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© 2004 The American Society for Nutritional Sciences J. Nutr. 134:2022S-2026S, August 2004

Supplement: WALTHAM International Science Symposium: Nature, Nurture, and the Case for Nutrition

Bacteria in the Gut: Friends and Foes and How to Alter the Balance1

R. A. Rastall2

Food and Bioprocessing Sciences Group, School of Food Biosciences, University of Reading, Whiteknights, Reading, RG6 6AP, UK

2 To whom correspondence should be addressed. E-mail: r.a.rastall{at}reading.ac.uk.


   ABSTRACT
TOP
 ABSTRACT
LITERATURE CITED
 
The activities of the bacteria resident in the colon of companion animals can have an impact upon the health of the host. Our understanding of this microbial ecosystem is presently increasing due to the development of DNA-based microbiological tools that allow identification and enumeration of nonculturable microorganisms. These techniques are changing our view of the bacteria that live in the gut, and they are facilitating dietary-intervention approaches to modulate the colonic ecosystem. This is generally achieved by the feeding of either live bacteria (probiotics) or nondigestible oligosaccharides (prebiotics) that selectively feed the indigenous probiotics. Feeding studies with a Lactobacillus acidophilus probiotic have shown positive effects on carriage of Clostridium spp. in canines and on recovery from Campylobacter spp. infection in felines. Immune function was improved in both species. Prebiotic feeding studies with lactosucrose and fructo-oligosaccharides in both cats and dogs have shown positive effects on the microflora balance. Recently synbiotic forms (a probiotic together with a prebiotic) targeted at canines have been developed that show promise as dietary-intervention tools.

KEY WORDS: • bacteria • colonic microflora • probiotics • prebiotics • Bifidobacterium • Lactobacillus

Introduction

The link between the barrier function of colonic microflora and susceptibility to disease (1) is an area of great interest. This has led to a vibrant, global, functional food industry that is introducing new products for gut health into markets targeted to humans and companion animals.

The gut microflora

Most of our knowledge of gut microflora comes from studies on humans (2). Microbiologically, the gut can be thought of in terms of three principal regions: the stomach, small intestine, and colon. In terms of microbial population, the stomach has very low bacterial numbers; facultative anaerobes such as lactobacilli, streptococci, and yeast are present at ~100 colony-forming units (CFU)3 per milliliter due to the low environmental pH.3 The small intestine has a larger bacterial load that consists of facultative anaerobes such as lactobacilli, streptococci, and enterobacteria as well as anaerobes such as Bifidobacterium spp., Bacteroides spp., and clostridia at levels of ~104–108 CFU/ml. The most heavily colonized region, however, is the colon, with a total population of 1011–1012 CFU/ml of contents (3). The colonic microflora is the predominant target for dietary intervention in the gut ecology, and it is this region that is the subject of this article. Consisting of higher levels of obligate anaerobes and lower levels of facultative aerobes (Fig. 1), the colonic microflora is very complex.



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  		FIGURE 1  Overview of the human colonic microflora. Bacterial genera were classified as health positive, health negative, or health neutral. Bacterial enumeration was by selective media (4). Ps, Pseudomonas.

 
The colonic microflora is dominated by strict anaerobes such as Bacteroides spp., the clostridia and other families within the Clostridium mega-genus (including Ruminococcus spp., Butyrovibrio spp., Fusobacterium spp., Eubacterium spp., and Peptostreptococcus), Bifidobacterium spp., Atopobium spp., and the peptococci. Facultative anaerobes occur in numbers ~1000-fold lower and include lactobacilli, enterococci, streptococci, and Enterobacteriaceae. Yeasts are present only at relatively low numbers of 102–104 CFU/ml (4).

In terms of health, the most significant organisms are believed to be the bifidobacteria (4). Bifidobacteria are the major component of the microbial barrier to infection. Bifidobacteria produce a range of antimicrobial agents that are active against Gram-positive and -negative organisms (5). Lactobacilli are also health positive and produce a range of antimicrobial agents, but they are present in much lower levels in the human colon. In addition to producing antimicrobial agents, a large population of beneficial bacteria competitively excludes pathogens by occupying receptor sites and competing for space, nutrients, etc.

Much of the information presently available regarding colonic microflora comes from studies that employed classical microbiological techniques based on agar plates. This poses a problem, however, as colonic microflora are thought to contain a high level of biodiversity including many species that cannot be cultured using present techniques. This unculturable flora can only be characterized using DNA-based microbiology methods (6–8). Most of these methods rely on amplification, detection, and/or sequencing of diagnostic regions of 16S rRNA genes (8).

This culture problem is particularly acute in studies on canines. A recent study (9) illustrates the unreliability of apparently selective agar media for enumeration of canine fecal bacteria: many of the selective media used did not support the growth of the target population (Table 1).


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  		TABLE 1 Selectivity of media used in canine colonic microflora studies1

 
Our image of the canine gut flora, then, is based largely on traditional methods of investigation (Fig. 2) (10). In one study (11), fluorescent in situ hybridization (FISH) was used to describe the flora on one Labrador dog (Fig. 2). The most significant aspect of the canine microflora is the much lower level of bifidobacteria found in canines than in other animals. The feline colonic flora (Fig. 3) is even less well characterized (12), and the bifidobacteria levels are probably even lower than in canines. In fact, bifidobacteria are only intermittently isolated from felines.



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  		FIGURE 2  Overview of the canine colonic microflora. In a study of healthy adult Shepherd dogs (A), bacterial groups were enumerated by selective media and confirmed by denaturing gradient gel electrophoresis (10). In a study of one healthy adult Labrador dog (B), bacterial groups were enumerated by FISH (11).

 


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  		FIGURE 3  Overview of the feline colonic microflora in a study of 8 healthy adult cats. Bacterial enumeration was by selective media (12).

 
Dietary tools for changing the balance

Probiotics have been investigated as dietary management tools for many years (13) in human as well as livestock animal studies. The concept is that ingestion of beneficial bacteria leads to colonization of the gut with the added strain, and this then strengthens the gastrointestinal (GI) barrier to disease. Although bifidobacteria are the most significant health-positive organism in the colon, their obligate anaerobic nature has hindered commercial development. Most commercial probiotics are lactobacilli, and several species have been developed for application in humans and livestock animals (13).

There are few studies on probiotics in companion animals. One recent study (14) investigated the application of Lactobacillus acidophilus DSM 13241 in canines. This strain was chosen on the basis of its growth characteristics, antimicrobial activity toward pathogens, and survival rate in gut models. Feeding of 2 x 109 CFU/d to 15 healthy dogs resulted in a significant increase in the population of recoverable lactobacilli in the feces with a concomitant decrease in the clostridia population (determined by FISH). The animals displayed no significant changes in blood biochemistry, body temperature, or fecal quality. Immune-function studies showed no significant changes in haptoglobin level or white blood cell count but significant increases in serum IgG, monocytes, and neutrophils. Significant decreases in plasma nitric oxide levels and the osmotic fragility of red blood cells were observed. The researchers concluded that feeding of the probiotic resulted in positive changes in the gut microbiology and in systemic effects that suggested immune system stimulation as observed in humans after they consumed Lactobacillus spp. (13).

An attractive alternative to the feeding of probiotics is the use of prebiotics. A prebiotic is a nondigestible food ingredient that is selectively metabolized by the indigenous probiotic bacteria in the gut (4). Presently all prebiotics are carbohydrates (15), and a range of carbohydrates exist on the market around the world (Table 2).


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  		TABLE 2 Oligosaccharide prebiotics on the world market

 
Using prebiotics is attractive, because we can avoid the drawbacks of using probiotic bacteria such as maintaining viability. Prebiotics can thus be incorporated into a wider range of products and are stable to heat treatment.

There is relatively little work published on the use of prebiotics in companion animals (16). Most of the research to date has focused on lactosucrose and fructo-oligosaccharides (FOS). Studies on feeding of lactosucrose have been performed on dogs (17) and cats (12). Feeding of 1.5 g of lactosucrose/d to 8 healthy dogs for 2 wk resulted in statistically significant desirable changes to the gut flora as determined by fecal microbiological analysis (based on selective media). A 0.5-log increase in bifidobacteria was seen together with a 1.6-log decrease in clostridia levels. Decreases were also seen in toxin levels and fecal odor. Lactosucrose was also fed to 8 healthy cats at a level of 750 mg/d for 2 wk (12). This resulted in an increase in the incidence of recovery of bifidobacteria and a significant 0.9-log increase in lactobacilli numbers. Significant decreases of 0.4 log were seen with levels of clostridia and Enterobacteriaceae. Toxin levels and fecal odor were also reduced.

Several studies are available on FOS consumption in companion animals (16). A representative study (18) fed FOS at 4 g/d to 20 adult dogs. Fecal bacteriology was investigated by selective media, and bacterial metabolites were measured. Statistically significant increases in bifidobacteria (0.58 log) and lactobacilli (0.86 log) numbers were seen together with a small but significant decrease in clostridia level of 0.11 log. Increases were seen in lactate and butyrate quantities but increases were also observed in ammonia, isovalerate, dimethylsulfide, and hydrogen sulfide levels.

A study in cats (19) was performed by feeding a diet that contained 0.75% FOS for 12 wk to 12 adult cats. Bacteriology was performed by selective media, and bacterial metabolites were measured. Only one isolation of Bifidobacterium sp. was made, but a significant increase in lactobacilli number was seen (0.22 log). Significant decreases in clostridia (1.47 log) and Escherichia coli (0.52 log) numbers and an increase in bacteroides level (0.56 log) were also noted.

The combination of a probiotic with a prebiotic to support its viability and activity has been termed a synbiotic (4). An exciting development in the field of companion animals is that of synbiotics targeted to particular species. This has been attempted for the first time with canine synbiotics. Five candidate lactobacilli, L. acidophilus, L. murinus, L. reuteri, L. mucosae, and L. rhamnosus were isolated from a Labrador dog (11). It is generally held in the context of human and livestock animal nutrition that probiotic strains should originate from the species in which they are to be used (13). Three of these strains, L. mucosae, L. acidophilus, and L. reuteri, were then evaluated (11) for their growth on various carbohydrates (Table 3) and antimicrobial activity (Table 4) against Salmonella enterica serotype Typhimurium, enteropathogenic E. coli, and the toxin-negative mutant of E. coli O157:H7. On the basis of these data, candidate synbiotic forms can be identified with activity against specific target pathogens (Table 5).


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  		TABLE 3 Growth rate of canine probiotics on various carbohydrates1

 

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  		TABLE 4 Antimicrobial activity of selected probiotics against GI pathogens1

 

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  		TABLE 5 Effective synbiotic combinations against GI pathogens1

 
This canine synbiotic concept was taken further in an attempt to manufacture a prebiotic targeted to a particular probiotic organism. Most of the prebiotic oligosaccharides presently on the market are synthesized using enzymatic methods (15). For instance, galacto-oligosaccharides, which are a mixture of ß-linked di- to pentasaccharides, are manufactured by the action of ß-galactosidase on lactose. The enzyme catalyzes a glycosyl transfer reaction at high lactose levels and transfers galactose (Gal) from lactose, thereby acting as a glycosyl donor to other lactose molecules (which are acting as glycosyl acceptors) and thus building up higher oligosaccharide levels (15). This manufacturing technology can be modified to utilize enzymes from bifidobacteria on the assumption that the resultant products might have enhanced selectivity for the producing organism (20). This approach has been used in an attempt to develop highly targeted synbiotics for canine application. One of the canine probiotics discussed above, L. reuteri, was selected for further study: {alpha}-galactosidase enzyme activity was extracted from cultures of the organism and used to synthesize oligosaccharide mixtures from melibiose (Gal{alpha}1 -> 4 glucose) as a glycosyl donor (21). The resultant oligosaccharide mixtures were then evaluated in mixed fecal pH-controlled batch cultures (22). The oligosaccharides were found to be prebiotic in their own right and to act synergistically with added L. reuteri to a greater extent than with added L. acidophilus (Fig. 4). Interestingly, the synbiotic of L. reuteri and its synthesized oligosaccharides also stimulated bifidobacterial populations, presumably by inhibiting other species that inhibit the bifidobacteria.



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  		FIGURE 4  Fermentation properties of canine-targeted synbiotics. Bars represent percentage changes from the inoculum level in triplicate pH-controlled fecal batch cultures. Bacterial groups were enumerated by FISH (22).

 
Concluding remarks

The bacterial population within the GI tract of mammals constitutes a metabolically active organ that acts as a significant barrier to infection by exogenous pathogenic microorganisms. At present, our picture of human GI-tract ecology is far from complete, even less so for companion animals such as cats and dogs. Rapid development of new DNA-based methods is under way for studying the composition of complex microbial ecosystems such as the colonic microflora, and these have not yet been systematically applied to the study of companion animals. Such studies are required if we are to realize the potential of dietary manipulation of this barrier effect.

Dietary-management tools already exist in the shape of probiotic microorganisms, prebiotic oligosaccharides, and synbiotic mixtures of the two. There is evidence that these tools do work in dogs and, to a lesser extent, in cats. However, much more research is needed on the effects of the various probiotic strains and prebiotic oligosaccharides in a wide range of breeds. Well-designed feeding studies are required that use molecular microbiology techniques ideally coupled with more fundamental studies on the mechanisms of action of these agents. Such studies will lead to many new product opportunities in the pet-care field.


   FOOTNOTES
 
1 Presented as part of the WALTHAM International Science Symposium: Nature, Nurture, and the Case for Nutrition held in Bangkok, Thailand, October 28–31, 2003. This symposium and the publication of the symposium proceedings were sponsored by the WALTHAM Centre for Pet Nutrition, a division of Mars, Inc. Symposium proceedings were published as a supplement to The Journal of Nutrition. Guest editors for this supplement were D'Ann Finley, James G. Morris, and Quinton R. Rogers, University of California, Davis. Back

3 Abbreviations used: CFU, colony-forming unit; FISH, fluorescent in situ hybridization; FOS, fructo-oligosaccharides; GI, gastrointestinal. Back


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2. Roberfroid, M. B., Van Loo, J. A. E. & Gibson, G. R. (1998) The bifidogenic nature of chicory inulin and its hydrolysis products. J. Nutr. 128: 11–19.[Abstract/Free Full Text]

3. Cummings, J. H., Gibson, G. R. & Macfarlane, G. T. (1989) Quantitative estimates of fermentation in the hind gut of man. Acta Vet. Scand. Suppl. 86: 76–82.[Medline]

4. Gibson, G. R. & Roberfroid, M. B. (1995) Dietary modulation of the human colonic microbiota: introducing the concept of prebiotics. J. Nutr. 125: 1401–1412.

5. Gibson, G. R. & Wang, X. (1994) Regulatory effects of bifidobacteria on the growth of other colonic bacteria. J. Appl. Bacteriol. 77: 412–420.[Medline]

6. McCartney, A. L., Wenzhi, W. & Tannock, G. W. (1996) Molecular analysis of the composition of the bifidobacterial and lactobacillus microflora of humans. Appl. Environ. Microbiol. 62: 4608–4613.[Abstract]

7. Langendijk, P. S., Schut, F., Jansen, G. J., Raangs, G. C., Kamphuis, G. R., Wilkinson, M. H. F. & Welling, G. W. (1995) Quantitative fluorescence in situ hybridization of Bifidobacterium spp. with genus-specific 16S rRNA-targeted probes and its application in fecal samples. Appl. Environ. Microbiol. 61: 3069–3075.[Abstract]

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11. Tzortzis, G., Baillon, M. L. A., Gibson, G. R. & Rastall, R.A. (2004) Modulation of anti-pathogenic activity in canine-derived Lactobacillus species by carbohydrate growth substrate. J. Appl. Microbiol. 96: 552–559.[Medline]

12. Terada, A., Hara, H., Kato, S., Kimura, T., Fujimori, I., Hara, K., Maruyama, T. & Mitsuoka, T. (1992) Effect of lactosucrose (4G-ß-D-galactosylsucrose) on fecal flora and fecal putrefactive products of cats. J. Vet. Med. Sci. 55: 291–295.

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14. Baillon, M. L. A., Marshall-Jones, Z. V. & Butterwick, R. F. (2004) Effects of probiotic Lactobacillus acidophilus strain DSM13241 in healthy adult dogs. Am. J. Vet. Res. 65: 338–343.[Medline]

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16. Flickinger, E. A., Van Loo, J. & Fahey, G. C. (2003) Nutritional responses to the presence of inulin and oligofructose in the diets of domesticated animals: a review. Crit. Rev. Food Sci. Nutr. 43: 19–60.[Medline]

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18. Swanson, K. S., Grieshop, C. M., Flickinger, E. A., Bauer, L. L., Healy, H. P., Dawson, K. A., Merchen, N. R. & Fahey, G. C. (2002) Supplemental fructo-oligosaccharides and mannanoligosaccharides influence immune function, ileal and total tract nutrient digestibilities, microbial populations, and concentrations of protein catabolites in the large bowel of dogs. J. Nutr. 132: 980–989.[Abstract/Free Full Text]

19. Sparkes, A. H., Papasouliotis, K., Sunvold, G., Werrett, G., Gruffydd-Jones, E. A., Egan, K., Gruffydd-Jones, T. J. & Reinhart, G. (1998) Effect of supplementation with fructo-oligosaccharides on fecal flora of healthy cats. Am. J. Vet. Res. 59: 436–440.[Medline]

20. Rabiu, B. A., Jay, A. J., Gibson, G. R. & Rastall, R. A. (2001) Synthesis and fermentation properties of novel galacto-oligosaccharides by ß-galactosidases from Bifidobacterium species. Appl. Environ. Microbiol. 67: 2526–2530.[Abstract/Free Full Text]

21. Tzortzis, G., Jay, A. J., Baillon, M. L. A., Gibson, G. R. & Rastall, R. A. (2003) Synthesis of {alpha}-galactooligosaccharides with {alpha}-galactosidase from Lactobacillus reuteri of canine origin. Appl. Microbiol. Biotechnol. 63: 286–292.[Medline]

22. Tzortzis, G., Goulas, A. K., Baillon, M. L. A., Gibson, G. R. & Rastall, R. A. (2004) In vitro evaluation of the fermentation properties of galactooligosaccharides synthesized by {alpha}-galactosidase from Lactobacillus reuteri. Appl. Microbiol. Biotechnol. 64: 106–111.[Medline]




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