proapoptotic pathways and caspase-1 TNF-alpha (Rojas et al.
1999). Exemplary cases of apoptosis induced by microbes
involved pathogenic Shigella and intracellular Salmonella.
To escape from macrophages, these bacteria excrete
effectors via a secretion system directly into the cytoplasm
of the host cell (Mills et al. 1997). Injected toxins activate
caspase-1 and induce apoptosis, thus facilitating systemic
infection as demonstrated in mice (Monack et al. 1998).
Functional Duality: Inhibition and Activation
of Apoptosis
Many microbial pathogens can use pro- and antiapoptotic
activities to promote their replication and diffusion in the
host cells. This seemingly paradoxical behavior is found in
many viruses and some bacteria such as Chlamydia and
mycobacteria (Miyairi and Byrne 2006). It is suggested
that the antiapoptotic activity allows replication and generation of enough infectious entities in primo-infected host cells.
Then, apoptosis is activated in the latter stages of infection to
facilitate the spread of microbes to other surrounding host
cells. Cell tropism has been observed in Chlamydia that has a
functional duality: inhibition of apoptosis occurs primarily in
phagocytic cells in which infectious agents multiply, while
apoptosis is activated in immune response cells such as T cells
(Miyairi and Byrne 2006). The complex interactions that
occur during the modulation of apoptosis often involve the
expression of many genes. However, in some cases the product of a single gene of microbial origin may be at the origin of
apoptosis modulation. First shown in baculovirus (Clarke and
Clem 2003), the involvement of a single gene in the control of
apoptosis has been found in many other pathogens. For example, in Photorhabdus, the mcf gene alone is sufficient to
induce apoptosis in insect cells (Daborn et al. 2002). In a
similar way, the nuoG virulence gene in Mycobacterium
tuberculosis is sufficient to inhibit apoptosis of host cells
(Velmurugan et al. 2007).
11.4.5 Applications in the Interactions Between
Microorganisms and Animals
11.4.5.1 Microbes and Nutrition
Microbes play an important role in human and animal nutrition. Following the ingestion of food by animals, the natural
process of digestion through the action of enzymes leads to
the conversion of macromolecules into simple compounds
(sugars, amino acids) that can be assimilated and that are
involved in the production of the energy needed for growth
and development of organisms. However, we have seen that
through digestive enzymes eukaryotic cells are unable to
degrade certain macromolecules of plant origin, such as
cellulose or chitin. This function is then performed by
microbial communities (bacteria, fungi, protozoa) of the
digestive tract.
The products of metabolism of the microbiota also contribute to animal nutrition, such as amino acids and various
vitamins (B, C), whose deficiencies are the cause of many
diseases such as anemia or beriberi. Food industries use
microorganisms to produce these essential compounds that
can be incorporated as dietary supplements.
Finally, microbial fermentation has been used since
antiquity in the manufacture of dairy products (milk, yogurt,
cheese), but also in the production of alcoholic beverages
such as beer and wine, which excessive consumption may in
turn adversely affect human health.
11.4.5.2 Microbes and Health
In the field of human and animal health, antibiotics of bacterial origin are the major molecules used in treatments against
nosocomial and community-acquired infections. The group
of Actinobacteria, in particular, the genus Streptomyces, is
the major producer of antibiotics (glycosides, penicillins,
tetracyclines, cf. Chap. 9), antifungals (nystatin, amphotericin B), and immunosuppressants such as cyclosporine
used during organ transplantations. Some anticancer
compounds are also produced by microbes. Other microbial
molecules used for public health purposes include dextrans,
a substitute for blood plasma, numerous steroids for hormone treatments. Microbes themselves, once inactivated by
heat or ionizing radiation, can be used in immunization.
Many infectious diseases are transmitted by bloodsucking arthropods such as mosquitoes, ticks, and lice. The
absence of a vaccine against many of these diseases renders
indispensable the control of arthropod vectors. Spores of the
bacterium Bacillus thuringiensis have been used since the
1950s to fight against a large number of crop pests and
vectors of animal and human pathogens. However, vectors
increasingly develop resistance toward the products used,
and, on the one hand, the production of new chemical
compounds is hindered by the financial costs for research
and development and, on the other hand, by the increasing
risk linked to the toxicity of these products, both for human
and animal populations but also for the environment. In this
context, the exploitation of microbial communities as
biopesticides for vector control, in addition to genetic and
chemical agents, is a strategy globally promoted worldwide.
Knowledge of interaction mechanisms and effector
molecules of pathogens can lead to the development of
antagonistic molecules that block infection or spreading of
infectious agents. It should be recalled that the commensal
microflora that colonizes the surface of a given organ such as
the skin can prevent by simple physical barrier the colonization by pathogens.
436
Y. Moe ¨nne-Loccoz et al.
1999). Exemplary cases of apoptosis induced by microbes
involved pathogenic Shigella and intracellular Salmonella.
To escape from macrophages, these bacteria excrete
effectors via a secretion system directly into the cytoplasm
of the host cell (Mills et al. 1997). Injected toxins activate
caspase-1 and induce apoptosis, thus facilitating systemic
infection as demonstrated in mice (Monack et al. 1998).
Functional Duality: Inhibition and Activation
of Apoptosis
Many microbial pathogens can use pro- and antiapoptotic
activities to promote their replication and diffusion in the
host cells. This seemingly paradoxical behavior is found in
many viruses and some bacteria such as Chlamydia and
mycobacteria (Miyairi and Byrne 2006). It is suggested
that the antiapoptotic activity allows replication and generation of enough infectious entities in primo-infected host cells.
Then, apoptosis is activated in the latter stages of infection to
facilitate the spread of microbes to other surrounding host
cells. Cell tropism has been observed in Chlamydia that has a
functional duality: inhibition of apoptosis occurs primarily in
phagocytic cells in which infectious agents multiply, while
apoptosis is activated in immune response cells such as T cells
(Miyairi and Byrne 2006). The complex interactions that
occur during the modulation of apoptosis often involve the
expression of many genes. However, in some cases the product of a single gene of microbial origin may be at the origin of
apoptosis modulation. First shown in baculovirus (Clarke and
Clem 2003), the involvement of a single gene in the control of
apoptosis has been found in many other pathogens. For example, in Photorhabdus, the mcf gene alone is sufficient to
induce apoptosis in insect cells (Daborn et al. 2002). In a
similar way, the nuoG virulence gene in Mycobacterium
tuberculosis is sufficient to inhibit apoptosis of host cells
(Velmurugan et al. 2007).
11.4.5 Applications in the Interactions Between
Microorganisms and Animals
11.4.5.1 Microbes and Nutrition
Microbes play an important role in human and animal nutrition. Following the ingestion of food by animals, the natural
process of digestion through the action of enzymes leads to
the conversion of macromolecules into simple compounds
(sugars, amino acids) that can be assimilated and that are
involved in the production of the energy needed for growth
and development of organisms. However, we have seen that
through digestive enzymes eukaryotic cells are unable to
degrade certain macromolecules of plant origin, such as
cellulose or chitin. This function is then performed by
microbial communities (bacteria, fungi, protozoa) of the
digestive tract.
The products of metabolism of the microbiota also contribute to animal nutrition, such as amino acids and various
vitamins (B, C), whose deficiencies are the cause of many
diseases such as anemia or beriberi. Food industries use
microorganisms to produce these essential compounds that
can be incorporated as dietary supplements.
Finally, microbial fermentation has been used since
antiquity in the manufacture of dairy products (milk, yogurt,
cheese), but also in the production of alcoholic beverages
such as beer and wine, which excessive consumption may in
turn adversely affect human health.
11.4.5.2 Microbes and Health
In the field of human and animal health, antibiotics of bacterial origin are the major molecules used in treatments against
nosocomial and community-acquired infections. The group
of Actinobacteria, in particular, the genus Streptomyces, is
the major producer of antibiotics (glycosides, penicillins,
tetracyclines, cf. Chap. 9), antifungals (nystatin, amphotericin B), and immunosuppressants such as cyclosporine
used during organ transplantations. Some anticancer
compounds are also produced by microbes. Other microbial
molecules used for public health purposes include dextrans,
a substitute for blood plasma, numerous steroids for hormone treatments. Microbes themselves, once inactivated by
heat or ionizing radiation, can be used in immunization.
Many infectious diseases are transmitted by bloodsucking arthropods such as mosquitoes, ticks, and lice. The
absence of a vaccine against many of these diseases renders
indispensable the control of arthropod vectors. Spores of the
bacterium Bacillus thuringiensis have been used since the
1950s to fight against a large number of crop pests and
vectors of animal and human pathogens. However, vectors
increasingly develop resistance toward the products used,
and, on the one hand, the production of new chemical
compounds is hindered by the financial costs for research
and development and, on the other hand, by the increasing
risk linked to the toxicity of these products, both for human
and animal populations but also for the environment. In this
context, the exploitation of microbial communities as
biopesticides for vector control, in addition to genetic and
chemical agents, is a strategy globally promoted worldwide.
Knowledge of interaction mechanisms and effector
molecules of pathogens can lead to the development of
antagonistic molecules that block infection or spreading of
infectious agents. It should be recalled that the commensal
microflora that colonizes the surface of a given organ such as
the skin can prevent by simple physical barrier the colonization by pathogens.
436
Y. Moe ¨nne-Loccoz et al.