Download Host Tissues May Actively Respond to Beneficial Microbes

Host Tissues May Actively
Respond to Beneficial Microbes
Some bacteria are viewed as having a dynamic and positive, rather than
passive and harmful, relationship with their hosts
Richard P. Darveau, Margaret McFall-Ngai, Edward Ruby, Sam Miller, and
Dennis F. Mangan
ome bacteria help to maintain health
Crohns and Colitis Foundation of America, the
and otherwise benefit multicellular orUniversity of Washington School of Dentistry,
ganisms. Traditionally, microbioloSonicare/Phillips Oral Healthcare, Inc., and the
gists attributed these benefits to pasHack Estate. The workshop agenda and other
sive and nonspecific effects—arguing,
information may be viewed at http://www
for example, that a host’s resistance to further
.adminw.com/bmw/.
bacterial colonization reflects occupation of
During the workshop, participants reexammost niches on tissue surfaces by benign bacined current knowledge of the physical organiteria. Other metabolic activities, such as those
zation of host-associated microbial communithat produce acids, also help to preclude
ties, the role of bacteria in host development and
colonization by pathogens and thus discourage
metabolism, innate host defense recognition,
infections. Yet another traditional
mechanisms of probiotics, and
belief holds that, as long as nontheories of host-microbe coevolupathogenic bacteria remain in certion. Many scientists working in
The past
tain niches and cause no harm,
this area believe that the past parparadigm is
the host immune system tolerates
adigm is shifting, with bacteria beshifting, with
their presence and does not elimiing viewed as having active and
nate them. However, recent dedynamic rather than passive relabacteria being
velopments indicate that the retionships with their hosts. Thus,
viewed as
lationships between the usual
they say that using the term “comhaving active
microbiota and the host are more
mensal” to refer to nonpathogenic
and dynamic
active and specific than previously
microbiota of animals perpetuates
rather than
thought.
a misleading and naive view of the
passive
A group of about 100 scientists
complexity of these interactions.
with backgrounds in evolutionary
NIH officials will draw on these
relationships
biology, ecology, pathogenic midiscussions for guidance when
with their hosts
crobiology, immunology, or cell
planning new programs.
and developmental biology convened in October 2001 at the
Consortia in Oral Cavity Could Typify
“Beneficial Microbial Workshop,” which was
Those Found More Widely
cochaired by Sigmund Socransky of the Forsyth
The many and varied microbial associations
Institute, Boston, Mass., and Margaret McFallwithin the oral cavity include some that appear
Ngai of the University of Hawaii (UH) in Honocritical for maintaining health, according to
lulu. The workshop was sponsored by the NaSocransky. For instance, DNA probe studies of
tional Institute of Dental and Craniofacial
40 taxa of oral bacteria obtained from subginResearch (NIDCR) of the National Institutes of
gival plaque samples of healthy individuals and
Health (NIH), VSL Pharmaceuticals, Inc., the
S
Richard P. Darveau
is a Research Professor at the University of Washington School of
Dentistry, Seattle;
Margaret McFallNgai is a Professor
and Edward Ruby is
Professor at the
University of Hawaii; Sam Miller is
Professor at the
University of Washington, Seattle; and
Dennis F. Mangan
is Chief, Infectious
Diseases and Immunity Branch at
National Institute of
Dental and Craniofacial Research,
Bethesda.
186 Y ASM News / Volume 69, Number 4, 2003
others with periodontal disease indicate
FIGURE 1
that several bacterial taxa, including
those associated with gingival health,
co-occur in the periodontal crevice.
This group, designated the “green cluster,” includes Capnocytophaga species,
Campylobacter concisus, Eubacterium
nodatum, and Streptococcus constellatus. Although the host-associated consortium within the oral cavity is one of
the best described, it includes many asyet unculturable and thus poorly characterized bacteria.
Bacteria adhere to tooth surfaces to
form dental plaque, an oral host-associated microbial biofilm, according to
Paul Kolenbrander of NIDCR and
Richard Lamont of the University of
Washington, Seattle. Highly specific interactions occur between bacterial adhesin proteins and carbohydrate or protein receptors found on the surfaces of
other bacteria. For example, a 26-amino-acid fragment of the SspB polypeptide made by Streptococcus gordonii is
critical for its association with Porphyromonas gingivalis, another bacterium
in the oral cavity. These and other interactions form complex arrays of different oral bacteria, which are also
linked to tooth surfaces (Fig. 1).
Microbial symbioses, including those
Innate host defense status in clinically normal periodontal tissue. Clinically healthy tissue
with humans, fall within a larger evoluexpresses low levels of E-selectin and a gradient of IL-8 (represented in shades of red)
tionary context, according to McFallthat facilitates the transit of neutrophils through the tissue and into the gingival crevice
where they protect the host from infection. These mediators are made in response to a
Ngai. If anything, such intimate,
highly organized bacterial biofilm, termed dental plaque. This representation is adapted
coevolved relationships with microorfrom Whittaker et al. 1996 and Tonetti et al., 1994.
ganisms are the rule rather than the
exception among animals. Among vertebrates, for example, 8 of the 10 major organ
lying tissue are actively involved in maintaining
systems have components in which bacterial
a stable, long-term relationship.
cells may outnumber host cells.
However, biologists rarely focus on such aniFor instance, such metabolic cooperativity is
mal-bacterial associations, despite their wideextensive within the female reproductive tract,
spread occurrence and apparent importance.
according to Sharon Hillier of the Magee-WomThis comparative neglect in large part reflects
an’s Research Institute in Pittsburgh, Pa. Cells
the way in which scientific fields developed
within the vaginal epithelium convert glycogen
historically, according to McFall-Ngai. For
to glucose, thereby providing nutrients useful
instance, bacteriology separately emphasized
for beneficial, lactic acid-producing microorpathogenic and environmental microbiology,
ganisms. These bacteria employ a set of highly
while immunology tended to focus on issues
specific molecular mechanisms to optimize
such as nonself recognition. Thus none of these
growth, indicating that these microorganisms
fields provided a natural home for studying coare not passively occupying those niches.
evolved, beneficial associations of bacteria with
Rather, the microbial community and its underanimal hosts. Emerging shifts in these concep-
Volume 69, Number 4, 2003 / ASM News Y 187
Representative models of monospecific beneficial bacterial associations
with invertebrates
Host species
Extracellular
Euprymna scolopes (sepiolid squid)
Hirudo medicinalis (medicinal
leech)
Heterorhabditis bacteriophora
(entomophagous nematode)
Bugula neritina (bryozoan)
Intracellular
Glossina spp. (tsetse fly)
Sitophilus spp. (weevil)
Bankia setacea (shipworm)
Bacterial species
Function
Vibrio fischeri
Aeromonas veronii
Bioluminescence
Nutrition (?)
Photorhabdus
luminescens
Endobugula sertula
Predation/antibiosis
Sodalis glossinidiu
Unnamed symbiont
Unnamed symbiont
Nutrition
Nutrition
Cellulose and N2 fixation
Polyketide synthesis
tual frameworks, along with new techniques
and models for studying cooperative interactions,
promise to open this vast frontier of biology.
Microbial Cooperativity with Host Tissues
Can Be Critical
Species within mixed microbial communities
communicate more extensively with one another than was previously appreciated. Many of
these host-associated microbial consortia are
recognized as highly organized communities
within biofilms rather than mere collections of
microorganisms.
For instance, microorganisms within such
biofilms demonstrate a high order of physical
and physiological organization, and may be divided between areas of high and low biomass
interlaced with water-filled channels of different
sizes. Moreover, these channels can serve as
conduits, bringing oxygen to otherwise anoxic
microbial communities.
Such physical structures also allow diverse
bacterial species to benefit from their juxtaposition, thereby facilitating a physiological cooperativity not seen in mixed populations of planktonic microorganisms, according to John Breznak
at Michigan State University, East Lansing. He
and his colleagues determined that hydrogen gas
transfers between members of the interacting
microbial populations in termite hindguts require spatial relationships which lead to predictable patterns of species distributions.
Optimizing spatial relationships among different microbial species depends on mechanisms
for sensing and responding to environmental
cues, according to Peter Greenberg of the University of Iowa, Iowa City. Thus, microorgan-
188 Y ASM News / Volume 69, Number 4, 2003
isms in biofilms depend on quorum
sensing to regulate gene expression and
cell growth. His group and others are
providing evidence that this mechanism
contributes significantly to the maintenance of such highly organized microbial communities.
Microbial-Host Associations
Result in Normal Functions and
Morphology
Studies using germ-free or gnotobiotic
mice provide dramatic evidence that
“commensal” bacteria can directly participate in normal tissue development
and functions. For instance, components of the
host immune system, including Peyer’s patches,
the lamina propria, and the intraepithelial
space, do not develop unless ordinary microbiota are allowed to colonize the intestines of
gnotobiotic mice.
Recent developments in genomics-based technologies are enabling microbiologists to study
more subtle but dramatic molecular effects of
such bacteria on mammalian tissue development. For example, when bacteria colonize the
intestinal lumen of germ-free mice, host genes
encoding specific sugar modifications are activated, leading the mouse epithelium to change.
For instance, fucose sugars are added to glycolipids in these tissues. Thus, enteric bacteria can
play an active and specific role in host tissue
maturation steps.
Specific interactions can occur between nonmammalian hosts and associated bacteria.
Among invertebrates, interactions with bacteria
are often binary—that is, involving only one
microbial symbiont. Just as fruit flies and nematodes provide relatively simple models for
studying development, binary symbiotic associations with invertebrates offer valuable opportunities for manipulating and studying host-microbe relationships (Table 1).
Consider the luminous bacterium Vibrio fischeri. It induces the development of the lightemitting organ of the bobtail squid (Fig. 2). The
bacteria trigger a number of specific developmental events in the squid, including the loss of
an external layer of ciliated epithelium and an
increase in the density of microvilli in the internal bacteria-associated tissue, as reviewed by
Edward Ruby at UH.
FIGURE 2
Symbiosis between the Hawaiian sepiolid squid Euprymna scolopes (A) and the luminous bacterium Vibrio fischeri (B). V. fischeri cells
colonize the bilobed light-emitting organ (C) located in the mantle of the bobtail squid. The bacteria provide the host with light for its
counterillumination behavior during nocturnal feeding. A large population of the extracellular bacterial symbionts resides within the organ
in crypt spaces lined by microvillus epithelial tissue (D). (Images from M. McFall-Ngai.)
Ongoing studies using bacterial mutants and
measuring host gene expression are uncovering
some of the mechanisms underlying the molecular dialog that initiates this development program in the squid. Understanding the basis for
the bacteria-directed effects on development and
morphology in this and other associations may
well lead to the discovery of additional bacteria
that are involved in normal tissue and organ
functions in humans as well as other species.
Detecting Host-Associated
Microbial Communities
Advances in our understanding of how the innate host immune system detects and responds
to bacteria are providing another impetus for
reevaluating how cooperative bacterial associations affect health and development. Host responses to microorganisms vary considerably
and depend upon the structures with which they
interact, according to Brain Henderson of the
Eastman Dental Institute in London, U.K., who
provided an overview of this subject.
Host cells depend on several key pattern-recognition receptors, including lipopolysaccharide-binding protein (LBP), CD14, and the tolllike family of membrane receptors (TLR),
according to Henderson. For example, different
host TLRs recognize specific microbial components: thus, TLR2 recognizes lipoteichoic acid
Volume 69, Number 4, 2003 / ASM News Y 189
and lipoproteins; TLR4 intereacts with lipopolysaccharide (LPS); TLR5, with bacterial flagella; and TLR9, with bacterial DNA motifs. In
addition, each such TLR interaction appears to
trigger a distinct type of host response, activating different host cell-signaling pathways. Cellsurface expression of TLR may vary, as may
serum-soluble microbial recognition host components such as LBP and sCD14s. They further
contribute to the exquisite microbial specificity
of the innate host response system.
When this innate system detects particular
host-associated microbial communities, it can
profoundly influence specific tissues where those
communities form, according to Richard
Darveau of the University of Washington, Seattle. For instance, a host surveillance system
tracks gingival tissues. Even when those tissues
are healthy, they are bathed in low levels of
E-selectin, intracellular adhesion molecule
(ICAM), and interleukin-8 (IL-8) (Fig. 1).
Among them, IL-8 forms a gradient of expression that is greatest near the bacteria/epithelial
cell interface and decreases deeper into periodontal tissues.
These molecular findings are consistent with
other observations indicating that neutrophils
steadily stream through normally functioning,
healthy gingival tissue, collect in the gingival
crevice, and prevent oral infections. However,
this flow of neutrophils can be disrupted when
the ordinary periodontal microbiota changes. It
appears that normal bacterial colonization of
the periodontium serves to maintain this tissue
as an effective, disease-preventing host interface.
Probiotics Represent a Practical
Application of Microbial Communities
Probiotics represent one area in which bacterial
interactions with hosts are being put to practical
and even therapeutic use. Probiotics consist of
live bacterial infusions that are administered
orally for the treatment of inflammatory bowel
disease, according to Balfour Sartor of the University of North Carolina, Chapel Hill, and
Claudio Fiocchi of Case Western Reserve University in Cleveland, Ohio. Thus, probiotics are
administered to patients—replacing pathogenic
microbial communities with beneficial ones—as
one way of modulating the destructive host response that occurs in inflammatory bowel disease.
190 Y ASM News / Volume 69, Number 4, 2003
However, the mechanisms underlying probiotic action are far from well understood. For
example, host responses may depend on specific
microbial signature compounds, and outcomes
from infusing patients with live microorganisms
may depend on those microbes releasing specific
chemicals necessary for altering dysfunctional
molecular dialogues between host tissues and
resident microbiota. Inducing beneficial hostbacterial interactions could provide opportunities for discovering immunomodulating drugs.
Bacteria May Coevolve with their Hosts
Some microbe-host associations apparently
evolved through coordinate adaptations between the partners, according to Vaughn Cooper of the University of Michigan, Ann Arbor.
Coevolutionary theory predicts that the species
with the shorter generation time in such pairs
will undergo a greater amount of adaptive
change over time. Therefore, a dynamic dialogue between bacteria and their hosts suggests
that the symbionts are continually adapting to
the biochemical and genetic environment of the
individual hosts with which they are associated.
Such adaptations may not always be beneficial
to the host and could contribute to common
diseases such as inflammatory bowel disease and
periodontitis, conditions that include both human and microbial genetic components.
The human immune system may have evolved
not so much to recognize nonself components
such as bacteria, but rather conditions that signify “danger,” according to Polly Matzinger of
NIH. While this danger concept was proposed
to explain how autoimmune disorders can develop, it now is also being applied to explain the
role of the immune system in the context of
beneficial associations of the host with bacteria.
No longer are scientists constrained to viewing these bacteria as merely foreign entities that
human (or other animal) tissues learn to tolerate. Instead, under certain circumstances host
tissues may promote associations with bacteria
because those interactions benefit the host and
thus do not represent a danger. Indeed, a key
function of microbial pattern recognition receptors involved in innate immunity may be to
maintain healthy relationships with our ordinary microbiota and to promote mature tissue
development, rather than to prevent infections.
ACKNOWLEDGMENTS
Richard Darveau was supported by an IPA assignment to the NIDCR during the organization of this workshop.
SUGGESTED READING
Akira, S., K. Takeda, and T. Kaisho. 2001. Toll-like receptors: critical proteins linking innate and acquired immunity. Nature
Immunol. 2:675– 680.
Costerton, J. R., Z. Lewandowski, D. E. Caldwell, et al. 1995. Microbial biofilms. Annu. Rev. Microbiol. 49:711–745.
Darveau, R. P., A. Tanner, and R. C. Page. 1997. The microbial challenge in periodontitis. Periodontology 14:12–32.
Hooper, L. V., M. H. Wong, A. Thelin, L. Hansson, P. G. Falk, and J. I. Gordon. 2001. Molecular analysis of commensal
host-microbial relationships in the intestine. Science 291:881– 884.
Kroes, I., P. W. Lepp, and D. A. Relman. 1999. Bacterial diversity within the human subgingival crevice. Proc. Natl. Acad. Sci.
USA 96:14547–14552.
Leadbetter, J. R., T. M. Schmidt, J. R. Graber, and J. A. Breznak. 1999. Acetogenesis from H2 plus CO2 by spirochetes from
termite guts. Science 283:686 – 689.
McFall-Ngai, M. J. 1998. The development of cooperative associations between animals and bacteria: establishing détente
among domains. Am. Zool. 38:593– 608.
Ruby, E. G. 1999. The Euprymna scolopes-Vibrio fischeri symbiosis: a biomedical model for the study of bacterial
colonization of animal tissue. J. Mol. Microbiol. Biotechnol. 1:13–21.
Savage, D. C. 1997. Microbial ecology of the gastrointestinal tract. Annu. Rev. Microbiol. 31:107–133.
Socransky, S. S., A. D. Haffajee, M. A. Cugini, C. Smith, and R. L. Kent, Jr. 1998. Microbial complexes in subgingival plaque.
J. Clin. Periodontol. 25:134 –144.
Tonetti, M. S., M. A. Imboden, and N. P. Lang. 1998. Neutrophil migration into the gingival sulcus is associated with
transepithelial gradients of interleukin-8 and ICAM-1. J. Periodontol. 69:1139 –1147.
Whittaker, C. J., C. M. Klier, and P. E. Kolenbrander. 1996. Mechanisms of adhesion by oral bacteria. Annu. Rev. Microbiol.
50:513–552.
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