fragilis in the EPS peaks of high and medium molar mass, we could not rule out changes in the amount of the smallest polymer, not considered in our study because the overlapping with a protein peak. Variations in this small polymer may provide a possible explanation for the increased metabolic activity evidenced in cultures of B. fragilis until 72 h of incubation in the presence of the EPS R1 fraction as compared to the control in MM without carbon sources added. The EPS R1 fraction is formed by glucose, galactose and rhamnose [24] and only the chemical structure of the high molar mass peak has been determined to date, which is composed by 50 % rhamnose [29]. Although we know that the presence of both EPS stimulates the production of ?-glucosidase by B. fragilis, the high molar mass polymer of the EPS R1 fraction lacks the ?-linkages targeted by this enzymatic activity [29]. This together with the inability of B. fragilis to ferment rhamnose [28], could pose difficulties for the use as fermentable substrate of the EPS R1 by this microorganism.

The ability of B. fragilis to use bifidobacterial EPS may provide this microorganism with a long-term available complex carbon source, thus enhancing its survival and conferring it a selective advantage in environments where nutrients are scarce, such as the case of the human large intestine. Bacteroides plays an important role in the utilization of indigestible dietary compounds and complex polymers secreted by other microorganisms [6]. Some EPS and capsular polysaccharides are involved in adhesion to eukaryotic cells, biofilm formation and protection of several species against the gastrointestinal stressing factors [14]. In this way the ability of Bacteroides to degrade these polymers may confer this microorganism a role in the regulation of microbial relationships in the gut ecosystem. Fermentation of EPS in faecal cultures lead to an increase in propionic acid production [17], most likely due to the metabolic activity of members from the genus Bacteroides.