9 Catabolism of Mucopolysaccharides, Plant Gums, and Maillard Products by Human Colonic Bacteroides ABIGAIL A. SALYERS, MILDRED O'BRIEN, and BARRY SCHMETTER University of Illinois, Department of Microbiology, Urbana, IL 61801 The mixture of complex carbohydrates which enters the human colon includes not only plant c e l l wall polysaccharides but also a variety of other types of carbohydrate such as mucopolysaccharides (from dietary meat or host secretions), plant gums which are added to foods as emulsifiers, and Maillard reaction products. Some strains of colonic Bacteroides which can degrade plant c e l l wall polysaccharides can also degrade one or more of these other types of carbohydrates. Bacterial strategies for degrading these substances differ. For example, the enzymes responsible for catabolism of mucopolysaccharides by Bacteroides are cell-associated rather than extracellular and are not associated with the bacterial membranes. By contrast, the catabolism of guar gum, a viscous plant galactomannan, involves both an extracellular enzyme and at least one membrane-bound enzyme. Breakdown of polygalacturonic acid involves a membrane-bound enzyme. A feature which is common to a l l three of these polysaccharide-degrading systems is that the enzymes are inducible, that is they are produced only when the appropriate substrate is encountered by the bacteria. The Maillard product, fructosylglycine, is not utilized by strains of Bacteroides which can ferment fructose and fructosyl compounds such as sucrose, presumably because of difficulties in hydrolyzing the C-N bond which binds fructose to glycine. Another Maillard product, isomaltol-galactoside, which contains a glycosidic bond, can be degraded and the galactose moeity u t i l i z e d . U t i l i z a t i o n of this compound, which is an analogue of lactose, appears to occur via the lactose system. The enzyme which degrades 0097-6156/83/0214-0123$06.00/0 © 1983 American Chemical Society
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isomaltolgalactoside, unlike the polysaccharidases which are produced by the same organism, is constitutive rather then inducible. Nutritional studies in humans have demonstrated quite clearly that some components of dietary fiber are degraded extensively during passage through the intestinal tract ( 1_). Degradation of dietary fiber appears to occur in the colon where component polysaccharides are fermented by saccharolytic bacteria which reside there. Although this fermentation occurs in the colon, i t may contribute to human nutrition since a significant percentage of the bacterial fermentation products can be absorbed through the colonic mucosa (1_, 2). Because of the potential contribution of dietary fiber components to host nutrition and also because of an increased awareness of the role of the bacterial flora of the colon in human disease, considerable attention has been devoted in recent years to the possible effects of dietary fiber on the colonic flora. The focus on bacterial fermentation of components of dietary fiber from conventional sources such as bran should not prevent us from considering other types of carbohydrate which may also be fermented by colon bacteria. For example, the host i t s e l f produces a variety of polysaccharides which reach the colon. These include mucins from the secretions which lubricate the intestinal tract and mucopolysaccharides from slonghed epithelial cells. It is d i f f i c u l t to obtain precise information concerning how much of this material is produced daily, but the amount could easily be comparable to the amount of polysaccharide which is ingested in the diet. Some of these hostproduced polysaccharides, particularly the mucopolysaccharides, are readily utilizable by some of the same bacterial species which ferment plant polysaccharides (3). Another potential source of carbohydrate for colon bacteria consists of polysaccharides which are added to foods as emulsifiers (e.g. guar gum, alginate) or which are used as stool softeners (e.g. psyllium). Plant polysaccharides such as guar gum are also being evaluated for use in the treatment of diabetes because they retard glucose absorption (4^, 5). The amount of polysaccharide from these sources w i l l vary from person to person but could be appreciable in some cases. Maillard products, associated with the browning of foods, could also provide some carbohydrate for intestinal bacteria. At present, this possibility is merely speculative. Many possible Maillard products exist (6^, jO. The Maillard products which are most likely to be utilizable by bacteria are the early products, such as glycosylamines and aryl-glycosides, which resemble fermentable disaccharides. Some Maillard products such as catechols or substituted pyridines could actually be toxic to
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colon b a c t e r i a . No estimates are yet a v a i l a b l e as to the amounts of these substances which a c t u a l l y reach the colon. Moreover, the number and v a r i e t y of M a i l l a r d products which can be formed i n foods makes i t d i f f i c u l t to decide which compounds to i n v e s tigate. Some M a i l l a r d products such as a r y l - g l y c o s i d e s could be cleaved by enzymes i n the host's small i n t e s t i n e , although t h i s might not be the case for i n d i v i d u a l s with d i g e s t i v e enzyme d e f i c i e n c i e s such as lactose i n t o l e r a n c e . Moreover, some recent n u t r i t i o n a l studies have shown that v a r y i n g amounts of compounds such as sucrose or starch,which are g e n e r a l l y considered to be completely d i g e s t i b l e , can escape d i g e s t i o n i n the small i n t e s t i n e and thus reach the colon (8_, 9). A c c o r d i n g l y , the p o s s i b i l i t y that colon b a c t e r i a are exposed to M a i l l a r d products should be considered. Polysaccharide U t i l i z a t i o n by Colon B a c t e r i a From these examples, i t can be seen that studies of the e f f e c t of d i e t on the i n t e s t i n a l f l o r a must take i n t o account the a v a i l a b i l i t y of a v a r i e t y of polysaccharides. Information about i n v i t r o fermentation of d i f f e r e n t polysaccharides by colon b a c t e r i a can help us to make p r e d i c t i o n s about what s o r t s of carbohydrate may be u t i l i z a b l e i n v i v o , but t h i s information r a i s e s s t i l l other questions. To see t h i s , consider some of the r e s u l t s of surveys of polysaccharide fermentation by numerically predominant species of colon b a c t e r i a (3, 10). Some of these r e s u l t s are summarized i n Table 1. One t h i n g that i s immediately apparent from Table 1 i s that the most v e r s a t i l e fermenters of polysaccharides are Bacteroides species. Moreover, many of these species can ferment s e v e r a l types of polysaccharides. This r a i s e s the question of which type of polysaccharide would be u t i l i z e d p r e f e r e n t i a l l y i n v i v o i f these b a c t e r i a were to be confronted by s e v e r a l u t i l i z a b l e polysaccharides at the same time. Not only do i n d i v i d u a l s t r a i n s or species e x h i b i t the a b i l i t y to ferment a v a r i e t y of polysaccharides, but i t i s a l s o the case that each polysaccharide can be fermented by more than one species. Do a l l of these species a c t u a l l y p a r t i c i p a t e i n degrading a p a r t i c u l a r polysaccharide i n v i v o or does one species predominate due to a more e f f i c i e n t u t i l i z a t i o n system? In short, r e s u l t s of surveys of polysaccharide fermentation allow us to p r e d i c t that many types of carbohydrate ( i n c l u d i n g d i e t a r y f i b e r components) w i l l be fermented i n the colon, but these r e s u l t s do not provide answers to other questions such as which organisms a c t u a l l y degrade a polysaccharide i n v i v o or whether a p a r t i c u l a r d i e t a r y carbohydrate (e.g. an e r a u l s i f i e r such as guar gum) might upset the normal patterns of polysaccharide catabolism i n the colon i f i t were present i n high enough concentrations i n the d i e t .
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Table 1. Species of i n t e s t i n a l b a c t e r i a which can degrade various i n d i g e s t i b l e polysaccharides (3, 10)
Polysaccharide
Species of c o l o n i c b a c t e r i a which can degrade i t
Chondroitin sulfate (mucopolysaccharide)
Bacteroides thetaiotaomicron Bacteroides ovatus Bacteroides '3452A'* Bacteroides f r a g i l i s subsp Bacteroides uniformis
Guar gum, locust bean gum (ga1ac t omannan)
Bacteroides ovatus Bacteroides uniformis Ruminococcus albus
Xylan
Bacteroides ovatus Bacteroides f r a g i l i s subsp.a Bacteroides e g g e r t h i i Bacteroides vulgatus Bifidobacterium adolescentis Bifidobacterium infantis
P e c t i n , p o l y g a l a c t u r o n i c acid
Bacteroides Bacteroides Bacteroides Bacteroides Bacteroides Eubacterium
Arabinogalactan
Bacteroides thetaiotaomicron Bacteroides ovatus Bacteroides 3452A * Bacteroides uniformis Bacteroides vulgatus Bacteroides 'T4-1'*
thetaiotaomicron ovatus '3452A'* f r a g i l i s subsp.a vulgatus eligens
,
Unnamed DNA homology groups (11).
t
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Our approach to these and s i m i l a r question has been to i n v e s t i g a t e the mechanisms by which pure c u l t u r e s of colon b a c t e r i a u t i l i z e i n d i v i d u a l polysaccharides i n v i t r o , with a view to determining what f a c t o r s a f f e c t the organism's d e c i s i o n to u t i l i z e a p a r t i c u l a r type of carbohydrate. This approach i s based on the assumption that information about the s p e c i f i c features and l i m i t a t i o n s of p o l y s a c c h a r i d e - c a t a b o l i z i n g systems i n colon b a c t e r i a w i l l permit us e i t h e r to make p r e d i c t i o n s about the extent to which catabolism of a p a r t i c u l a r c l a s s of polysaccharides can occur i n v i v o or to develop s p e c i f i c methods for d e t e c t i n g metabolic states i n b a c t e r i a i n v i v o . S t r a t e g i e s f o r Polysaccharide
Metabolism
To i l l u s t r a t e some c h a r a c t e r i s t r i c s of polysaccharide u t i l i z a t i o n systems of Bacteroides species, we w i l l compare the systems involved i n u t i l i z a t i o n of a host-produced mucopolysaccharide ( c h o n d r o i t i n s u l f a t e ) , a plant galactomannan (guar gum) and p o l y g a l a c t a r o n i c a c i d . Bacteroides thetaiotaomicron ferments not only c h o n d r o i t i n s u l f a t e but a l s o s t r u c t u r a l l y s i m i l a r mucopolysaccharides such as hyaluronic a c i d and heparin. The degradative enzymes are i n d u c i b l e , i . e . they are only produced i n appreciable amounts only when the organism i s exposed to the appropriate substrate (12). The rate at which the enzymes are produced f o l l o w i n g exposure to the polysacchar i d e and the amount of enzyme produced are both p r o p o r t i o n a l to the concentration of polysaccharide i n the medium. Thus the higher the concentration of a polysaccharide the more l i k e l y i t i s to be u t i l i z e d . Induction of enzyme a c t i v i t y i s f a i r l y s p e c i f i c . Growth on c h o n d r o i t i n s u l f a t e i s accompanied by production of enzymes which attack c h o n d r o i t i n s u l f a t e and those mucopolysaccharides which c l o s e l y resemble c h o n d r o i t i n s u l f a t e (e.g. hyaluronic a c i d ) . Enzymes for degrading heparin, a mucopolysaccharide which contains a d i f f e r e n t g l y c o s i d i c linkage, are not induced by c h o n d r o i t i n s u l f a t e . B a c t e r i a which are growing on heparin produce enzymes which attack heparin but not c h o n d r o i t i n s u l f a t e or hyaluronic a c i d (K. Voss and A. S a l y e r s , unpublished r e s u l t s ) . None of the enzymes which are involved i n the breakdown of c h o n d r o i t i n s u l f a t e by IB. thetaiotaomicron are e x t r a c e l l u l a r . In f a c t , the f i r s t degradative enzyme, c h o n d r o i t i n lyase, appears to be located i n the periplasmic space between the inner and outer membranes of these gram negative organisms (13). Chondroitin lyase breaks c h o n d r o i t i n s u l f a t e i n t o disaccharides and these disaccharides are f u r t h e r degraded by enzymes which are located i n the cytoplasm. None of these enzymes i s membrane-bound. However there are probably some membrane proteins involved, e.g. receptors on the c e l l surface which b r i n g the c h o n d r o i t i n s u l f a t e i n t o contact with the lyase and
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proteins i n the cytoplasmic membrane which transport the disaccharides i n t o the cytoplasm. The c h o n d r o i t i n sulfate-degrading systems of two species of colon b a c t e r i a which are c l o s e l y r e l a t e d to B^. t h e t a i o t a o micron, Bacteroides ovatus and Bacteroides 3452A (an unnamed DNA homology group of B a c t e r o i d e s ) , appear to be s i m i l a r to the c h o n d r o i t i n sulfate-degrading system of B^. thetaiotaomicron i n that the degradative enzymes are i n d u c i b l e and are c e l l - a s s o c i a t e d rather than e x t r a c e l l u l a r (A. Salyers and M. O'Brien, unpublished data). Moreover, none of these enzymes i n B. ovatus or B^. 3452A are membrane-bound. I t i s i n t e r e s t i n g to note that DNA homology data i n d i c a t e s that B^. ovatus and B^. 3452A have 30-45% cross homology with thetaiotaomicron (11). We are c u r r e n t l y i n v e s t i g a t i n g the p o s s i b i l i t y that some of t h i s shared genetic m a t e r i a l contains the genes for chondroi t i n s u l f a t e u t i l i z a t i o n , i . e . that these three species have i d e n t i c a l systems for degrading c h o n d r o i t i n s u l f a t e . Guar gum, a galactomannan which i s used as an e m u l s i f i e r i n processed foods, i s u t i l i z e d by a d i f f e r e n t sort of system. When B^. ovatus i s growing on guar gum, there i s an enzymatic a c t i v i t y i n the e x t r a c e l l u l a r f l u i d which decreases the v i s c o s i t y of guar gum (14). This enzyme reduces the molecular weight of guar gum from 10^ to 4 χ 10 , but does not produce small carbohydrate fragments. In a d d i t i o n to t h i s enzyme, there i s a l s o c e l l - a s s o c i a t e d galactomannanase a c t i v i t y , most of which stays with the c e l l membranes even a f t e r treatment with 0.5M NaCl. This i n d i c a t e s that the enzyme i s an i n t e g r a l membrane p r o t e i n . The product of t h i s galactomannanase appears to be a t r i s a c c h a r i d e (15). Thus the galactomannan-degrading system d i f f e r s from the c h o n d r o i t i n sulfate-degrading system i n the l o c a t i o n of the i n i t i a l degradative enzymes. However, these two systems resemble each other i n that the degradative enzymes are inducible. B^. thetaiotaomicron does not ferment guar gum. However i t does ferment another viscous polysaccharide, p o l y g a l a c t u r o n i c a c i d . Recent i n v e s t i g a t i o n s of p o l y g a l a c t u r o n i c a c i d breakdown by B. thetaiotaomicron have shown that t h i s organism produces an i n d u c i b l e membrane-bound polygalacturonase ( J . Leedle and A. S a l y e r s , unpublished r e s u l t s ) . No e x t r a c e l l u l a r enzyme was detected. Thus c o l o n i c Bacteriodes appear to have d i f f e r e n t s t r a t e gies for degrading d i f f e r e n t polysaccharides. It may be the case that i n general the type of mechanisms used for degrading a p a r t i c u l a r polysaccharide depends on the type of polysaccharide being u t i l i z e d rather than on the species of the organism. 1
f
1
1
f
1
U t i l i z a t i o n of M a i l l a r d
Products
At present there i s l i t t l e information concerning what types of M a i l l a r d products reach the colon. The l i g n i n - l i k e
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polymers which are the end products of the M a i l l a r d r e a c t i o n reach the colon but are not degraded by colon b a c t e r i a (Van Soest, p r i v a t e communication). I t i s not known whether e a r l i e r products i n the M a i l l a r d r e a c t i o n (e.g. a r y l - g l y c o s i d e s , glycosylamines) are digested i n the small i n t e s t i n e . However, we can determine whether these M a i l l a r d products could be u t i l i z e d by s a c c h a r o l y t i c b a c t e r i a i f they were to reach the colon. We have i n v e s t i g a t e d the u t i l i z a t i o n of two M a i l l a r d compounds, f r u c t o s y l - g l y c i n e and i s o m a l t o l - g a l a c t o s i d e , by Bacteroides species. F r u c t o s y l - g l y c i n e i s formed during condensation of glucose with g l y c i n e (]_, 16). Isomaltol-galactoside i s formed as a consequence of the i n t e r a c t i o n of an amine with l a c t o s e . The r e s u l t i n g r e a c t i o n s culminate i n dehydration of the glucose moeity to produce isomaltol (17). Isomaltol and i s o m a l t o l - g a l a c t o s i d e are found i n baked breads and contribute to the c h a r a c t e r i s t i c odor and f l a v o r of these foods (18). The structures of f r u c t o s y l - g l y c i n e and i s o m a l t o l - g a l a c t o side are shown i n Figure 1. F r u c t o s y l - g l y c i n e could be considered a s t r u c t u r a l analogue of sucrose or of other f r u c t o s y l compounds such as i n u l i n . We tested two species of c o l o n i c Bacteroides (B^. ovatus and B^. thetaiotaomicron) f o r the a b i l i t y to grow i n medium which contained f r u c t o s y l - g l y c i n e as the sole carbon source. Both of these species ferment sucrose and B^. ovatus also ferments i n u l i n , but neither could u t i l i z e the f r u c tose i n f r u c t o s y l - g l y c i n e . Moreover, crude e x t r a c t s from s o n i c a l l y d i s r u p t e d B^. ovatus or B^. thetaiotaomicron (grown on f r u c tose, sucrose or i n u l i n ) d i d not degrade f r u c t o s y l - g l y c i n e . The i n a b i l i t y of these Bacteroides species to u t i l i z e f r u c t o r y l - g l y cine as a carbon source does not r u l e out the p o s s i b l i t y that other i n t e s t i n a l b e c t e r i a might be able to u t i l i z e i t . Although f r u c t o s y l - g l y c i n e was not u t i l i z e d as a carbon source by 15. ovatus, i t d i d appear to i n h i b i t growth of B. ovatus on other carbohydrates (Figure 2). This i n h i b i t o r y e f f e c t occurred with glucose as well as with fructose and f r u c t o s y l compounds. Note that t h i s e f f e c t was not very pronounced and that i t required high concentrations of f r u c t o s y l - g l y c i n e . Thus i t i s not l i k e l y to be important i n the colon where concent r a t i o n s of compounds such as f r u c t o s y l - g l y c i n e would be expected to be much lower. F r u c t o s y l - g l y c i n e did not i n h i b i t the growth of B^. thetaiotaomicron on f r u c t o s e , glucose or sucrose. So the e f f e c t of f r u c t o s y l g l y c i n e on growth may be l i m i t e d to c e r t a i n species. I n v e s t i g a t i o n s , such as those described above, o f the e f f e c t of compounds l i k e f r u c t o s y l - g l y c i n e on growth rate o f b a c t e r i a have a major drawback. The compound being tested must be incubated for 10-20h at 37°C i n growth medium. Although f r u c t o s y l - g l y c i n e i s r e l a t i v e l y stable and there was no v i s i b l e darkening of the medium during the incubation period, we were able to detect by t h i n layer chômâtography traces of compounds
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Fructosyl -glycine H C-NH — C H 2
2
—C0 H 2
C= 0 I HOCH !
HCOH I HCOH I CH OH 2
Isomaltol
galactoside
" galactose HC
C
\
/ \
Ο Figure 1. Structures of jructosyl-glycine and isomaltol galactoside.
c= I CH
3
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Inulin (0.2%)
Sucrose (0.2%) 0.7
/ ρ If 1 ί
0.5
0.3
Ι
° If
ι
ν 6
E c
S
Γ
CO
I
1' ·
°·1
û
S
α Ο
Fructose (0.2%)
Glucose(0.2%)
0.7 0.5
/
/
ρ
I> 0.3
0.1
-
β
f
10
20
0
10
20
Time (h) Figure 2. Effect of fructosyl-glycine on the growth of Β. ovatus in a medium that contained glucose, fructose, sucrose, or inulin as the sole source of carbohydrate. The basal medium has been described previously (12). The carbohydrate and fructosyl-glycine werefilter-sterilizedand added to the basal medium after autoclaving. The initial concentration of both the carbohydrate and fructosyl-glycine was 2 mg/mL. Cultures grew more slowly in the medium that contained fructosyl-glycine plus the fermentable sugar (O) than in the medium that contained only the fermentable sugar (φ).
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other than f r u c t o s y l - g l y c i n e i n uninoculated medium which had been incubated for 12h at 37°C. This r a i s e s the question of whether f r u c t o s y l - g l y c i n e i t s e l f or a f u r t h e r r e a c t i o n product was i n fact responsible for the e f f e c t on IB. ovatus. We tested the e f f e c t of f r u c t o s y l - g l y c i n e on uptake of glucose, fructose and sucrose by B^. ovatus. We found no s i g n i f i c a n t e f f e c t of f r u c t o s y l - g l y c i n e on uptake of any of these compounds. However, the uptake of [ % ] s u c r o s e was so slow that we could have f a i l e d to detect reductions i n the rate of uptake which were on the order of 20% or l e s s . We a l s o tested the e f f e c t of f r u c t o s y l - g l y c i n e on sucrase a c t i v i t y i n crude e x t r a c t s from disrupted b a c t e r i a . F r u c t o s y l - g l y c i n e did i n h i b i t sucrase a c t i v i t y . Further i n v e s t i g a t i o n s along these l i n e s are needed to e s t a b l i s h whether f r u c t o s y l - g l y c i n e has any other e f f e c t s on b a c t e r i a l metabolism which might e x p l a i n why i t i n h i b i t s growth. The r e s u l t s of our i n v e s t i g a t i o n s of the e f f e c t s of another M a i l l a r d product, i s o m a l t o l - g a l a c t o s i d e , can be summarized as follows (B. Schmetter and A. Salyers, unpublished data): B_. thetaiotaomicron u t i l i z e d the galactose i n i s o m a l t o l - g a l a c t o s i d e . During u t i l i z a t i o n , i s o m a l t o l accumulated i n the e x t r a cellular fluid. Isomaltol was not u t i l i z e d , but at high enough concentrations (4-5 mg/ml) i t i n h i b i t e d growth. Thus i s o m a l t o l i s t o x i c to Bacteriodes, but only at concentrations much higher than those which are l i k e l y to be found iti v i v o . Isomaltol d i d not a f f e c t uptake of simple sugars on amino a c i d s . However, i t stopped b a c t e r i a l synthesis of p r o t e i n and DNA. Since [**H]-labeled i s o m a l t o l was not taken up by the b a c t e r i a i n detectable q u a n t i t i e s , i t i s not c l e a r how i s o m a l t o l exerts t h i s e f f e c t . One p o s s i b i l i t y i s that i t binds to membranes. Isomaltol-galactoside appears to be u t i l i z e d v i a the l a c t o s e system of B^. thetaiotaomicron. Isomaltolgalactoside i n h i b i t e d uptake of [ ^ C ] - l a c t o s e . Moreover mutants which were d e f i c i e n t i n lactose uptake did not u t i l i z e i s o m a l t o l g a l a c t o s i d e . Isomaltol-galactoside was hydrolyzed almost as r a p i d l y by b a c t e r i a l enzymes as l a c t o s e . Whether or not i s o m a l t o l - g a l a c t o s i d e i t s e l f i s a c t u a l l y present i n s i g n i f i c a n t q u a n t i t i e s i n the colon, i t i s i n t e r e s t i n g to note that l a c t o s e analogues of t h i s sort are r e a d i l y u t i l i z e d by a l a c t o s e - f e r menting organism. Other M a i l l a r d a r y l - g l y c o s i d e s may also be u t i l i z e d by b a c t e r i a which normally ferment s t r u c t u r a l l y s i m i l a r saccharides. The enzyme a c t i v i t y which was associated with the breakdown of i s o m a l t o l - g a l a c t o s i d e was c o n s t i t u t i v e . In t h i s respect, the c a t a b o l i c system for i s o m a l t o l - g a l a c t o s i d e d i f f e r s from the c a t a b o l i c systems for degrading poysaccharides. It also d i f f e r s from the lactose system i n E_. c o l i which i s an i n d u c i b l e system. The fact that i s o m a l t o l - g a l a c t o s i d e breakdown can occur without
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p r i o r adaptation of the b a c t e r i a i n d i c a t e s that these would be r a p i d l y u t i l i z e d i i i v i v o .
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Conclusions Human c o l o n i c b a c t e r i a are q u i t e v e r s a t i l e with respect to t h e i r a b i l i t y to ferment polysaccharides. Moreover, i n d i v i d u a l species do not seem to have a c o n s i s t e n t strategy for degrading polysaccharides. In some cases, c e l l - a s s o c i a t e d periplasmic enzymes are used. In other cases the enzymes are e x t r a c e l l u l a r or even membrane-bound. I t may turn out to be the case that the c a t a b o l i c strategy i s s p e c i f i c for the type polysaccharide rather than f o r the species, i . e . d i f f e r e n t species may u t i l i z e a p a r t i c u l a r type of polysaccharide i n the same way. Because of the v a r i e t y of polysaccharides which can be fermented by some Bacteroides species, i t i s d i f f i c u l t to pre d i c t with c e r t a i n t y which polysaccharides i n the complex mixture of d i e t a r y and host-produced carbohydrates that enter the colon w i l l be degraded most r a p i d l y and most e x t e n s i v e l y . Further information about how these organisms make choices between d i f f e r e n t polysaccharides i i i v i t r o may help to c l a r i f y t h i s i s s u e . However, n u t r i t i o n i s t s who are i n t e r e s t e d i n catabolism of d i e t a r y f i b e r components i i i v i v o should be aware that the b a c t e r i a may p r e f e r other sources of carbohydrate, such as mucopolysaccharides from host s e c r e t i o n s or even M a i l l a r d products, to the d i e t a r y polysaccharide under study, and that t h i s preference may i n f l u e n c e catabolism of a p a r t i c u l a r polysaccharide i n ways which we cannot at present p r e d i c t . E f f e c t s of t h i s sort may be r e s p o n s i b l e f o r some of the i n d i v i d u a l - t o - i n d i v i d u a l v a r i a t i o n which i s encountered i n n u t r i t i o n a l studies of d i e t a r y f i b e r u t i l i z a t i o n .
Acknowledgment s T h i s work was supported by grant PFR 79-19126 from the Problem-Focused Research S e c t i o n of the N a t i o n a l Science Foun d a t i o n and by grant AI 17876 from the N a t i o n a l I n s t i t u t e s of Health.
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