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Department of Biological Sciences, Mississippi State University, Mississippi State, MS 39762
2To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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KEY WORDS: • gastrointestinal • bacteria • diet • ecosystem • fiber • oligofructose • inulin
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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).
Although diet is known to influence the healthy intestine, much less is
known about the responses to diet during disease states. Diarrhea,
particularly chronic forms with associated enteritis, disturb
intestinal structure, functions and the resident bacteria, and often
lead to secondary systemic infections and malnutrition. The effect of
diarrhea is greatest for infants and young farm animals because they
suffer the highest incidences of morbidity and mortality. In light of
this, there is a need to develop more effective nutritional and
therapeutic approaches that will accelerate recovery of the
gastrointestinal tract (GIT).
The GIT of mammals has several regions that can be considered as
distinct habitats with assemblages of microorganisms that are adapted
to local physical, chemical and biotic characteristics. Although the
GIT meets the general tenets of an ecosystem, as proposed by
Tansley (1935)
, the use of the term ecosystem has been
questioned (Santini and Palka 1997
). Whether or not the
GIT is an ecosystem is not as important as the application of
ecological principles to understand the complex and poorly understood
interactions among dietary inputs, functions and resident organisms of
the GIT, and the implications to health. It is appropriate to view the
GIT as an ecological system and that by applying ecological principles,
a better understanding of the distributions and interactions of
organisms can be achieved.
The principles of stream ecology are appropriate and very relevant for determining whether dietary inputs, particularly fermentable fibers, can be used to manage the GIT ecosystem in health and for accelerating recovery from disease states such as diarrhea. The following sections will describe ecosystem concepts with an emphasis on river ecosystems and the similarities shared with the GIT of mammals. We focus on the physical and chemical features of stream systems and the GIT, and their relations with the biological components. Subsequent sections characterize disturbances of the GIT caused by diarrhea and how fermentable fibers may be useful for accelerating recovery. A concluding section presents our perspectives. We did not set out to provide an exhaustive review of the literature. Instead, we selected publications that will allow readers to become familiar with the relevant principles of ecology. It is our hope that readers will recognize that an interdisciplinary approach will be essential for understanding the complex relations between diet and the GIT ecosystem.
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Ecosystem concepts and river systems |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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, and there have been various refinements since then.
Ecosystems were originally described as including biotic and abiotic
components, which constitute structural elements, whereas the flow and
cycling of energy and matter that maintain the ecosystem are considered
functional elements (Aber and Melillo 1991
, Odum 1971
). Studies of ecosystems usually focus on either the
movement of energy and matter among the different components
(Odum 1971 and 1968
) or on the feeding behaviors of the
organisms (Pimm 1982
). Recent studies have revealed
interesting relations among the types (i.e., diversity), densities and
functional roles of organisms and the functional components of
ecosystems (Hooper and Vitousek 1997
, Schulze and Mooney 1994
, Tilman et al. 1997
).
River ecosystems are continua with a series of changes in structural
and functional elements (Cummins 1974
). This is
immediately obvious when headwaters of rivers are compared with the
eventual outflow into the ocean. The changes along the continuum result
in a series of broad divisions (e.g., upper, middle and lower
sections), with the changes between each division ranging from gradual
to very abrupt.
Usually the upper sections of rivers have high water velocity, are
heterotrophic and are well oxygenated. Many, but not all components of
the biota are dependent on allochthonous organic inputs (Wallace et al. 1997
). The wider middle regions tend to have slower
water movement, are autotrophic and have high species richness
(diversity is usually maximum here). The lower regions are again
heterotrophic and are characterized by large volumes of slowly moving
water with high sediment levels and low oxygen content; they have lower
species diversity than the mid-region.
The hydrologic regime is a critical determinant of the structure and
functions of a river ecosystem (Angradi 1997
,
Dynesius and Nilsson 1994
, Naiman and Decamps 1997
, Nilsson et al. 1991
). The disturbances
caused by floods create heterogeneity by slowing competitive exclusion
and by producing microhabitats for regeneration. Corresponding with
this, the frequency and magnitude of flooding are important factors
influencing stream ecosystems and the recovery time (Fisher et al. 1999
). Seasonal, tidal and other small floods are
considered necessary for maintaining the health and diversity of river
ecosystems (Dynesius and Nilsson 1994
, Nilsson et al. 1991
). Larger floods are less frequent, but have major
influences on ecosystem structure that effectively "reset" the
ecosystem back to an earlier successional stage (Angradi 1997
, Auble et al. 1994
, Toner and Keddy 1997
).
The water column of rivers is separated from the terrestrial ecosystem
by the riparian zone. This component of rivers can be considered as a
semipermeable membrane that regulates the exchange of materials between
aquatic and terrestrial ecosystems (Naiman and Decamps 1997
). The riparian zone harbors a diverse mixture of aquatic
and terrestrial characteristics and can act as a refuge for some
organisms during floods. However, the riparian zone is particularly
sensitive to environmental disturbances (Naiman and Decamps 1997
).
The ability of ecosystems to resist invasion by exotic (nonnative)
species is of great interest. Although invading exotic species can
reduce diversity, only rarely are native species completely removed or
replaced (Mooney and Drake 1986
, Planty-Tabacchi et al. 1996
). Interestingly, the very factors that are thought
to promote high diversity (e.g., frequent, small disturbances, moderate
resource limitation) make an ecosystem inherently more susceptible to
invasion (Lodge 1993
, Planty-Tabacchi et al. 1996
).
It is important from ecological and management perspectives to restore river ecosystems to the "natural state" after a disturbance. In general, ecological restoration is based on the maintenance and restoration of ecological processes. Returning the hydrologic state (rate and variability of flow) to "normal" is necessary for recovery of ecosystem structure and function. Thereafter, several strategies can be used to hasten the process for the resident organisms. Pesticides have been used infrequently if the recovery of a system is hindered by the invasion of an exotic species, but this management tool is considered as a last resort. More commonly, native species that have been suppressed or extirpated are reintroduced in an attempt to hasten their recovery. Another set of management tools includes manipulating the hydrology and adjusting chemical characteristics.
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The gastrointestinal tracts of mammals as "rivers" |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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The various GIT regions have different structural and functional elements that provide a continuum of habitat types. The differences between adjacent regions can be dramatic (stomach vs. small intestine) or gradual (e.g., jejunum vs. ileum). The upper reaches of the GIT river system originate at the pyloric sphincter, much the way many river systems originate from a lake or reservoir. Regulating the tonicity of the pyloric sphincter allows contents of the stomach to enter the intestine, much as dams regulate the flow of water into rivers.
Digesta move at a higher velocity in the proximal small intestine, just
as the headwaters of rivers do, and the digesta have a higher oxygen
content than those in more distal segments. Initially, composition of
the digesta is determined largely by dietary inputs and secretions from
the stomach, intestine, pancreas and gall bladder. As digesta proceed
distally, flow rates and oxygen content decline, and the composition
changes as a result of digestive processes (hydrolysis and absorption)
and microbial metabolism. The ileocolonic junction regulates the flow
of digesta into the colon (Faussone-Pellegrinni et al.
1993
), and when it is removed, such as in ileostomy patients,
digesta move faster. The contents of the colon move even more slowly,
have a higher dry matter content (i.e., suspended solids) and are
virtually anaerobic.
The mucosa of the GIT is much like the riparian zone of rivers in that
it effectively "traps" nutrients and transfers them to the
organism. It is effectively a semipermeable membrane that acts as an
interface that effectively regulates the exchange of materials between
the organism and the lumenal contents. The mucosa is metabolically very
active (Cant et al. 1996
) and influences the composition
of the digesta by its secretory and digestive functions. Furthermore,
the secretions and the proteins of the brush border membrane influence
the adherence and metabolic activities of bacteria (Kelly et al. 1994
). There is also regional variation in mucosal
architecture. The villi shorten from the proximal to distal small
intestine and are barely present in the colon. This influences the
amount of surface area available for digestion and bacterial
attachment, as well as the depth of the unstirred layer.
The biotic components.
The >400 different species of bacteria from numerous genera that can
be recovered from the GIT of most mammals include both resident species
and those that are transient (Simon and Gorbach 1986
).
On the basis of their interactions with the host and their metabolic
activities, the different bacteria can be further categorized into
those that are perceived as being beneficial and those that have the
potential of detrimentally influencing the host (Gibson and Roberfroid 1995
). Although the importance of the bacteria
resident in the GIT in health and disease is well recognized
(Simon and Gorbach 1987
), little is understood about the
complex interactions between the host and the bacteria, and the
implications for health and disease (Bry et al. 1996
).
Just as biotic components vary in the different segments of rivers
(horizontal zonation), the assemblages of bacteria differ among regions
of the GIT. Bacterial densities increase from the stomach to the colon.
As with any ecosystem, the distribution and abundance of organisms in
the GIT are not static. This is evident from the different densities
and metabolic activities of bacteria in subjects fed different diets
(Moore et al. 1987
) and in patients with short-bowel
syndrome (Kaneko et al. 1997
).
The qualitative and quantitative differences in bacteria resident in the different GIT regions probably reflect adaptation of bacteria to specific environmental conditions. This is evident from the increasing proximal to distal abundance of obligate anaerobes, corresponding with declining oxygen tensions and increasing redox potentials. The restriction of Helicobacter pylori to the gastric regions provides another example. Additionally, if the GIT is truly similar to a river, then the lower small intestine should have the highest species diversity, but not necessarily the highest density, and may be more susceptible to invasion by exotic (pathogenic) species. This interesting possibility has not been adequately explored.
In addition to the horizontal zonation of bacteria and other organisms
along the GIT, there are vertical gradients of species distribution.
The mucosa, like the riparian zone of rivers, provides an environment
that differs physically and chemically from the digesta in the lumen.
It is not surprising that bacterial populations associated with the
mucosa differ from those of the digesta. It is also recognized that the
bacteria associated with the mucosa are likely to have a greater
potential to influence the host than those present in the lumen
(Poxton et al. 1997
). Moreover, the adherent bacteria
influence mucosal and enterocyte architecture, the expression of genes
and processing of gene products (Bry et al. 1996
,
Hill and Cowley 1989
). Despite these interactions, much
less is known about the assemblages of mucosal bacteria that are
attached to the mucosa than those of the lumenal contents.
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Dietary management of the GIT ecosystem |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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). However, the
influences of diet are complex; they involve several possible
mechanisms of action, all of which are not yet well understood. Dietary
inputs can serve as substrates for the bacteria. Some components of the
diet can alter the assemblages of bacteria, just as adding certain
fertilizers to aquatic systems can change the composition and
interactions of the resident organisms. For example, some products of
the hydrolysis of casein stimulate the proliferation of bifidobacteria
(Poch and Bezkorovainy 1991
). Moreover, the fermentation
of dietary inputs by lactic acid bacteria results in short-chain
fatty acids and other metabolites that reduce pH and inhibit the growth
of many species of bacteria (Russell and Diez-Gonzalez 1998
). Other dietary components can reduce the ability of some
bacteria to adhere to enterocytes (Oyofo et al. 1988
). Alternatively, diet can indirectly influence the bacteria by modulating GIT structure and associated functions. The resulting changes in the physical and chemical characteristics of the environments in the different GIT regions can be expected to alter the densities, relative proportions and metabolic characteristics of the resident bacteria.
Fermentable fibers, such as oligofructose and inulin, selectively
increase the abundance of lactic acid bacteria while decreasing the
percentages of potential pathogens and putrefactive bacteria in several
species (Gibson and Wang 1994
, Wang and Gibson 1993
, Williams et al. 1994
). They also
influence the metabolic activities of the bacteria (Buddington et al. 1996
).
There is less known about the responses of GIT structure and associated
functions to fermentable fiber. Adding oligofructose and beet pulp,
which include fermentable components, to the diet of dogs resulted in
longer intestines with more surface area and greater mucosal mass
compared with the intestines of dogs fed a diet with cellulose, which
is poorly fermented (Buddington et al. 1999
). Moreover,
inclusion of fermentable fiber into a diet for dogs caused an increase
in rates of active glucose transport by the apical membrane
(McBurney et al. 1998
), and the responses were more
profound in the proximal small intestine (Buddington et al. 1999
). Similar responses occurred in mice fed diets with
oligofructose and inulin compared with those fed a diet with cellulose
(our unpublished data).
Because mammals are unable to digest fermentable fibers, the increases
in intestinal dimensions and functional capacities provide evidence for
interactions among the diet, the resident bacteria and GIT
characteristics. Recent findings indicate that bacterial fermentation
of fiber triggers the release of glucagon-like peptides 1 and 2,
gastric inhibitory peptide and possibly other enteric hormones.
These then stimulate mucosal growth and upregulation of transport
processes in the proximal intestine (McBurney et al. 1998
). Therefore, the GIT bacteria, much like many organisms in
other ecosystems, are able to modify their environment (Hill and Cowley 1990
). By doing so, they can act like the feedback
agents described for other ecosystems (Jones et al. 1994
, Pahl-Wostl 1995
). The GIT ecosystem is
unique in that the bacteria are able to trigger changes
"upstream" via neuroendocrine responses, which are particularly
evident in diarrhea.
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The impact of diarrhea on the GIT |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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).
Species with the shortest generation times recover faster after floods.
However, such species are often considered as "weeds" and are
less desirable. The same appears to be true for the GIT after diarrhea
(Oli et al. 1998
). Antibiotics also disturb the GIT
bacterial assemblages (Jackson et al. 1989
), and this
can affect the structure and functions of the mucosa. The changes in
the microenvironment can lead to the proliferation of some pathogens,
such as Clostridium difficile (Wilson 1993
).
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Dietary management of the GIT during recovery from diarrhea |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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There are two therapeutic strategies that do not involve antibiotics
that can be applied to accelerate the recovery of the GIT ecosystem and
restore the "normal" bacterial assemblages. The first uses
probiotics to "seed" the GIT with bacteria perceived as
beneficial and by doing so, competitively exclude pathogens
(Stavric et al. 1991
). The probiotic approach alters the
composition and metabolism of the GIT bacterial assemblages
(Djouzi et al. 1997
, Jiang and Saviano 1997
) and can alter the transfer of nutrients from the
intestine to the blood (Rychen and Nunes 1995
). However,
the benefits are transient, lasting only for as long as the time the
bacteria are administered. Usually, the probiotic bacteria are not able
to establish and maintain significant populations in the GIT. As a
result, they rapidly diminish after the probiotic is stopped, probably
due to competitive exclusion by species already present in the GIT and
adapted for existing conditions.
The prebiotic approach uses diet components to selectively encourage
the growth of beneficial species, and although it is not as frequently
used, this approach is gaining in popularity. The ability to
selectively encourage the proliferation of beneficial bacteria is well
established for oligofructose and inulin (Gibson et al. 1995
) as well as other fermentable fibers (e.g., lactosucrose;
Kumemura et al. 1994
). Recently, there has been interest
in using fermentable fiber as a management tool that will accelerate
recovery of the GIT during and after diarrhea. One of the desired
consequences of adding fermentable fiber to oral electrolyte solutions
is the decline in the relative abundance of potential pathogens, even
though they tend to recover faster after diarrhea (Oli et al. 1998
). Other potential benefits include production of
metabolites that are beneficial to the host (e.g., short-chain
fatty acids and vitamins) or reduce the growth of pathogens
(Wang and Gibson 1993
), and faster recovery of mucosal
mass and digestive capacities, possibly including immune functions. The
influence of fermentable fiber on the densities of nutrient
transporters may have profound clinical relevance. The apical
sodium-dependent glucose transporter, SGLT-1, appears to have a
dual function as water carrier (Loo et al. 1996
), and
this may be shared by transporters for other nutrients. Therefore, any
therapeutic approach that increases densities of nutrient transporters
may enhance rehydration.
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CONCLUSIONS AND PERSPECTIVES |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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). GIT characteristics are set by genetic
determinants that match digestive abilities with the natural diet. The
bacterial assemblages found in the mammalian GIT are also highly
variable and species specific. For example, bifidobacteria are not
present at significant densities in all mammals (Buddington and Sunvold 1998
). Furthermore, the ability of different species of
bifidobacteria to bind to enterocytes varies among species of mammals.
This may influence the competitive interactions between beneficial and
pathogenic bacteria (Bernet et al. 1993
).
Even the bacteria present in GIT ecosystems of closely related
individuals can differ (Hinton and Linton 1987
), just as
rivers differ in abiotic and biotic characteristics, even when they are
located in the same geographical area. As a consequence, a successful
management tool for one species, or even individual, may not provide
the same benefits for another. Therefore, it can be predicted that
adding fermentable fiber to the diet will cause varying responses among
individuals, species, life history stages and states of health.
In clinical settings, it is difficult to obtain samples from the
various regions of the GIT, and diagnoses are generally limited to
bacteriologic analyses of stool samples. However, bacterial populations
and metabolism vary spatially along the GIT, and the interactions that
occur in proximal regions of the GIT (e.g., small intestine) among the
bacteria, the host and dietary inputs may have a greater effect on
health than what is evident from stool samples. For example, bacterial
responses to fermentable fiber appear to be greater in the proximal
bowel than in the colon (McBain and MacFarlane 1997
).
Corresponding with this, we have found that the influences of
oligofructose on bacterial populations are more pronounced in the small
intestine and proximal colon of suckling pigs compared with fecal
samples (unpublished data). As a consequence, stool samples, like the
water draining into the ocean, can provide only limited insights about
events and processes occurring "upstream" in more proximal
regions of the GIT. Therefore, just as ecologists use key indicators to
monitor ecosystems, there is a need to identify species of bacteria or
other factors that can be examined in stool samples and that will
provide insights about events and processes throughout the GIT.
The "age" of an ecosystem also influences responses to floods and
management strategies. It is uncertain if the age of the GIT is
similarly an important determinant of the responses to dietary inputs.
During the first weeks and months after birth, digestive functions
mature (e.g., onset of gastric secretion, changes in rates of
absorption for various nutrients), which alters GIT microenvironments
and coincides with shifts in the composition of the bacterial
assemblages (Swords et al. 1993
). Although infants are
at greater risk from complications caused by diarrhea than adults, it
is unknown if this applies to the developing GIT.
If more effective management tools and protocols (e.g., prebiotics, probiotics, synbiotics or antibiotics) are to be developed, future research must be directed at answering several questions. These include, but are not restricted to, identifying the key environmental factors that control the composition of the GIT microbiotic community, determining species diversity along the GIT and the relations with stability and resistance to invasion, and understanding which bacterial species should be managed to promote optimal health and where in the GIT such management strategies should be targeted. Although the lactic acid bacteria are considered as beneficial, it is of importance to decide which species or strains provide the most benefits for various hosts and what the benefits are. It is our contention that applying relevant ecological theory for river systems to the GIT will facilitate understanding about how fermentable fibers and other dietary components can be used to manage the GIT in health and disease, as well as to define potential limitations.
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONEcosystem concepts and river...The gastrointestinal tracts of...Dietary management of the...The impact of diarrhea...Dietary management of the...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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