The supposed selectivity of FOS and inulin: an important point against it

[Guest guest]

For those who don't want to slog through the whole abstract, I'll boil down

the essence: even if certain undesirable bacteria cannot feed on inulin

itself, they most certainly can flourish on the byproducts formed when

other species and strains do begin to digest it.

And this is completely aside from the fact that bacteria are constantly

swapping plasmids and learning to digest new things, a phenomenon which

also wouldn't be observed in simple in vitro experiments.

Appl Environ Microbiol. 2003 Feb;69(2):1136-42.

Effects of alternative dietary substrates on competition between human

colonic bacteria in an anaerobic fermentor system.

Duncan SH, KP, Ramsay AG, Harmsen HJ, Welling GW, CS, Flint HJ.

Gut Microbiology and Immunology Division, Rowett Research Institute,

Bucksburn, Aberdeen AB21 9SB, United Kingdom.

Duplicate anaerobic fermentor systems were used to examine changes in a

community of human fecal bacteria supplied with different carbohydrate

energy sources. A panel of group-specific fluorescent in situ hybridization

probes targeting 16S rRNA sequences revealed that the fermentors supported

growth of a greater proportion of Bacteroides and a lower proportion

of gram-positive anaerobes related to Faecalibacterium prausnitzii,

Ruminococcus flavefaciens-Ruminococcus bromii, Eubacterium

rectale-Clostridium coccoides, and Eubacterium cylindroides than the

proportions in the starting fecal inoculum. Nevertheless, certain

substrates, such as dahlia inulin, caused a pronounced increase in the

number of bacteria related to R. flavefaciens-R. bromii and E.

cylindroides. The ability of three strictly anaerobic, gram-positive

bacteria to compete with the complete human fecal flora was tested in the

same experiment by using selective plating to enumerate the introduced

strains. The Roseburia-related strain A2-183(F) was able to grow on all

substrates despite the fact that it was unable to utilize complex

carbohydrates in pure culture, and it was assumed that this organism

survived by cross-feeding. In contrast, Roseburia intestinalis L1-82 ®

and Eubacterium sp. strain A2-194® survived less well despite the fact

that they were able to utilize polysaccharides in pure culture, except that

A2-194® was stimulated 100-fold by inulin. These results suggest

that many low-G+C-content gram-positive obligate anaerobes may be selected

against during in vitro incubation, although several groups were stimulated

by inulin. Thus, considerable caution is necessary when workers attempt to

predict the in vivo effects of probiotics and prebiotics from their effects

in vitro.

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