2005; Ley et al. 2006). Commensal bacteria, symbiotic bacteria (Bacteroides, Lactococcus), or pathogens such as
enteropathogenic
Escherichia
coli,
Pseudomonas
aeruginosa, Streptococcus, or Chlamydia are those most
often found (Ba ¨ckhed et al. 2005). Diversity and structure
of the microbiota reflect contrasting types of evolutionary
selection pressures and the nature of interacting organisms
(Ley et al. 2006).
Invertebrates (arthropods and nematodes) consist of phyla
including a large number of species, 1 million species
described for 4–5 million estimated for the single group of
insects (Novotny et al. 2002). Invertebrates harbor diverse
microbial communities, some of which contribute to their
development and sometimes play an important role in their
ability to adapt to extreme environments (Buchner 1965;
Nardon and Charles 2001). Most wood-feeding termites
depend on microbial partners (bacteria, fungi, protozoan
flagellates) to degrade cellulose and lignified compounds in
their intestinal tract (Brune 2003). More than 700 species of
bacteria belonging to different phyla (Spirochetes,
Bacteroidetes, Proteobacteria, Actinobacteria, Mycoplasma)
have been described in the gut of termites Reticulitermes
speratus and Macrotermes gilvus. This diversity can vary
from one termite type to another (Hongoh et al. 2006; Fall
et al. 2007). Drosophila, the fruit fly very familiar to
geneticists, hosts Proteobacteria (E. coli, Pseudomonas,
Sphingomonas), Bacteroidetes, and Mycoplasma (Mateos
et al. 2006). Besides the parasites (Plasmodium) or viruses
(dengue or Chikungunya) they transmit, mosquitoes belonging
to genera Anopheles and Aedes harbor many acetic acid bacteria including Asaia, Acetobacter, Gluconobacter, and
Sphingomonas (Favia et al. 2007) or even genera Aeromonas,
Acidovorax, Bacillus, Paenibacillus, Pseudomonas, and
Stenotrophomonas (Lindh et al. 2005).
Although little studied, worms also host microbes, especially bacteria. For example, filarial nematodes such as
Onchocerca volvulus (etiologic agent of onchocerciasis or
river blindness) and Brugia malayi (etiologic agent of lymphatic filariasis such as elephantiasis) shelter in ovarian
tissues the bacteria of genus Wolbachia that synthesize
polysaccharides in infected animals, which constitutes an
inflammatory factor aggravating disease (Saint Andre ´ et al.
2002; Taylor 2003). As it will be described later on,
the entomophagous nematodes (feeding on arthropods)
of the genera Steinernema and Heterorhabditis host
enterobacteria of the genera Xenorhabdus and
Photorhabdus, respectively, with which they establish
symbiotic associations (Forst et al. 1997).
11.4.2.2 Abundance and Location of Microbes
Microbial populations vary in size according to their intracellular or extracellular localization, the surface or inner organ
colonized, and depending on the host considered. These
numbers can reach relatively high values in certain organs
of human and animals. In healthy adult men, densities are
10
6–7 bacteria per cm
2 of skin (Roth and James 1988), 10
11–12
bacteria per ml in the colon (Whitman et al. 1998), and 10
14
bacteria in the entire intestinal tract. The microbes of the
human body account for 10 times the total number of somatic
and germ cells. The bacterial densities can greatly increase in
cases of bacteremia or sepsis.
Cytological observations showed that in some insects,
bacteria can also be localized in specialized cells called
bacteriocytes (or bacteriomes) and in oocytes, which
promotes vertical transmission from parents to offspring
(Baumann et al. 2000). This is the case of bacteria Buchnera
and Rickettsia in aphids or Wolbachia and Spiroplasma in
many Diptera and Hymenoptera (Fig. 11.13). The whitefly
Bemisia tabaci, the vector of viral pathogens of many plants,
can host in a single cell many different bacterial genera,
including Arsenophonus, Cardinium, Hamiltonella,
Portiera, Rickettsia, and Wolbachia (Gottlieb et al. 2006).
The bacteriocytes of the beetle Sitophilus oryzae
(Fig. 11.13), a pest of stored grain, contain an average of
10
3 bacteria per cell or 10
6 bacteria per insect (Heddi et al.
1999). Apart from these few fragmentary data on insect
models, investigations on the associated microbiota remain
very limited despite their important role in the adaptation
and development of invertebrates. A large effort is under
way to study the microbes of insects since many of the latter
are devastating pests of crops or vectors of pathogens that
cause serious diseases in plants, animals, and humans.
11.4.3 Types of Interactions Between
Microorganisms and Animals/Humans
Microbes that establish interactions with animals mostly
have environmental origins (air, water, soil, food), but
some of them move from one animal to another. An exemplary case of inter-animal microbial traffic is the transmission of the microbiota from mother to newborn. The vast
majority of microorganisms are harmless and some can even
be used as nutrients for animals. Fungivorous nematodes eat
fungi, while bacterivorous ones feed on bacteria, up to 10
6
bacterial cells per day (Blanc et al. 2006). In soils, this
predation may significantly alter the composition and structure of microbial communities and therefore the biological
functioning of soil, as microorganisms play an important
role in various processes such as decomposition of organic
matter, solubilization of phosphates, nitrogen-fixing symbiosis, or denitrification. Other microorganisms have spectacular effects on their host, such as light organ differentiation
in the squid Euprymna scolopes or manipulation of invertebrate reproduction by the bacterium Wolbachia.
11 Microorganisms and Biotic Interactions
423
enteropathogenic
Escherichia
coli,
Pseudomonas
aeruginosa, Streptococcus, or Chlamydia are those most
often found (Ba ¨ckhed et al. 2005). Diversity and structure
of the microbiota reflect contrasting types of evolutionary
selection pressures and the nature of interacting organisms
(Ley et al. 2006).
Invertebrates (arthropods and nematodes) consist of phyla
including a large number of species, 1 million species
described for 4–5 million estimated for the single group of
insects (Novotny et al. 2002). Invertebrates harbor diverse
microbial communities, some of which contribute to their
development and sometimes play an important role in their
ability to adapt to extreme environments (Buchner 1965;
Nardon and Charles 2001). Most wood-feeding termites
depend on microbial partners (bacteria, fungi, protozoan
flagellates) to degrade cellulose and lignified compounds in
their intestinal tract (Brune 2003). More than 700 species of
bacteria belonging to different phyla (Spirochetes,
Bacteroidetes, Proteobacteria, Actinobacteria, Mycoplasma)
have been described in the gut of termites Reticulitermes
speratus and Macrotermes gilvus. This diversity can vary
from one termite type to another (Hongoh et al. 2006; Fall
et al. 2007). Drosophila, the fruit fly very familiar to
geneticists, hosts Proteobacteria (E. coli, Pseudomonas,
Sphingomonas), Bacteroidetes, and Mycoplasma (Mateos
et al. 2006). Besides the parasites (Plasmodium) or viruses
(dengue or Chikungunya) they transmit, mosquitoes belonging
to genera Anopheles and Aedes harbor many acetic acid bacteria including Asaia, Acetobacter, Gluconobacter, and
Sphingomonas (Favia et al. 2007) or even genera Aeromonas,
Acidovorax, Bacillus, Paenibacillus, Pseudomonas, and
Stenotrophomonas (Lindh et al. 2005).
Although little studied, worms also host microbes, especially bacteria. For example, filarial nematodes such as
Onchocerca volvulus (etiologic agent of onchocerciasis or
river blindness) and Brugia malayi (etiologic agent of lymphatic filariasis such as elephantiasis) shelter in ovarian
tissues the bacteria of genus Wolbachia that synthesize
polysaccharides in infected animals, which constitutes an
inflammatory factor aggravating disease (Saint Andre ´ et al.
2002; Taylor 2003). As it will be described later on,
the entomophagous nematodes (feeding on arthropods)
of the genera Steinernema and Heterorhabditis host
enterobacteria of the genera Xenorhabdus and
Photorhabdus, respectively, with which they establish
symbiotic associations (Forst et al. 1997).
11.4.2.2 Abundance and Location of Microbes
Microbial populations vary in size according to their intracellular or extracellular localization, the surface or inner organ
colonized, and depending on the host considered. These
numbers can reach relatively high values in certain organs
of human and animals. In healthy adult men, densities are
10
6–7 bacteria per cm
2 of skin (Roth and James 1988), 10
11–12
bacteria per ml in the colon (Whitman et al. 1998), and 10
14
bacteria in the entire intestinal tract. The microbes of the
human body account for 10 times the total number of somatic
and germ cells. The bacterial densities can greatly increase in
cases of bacteremia or sepsis.
Cytological observations showed that in some insects,
bacteria can also be localized in specialized cells called
bacteriocytes (or bacteriomes) and in oocytes, which
promotes vertical transmission from parents to offspring
(Baumann et al. 2000). This is the case of bacteria Buchnera
and Rickettsia in aphids or Wolbachia and Spiroplasma in
many Diptera and Hymenoptera (Fig. 11.13). The whitefly
Bemisia tabaci, the vector of viral pathogens of many plants,
can host in a single cell many different bacterial genera,
including Arsenophonus, Cardinium, Hamiltonella,
Portiera, Rickettsia, and Wolbachia (Gottlieb et al. 2006).
The bacteriocytes of the beetle Sitophilus oryzae
(Fig. 11.13), a pest of stored grain, contain an average of
10
3 bacteria per cell or 10
6 bacteria per insect (Heddi et al.
1999). Apart from these few fragmentary data on insect
models, investigations on the associated microbiota remain
very limited despite their important role in the adaptation
and development of invertebrates. A large effort is under
way to study the microbes of insects since many of the latter
are devastating pests of crops or vectors of pathogens that
cause serious diseases in plants, animals, and humans.
11.4.3 Types of Interactions Between
Microorganisms and Animals/Humans
Microbes that establish interactions with animals mostly
have environmental origins (air, water, soil, food), but
some of them move from one animal to another. An exemplary case of inter-animal microbial traffic is the transmission of the microbiota from mother to newborn. The vast
majority of microorganisms are harmless and some can even
be used as nutrients for animals. Fungivorous nematodes eat
fungi, while bacterivorous ones feed on bacteria, up to 10
6
bacterial cells per day (Blanc et al. 2006). In soils, this
predation may significantly alter the composition and structure of microbial communities and therefore the biological
functioning of soil, as microorganisms play an important
role in various processes such as decomposition of organic
matter, solubilization of phosphates, nitrogen-fixing symbiosis, or denitrification. Other microorganisms have spectacular effects on their host, such as light organ differentiation
in the squid Euprymna scolopes or manipulation of invertebrate reproduction by the bacterium Wolbachia.
11 Microorganisms and Biotic Interactions
423